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  [Illustration: Photo, Mt. Wilson Solar Observatory

  _An Active Prominence of the Sun, 140,000 Miles High,
  photographed July 9, 1917._]




  ASTRONOMY

  THE SCIENCE OF THE HEAVENLY BODIES

  BY

  DAVID TODD

  DIRECTOR EMERITUS, AMHERST COLLEGE OBSERVATORY

  [Illustration: Harper & Brothers logo]

  NEW YORK AND LONDON
  HARPER & BROTHERS
  PUBLISHERS MCMXXII




  Copyright 1922

  BY P. F. COLLIER & SON COMPANY


Sir William Rowan Hamilton, the eminent mathematician of Dublin, has, of
all writers ancient and modern, most fittingly characterized the ideal
science of astronomy as man's golden chain connecting the heavens to the
earth, by which we "learn the language and interpret the oracles of the
universe."

The oldest of the sciences, astronomy is also the broadest in its
relations to human knowledge and the interests of mankind. Many are the
cognate sciences upon which the noble structure of astronomy has been
erected: foremost of all, geometry and the higher mathematics, which
tell us of motions, magnitudes and distances; physics and chemistry, of
the origin, nature, and destinies of planets, sun, and star;
meteorology, of the circulation of their atmospheres; geology, of the
structure of the moon's surface; mineralogy, of the constitution of
meteorites; while, if we attack, even elementally, the fascinating,
though perhaps forever unsolvable, problem of life in other worlds, the
astronomer must invoke all the resources that his fellow biologists and
their many-sided science can afford him.

The progress of astronomy from age to age has been far from
uniform--rather by leaps and bounds: from the earliest epoch when man's
planet earth was the center about which the stupendous cosmos wheeled,
for whom it was created, and for whose edification it was
maintained--down to the modern age whose discoveries have ascertained
that even our stellar universe, the vast region of the solar domain, is
but one of the thousands of island universes that tenant the
inconceivable immensities of space.

Such results have been attainable only through the successful
construction and operation of monster telescopes that bring to the eye
and visualize on photographic plates the faintest of celestial objects
which were the despair of astronomers only a few years ago.

But the end is not yet; astronomy to-day is but passing from infancy to
youth. And with new and greater telescopes, with new photographic
processes of higher sensitivity, with the help of modern invention in
overcoming the obstacle of the air--that constant foe of the
astronomer--who will presume to set down any limit to the leaps and
bounds of astronomy in the future?

So rapid, indeed, has been the progress of astronomy in very recent
years that the present is especially favorable for setting forth its
salient features; and this book is an attempt to present the wide range
of astronomy in readable fashion, as if a story with a definite plot,
from its origin with the shepherds of ancient Chaldea down to
present-day ascertainment of the actual scale of the universe, and
definite measures of the huge volume of supersolar giants among the
stars.

                                                          DAVID TODD
  AMHERST COLLEGE OBSERVATORY
  November, 1921




CONTENTS


   CHAPTER                                                      PAGE

        I. ASTRONOMY A LIVING SCIENCE                              9

       II. THE FIRST ASTRONOMERS                                  19

      III. PYRAMID, TOMB, AND TEMPLE                              23

       IV. ORIGIN OF GREEK ASTRONOMY                              27

        V. MEASURING THE EARTH--ERATOSTHENES                      30

       VI. PTOLEMY AND HIS GREAT BOOK                             33

      VII. ASTRONOMY OF THE MIDDLE AGES                           37

     VIII. COPERNICUS AND THE NEW ERA                             42

       IX. TYCHO, THE GREAT OBSERVER                              45

        X. KEPLER, THE GREAT CALCULATOR                           49

       XI. GALILEO, THE GREAT EXPERIMENTER                        53

      XII. AFTER THE GREAT MASTERS                                57

     XIII. NEWTON AND MOTION                                      62

      XIV. NEWTON AND GRAVITATION                                 66

       XV. AFTER NEWTON                                           73

      XVI. HALLEY AND HIS COMET                                   83

     XVII. BRADLEY AND ABERRATION                                 90

    XVIII. THE TELESCOPE                                          93

      XIX. REFLECTORS--MIRROR TELESCOPES                         102

       XX. THE STORY OF THE SPECTROSCOPE                         111

      XXI. THE STORY OF ASTRONOMICAL PHOTOGRAPHY                 125

     XXII. MOUNTAIN OBSERVATORIES                                139

    XXIII. THE PROGRAM OF A GREAT OBSERVATORY                    152

     XXIV. OUR SOLAR SYSTEM                                      162

      XXV. THE SUN AND OBSERVING IT                              165

     XXVI. SUN SPOTS AND PROMINENCES                             174

    XXVII. THE INNER PLANETS                                     189

   XXVIII. THE MOON AND HER SURFACE                              193

     XXIX. ECLIPSES OF THE MOON                                  206

      XXX. TOTAL ECLIPSES OF THE SUN                             209

     XXXI. THE SOLAR CORONA                                      219

    XXXII. THE RUDDY PLANET                                      227

   XXXIII. THE CANALS OF MARS                                    235

    XXXIV. LIFE IN OTHER WORLDS                                  242

     XXXV. THE LITTLE PLANETS                                    254

    XXXVI. THE GIANT PLANET                                      260

   XXXVII. THE RINGED PLANET                                     264

  XXXVIII. THE FARTHEST PLANETS                                  267

    XXXIX. THE TRANS-NEPTUNIAN PLANET                            270

       XL. COMETS--THE HAIRY STARS                               273

      XLI. WHERE DO COMETS COME FROM?                            279

     XLII. METEORS AND SHOOTING STARS                            283

    XLIII. METEORITES                                            290

     XLIV. THE UNIVERSE OF STARS                                 294

      XLV. STAR CHARTS AND CATALOGUES                            300

     XLVI. THE SUN'S MOTION TOWARD LYRA                          304

    XLVII. STARS AND THEIR SPECTRAL TYPE                         307

   XLVIII. STAR DISTANCES                                        311

     XLIX. THE NEAREST STARS                                     319

        L. ACTUAL DIMENSIONS OF THE STARS                        321

       LI. THE VARIABLE STARS                                    324

      LII. THE NOVÆ, OR NEW STARS                                331

     LIII. THE DOUBLE STARS                                      334

      LIV. THE STAR CLUSTERS                                     336

       LV. MOVING CLUSTERS                                       341

      LVI. THE TWO STAR STREAMS                                  345

     LVII. THE GALAXY OR MILKY WAY                               350

    LVIII. STAR CLOUDS AND NEBULÆ                                357

      LIX. THE SPIRAL NEBULÆ                                     361

       LX. COSMOGONY                                             366

      LXI. COSMOGONY IN TRANSITION                               380




LIST OF ILLUSTRATIONS


  ACTIVE PROMINENCE OF THE SUN, 140,000 MILES HIGH    _Frontispiece_
                                                         FACING PAGE
  NICHOLAS COPERNICUS                                             64

  GALILEO GALILEI                                                 64

  JOHANN KEPLER                                                   65

  SIR ISAAC NEWTON                                                65

  THE HUNDRED-INCH REFLECTING TELESCOPE AT MOUNT WILSON           96

  THE FORTY-INCH REFRACTING TELESCOPE, YERKES OBSERVATORY         96

  150-FOOT TOWER, MOUNT WILSON, A DIAGRAM OF TOWER AND PIT        97

  150-FOOT TOWER--EXTERIOR VIEW                                   97

  VIEW LOOKING DOWN INTO THE PIT BENEATH 150-FOOT TOWER           97

  MOUNT WILSON SOLAR OBSERVATORY--THE 100-FOOT DOME              128

  MOUNT CHIMBORAZO, THE BEST SITE IN THE WORLD FOR
     AN OBSERVATORY                                              128

  LICK OBSERVATORY, MOUNT HAMILTON, CALIFORNIA                   129

  PHOTOGRAPHING WITH THE 40-INCH REFRACTOR                       129

  GREAT SUNSPOT GROUP OF AUGUST 8, 1917                          160

  CALCIUM FLOCCULI ON THE SUN                                    161

  ECLIPSE OF THE MOON, WITH THE LUNAR SURFACE VISIBLE            161

  MOON'S SURFACE IN THE REGION OF COPERNICUS                     192

  SOUTH CENTRAL PORTION OF THE MOON, AT LAST QUARTER             193

  CORONA OF THE SUN DURING AN ECLIPSE                            224

  VENUS, IN THE CRESCENT PHASE                                   225

  MARS, SHOWING BRIGHT POLAR CAP                                 225

  JUPITER, THE GIANT PLANET                                      256

  NEPTUNE AND ITS SATELLITES                                     256

  SATURN, WITH EDGE OF RINGS ONLY IN VIEW                        257

  SATURN, WITH RINGS DISPLAYED TO FULLEST EXTENT                 257

  TWO VIEWS OF HALLEY'S COMET                                    288

  SWIFT'S COMET, WHICH SHOWED REMARKABLE TRANSFORMATIONS         288

  METEOR TRAIL IN FIELD WITH FINE NEBULÆ                         289

  RING NEBULA IN LYRA                                            320

  DUMB-BELL NEBULA                                               321

  STAR CLOUDS AND BLACK HOLES IN SAGITTARIUS                     352

  GREAT NEBULA IN ANDROMEDA                                      353




CHAPTER I

ASTRONOMY A LIVING SCIENCE


Like life itself we do not know when astronomy began; we cannot conceive
a time when it was not. Man of the early stone age must have begun to
observe sun, moon, and stars, because all the bodies of the cosmos were
there, then as now. With his intellectual birth astronomy was born.

Onward through the childhood of the race he began to think on the things
he observed, to make crude records of times and seasons; the Chaldeans
and Chinese began each their own system of astronomy, the causes of
things and the reasons underlying phenomena began to attract attention,
and astronomy was cultivated not for its own sake, but because of its
practical utility in supplying the data necessary to accurate
astrological prediction. Belief in astrology was universal.

The earth set in the midst of the wonders of the sky was the reason for
it all. Clearly the earth was created for humanity; so, too, the heavens
were created for the edification of the race. All was subservient to
man; naturally all was geocentric, or earth-centered. From the savage
who could count only to five, the digits of one hand, civilized man very
slowly began to evolve; he noted the progress of the seasons; the old
records of eclipses showed Thales, an early Greek, how to predict their
happenings, and true science had its birth when man acquired the power
to make forecasts that always came true.

Few ancient philosophers were greater than Pythagoras, and his
conceptions of the order of the heavens and the shape and motion of the
earth were so near the truth that we sometimes wonder how they could
have been rejected for twenty centuries. We must remember, however, that
man had not yet learned the art of measuring things, and the world could
not be brought into subjection to him until he had. To measure he must
have tools--instruments; to have instruments he must learn the art of
working in metals, and all this took time; it was a slow and in large
part imperceptible process; it is not yet finished.

The earliest really sturdy manifestation of astronomical life came with
the birth of Greek science, culminating with Aristarchus, Hipparchus and
Ptolemy. The last of these great philosophers, realizing that only the
art of writing prevents man's knowledge from perishing with him, set
down all the astronomical knowledge of that day in one of the three
greatest books on astronomy ever written, the Almagest, a name for it
derived through the Arabic, and really meaning "the greatest."

The system of earth and heaven seemed as if finished, and the authority
of Ptolemy and his Almagest were as Holy Writ for the unfortunate
centuries that followed him. With fatal persistence the fundamental
error of his system delayed the evolutionary life of the science through
all that period.

But man had begun to measure. Geometry had been born and Eratosthenes
had indeed measured the size of the earth. Tools in bronze and iron
were fashioned closely after the models of tools of stone; astrolabes
and armillary spheres were first built on geometric spheres and circles;
and science was then laid away for the slumber of the Dark Ages.

Nevertheless, through all this dreary period the life of the youthful
astronomical giant was maintained. Time went on, the heavens revolved;
sun, moon, and stars kept their appointed places, and Arab and Moor and
the savage monarchs of the East were there to observe and record, even
if the world-mind was lying fallow, and no genius had been born to
inspire anew that direction of human intellect on which the later growth
of science and civilization depends. With the growth of the collective
mind of mankind, from generation to generation, we note that ordered
sequence of events which characterizes the development of astronomy from
earliest peoples down to the age of Newton, Herschel, and the present.
It is the unfolding of a story as if with a definite plot from the
beginning.

Leaving to philosophical writers the great fundamental reason underlying
the intellectual lethargy of the Dark Ages, we only note that astronomy
and its development suffered with every other department of human
activity that concerned the intellectual progress of the race. To
knowledge of every sort the medieval spirit was hostile. But with the
founding and growth of universities, a new era began. The time was ripe
for Copernicus and a new system of the heavens. The discovery of the New
World and the revival of learning through the universities added that
stimulus and inspiration which marked the transition from the Middle
Ages to our modern era, and the life of astronomy, long dormant, was
quickened to an extraordinary development.

It fell to the lot of Copernicus to write the second great book on
astronomy, "De Revolutionibus Orbium Coelestium." But the new
heliocentric or sun-centered system of Copernicus, while it was the true
system bidding fair to replace the false, could not be firmly
established except on the basis of accurate observation.

How fortunate was the occurrence of the new star of 1572, that turned
the keen intellect of Tycho Brahe toward the heavens! Without the
observational labors of Tycho's lifetime, what would the mathematical
genius of Kepler have availed in discovery of his laws of motion of the
planets?

Historians dwell on the destruction and violent conflicts of certain
centuries of the Middle Ages, quite overlooking the constructive work in
progress through the entire era. Much of this was of a nature absolutely
essential to the new life that was to manifest itself in astronomy. The
Arabs had made important improvements in mathematical processes,
European artisans had made great advances in the manufacture of glass
and in the tools for working in metals.

Then came Galileo with his telescope revealing anew the universe to
mankind. It was the north of Italy where the Renaissance was most
potent, recalling the vigorous life of ancient Greece. Copernicus had
studied here; it was the home of Galileo. Columbus was a Genoese, and
the compass which guided him to the Western World was a product of deft
Italian artisans whose skill with that of their successors was now
available to construct the instruments necessary for further progress in
the accurate science of astronomical observation. Even before
Copernicus, Johann Müller, better known as Regiomontanus, had imbibed
the learning of the Greeks while studying in Italy, and founded an
observatory and issued nautical almanacs from Nuremberg, the basis of
those by which Columbus was guided over untraversed seas.

About this time, too, the art of printing was invented, and the
interrelation of all the movements then in progress led up to a general
awakening of the mind of man, and eventually an outburst in science and
learning, which has continued to the present day. Naturally it put new
life into astronomy, and led directly up from Galileo and his
experimental philosophy to Newton and the _Principia_, the third in the
trinity of great astronomical books of all time.

To get to the bottom of things, one must study intimately the history of
the intellectual development of Europe through the fifteenth and
sixteenth centuries. Many of the western countries were ruled by
sovereigns of extraordinary vigor and force of character, and their
activities tended strongly toward that firm basis on which the
foundations of modern civilization were securely laid.

Contemporaneously with this era, and following on through the
seventeenth century, came the measurements of the earth by French
geodesists, the construction of greater and greater telescopes and the
wonderful discoveries with them by Huygens, Cassini, and many others.

Most important of all was the application of telescopes to the
instruments with which angles are measured. Then for the first time man
had begun to find out that by accurate measures of the heavenly bodies,
their places among the stars, their sizes and distances, he could attain
to complete knowledge of them and so conquer the universe.

But he soon realized the insufficiency of the mathematical tools with
which he worked--how unsuited they were to the solution of the problem
of three bodies (sun, earth, and moon) under the Newtonian law of
gravitation, let alone the problem of n-bodies, mutually attracting each
the other; and every one perturbing the motion of every other one. So
the invention of new mathematical tools was prosecuted by Newton and his
rival Leibnitz, who, by the way, showed himself as great a man as
mathematician: "taking mathematics," wrote Leibnitz, "from the beginning
of the world to the times when Newton lived, what he had done was much
the better half." Newton was the greatest of astronomers who, since the
revival of learning, had observed the motions of the heavenly bodies and
sought to find out why they moved.

Copernicus, Tycho Brahe, Galileo, Kepler, Newton, all are bound together
as in a plot. Not one of them can be dissociated from the greatest of
all discoveries. But Newton, the greatest of them all, revealed his
greatness even more by saying: "If I have seen further than other men,
it is because I have been standing on the shoulders of giants."
Elsewhere he says: "All this was in the two plague years of 1665 and
1666 [he was then but twenty-four], for in those days I was in the prime
of my age for invention, and minded mathematics and philosophy more than
at any time since." All school children know these as the years of the
plague and the fire; but very few, in school or out, connect these years
with two other far-reaching events in the world's history, the
invention of the infinitesimal calculus and the discovery of the law of
gravitation.

We have passed over the name of Descartes, almost contemporary with
Galileo, the founder of modern dynamics, but his initiation of one of
the greatest improvements of mathematical method cannot be overlooked.
This era was the beginning of the Golden Age of Mathematics that
embraced the lives of the versatile Euler, equally at home in dynamics
and optics and the lunar theory; of La Grange, author of the elegant
"Mécanique Analytique"; and La Place, of the unparalleled "Mécanique
Céleste." With them and a fully elaborated calculus Newton's universal
law had been extended to all the motions of the cosmos. Even the tides
and precession of the equinoxes and Bradley's nutation were accounted
for and explained. Mathematical or gravitational astronomy had attained
its pinnacle--it seemed to be a finished science: all who were to come
after must be but followers.

The culmination of one great period, however, proved to be but the
inception of another epoch in the development of the living science.

The greatest observer of all time, with a telescope built by his own
hands, had discovered a great planet far beyond the then confines of the
solar system. Mathematicians would take care of Uranus, and Herschel was
left free to build bigger telescopes still, and study the construction
of the stellar universe. Down to his day astronomy had dealt almost
wholly with the positions and motions of the celestial bodies--astronomy
was a science of _where_. To inquire _what_ the heavenly bodies _are_,
seemed to Herschel worthy of his keenest attention also. While "a
knowledge of the construction of the heavens has always been the
ultimate object of my observations," as he said, and his ingenious
method of star-gauging was the first practicable attempt to investigate
the construction of the sidereal universe, he nevertheless devoted much
time to the description of nebulæ and their nature, as well as their
distribution in space. He was the founder of double-star astronomy, and
his researches on the light of the stars by the simple method of
sequences were the inception of the vast fields of stellar photometry
and variable stars. The physics of the sun, also, was by no means
neglected; and his lifework earned for him the title of father of
descriptive astronomy.

While progress and discovery in the earlier fields of astronomy were
going on, the initial discoveries in the vast group of small planets
were made at the beginning of the nineteenth century. The great Bessel
added new life to the science by revolutionizing the methods and
instruments of accurate observation, his work culminating in the measure
of the distance of 61 Cygni, first of all the stars whose distance from
the sun became known.

Wonderful as was this achievement, however, a greater marvel still was
announced just before the middle of the century--a new planet far beyond
Uranus, whose discovery was made as a direct result of mathematical
researches by Adams and Le Verrier, and affording an extraordinary
verification of the great Newtonian law. These were the days of great
discoveries, and about this time the giant of all the astronomical tools
of the century was erected by Lord Rosse, the "Leviathan" reflector with
a speculum six feet in diameter, which remained for more than half a
century the greatest telescope in the world, and whose epochal
discovery of spiral nebulæ has greater significance than we yet know or
perhaps even surmise.

The living science was now at the height of a vigorous development, when
a revolutionary discovery was announced by Kirchhoff which had been
hanging fire nearly half a century--the half century, too, which had
witnessed the invention of photography, the steam engine, the railroad,
and the telegraph: three simple laws by which the dark absorption lines
of a spectrum are interpreted, and the physical and chemical
constitution of sun and stars ascertained, no matter what their distance
from us.

Huggins in England and Secchi in Italy were quick to apply the discovery
to the stars, and Draper and Pickering by masterly organization have
photographed and classified the spectra of many hundred thousand stars
of both hemispheres, a research of the highest importance which has
proved of unique service in studies of stellar movements and the
structure of the universe by Eddington and Shapley, Campbell and
Kapteyn, with many others who are still engaged in pushing our knowledge
far beyond the former confines of the universe.

Few are the branches of astronomy that have not been modified by
photography and the spectroscope. It has become a measuring tool of the
first order of accuracy; measuring the speed of stars and nebulæ toward
and from us; measuring the rotational speed of sun and planets, corona
and Saturnian ring; measuring the distances of whole classes of stars
from the solar system; measuring afresh even the distance of the
sun--the yardstick of our immediate universe; measuring the drift of the
sun with his entire family of planets twelve miles every second in the
direction of Alpha Lyræ; and discovering and measuring the speed of
binary suns too close together for our telescopes, and so making real
the astronomy of the invisible.

Impatient of the handicap of a turbulent atmosphere, the living science
has sought out mountain tops and there erected telescopes vastly greater
than the "Leviathan" of a past century. There the sun in every detail of
disk and spectrum is photographed by day, and stars with their spectra
and the nebulæ by night. Great streams of stars are discovered and the
speed and direction of their drift ascertained. The marvels of the
spiral nebulæ are unfolded, their multitudinous forms portrayed and
deciphered.

And their distances? And the distances of the still more wonderful
clusters? Far, inconceivably far beyond the Milky Way. And are they
"island universes"? And can man, the measurer, measure the distance of
the "mainland" beyond?




CHAPTER II

THE FIRST ASTRONOMERS


Who were the first astronomers? And who wrote the first treatise on
astronomy, oldest of the sciences?

Questions not easy to answer in our day. With the progress of
archæological research, or inquiry into the civilization and monuments
of early peoples, it becomes certain that man has lived on this planet
earth for tens of thousands of years in the past as an intelligent,
observing, intellectual being; and it is impossible to assign any time
so remote that he did not observe and philosophize upon the firmament
above.

We can hardly imagine a people so primitive that they would fail to
regard the sun as "Lord of the Day," and therefore all important in the
scheme of things terrestrial. Says Anne Bradstreet of the sun in her
"Contemplations":

    What glory's like to thee?
    Soul of this world, this universe's eye,
    No wonder some made thee deity.

To the Babylonians belongs the credit of the oldest known work on
astronomy. It was written nearly six thousand years ago, about B. C.
3800, by their monarch Sargon the First, King of Agade. Only the merest
fragments of this historic treatise have survived, and they indicate the
reverence of the Babylonians for the sun. Another work by Sargon is
entitled "Omens," which shows the intimate relationship of astronomy to
mysticism and superstitious worship at this early date, and which
persists even at the present day.

As remotely as B. C. 3000, the sun-god Shamash and his wife Aya are
carved upon the historic cylinders of hematite and lapis lazuli, and one
of the oldest designs on these cylinders represents the sun-god coming
out of the Door of Sunrise, while a porter is opening the Gate of the
East. The Semitic religion had as its basis a reverence for the bodies
of the sky; and Samson, Hebrew for sun, was probably the sun-god of the
Hebrews. The Phoenician deity, Baal, was a sun-god under differing
designations; and at the epoch of the Shepherd Kings, about B. C. 1500,
during the Hyksos dynasty, the sun-god was represented by a circle or
disk with extended rays ending in hands, possibly the precursor of the
frequently recurring Egyptian design of the winged disk or winged solar
globe. Hittites, Persians, and Assyrians, as well as the Phoenicians,
frequently represented the sun-god in similar fashion in their sacred
glyphs or carvings.

For a long period in early human history, astronomy and astrology were
pretty much the same. We can trace the history of astrology back as far
as B. C. 3000 in ancient Babylonia. The motions of the sun, moon, and
the five lucid planets of that time indicated the activity of the
various gods who influenced human affairs. So the Babylonian priests
devised an elaborate system of interpreting the phenomena of the
heavens; and attaching the proper significance in human terms to
everything that took place in the sky. In Babylonia and Assyria it was
the king and his people for whom the prognostications were made out. It
was the same in Egypt. Later, about the fifth century B. C., astrology
spread through Greece, where astrologers developed the idea of the
influence of planets upon individual concerns. Astrology persisted
through the Dark Ages, and the great astronomers Copernicus, Tycho,
Kepler, Gassendi, and Huygens were all astrologers as well. Milton makes
many references to planetary influence, our language has many words with
a direct origin in astrology, and in our great cities to-day are many
astrologers who prepare individual horoscopes of more than ordinary
interest.

It is difficult to assign the antiquity of the Chinese astronomy with
any approach to definiteness. Their earliest records appear to have been
total eclipses of the sun, going back nearly 2,200 years before the
Christian era; and nearly a thousand years earlier the Hindu astronomy
sets down a conjunction of all the planets, concerning which, however,
there is doubt whether it was actually observed or merely calculated
backward. Owing to a colossal misfortune, the burning of all native
scientific books by order of the Emperor Tsin-Chi-Hwang-Ti, in B. C.
221, excepting only the volumes relating to agriculture, medicine, and
astrology, the Chinese lost a precious mass of astronomical learning,
accumulated through the ages. No less an authority than Wells Williams
credits them with observing 600 solar eclipses between B. C. 2159 and A.
D. 1223, and there must have been some centuries of eclipses observed
and recorded anterior to B. C. 2159, as this is the date assigned to the
eclipse which came unheralded by the astronomers royal, Hi and Ho, who
had become intoxicated and forgot to warn the Court, in accord with
their duty. China was thereby exposed to the anger of the gods, and Hi
and Ho were executed by his Majesty's command. It is doubtful if there
is an earlier record of any celestial phenomenon.




CHAPTER III

PYRAMID, TOMB, AND TEMPLE


Inquiry into the beginnings of astronomy in ancient Egypt reveals most
interesting relations of the origins of the science to the life and work
and worship of the people. Their astronomers were called the "mystery
teachers of heaven"; their monuments indicate a civilization more or
less advanced; and their temples were built on astronomical principles
and dedicated to purpose of worship. The Egyptian records carry us back
many thousands of years, and we find that in Egypt, as in other early
civilizations, observation of the heavenly bodies may be embraced in
three pretty distinct stages. Awe, fear, wonder and worship were the
first. Then came utility: a calendar was necessary to tell men when "to
plow and sow, to reap and mow," and a calendar necessitated astronomical
observations of some sort. Following this, the third direction required
observations of celestial positions and phenomena also, because
astrology, in which the potentates of every ancient realm believed,
could only thrive as it was based on astronomy.

Sun worship was preeminent in early Egypt as in India, where the primal
antithesis between night and day struck terror in the unformed mind of
man. In one of the Vedas occurs this significant song to the god of day:
"Will the Sun rise again? Will our old friend the Dawn come back again?
Will the power of Darkness be conquered by the God of Light?"

Quite different from India, however, is Egypt in matters of record: in
India, records in papyrus, but no monuments of very great antiquity; in
Egypt, no papyrus, but monuments of exceeding antiquity in abundance.
Herodotus and Pliny have told us of the great antiquity of these
monuments, even in their own day, and research by archæologist and
astronomer has made it certain that the pyramids were built by a race
possessing great knowledge of astronomy. Their temples, too, were
constructed in strict relation to stars. Not only are the temples, as
Edfu and Denderah, of exceeding interest in themselves, but associated
with them are often huge monoliths of syenite, obelisks of many hundred
tons in weight, which the astronomer recognizes as having served as
observation pillars or gnomons. Specimens of these have wandered as far
from home as Central Park and the bank of the Thames. But there is an
even more remarkable wealth of temple inscriptions, zodiacs especially.

Next to the sun himself was the worship of the Dawn and Sunrise, the
great revelations of nature. There were numerous hymns to the still more
numerous sun-gods and the powers of sunlight. Ra was the sun-god in his
noontide strength; Osiris, the dying sun of sunset. Only two gods were
associated with the moon, and for the stars a special goddess, Sesheta.
Sacrifices were made at day-break; and the stars that heralded the dawn
were the subjects of careful observation by the sacrificial priests, who
must therefore have possessed a good knowledge of star places and names,
doubtless in belts of stars extending clear around the heavens. These
decans, as they were called, are the exact counterparts of the moon
stations devised by the Arabians, Indians, and other peoples for a like
purpose.

The plane or circle of observation, both in Egypt and India, was always
the horizon, whether the sun was observed or moon or stars. So the sun
was often worshiped by the ancient Egyptians as the "Lord of the Two
Horizons." It is sometimes difficult to keep in mind the fact, in regard
to all temples of the ancients, whether in Egypt or elsewhere, that in
studying them we must deal with the risings or settings of the heavenly
bodies in quite different fashion from that of the astronomer of to-day,
who is mainly concerned only with observing them on the meridian. The
axis of the temple shows by its direction the place of rising or
setting: if the temple faces directly east or west, its amplitude is 0.
Now the sun, moon, and planets are, as everyone knows, very erratic as
to their amplitudes (i. e., horizon points) of rising and setting; so it
must have been the stars that engrossed the attention of the earliest
builders of temples. After that, temples were directed to the rising
sun, at the equinox or solstices. Then came the necessity of finding out
about the inclination or obliquity of the ecliptic, and this is where
the gnomon was employed.

At Karnak are many temples of the solstitial order: the wonderful temple
of Amen-Ra is so oriented that its axis stands in amplitude 26 degrees
north of west, which is the exact amplitude of the sun at Thebes at
sunset of the summer solstice. The axis of a lesser temple adjacent
points to 26 degrees south of east, which is the exact amplitude of
sunrise at the winter solstice. At Gizeh we find the temples oriented,
not solstitially, but by the equinoxes, that is, they face due east and
west. Peoples who worshiped the sun at the solstice must have begun
their year at the solstice; and Sir Norman Lockyer shows how the rise of
the Nile, which took place at the summer solstice, dominated not only
the industry but the astronomy and religion of Egypt.

Looking into the question of temple orientation in other countries, as
China, for example, Lockyer finds that the most important temple of that
country, the Temple of the Sun at Peking, is oriented to the winter
solstice; and Stonehenge, as has long been known, is oriented to sunrise
at the summer solstice.

In like fashion the rising and setting of many stars were utilized by
the Egyptians, in both temple and pyramid; and no astronomer who has
ever seen these ancient structures and studied their orientations can
doubt that they were built by astronomers for use by astronomers of that
day. The priests were the astronomers, and the temples had a deep
religious significance, with a ceremony of exceeding magnificence
wherever observations of heavenly bodies were undertaken, whether of sun
or stars.

Hindu and Persian astronomy must be passed over very briefly.
Interesting as their systems are historically, there were few, if any,
original contributions of importance, and the Indian treatises bear
strong evidence of Greek origin.




CHAPTER IV

ORIGIN OF GREEK ASTRONOMY


While the Greeks laid the foundations of modern scientific astronomy,
they were not as a whole observers: rather philosophers, we should say.
The later representatives of the Greek School, however, saw the
necessity of observation as a basis of true induction; and they
discovered that real progress was not possible unless their speculative
ideas were sufficiently developed and made definite by the aid of
geometry, so that they became capable of detailed comparison with
observation. This was the necessary and ultimate test with them, and the
same is true to-day. The early Greek philosophers were, however, mainly
interested, not in observations, but in guessing the causes of
phenomena.

Thales of Miletus, founder of the Ionian School, introduced the system
of Egyptian astronomy into Greece, about the end of the seventh century
B. C. He is universally known as the first astronomer who ever predicted
a total eclipse of the sun that happened when he said it would: the
eclipse of B. C. 585. This he did by means of the Chaldean eclipse cycle
of 18 years known as the Saros.

Aristarchus of Samos was the first and most eminent of the Alexandrian
astronomers, and his treatise "On the Magnitudes and Distances of the
Sun and Moon" is still extant. This method of ascertaining how many
times farther the sun is than the moon is very simple, and
geometrically exact. Unfortunately it is impossible, even to-day, to
observe with accuracy the precise time when the moon "quarters," (an
observation essential to his method), because the moon's terminal, or
line between day and night, is not a straight line as required by
theory, but a jagged one. By his observation, the sun was only twenty
times farther away than the moon, a distance which we know to be nearly
twenty times too small.

His views regarding other astronomical questions were right, although
they found little favor among contemporaries. Not only was the earth
spherical, he said, but it rotated on its axis and also traveled round
the sun. Aristarchus was, indeed, the true originator of the modern
doctrine of motions in the solar system, and not Copernicus, seventeen
centuries later; but Seleucus appears to have been his only follower in
these very advanced conceptions. Aristarchus made out the apparent
diameters of sun and moon as practically equal to one another, and
inferred correctly that their real diameters are in proportion to their
distances from the earth. Also he estimated, from observations during an
eclipse of the moon, that the moon's diameter is about one-third that of
the earth. Aristarchus appears to have been one of the clearest and most
accurate thinkers among the ancient astronomers; even his views
concerning the distances of the stars were in accord with the fact that
they are immeasurably distant as compared with the distances of the sun,
moon, and planets.

Practically contemporary with Aristarchus were Timocharis and
Aristillus, who were excellent observers, and left records of position
of sun and planets which were exceedingly useful to their successors,
Hipparchus and Ptolemy in particular. Indeed their observations of star
positions were such that, in a way, they deserve the fame of having made
the first catalogue, rather than Hipparchus, to whom is universally
accorded that honor.

Spherical astronomy had its origin with the Alexandrian school, many
famous geometers, and in particular Euclid, pointing the way. Spherics,
or the doctrine of the sphere, was the subject of numerous treatises,
and the foundations were securely laid for that department of
astronomical research which was absolutely essential to farther advance.
The artisans of that day began to build rude mechanical adaptations of
the geometric conceptions as concrete constructions in wood and metal,
and it became the epoch of the origin of astrolabes and armillary
spheres.




CHAPTER V

MEASURING THE EARTH--ERATOSTHENES


All told, the Greek philosophers were probably the keenest minds that
ever inhabited the planet, and we cannot suppose them so stupid as to
reject the doctrine of a spherical earth. In fact so certain were they
that the earth's true figure is a sphere that Eratosthenes in the third
century B. C. made the first measure of the dimensions of the
terrestrial sphere by a method geometrically exact.

At Syene in Upper Egypt the sun at the summer solstice was known to pass
through the zenith at noon, whereas at Alexandria Eratosthenes estimated
its distance as seven degrees from the zenith at the same time. This
difference being about one-fiftieth of the entire circumference of a
meridian, Eratosthenes correctly inferred that the distance between
Alexandria and Syene must be one-fiftieth of the earth's circumference.
So he measured the distance between the two and found it 5,000 stadia.
This figured out the size of the earth with a percentage of error
surprisingly small when we consider the rough means with which
Eratosthenes measured the sun's zenith distance and the distance between
the two stations.

Greatest of all the Greek astronomers and one of the greatest in the
history of the science was Hipparchus who had an observatory at Rhodes
in the middle of the second century B. C. His activities covered every
department of astronomy; he made extensive series of observations which
he diligently compared with those handed down to him by the earlier
astronomers, especially Aristillus and Timocharis. This enabled him to
ascertain the motion of the equinoxial points, and his value of the
constant of precession of the equinoxes is exceedingly accurate for a
first determination.

In 134 B. C. a new star blazed out in the constellation Scorpio, and
this set Hipparchus at work on a catalogue of the brighter stars of the
firmament, a monumental work of true scientific conception, because it
would enable the astronomers of future generations to ascertain what
changes, if any, were taking place in the stellar universe. There were
1,080 stars in his catalogue, and he referred their positions to the
ecliptic and the equinoxes. Also he originated the present system of
stellar magnitudes or orders of brightness, and his catalogue was in use
as a standard for many centuries.

Hipparchus was a great mathematician as well, and he devoted himself to
the improvement of the method of applying numerical calculations to
geometrical figures: trigonometry, both plane and spherical, that is;
and by some authorities he is regarded as the inventor of original
methods in trigonometry. The system of spheres of Eudoxus did not
satisfy him, so he devised a method of representing the paths of the
heavenly bodies by perfectly uniform motion in circles. There is slight
evidence that Apollonius of Perga may have been the originator of the
system, but it was reserved for Hipparchus to work it out in final form.
This enabled him to ascertain the varying length of the seasons, and he
fixed the true length of the year as 365-1/4 days. He had almost equal
success in dealing with the irregularities of the moon's motion,
although the problem is much more complicated. The distance and size of
the moon, by the method of Aristarchus, were improved by him, and he
worked out, for the distance of the sun, 1,200 radii of the earth--a
classic for many centuries.

Hipparchus devoted much attention to eclipses of both sun and moon, and
we owe to him the first elucidation of the subject of parallax, or the
effect of difference of position of an observer on the earth's surface
as affecting the apparent projection of the moon against the sun when a
solar eclipse takes place; whereas an eclipse of the moon is unaffected
by parallax and can be seen at the same time by observers everywhere, no
matter what their location on the earth. Indeed, with all that
Hipparchus achieved, we need not be surprised that astronomy was
regarded as a finished science, and made practically no progress
whatever for centuries after his time.

Then came Claudius Ptolemæus, generally known as Ptolemy, the last great
name in Greek astronomy. He lived in Alexandria about the middle of the
second century A. D. and wrote many minor astronomical and astrological
treatises, also works on geography and optics, in the last of which the
atmospheric refraction of rays of light from the heavenly bodies,
apparently elevating them toward the zenith, is first dealt with in true
form.




CHAPTER VI

PTOLEMY AND HIS GREAT BOOK


Ptolemy was an observer of the heavens, though not of the highest order;
but he had all the work of his predecessors, best of all Hipparchus, to
build upon. Ptolemy's greatest work was the "Megale Syntaxis," generally
known as the Almagest. It forms a nearly complete compendium of the
ancient astronomy, and although it embodies much error, because built on
a wrong theory, the Almagest nevertheless is competent to follow the
motions of all the bodies in the sky with a close approach to accuracy,
even at the present day. This marvelous work written at this critical
epoch became as authoritative as the philosophy of Aristotle, and for
many centuries it was the last word in the science. The old astrology
held full sway, and the Ptolemaic theory of the universe supplied
everything necessary: further progress, indeed, was deemed impossible.

The Almagest comprises in all thirteen books, the first two of which
deal with the simpler observations of the celestial sphere, its own
motion and the apparent motions of sun, moon, and planets upon it. He
discusses, too, the postulates of his system and exhibits great skill as
an original geometer and mathematician. In the third book he takes up
the length of the year, and in the fourth book similarly the moon and
the length of the month. Here his mathematical powers are at their best,
and he made a discovery of an inequality in the moon's motion known as
the evection. Book five describes the construction and use of the
astrolabe, a combination of graduated circles with which Ptolemy made
most of his observations. In the sixth book he follows mainly Hipparchus
in dealing with eclipses of sun and moon. In the seventh and eighth
books he discusses the motion of the equinox, and embodies a catalogue
of 1,028 stars, substantially as in Hipparchus. The five remaining books
of the Almagest deal with the planetary motions, and are the most
important of all of Ptolemy's original contributions to astronomy.
Ptolemy's fundamental doctrines were that the heavens are spherical in
form, all the heavenly motions being in circles. In his view, the earth
too is spherical, and it is located at the center of the universe, being
only a point, as it were, in comparison. All was founded on mere
appearance combined with the philosophical notion that the circle being
the only perfect curve, all motions of heavenly bodies must take place
in earth-centered circles. For fourteen or fifteen centuries this false
theory persisted, on the authority of Ptolemy and the Almagest,
rendering progress toward the development of the true theory impossible.

Ptolemy correctly argued that the earth itself is a sphere that is
curved from east to west, and from north to south as well, clinching his
argument, as we do to-day, by the visibility of objects at sea, the
lower portions of which are at first concealed from our view by the
curved surface of the water which intervenes. To Ptolemy also the earth
is at the center of the celestial sphere, and it has no motion of
translation from that point; but his argument fails to prove this. Truth
and error, indeed, are so deftly intermingled that one is led to wonder
why the keen intelligence of this great philosopher permitted him to
reject the simple doctrine of the earth's rotation on its axis. But if
we reflect that there was then no science of natural philosophy or
physics proper, and that the age was wholly undeveloped along the lines
of practical mechanics, we shall see why the astronomers of Ptolemy's
time and subsequent centuries were content to accept the doctrines of
the heavens as formulated by him.

When it came to explaining the movements of the "wandering stars," or
planets, as we term them, the Ptolemaic theory was very happy in so far
as accuracy was concerned, but very unhappy when it had to account for
the actual mechanics of the cosmos in space. Sun and moon were the only
bodies that went steadily onward, easterly: whereas all the others,
Mercury, Venus, Mars, Jupiter, Saturn, although they moved easterly most
of the time, nevertheless would at intervals slow down to stationary
points, where for a time they did not move at all, and then actually go
backward to the west, or retrograde, then become stationary again,
finally resuming their regular onward motion to the east.

To help out of this difficulty, the worst possible mechanical scheme was
invented, that known as the epicycle. Each of the five planets was
supposed to have a fictitious "double," which traveled eastward with
uniformity, attached to the end of a huge but mechanically impossible
bar. The earth-centered circle in which this traveled round was called
the "deferent." What this bar was made of, what stresses it would be
subjected to, or what its size would have to be in order to keep from
breaking--none of these questions seems to have agitated the ancient and
medieval astronomers, any more than the flat-earth astronomy of the
Hindu is troubled by the necessity of something to hold up the tortoise
that holds up the elephant that holds up the earth.

But at the end of this bar is jointed or swiveled another shorter bar,
to the revolving end of which is attached the actual planet itself; and
the second bar, by swinging once round the end of the primary advancing
bar, would account for the backward or retrograde motion of the planet
as seen in the sky. For every new irregularity that was found, in the
motion of Mars, for instance, a new and additional bar was
requisitioned, until interplanetary space was hopelessly filled with
revolving bars, each producing one of the epicycles, some large, some
small, that were needed to take up the vagaries of the several planets.

The Arabic astronomers who kept the science alive through the Middle
Ages added epicycle to epicycle, until there was every justification for
Milton's verses descriptive of the sphere:

    With Centric and Eccentric scribbled o'er,
    Cycle and Epicycle, Orb in Orb.




CHAPTER VII

ASTRONOMY OF THE MIDDLE AGES


With the fall of Alexandria and the victory of Mohammed throughout the
West, and a consequent decline in learning, supremacy in science passed
to the East and centered round the caliphs of Bagdad in the seventh and
eighth centuries. They were interested in astronomy only as a practical,
and to them useful, science, in adjusting the complicated lunar calendar
of the Mohammedans, in ascertaining the true direction of Mecca which
every Mohammedan must know, and in the revival of astrology, to which
the Greeks had not attached any particular significance.

Harun al-Rashid ordered the Almagest and many other Greek works
translated, of which the modern world would otherwise no doubt never
have heard, as the Greek originals are not extant.

Splendid observatories were built at Damascus and Bagdad, and fine
instruments patterned after Greek models were continuously used in
observing. The Arab astronomers, although they had no clocks, were
nevertheless so fully impressed with the importance of time that they
added extreme value to their observations of eclipses, for example, by
setting down the altitudes of sun or stars at the same time. On very
important occasions the records were certified on oath by a body of
barristers and astronomers conjointly--a precedent which fortunately has
never been followed.

About the middle of the ninth century, the Caliph Al-Mamun directed his
astronomers to revise the Greek measures of the earth's dimensions, and
they had less reverence for the Almagest than existed in later
centuries: indeed, Tabit ben Korra invented and applied to the tables of
the Almagest a theoretical fluctuation in the position of the ecliptic
which he called "trepidation," which brought sad confusion into
astronomical tables for many succeeding centuries.

Albategnius was another Arab prince whose record in astronomy in the
ninth and tenth centuries was perhaps the best: the Ptolemaic values of
the precession of the equinoxes and of the obliquity of the ecliptic
were improved by new observations, and his excellence as mathematician
enabled him to make permanent improvements in the astronomical
application of trigonometry.

Abul Wefa was the last of the Bagdad astronomers in the latter half of
the tenth century, and his great treatise on astronomy known as the
Almagest is sometimes confused with Ptolemy's work. Following him was
Ibn Yunos of Cairo, whose labors culminated in the famous Hakemite
Tables, which became the standard in mathematical and astronomical
computations for several centuries.

Mohammedan astronomy thrived, too, in Spain and northern Africa.
Arzachel of Toledo published the Toledan Tables, and his pupils made
improvements in instruments and the methods of calculation. The Giralda
was built by the Moors in Seville in 1196, the first astronomical
observatory on the continent of Europe; but within the next half century
both Seville and Cordova became Christian again, and Arab astronomy was
at an end.

Through many centuries, however, the science had been kept alive, even
if no great original advances had been achieved; and Arab activities
have modified our language very materially, adding many such words as
almanac, zenith, and radii, and a wealth of star names, as Aldebaran,
Rigel, Betelgeuse, Vega, and so on.

Meanwhile, other schools of astronomy had developed in the East, one at
Meraga near the modern Persia, where Nassir Eddin, the astronomer of
Hulagu Khan, grandson of the Mongol emperor Genghis Khan, built and used
large and carefully constructed instruments, translated all the Greek
treatises on astronomy, and published a laborious work known as the
Ilkhanic Tables, based on the Hakemite Tables of Ibn Yunos.

More important still was the Tartar school of astronomy under Ulugh Beg,
a grandson of Tamerlane, who built an observatory at Samarcand in 1420,
published new tables of the planets, and made with his excellent
instruments the observations for a new catalogue of stars, the first
since Hipparchus, the star places being recorded with great precision.

The European astronomy of the Middle Ages amounted to very little
besides translation from the Arabic authors into Latin, with
commentaries. Astronomers under the patronage of Alfonso X of Leon and
Castile published in 1252 the Alfonsine Tables, which superseded the
Toledan tables and were accepted everywhere throughout Europe. Alfonso
published also the "Libros del Saber," perhaps the first of all
astronomical cyclopedias, in which is said to occur the earliest diagram
representing a planetary orbit as an ellipse: Mercury's supposed path
round the earth as a center.

Purbach of Vienna about the middle of the 15th century began his
"Epitome of Astronomy" based on the "Almagest" of Ptolemy, which was
finished by his collaborator Regiomontanus, who was an expert in
mathematics and published a treatise on trigonometry with the first
table of sines calculated for every minute from 0° to 90°, a most
helpful contribution to theoretical astronomy.

Regiomontanus had a very picturesque career, finally taking up his
residence in Nuremberg, where a wealthy citizen named Walther became his
patron, pupil, and collaborator. The artisans of the city were set at
work on astronomical instruments of the greatest accuracy, and the comet
of 1472 was the first to be observed and studied in true scientific
fashion. Regiomontanus was very progressive and the invention of the new
art of printing gave him an opportunity to publish Purbach's treatise,
which went through several editions and doubtless had much to do in
promoting dissatisfaction with the ancient Ptolemaic system, and was
thus most significant in preparing a background for the coming of the
new Copernican order.

The Nuremberg presses popularized astronomy in other important ways,
issuing almanacs, the first precursors of our astronomical Ephemerides.
Regiomontanus was practical as well, and invented a new method of
getting a ship's position at sea, with tables so accurate that they
superseded all others in the great voyages of discovery, and it is
probable that they were employed by Columbus in his discovery of the
American continent. Regiomontanus had died several years earlier, in
1475 at Rome, where he had gone by invitation of the Pope to effect a
reformation in the calendar. He was only forty, and his patron Walther
kept on with excellent observations, the first probably to be corrected
for the effect of atmospheric refraction, although its influence had
been known since Ptolemy. The Nuremberg School lasted for nearly two
centuries.

Nearly contemporary with Regiomontanus were Fracastoro and Peter Apian,
whose original observations on comets are worthy of mention because they
first noticed that the tails of these bodies always point away from the
sun. Leonardo da Vinci was the first to give the true explanation of
earth-shine on the moon, and similarly the moon-illumination of the
earth; and this no doubt had great weight in disposing of the popular
notion of an essential difference of nature between the earth and
celestial bodies--all of which helped to prepare the way for Copernicus
and the great revolution in astronomical thought.




CHAPTER VIII

COPERNICUS AND THE NEW ERA


Throughout the Middle Ages the progress of astronomy was held back by a
combination of untoward circumstances. A prolonged reaction from the
heights attained by the Greek philosophers was to be expected. The
uprising of the Mohammedan world, and the savage conquerors in the East
did not produce conditions favorable to the origin and development of
great ideas.

At the birth of Copernicus, however, in 1473, the time was ripening for
fundamental changes from the ancient system, the error of which had
helped to hold back the development of the science for centuries. The
fifteenth century was most fruitful in a general quickening of
intelligence, the invention of printing had much to do with this, as it
spread a knowledge of the Greek writers, and led to conflict of
authorities. Even Aristotle and Ptolemy were not entirely in harmony,
yet each was held inviolate. It was the age of the Reformation, too, and
near the end of the century the discovery of America exerted a powerful
stimulus in the advance of thought.

Copernicus searched the works of the ancient writers and philosophers,
and embodied in this new order such of their ideas as commended
themselves in the elaboration of his own system.

Pythagoras alone and his philosophy looked in the true direction. Many
believe that he taught that the sun, not the earth, is at the center of
our solar system; but his views were mingled with the speculative
philosophy of the Greeks, and none of his writings, barring a few meager
fragments, have come down to our modern age.

To many philosophers, through all these long centuries, the true theory
of the celestial motions must have been obvious, but their views were
not formulated, nor have they been preserved in writing. So the fact
remains that Copernicus alone first proved the truth of the system which
is recognized to-day. This he did in his great treatise entitled "De
Revolutionibus Orbium Coelestium," the first printed copy of which was
dramatically delivered to him on his deathbed, in May, 1543. The seventy
years of his life were largely devoted to the preparation of this work,
which necessitated many observations as well as intricate calculations
based upon them. Being a canon in the church, he naturally hesitated
about publishing his revolutionary views, his friend Rheticus first
doing this for him in outline in 1540.

So simple are the great principles that they may be embodied in very few
words; what appears to us as the daily revolution of the heavens is not
a real motion, but only an apparent one; that is, the heavens are at
rest, while the earth itself is in motion, turning round an axis which
passes through its center. And the second proposition is that the earth
is simply one of the six known planets; and they all revolve round the
sun as the true center. The solar system, therefore, is "heliocentric,"
or sun-centered, not "geocentric" or earth-centered, as taught by the
Ptolemaic theory.

Copernicus demonstrates clearly how his system explains the retrograde
motion of the planets and their stationary points, no matter whether
they are within the orbit of the earth, as Mercury and Venus, or outside
of it, as Mars, Jupiter, and Saturn. His system provides also the means
of ascertaining with accuracy the proportions of the solar system, or
the relative distances of the planets from the sun and from each other.
In this respect also his system possessed a vast advantage over that of
Ptolemy, and the planetary distances which Copernicus computed are very
close approximations to the measures of the present day.

Reinhold revised the calculations of Copernicus and prepared the "Tabulæ
Prutenicæ," based on the "De Revolutionibus," which proved far superior
to the Alfonsine Tables, and were only supplanted by the Rudolphine
Tables of Kepler. On the whole we may regard the lifework of Copernicus
as fundamentally the most significant in the history and progress of
astronomy.




CHAPTER IX

TYCHO, THE GREAT OBSERVER


Clear as Copernicus had made the demonstration of the truth of his new
system, it nevertheless failed of immediate and universal acceptance.
The Ptolemaic system was too strongly intrenched, and the motions of all
the bodies in the sky were too well represented by it. Accurate
observations were greatly needed, and the Landgrave William IV. of Hesse
built the Cassel Observatory, which made a new catalogue of stars, and
introduced the use of clocks to carry on the time as measured by the
uniform motion of the celestial sphere. Three years after the death of
Copernicus, Tycho Brahe was born, and when he was 30 the King of Denmark
built for him the famous observatory of Uraniborg, where the great
astronomer passed nearly a quarter of a century in critically observing
the positions of the stars and planets. Tycho was celebrated as a
designer and constructor of new types of astronomical instruments, and
he printed a large volume of these designs, which form the basis of many
in use at the present day. Unfortunately for the genius of Tycho and the
significance of his work, the invention of the telescope had not yet
been made, so that his observations had not the modern degree of
accuracy. Nevertheless, they were destined to play a most important part
in the progress of astronomy.

Tycho was sadly in error in his rejection of the Copernican system,
although his reasons, in his day, seemed unanswerable. If the outer
planets were displaced among the stars by the annual motion of the earth
round the sun, he argued, then the fixed stars must be similarly
displaced--unless indeed they be at such vast distances that their
motions would be too slight to be visible. Of course we know now that
this is really true, and that no instruments that Tycho was able to
build could possibly have detected the motions, the effects of which we
now recognize in the case of the nearer fixed stars in their annual, or
parallactic, orbits.

The remarkably accurate instruments devised by Tycho Brahe and employed
by him in improving the observations of the positions of the heavenly
bodies were no doubt built after descriptions of astrolabes such as
Hipparchus used, as described by Ptolemy. In his "Astronomiæ Instauratæ
Mechanica" we find illustrations and descriptions of many of them.

One is a polar astrolabe, mounted somewhat as a modern equatorial
telescope is, and the meridian circle is adjustable so that it can be
used in any place, no matter what its latitude might be. There is a
graduated equatorial ring at right angles to the polar axis, so that the
astrolabe could be used for making observations outside the meridian as
well as on it. This equatorial circle slides through grooves, and is
furnished with movable sights, and a plumb line from the zenith or
highest point of the meridian circle makes it possible to give the
necessary adjustment in the vertical. Screws for adjustment at the
bottom are provided, just as in our modern instruments, and two
observers were necessary, taking their sights simultaneously; unless,
as in one type of the instrument, a clock, or some sort of measure of
time, was employed.

Another early type of instrument is called by Tycho the ecliptic
astrolabe (_Armillæ Zodiacales_, or the Zodiacal Rings). It resembles
the equatorial astrolabe somewhat, but has a second ring inclined to the
equatorial one at an angle equal to the obliquity of the ecliptic. In
observing, the equatorial ring was revolved round till the ecliptic ring
came into coincidence with the plane of the ecliptic in the sky. Then
the observation of a star's longitude and latitude, as referred to the
ecliptic plane, could be made, quite as well as that of right ascension
and declination on the equatorial plane. But it was necessary to work
quickly, as the adjustment on the ecliptic would soon disappear and have
to be renewed.

Tycho is often called the father of the science of astronomical
observation, because of the improvements in design and construction of
the instruments he used. His largest instrument was a mural quadrant, a
quarter-circle of copper, turning parallel to the north-and-south face
of a wall, its axis turning on a bearing fixed in the wall. The radius
of this quadrant was nine feet, and it was graduated or divided so as to
read the very small angle of ten seconds of arc--an extraordinary degree
of precision for his day.

Tycho built also a very large alt-azimuth quadrant, of six feet radius.
Its operation was very much as if his mural quadrant could be swung
round in azimuth. At several of the great observatories of the present
day, as Greenwich and Washington, there are instruments of a similar
type, but much more accurate, because the mechanical work in brass and
steel is executed by tools that are essentially perfect, and besides
this the power of the telescope is superadded to give absolute
direction, or pointing on the object under observation.

Excellent clocks are necessary for precise observation with such an
instrument; but neither Tycho Brahe, nor Hevelius was provided with such
accessories. Hevelius did not avail himself of the telescope as an aid
to precision of observation, claiming that pinhole sights gave him more
accurate results. It was a dispute concerning this question that Halley
was sent over from London to Danzig to arbitrate.

There could be but one way to decide; the telescope with its added power
magnifies any displacement of the instrument, and thereby enables the
observer to point his instrument more exactly. So he can detect smaller
errors and differences of direction than he can without it. And what is
of great importance in more modern astronomy, the telescope makes it
possible to observe accurately the position of objects so faint that
they are wholly invisible to the naked eye.




CHAPTER X

KEPLER, THE GREAT CALCULATOR


Most fortunate it was for the later development of astronomical theory
that Tycho Brahe not only was a practical or observational astronomer of
the highest order, but that he confined himself studiously for years to
observations of the places of the planets. Of Mars he accumulated an
especially long and accurate series, and among those who assisted him in
his work was a young and brilliant pupil named Johann Kepler.

Strongly impressed with the truth of the Copernican System, Kepler was
free to reject the erroneous compromise system devised by Tycho Brahe,
and soon after Tycho's death Kepler addressed himself seriously to the
great problem that no one had ever attempted to solve, viz: to find out
what the laws of motion of the planets round the sun really are. Of
course he took the fullest advantage of all that Ptolemy and Copernicus
had done before him, and he had in addition the splendid observations of
Tycho Brahe as a basis to work upon.

Copernicus, while he had effected the tremendous advance of substituting
the sun for the earth as the center of motion, nevertheless clung to the
erroneous notion of Ptolemy that all the bodies of the sky must perforce
move at uniform speeds, and in circular curves, the circle being the
only "perfect curve." Kepler was not long in finding out that this
could not be so, and he found it out because Tycho Brahe's observations
were much more accurate than any that Copernicus had employed.

Naturally he attempted the nearest planet first, and that was Mars--the
planet that Tycho had assigned to him for research. How fortunate that
the orbit of Mars was the one, of all the planets, to show practically
the greatest divergence from the ancient conditions of uniform motion in
a perfectly circular orbit! Had the orbit of Mars chanced to be as
nearly circular as is that of Venus, Kepler might well have been driven
to abandon his search for the true curve of planetary motion.

However, the facts of the cosmos were on his side, but the calculations
essential in testing his various hypotheses were of the most tedious
nature, because logarithms were not yet known in his day. His first
discovery was that the orbit of Mars is certainly not a circle, but oval
or elliptic in figure. And the sun, he soon found, could not be in the
center of the ellipse, so he made a series of trial calculations with
the sun located in one of the foci of the ellipse instead.

Then he found he could make his calculated places of Mars agree quite
perfectly with Tycho Brahe's observed positions, if only he gave up the
other ancient requisite of perfectly uniform motion. On doing this, it
soon appeared that Mars, when in perihelion, or nearest the sun, always
moved swiftest, while at its greatest distance from the sun, or
aphelion, its orbital velocity was slowest.

Kepler did not busy himself to inquire why these revolutionary
discoveries of his were as they were; he simply went on making enough
trials on Mars, and then on the other planets in turn, to satisfy
himself that all the planetary orbits are elliptical, not circular in
form, and are so located in space that the center of the sun is at one
of the two foci of each orbit. This is known as Kepler's first law of
planetary motion.

The second one did not come quite so easy; it concerned the variable
speed with which the planet moves at every point of the orbit. We must
remember how handicapped he was in solving this problem: only the
geometry of Euclid to work with, and none of the refinements of the
higher mathematics of a later day. But he finally found a very simple
relation which represented the velocity of the planet everywhere in its
orbit. It was this: if we calculate the area swept, or passed over, by
the planet's radius vector (that is, the line joining its center to the
sun's center) during a week's time near perihelion, and then calculate
the similar area for a week near aphelion, or indeed for a week when
Mars is in any intermediate part of its orbit, we shall find that these
areas are all equal to each other. So Kepler formulated his second great
law of planetary motion very simply: the radius vector of any planet
describes, or sweeps over, equal areas in equal times. And he found this
was true for all the planets.

But the real genius of the great mathematician was shown in the
discovery of his third law, which is more complex and even more
significant than the other two--a law connecting the distances of the
planets from the sun with their periods of revolution about the sun.
This cost Kepler many additional years of close calculation, and the
resulting law, his third law of planetary motion is this: The cubes of
the mean or average distances of the planets from the sun are
proportional to the squares of their times of revolution around him.

So Kepler had not only disposed of the sacred theories of motion of the
planets held by the ancients as inviolable, but he had demonstrated the
truth of a great law which bound all the bodies of the solar system
together. So accurately and completely did these three laws account for
all the motions, that the science of astronomy seemed as if finished;
and no matter how far in the future a time might be assigned, Kepler's
laws provided the means of calculating the planet's position for that
epoch as accurately as it would be possible to observe it. Kepler paused
here, and he died in 1630.




CHAPTER XI

GALILEO, THE GREAT EXPERIMENTER


The fifteenth and sixteenth centuries, containing the lives and work of
Copernicus, Tycho, Galileo, Kepler, Huygens, Halley, and Newton, were a
veritable Golden Age of astronomy. All these men were truly great and
original investigators.

None had a career more picturesque and popular than did Galileo. Born a
few years earlier and dying a few years later than Kepler, the work of
each of these two great astronomers was wholly independent of the other
and in entirely different fields. Kepler was discovering the laws of
planetary motion, while Galileo was laying the secure foundations of the
new science of dynamics, in particular the laws of falling bodies, that
was necessary before Kepler's laws could be fully understood. When only
eighteen Galileo's keen power of observation led to his discovery of the
laws of pendulum motion, suggested by the oscillation to and fro of a
lamp in the cathedral of Pisa.

The world-famous leaning tower of this place, where he was born, served
as a physical laboratory from the top of which he dropped various
objects, and thus was led to formulate the laws of falling bodies. He
proved that Aristotle was all wrong in saying that a heavy body must
fall swifter in proportion to its weight than a lighter one. These and
other discoveries rendered him unpopular with his associates, who
christened him the "Wrangler."

The new system of Copernicus appealed to him; and when he, first of all
men, turned a telescope on the heavenly bodies, there was Venus with
phases like those of the moon, and Jupiter with satellites traveling
about it--a Copernican system in miniature. Nothing could have happened
that would have provided a better demonstration of the truth of the new
system and the falsity of the old. His marvelous discoveries caused the
greatest excitement--consternation even, among the anti-Copernicans.
Galileo published the "Sidereus Nuncius," with many observations and
drawings of the moon, which he showed to be a body not wholly dissimilar
to the earth: this, too, was obviously of great moment in corroboration
of the Copernican order and in contradiction to the Ptolemaic, which
maintained sharp lines of demarcation between things terrestrial and
things celestial.

His telescopes, small as they were, revealed to him anomalous
appearances on both sides of the planet Saturn which he called _ansæ_,
or handles. But their subsequent disappearance was unaccountable to him,
and later observers, who kept on guessing ineffectively till Huygens,
nearly a half century after, showed that the true nature of the
appendage was a ring. Spots on the sun were frequently observed by
Galileo and led to bitter controversies. He proved, however, that they
were objects on the sun itself, not outside it, and by noticing their
repeated transits across the sun's disk, he showed that the sun turned
round on his axis in a little less than a month--another analogy to the
like motion of the earth on the Copernican plan.

Galileo's appointment in 1610 as "First Philosopher and Mathematician"
to the Grand Duke of Tuscany gave him abundant time for the pursuit of
original investigations and the preparation of books and pamphlets. His
first visit to Rome the year following was the occasion of a reception
with great honor by many cardinals and others of high rank. His lack of
sympathy with others whose views differed from his, and his naturally
controversial spirit, had begun to lead him headlong into controversies
with the Jesuits and the church, which culminated in his censure by the
authorities of the church and persecution by the Inquisition.

In 1618 three comets appeared, and Galileo was again in controversial
hot water with the Jesuits. But it led to the publication five years
later of "Il Saggiatore" (The Assayer), of no great scientific value,
but only a brilliant bit of controversial literature dedicated to the
newly elevated Pope, Urban VIII. Later he wrote through several years a
great treatise, more or less controversial in character, entitled a
"Dialogue on the Two Chief Systems of the World" between three speakers,
and extending through four successive days. Simplicio argues for the
Aristotelians, Salviati for the Copernicans, while Sagredo does his best
to be neutral. It will always be a very readable book, and we are
fortunate to have a recent translation by Professor Crew of Evanston.

Here we find the first suggestion of the modern method of getting
stellar parallaxes, the relative parallax, that is, of two stars in the
same field--a method not put into service till Bessel's time, two
centuries later. But the most important chapters of the "Dialogue" deal
with Galileo's investigations of the laws of motion of bodies in
general, which he applied to the problem of the earth's motion. In this
he really anticipated Newton in the first of his three laws of motion,
and in a subsequent work, dealing with the theory of projectiles, he
reaches substantially the results of Newton's second law of motion,
although he gave no general statement of the principle. Nevertheless, in
the epoch where his life was lived and his work done, his telescopic
discoveries, combined with his dynamic researches in untrodden fields,
resulted in the complete and final overthrow of the ancient system of
error, and the secure establishment of the Copernican system beyond
further question and discussion. Only then could the science of
astronomy proceed unhampered to the fullest development by the master
minds of succeeding centuries.




CHAPTER XII

AFTER THE GREAT MASTERS


Following Kepler and Galileo was a half century of great astronomical
progress along many lines laid out by the work of the great masters. The
telescope seemed only a toy, but its improvement in size and quality
showed almost inconceivable possibilities of celestial discoveries.

Hevelius of Danzig took up the study of the moon, and his
"Selenographia" was finely illustrated by plates which he not only drew
but engraved himself. Lunar names of mountains, plains, and craters we
owe very largely to him. Also he published among other works two on
comets, the second of which was published in 1668 and called the
"Cometographia," the first detailed account of all the comets observed
and recorded to date.

Many were the telescopes turned on the planet Saturn, and every variety
of guess was made as to the actual shape and physical nature of the
weird appendages discovered by Galileo. The true solution was finally
reached by Huygens, whose mechanical genius had enabled him to grind and
polish larger and better lenses than his contemporaries; in 1659 he
published the "Systema Saturnium" interpreting the ring and the cause of
its various configurations, and the first discovery of a Saturnian
satellite is due to him.

Gascoigne in England about 1640 was the first to make the important
application of the micrometer to enhance the accuracy of measurement of
small angles in the telescopic field; an invention made and applied
independently many years later by Huygens in Holland and Auzout and
Picard in France, where the instrument was first regularly employed as
an accessory in the work of an observatory.

Another Englishman, Jeremiah Horrocks, was the first observer of a
transit of Venus over the disk of the sun, in 1639. Horrocks was
possessed of great ability in calculational astronomy also. This was
about the time of the invention of the pendulum clock by Huygens, which
in conjunction with the later invention of the transit instrument by
Roemer wrought a revolution in the exacting art of practical astronomy.
This was because it enabled the time to be carried along continuously,
and the revolution of the earth could be utilized in making precise
measures of the position of sun, moon, and stars. Louis XIV had just
founded the new Observatory at Paris in 1668, and Picard was the first
to establish regular time-observations there.

Huygens followed up the motion of the pendulum in theory as well as
practice in his "Horologium Oscillatorium" (1673), showing the way to
measure the force of gravity, and his study of circular motion showed
the fundamental necessity of some force directed toward the center in
planetary motions.

The doctrine of the sphericity of the earth being no longer in doubt,
the great advance in accuracy of astronomical observation indicated to
Willebrord Snell in Holland the best way to measure an arc of meridian
by triangulation. Picard repeated the measurements near Paris with even
greater accuracy, and his results were of the utmost significance to
Newton in establishing his law of gravitation.

Domenico Cassini, an industrious observer, voluminous writer, and a
strong personality, devised telescopes of great size, discovered four
Saturnian satellites and the main division in the ring of Saturn,
determined the rotation periods of Mars and Jupiter, and prepared tables
of the eclipses of Jupiter's satellites. At his suggestion Richer
undertook an expedition to Cayenne in latitude 5 degrees north, where it
was found that the intensity of gravity was less than at Paris, and his
clock therefore lost time, thus indicating that the earth was not a
perfect sphere as had been thought, but a spheroid instead.

The planet Mars passed a near opposition, and Richer's observations of
it from Cayenne, when combined with those of Cassini and others in
France, gave a new value of the sun's parallax and distance, really the
first actual measurement worth the name in the history of astronomy.

To close this era of signal advance in astronomy we may cite a discovery
by Roemer of the first order: no less than that of the velocity of
transmission of light through space. At the instigation of Picard,
Roemer in studying the motions of Jupiter's satellites found that the
intervals between eclipses grew less and less as Jupiter and the earth
approached each other, and greater and greater than the average as the
two planets separated farther and farther. Roemer correctly attributed
this difference to the progressive motion of light and a rough value of
its velocity was calculated, though not accepted by astronomers
generally for more than a century.

Why the laws of Kepler should be true, Kepler himself was unable to say.
Nor could anyone else in that day answer these questions: (1) The
planets move in orbits that are elliptical not circular--why should they
move in an imperfect curve, rather than the perfect one in which it had
always been taught that they moved? (2) Why should our planet vary its
velocity at all, and travel now fast, now slow; especially why should
the speed so vary that the line of varying length, joining the planet to
the sun, always passes over areas proportional to the time of describing
them? And (3) Why should there be any definite relation of the distances
of planets from the sun to their times of revolution about him? Why
should it be exactly as the cube of one to the square of the other?

We must remember that the Copernican system itself was not yet, in the
beginning of the seventeenth century, accepted universally; and the
great minds of that period were most concerned in overturning the
erroneous theory of Ptolemy.

The next step in logical order was to find a basic explanation of the
planetary motions, and Descartes and his theory of vortices are worthy
of mention, among many unsuccessful attempts in this direction.
Descartes was a brilliant French philosopher and mathematician, but his
hypothesis of a multitude of whirlpools in the ether, while ingenious in
theory, was too vague and indefinite to account for the planetary
motions with any approach to the precision with which the laws of Kepler
represented them.

Another great astronomer whose labors helped immensely in preparing the
way for the signal discoveries that were soon to come was Huygens, a man
of versatility as natural philosopher, mechanician, and astronomical
observer. Huygens was born thirteen years before the death of Galileo,
and to the discovery of the laws of motion by the latter Huygens added
researches on the laws of action of centrifugal forces. Neither of them,
however, appeared to see the immediate bearing on the great general
problem of celestial motions in its true light, and it was reserved for
another generation, and an astronomer of another country, to make the
one fundamental discovery that should explain the whole by a single
simple law.




CHAPTER XIII

NEWTON AND MOTION


"How is it that you are able to make these great discoveries?" was once
asked of Sir Isaac Newton, _facile princeps_ of all philosophers, and
the discoverer of the great law of universal gravitation.

"By perpetually thinking about them," was Newton's terse and
illuminating reply. He had set for himself the definite problem of
Kepler's laws: why is it that they are true, and is there not some
single, general law that will embody all the circumstances of the
planetary motions?

Newton was born in 1643, the year after the death of Galileo. He had a
thorough training in the mathematics of his day, and addressed himself
first to an investigation and definite formulation of the general laws
of motion, which he found to be three in number, and which he was able
to put in very simple terms. The first one is: Any body, once it is set
in motion, will continue to move forward in a straight line with a
uniform velocity forever, provided it is acted upon by no force
whatever. In other words, a state of motion is as natural as a state of
rest (rest in relation to things everywhere adjacent) in which we find
all things in general.

Here on earth where gravity itself pulls all objects downward toward the
earth, and where resistance of the air tends to hold a moving body back
and bring it to rest, and where friction from contact with whatever
material substance may be in its path is perpetually tending to
neutralize all motion--with all three of these forces always at work to
stop a moving body, the truth of this first and fundamental law of
motion was not apparent on the surface.

Till Galileo's time everyone had made the mistake of supposing that some
force or other must be acting continually on every moving body to keep
it in motion. Ptolemy, Copernicus, Kepler, Leonardo da Vinci--all failed
to see the truth of this law which Newton developed in the immortal
_Principia_. And at the present day it is not always easy to accept at
first, although the progress of mechanical science, by reducing friction
and resistance, has produced machines in which motion of large masses
may be kept up indefinitely with the application of only the merest
minimum of force.

Once a planet is set in motion round the sun, it would go on forever
through frictionless, non-resistant space; but there must be a central
force, as Huygens saw clearly, to hold it in its orbit. Otherwise it
would at any moment take the direction of a tangent to the orbit. Here
is where Newton's second law of motion comes in, and he formulated it
with great definiteness. When any force acts on a moving body, its
deviation from a straight line will be in the direction of the force
applied and proportional to that force.

In accord with this law, Newton first began to inquire whether the force
of attraction here on earth, which everyone commonly recognizes as
gravity, drawing all things down toward the center of the earth, might
not extend upward indefinitely. It is found in operation on the summits
of mountain peaks, and the clouds above them and the rain falling from
them are obviously drawn downward by the same force. May it not extend
outward into space, even as far as the moon?

This was an audacious question, but Newton not only asked, but tried to
answer it in the year 1665, when he was only twenty-three. On the
surface of the earth this attraction is strong enough to draw a falling
body downward through a vertical space of sixteen feet in a second of
time. What ought it to be at the distance of the moon. The distance of
the moon in Newton's time was better known in terms of the earth's size
than was the size of the earth itself: the earth's radius was known to
be one-sixtieth of the moon's distance, but the earth's diameter was
thought to be something under 7,000 miles, so that Newton's first
calculations were most disappointing, and he laid them aside for nearly
twenty years.

Meanwhile the French astronomers led by Picard had measured the earth
anew, and showed it to be nearly 8,000 miles in diameter. As soon as
Newton learned of this, he revised his calculations, and found that by
the law of the inverse square the moon, in one second, should fall away
from a tangent to its orbit one thirty-six hundredth of sixteen feet.

This accorded exactly with his original supposition that the earth's
attraction extended to the moon. So he concluded that the force which
makes a stone fall, or an apple, as the story goes, is the same force
that holds the moon in its orbit, and that this force diminishes in the
exact proportion that the square of the distance from the earth's center
increases. The moon, indeed, becomes a falling body; only, as Kingdon
Clifford puts it: "She is going so fast and is so far off that she falls
quite around to the other side of the earth, instead of hitting it; and
so goes on forever."

    [Illustration: NICHOLAS COPERNICUS]

    [Illustration: GALILEO GALILEI]

    [Illustration: JOHANN KEPLER]

    [Illustration: SIR ISAAC NEWTON]

Newton goes on in the _Principia_ to explain the extension of
gravitation to the other bodies of the solar system beyond the earth and
moon. Clearly the same gravitation that holds the moon in its orbit
round the earth, must extend outward from the sun also, and hold all the
planets in their orbits centered about him. Newton demonstrates by
calculation based on Kepler's third law that (1) the forces drawing the
planets toward the sun are inversely as the squares of their mean
distances from him; and (2) if the force be constantly directed toward
the sun, the radius vector in an elliptic orbit must pass over equal
areas in equal times.




CHAPTER XIV

NEWTON AND GRAVITATION


So all of Kepler's laws could be embodied in a single law of gravitation
toward a central body, whose force of attraction decreases outward in
exact proportion as the square of the distance increases.

Only one farther step had to be taken, and this the most complicated of
all: he must make all the bodies of the sky conform to his third law of
motion. This is: Action and reaction are equal, or the mutual actions of
any two bodies are always equal and oppositely directed. There must be
mutual attractions everywhere: earth for sun as well as sun for earth,
moon for sun and sun for moon, earth for Venus and Venus for earth,
Jupiter for Saturn and Saturn for Jupiter, and so on.

The motions of the planets in the undisturbed ellipses of Kepler must be
impossible. As observations of the planets became more accurate, it was
found that they really did fail to move in exact accord with Kepler's
laws unmodified. Newton was unable, with the imperfect processes of the
mathematics of his day to ascertain whether the deviations then known
could be accounted for by his law of gravitation; but he nevertheless
formulated the law with entire precision, as follows:

Every particle of matter in the universe attracts every other particle
with a force exactly proportioned to the product of their masses, and
inversely as the square of the distance between their centers.

The centuries of astronomical research since Newton's day, however, have
verified the great law with the utmost exactness. Practically every
irregularity of lunar and planetary motion is accounted for; indeed, the
intricacies of the problems involved, and the nicety of their solution,
have led to the invention of new mathematical processes adequate to the
difficulties encountered.

And about the middle of the last century, when Uranus departed from the
path laid out for it by the mathematical astronomers, its orbital
deviations were made the basis of an investigation which soon led to the
assignment of the position where a great planet could be found that
would account for the unexplained irregularities of the motion of
Uranus. And the immediate discovery of this planet, Neptune, became the
most striking verification of the Newtonian law that the solar system
could possibly afford.

The astronomers of still later days investigating the statelier motions
of stellar systems find the Newtonian law regnant everywhere among the
stars where our most powerful telescopes have as yet reached. So that
Newton's law is known as the law of Universal Gravitation, and its
author is everywhere held as the greatest scientist of the ages.

Newton's _Principia_ may be regarded as the culminating research of the
inductive method, and further outline of its contents is desirable. It
is divided into three books following certain introductory sections. The
first book treats of the problems of moving bodies, the solutions being
worked out generally and not with special reference to astronomy. The
second book deals with the motion of bodies through resistant media, as
fluids, and has very little significance in astronomy. The third book is
the all important one, and applies his general principles to the case of
the actual solar system, providing a full explanation of the motions of
all the bodies of the system known in his day. Anyone who critically
reads the _Principia_ of Newton will be forced to conclude that its
author was a genius in the highest sense of the word. The elegance and
thoroughness of the demonstrations, and the completeness of application
of the law of gravitation are especially impressive.

The universality of his new law was the feature to which he gave
particular attention. It was clear to him that the gravitation of a
planet, although it acted as if wholly concentrated at the center, was
nevertheless resident in every one of the particles of which the planet
is composed. Indeed, his universal law was so formulated as to make
every particle attract every other particle; and an investigation known
as the Cavendish experiment--a research of great delicacy of
manipulation--not only proves this, but leads also to a measurement of
the earth's mean density, from which we can calculate approximately how
much the earth actually weighs.

Another way to attack the same problem is by measuring the attraction of
mountains, as Maskelyne, Astronomer Royal of Scotland did on Mount
Schehallien in Scotland, which was selected because of its sheer
isolation. The attraction of the mountain deflected the plumb-lines by
measurable amounts, the volume of the mountain was carefully
ascertained by surveys, and geologists found out what rocks composed it.
So the weight of the entire mountain became pretty well known, and
combining this with the observed deflection, an independent value of the
earth's weight was found.

Still other methods have been applied to this question, and as an
average it is found that the materials composing the earth are about
five and a half times as heavy as water, and the total weight of the
earth is something like six sextillions of tons.

What is the true shape of the earth? And does the earth's turning round
on its axis affect this shape? Newton saw the answer to these questions
in his law of gravitation. A spherical figure followed as a matter of
course from the mutual attraction of all materials composing the earth,
providing it was at rest, or did not turn round on its axis. But
rotation bulges it at the equator and draws it in at the poles, by an
amount which calculation shows to be in exact agreement with the amount
ascertained by actual measurement of the earth itself.

Another curious effect, not at first apparent, was that all bodies
carried from high latitudes toward the equator would get lighter and
lighter, in consequence of the centrifugal force of rotation. This was
unexpectedly demonstrated by Richer when the French Academy sent him
south to observe Mars in 1672. His clock had been regulated exactly in
Paris, and he soon found that it lost time when set up at Cayenne. The
amount of loss was found by observation, and it was exactly equal to the
calculated effect that the reduction of gravity by centrifugal action
should produce.

Also Newton saw that his law of gravitation would afford an explanation
of the rise and fall of the tides. The water on the side of the earth
toward the moon, being nearer to the moon, would be more strongly
attracted toward it, and therefore raised in a tide. And the water on
the farther side of the earth away from the moon, being at a greater
distance than the earth itself, the moon would attract the earth more
strongly than this mass of water, tending therefore to draw the earth
away from the water, and so raising at the same time a high tide on the
side of the earth away from the moon. As the earth turns round on its
axis, therefore, two tidal waves continually follow each other at
intervals of about twelve hours.

The sun, too, joins its gravitating force with that of the moon, raising
tides nearly half as high as those which the moon produces, because the
sun's vaster mass makes up in large part for its much greater distance.
At first and third quarters of the moon, the sun acts against the moon,
and the difference of their tide-producing forces gives us "neap tides";
while at new moon and full, sun and moon act together, and produce the
maximum effect known as "spring tides."

Newton passed on to explain, by the action of gravitation also, the
precession of the equinoxes, a phenomenon of the sky discovered by
Hipparchus, who pretty well ascertained its amount, although no reason
for it had ever been assigned. The plane of the earth's equator extended
to the celestial sphere marks out the celestial equator, and the two
opposite points where it intersects the plane of the ecliptic, or the
earth's path round the sun, are called the equinoctial points, or simply
the equinoxes. And precession of the equinoxes is the motion of these
points westward or backward, about 50 seconds each year, so that a
complete revolution round the ecliptic would take place in about 26,000
years.

Newton saw clearly how to explain this: it is simply due to the
attraction of the sun's gravitation upon the protuberant bulge around
the earth's equator, acting in conjunction with the earth's rotation on
its axis, the effect being very similar to that often seen in a spinning
top, or in a gyroscope. The moon moving near the ecliptic produces a
precessional effect, as also do the planets to a very slight degree; and
the observed value of precession is the same as that calculated from
gravitation, to a high degree of precision.

Newton died in 1727, too early to have witnessed that complete and
triumphant verification of his law which ultimately has accounted for
practically every inequality in the planetary motions caused by their
mutual attractions. The problems involved are far beyond the complexity
of those which the mathematical astronomer has to deal with, and the
mathematicians of France deserve the highest credit for improving the
processes of their science so that obstacles which appeared insuperable
were one after another overcome.

Newton's method of dealing with these problems was mainly geometric, and
the insufficiency of this method was apparent. Only when the French
mathematicians began to apply the higher methods of algebra was progress
toward the ultimate goal assured. D'Alembert and Clairaut for a time
were foremost in these researches, but their places were soon taken by
Lagrange, who wrote the "Mécanique Analytique," and Laplace, whose
"Mécanique Céleste" is the most celebrated work of all. In large part
these works are the basis of the researches of subsequent mathematical
astronomers who, strictly speaking, cannot as yet be said to have
arrived at a complete and rigorous solution of all the problems which
the mutual attractions of all the bodies of the solar system have
originated.

It may well be that even the mathematics of the present day are
incompetent to this purpose. When the brilliant genius of Sir William
Hamilton invented quaternion analysis and showed the marvelous facility
with which it solved the intricate problems of physics, there was the
expectation that its application to the higher problems of mathematical
astronomy might effect still greater advances; but nothing in that
direction has so far eventuated. Some astronomers look for the invention
of new functions with numerical tables bearing perhaps somewhat the
relation to present tables of logarithms, sines, tangents, and so on,
that these tables do to the simple multiplication table of Pythagoras.




CHAPTER XV

AFTER NEWTON


We have said that practically all the motions in the solar system have
been accounted for by the Newtonian law of gravitation. It will be of
interest to inquire into the instances that lead to qualification of
this absolute statement.

One relates to the planet Mercury, whose orbit or path round the sun is
the most elliptical of all the planetary orbits. This will be explained
a little later.

The moon has given the mathematical astronomers more trouble than any
other of the celestial bodies, for one reason because it is nearest to
us and very minute deviations in its motion are therefore detectible.
Halley it was who ascertained two centuries ago that the moon's motion
round the earth was not uniform, but subject to a slight acceleration
which greatly puzzled Lagrange and Laplace, because they had proved
exactly this sort of thing to be impossible, unless indeed the body in
question should be acted on by some other force than gravitation. But
Laplace finally traced the cause to the secular or very slow reduction
in the eccentricity of the earth's own orbit. The sun's action on the
moon was indeed progressively changing from century to century in such
manner as to accelerate the moon's own motion in its orbit round the
earth.

Adams, the eminent English astronomer, revised the calculations of
Laplace, and found the effect in question only half as great as Laplace
had done; and for years a great mathematical battle was on between the
greatest of astronomical experts in this field of research. Adams, in
conjunction with Delaunay, the greatest of the French mathematicians a
half century ago, won the battle in so far as the mathematical
calculations were concerned; but the moon continues to the present day
her slight and perplexing deviation, as if perhaps our standard
time-keeper, the earth, by its rotation round its axis, were itself
subject to variation. Although many investigations have been made of the
uniformity of the earth's rotation, no such irregularity has been
detected, and this unexplained variation of the moon's motion is one of
the unsolved problems of the gravitational astronomer of to-day.

But we are passing over the most impressive of all the earlier
researches of Lagrange and Laplace, which concerned the exceedingly slow
changes, technically called the secular variations of the elements of
the planetary orbits. These elements are geometrical relations which
indicate the form of the orbit, the size of the orbit, and its position
in space; and it was found that none of these relations or quantities
are constant in amount or direction, but that all, with but one
exception, are subject to very slow, or secular, change, or oscillation.

This question assumed an alarming significance at an early day,
particularly as it affected the eccentricity of the earth's orbit round
the sun. Should it be possible for this element to go on increasing for
indefinite ages, clearly the earth's orbit would become more and more
elliptical, and the sun would come nearer and nearer at perihelion, and
the earth would drift farther and farther from the sun at aphelion,
until the extremes of temperature would bring all forms of life on the
earth to an end. The refined and powerful analysis of Lagrange, however,
soon allayed the fears of humanity by accounting for these slow
progressive changes as merely part of the regular system of mere
oscillations, in entire accord with the operation of the law of
gravitation; and extending throughout the entire planetary system.
Indeed, the periods of these oscillations were so vast that none of them
were shorter than 50,000 years, while they ranged up to two million
years in length--"great clocks of eternity which beat ages as ours beat
seconds."

About a century ago, an eminent lecturer on astronomy told his audience
that the problem of weighing the planets might readily be one that would
seem wholly impossible to solve. To measure their sizes and distances
might well be done, but actually to ascertain how many tons they
weigh--never!

Yet if a planet is fortunate enough to have one satellite or more, the
astronomer's method of weighing the planet is exceedingly simple; and
all the major planets have satellites except the two interior ones,
Mercury and Venus. As the satellite travels round its primary, just as
the moon does round the earth, two elements of its orbit need to be
ascertained, and only two. First, the mean distance of the satellite
from its primary, and second the time of revolution round it.

Now it is simply a case of applying Kepler's third law. First take the
cube of the satellite's distance and divide it by the square of the
time of revolution. Similarly take the cube of the planet's distance
from the sun and divide by the square of the planet's time of revolution
round him. The proportion, then, of the first quotient to the second
shows the relation of the mass (that is the weight) of the planet to
that of the sun. In the case of Jupiter, we should find it to be 1,050,
in that of Saturn 3,500, and so on.

The range of planetary masses, in fact, is very curious, and is
doubtless of much significance in the cosmogony, with which we deal
later. If we consider the sun and his eight planets, the mass or weight
of each of the nine bodies far exceeds the combined mass of all the
others which are lighter than itself.

To illustrate: suppose we take as our unit of weight the one-billionth
part of the sun's weight; then the planets in the order of their masses
will be Mercury, Mars, Venus, Earth, Uranus, Neptune, Saturn, and
Jupiter. According to their relative masses, then, Mercury being a
five-millionth part the weight of the sun will be represented by 200;
similarly Venus, a four hundred and twenty-five thousandth part by
2,350, and so on. Then we have

  Mercury                                             200
  Mars                                                340
                                                   ------
    Sum of weights of Mercury and Mars                540
  Venus                                             2,350
                                                   ------
    Sum of weights of Mercury, Mars, and Venus      2,890
  The Earth                                         3,060
                                                   ------
    Sum of weights of four inner planets            5,950

  Uranus                                           44,250
                                                   ------
    Sum of weights of five planets                 50,200
  Neptune                                          51,600
                                                  -------
    Sum of weights of six planets                 101,800
  Saturn                                          285,580
                                                ---------
    Sum of weights of seven planets               387,380
  Jupiter                                         954,300
                                                ---------
    Sum of weights of all the planets           1,341,680
  Mass or weight of the sun                 1,000,000,000

Curious and interesting it is that Saturn is nearly three times as heavy
as the six lighter planets taken together, Jupiter between two and three
times heavier than all the other planets combined, while the sun's mass
is 750 times that of all the great planets of his system rolled into
one.

All the foregoing masses, except those of Mercury and Venus, are pretty
accurately known because they were found by the satellite method just
indicated. Mercury's mass is found by its disturbing effects on Encke's
comet whenever it approaches very near. The mass of Venus is ascertained
by the perturbations in the orbital motion of the earth. In such cases
the Newtonian law of gravitation forms the basis of the intricate and
tedious calculations necessary to find out the mass by this indirect
method.

Its inferiority to the satellite method was strikingly shown at the
Observatory in Washington soon after the satellites of Mars were
discovered in 1877. The inaccurate mass of that planet, as previously
known by months of computation based upon years and years of
observation, was immediately discarded in favor of the new mass derived
from the distance and period of the outer satellite by only a few
minutes' calculation.

In weighing the planets, astronomers always use the sun as the unit.
What then is the sun's own weight? Obviously the law of gravitation
answers this question, if we compare the sun's attraction with the
earth's at equal distances. First we conceive of the sun's mass as if
all compressed into a globe the size of the earth, and calculate how far
a body at the surface of this globe would fall in one second. The
relation of this number to 16.1 feet, the distance a body falls in one
second on the actual earth, is about 330,000, which is therefore the
number of times the sun's weight exceeds that of the earth.

A word may be added regarding the force of gravitation and what it
really is. As a matter of fact Newton did not concern himself in the
least with this inquiry, and says so very definitely. What he did was to
discover the law according to which gravitation acts everywhere
throughout the solar system. And although many physicists have
endeavored to find out what gravitation really is, its cause is not yet
known. In some manner as yet mysterious it acts instantaneously over
distances great and small alike, and no substance has been found which,
if we interpose it between two bodies, has in any degree the effect of
interrupting their gravitational tendency toward each other.

While the Newtonian law of gravitation has been accepted as true because
it explained and accounted for all the motions of the heavenly bodies,
even including such motions of the stars as have been subjected to
observation, astronomers have for a long time recognized that quite
possibly the law might not be absolutely exact in a mathematical sense,
and that deviations from it would surely make their appearance in time.

A crude instance of this was suggested about a century ago, when the
planet Uranus was found to be deviating from the path marked out for it
by Bouvard's tables based on the Newtonian law; and the theory was
advocated by many astronomers that this law, while operant at the medium
distances from the sun where the planets within Jupiter and Saturn
travel, could not be expected to hold absolutely true at the vast
distance of Uranus and beyond. The discovery of Neptune in 1846,
however, put an end to all such speculation, and has universally been
regarded as an extraordinary verification of the law, as indeed it is.

When, however, Le Verrier investigated the orbit of Mercury he found an
excess of motion in the perihelion point of the planet's orbit which
neither he nor subsequent investigators have been able to account for by
Newtonian gravitation, pure and simple. If Newton's theory is absolutely
true, the excess motion of Mercury's perihelion remains a mystery.

Only one theory has been advanced to account for this discrepancy, and
that is the Einstein theory of gravitation. This ingenious speculation
was first propounded in comprehensive form nearly fifteen years ago, and
its author has developed from it mathematical formulæ which appear to
yield results even more precise than those based on the Newtonian
theory.

In expressing the difference between the law of gravitation and his own
conception, Einstein says: "Imagine the earth removed, and in its place
suspended a box as big as a moon or a whole house and inside a man
naturally floating in the center, there being no force whatever pulling
him. Imagine, further, this box being, by a rope or other contrivance,
suddenly jerked to one side, which is scientifically termed 'difform
motion,' as opposed to 'uniform motion.' The person would then naturally
reach bottom on the opposite side. The result would consequently be the
same as if he obeyed Newton's law of gravitation, while, in fact, there
is no gravitation exerted whatever, which proves that difform motion
will in every case produce the same effects as gravitation.... The term
relativity refers to time and space. According to Galileo and Newton,
time and space were absolute entities, and the moving systems of the
universe were dependent on this absolute time and space. On this
conception was built the science of mechanics. The resulting formulas
sufficed for all motions of a slow nature; it was found, however, that
they would not conform to the rapid motions apparent in
electrodynamics.... Briefly the theory of special relativity discards
absolute time and space, and makes them in every instance relative to
moving systems. By this theory all phenomena in electrodynamics, as well
as mechanics, hitherto irreducible by the old formulæ, were
satisfactorily explained."

Natural phenomena, then, involving gravitation and inertia, as in the
planetary motions, and electro-magnetic phenomena, including the motion
of light, are to be regarded as interrelated, and not independent of one
another. And the Einstein theory would appear to have received a
striking verification in both these fields. On this theory the
Newtonian dynamics fails when the velocities concerned are a near
approach to that of light. The Newtonian theory, then, is not to be
considered as wrong, but in the light of a first approximation. Applying
the new theory to the case of the motion of Mercury's perihelion, it is
found to account for the excess quite exactly.

On the electro-magnetic side, including also the motion of light, a
total eclipse of the sun affords an especially favorable occasion for
applying the critical test, whether a huge mass like the sun would or
would not deflect toward itself the rays of light from stars passing
close to the edge of its disk, or limb. A total eclipse of exceptional
duration occurred on May 29, 1919, and the two eclipse parties sent out
by the Royal Society of London and the Royal Astronomical Society were
equipped especially with apparatus for making this test. Their stations
were one on the east coast of Brazil and the other on the west coast of
Africa.

Accurate calculation beforehand showed just where the sun would be among
the stars at the time of the eclipse; so that star plates of this region
were taken in England before the expeditions went out. Then, during the
total eclipse, the same regions were photographed with the eclipsed sun
and the corona projected against them. To make doubly sure, the stars
were a third time photographed some weeks after the eclipse, when the
sun had moved away from that particular region.

Measuring up the three sets of plates, it was found that an appreciable
deflection of the light of the stars nearest alongside the sun actually
exists; and the amount of it is such as to afford a fair though not
absolutely exact verification of the theory. The observed deflection
may of course be due to other causes, but the English astronomers
generally regard the near verification as a triumph for the Einstein
theory. Astronomers are already beginning preparations for a repetition
of the eclipse programme with all possible refinement of observation,
when the next total eclipse of the sun occurs, September 20, 1922,
visible in Australia and the islands of the Indian Ocean.

A third test of the theory is perhaps more critical than either of the
others, and this necessitates a displacement of spectral lines in a
gravitational field toward the red end of the spectrum; but the experts
who have so far made measures for detecting such displacement disagree
as to its actual existence. The work of St. John at Mt. Wilson is
unfavorable to the theory, as is that of Evershed of Kodiakanal, who has
made repeated tests on the spectrum of Venus, as well as in the cyanogen
bands of the sun.

The enthusiastic advocates of the Einstein theory hold that, as Newton
proved the three laws of Kepler to be special cases of his general law,
so the "universal relativity theory" will enable eventually the
Newtonian law to be deduced from the Einstein theory. "This is the way
we go on in science, as in everything else," wrote Sir George Airy,
Astronomer Royal; "we have to make out that something is true; then we
find out under certain circumstances that it is not quite true; and then
we have to consider and find out how the departure can be explained."
Meanwhile, the prudent person keeps the open mind.




CHAPTER XVI

HALLEY AND HIS COMET


Halley is one of the most picturesque characters in all astronomical
history. Next to Newton himself he was most intimately concerned in
giving the Newtonian law to the world.

Edmund Halley was born (1656) in stirring times. Charles I. had just
been executed, and it was the era of Cromwell's Lord Protectorate and
the wars with Spain and Holland. Then followed (1660) the promising but
profligate Charles II. (who nevertheless founded at Greenwich the
greatest of all observatories when Halley was nineteen), the frightful
ravages of the Black Plague, the tyrannies of James II., and the
Revolution of 1688--all in the early manhood of Halley, whose scientific
life and works marched with much of the vigor of the contending
personalities of state.

The telescope had been invented a half century earlier, and Galileo's
discoveries of Jupiter's moons and the phases of Venus had firmly
established the sun-centered theory of Copernicus.

The sun's distance, though, was known but crudely; and why the stars
seemed to have no yearly orbits of their own corresponding to that of
the earth was a puzzle. Newton was well advanced toward his supreme
discovery of the law of universal gravitation; and the authority of
Kepler taught that comets travel helter-skelter through space in
straight lines past the earth, a perpetual menace to humanity.

"Ugly monsters," that comets always were to the ancient world, the
medieval church perpetuated this misconception so vigorously that even
now these harmless, gauzy visitors from interstellar space possess a
certain "wizard hold upon our imagination." This entertaining phase of
the subject is excellently treated in President Andrew D. White's
"History of the Doctrine of Comets," in the Papers of the American
Historical Association. Halley's brilliant comet at its earlier
apparitions had been no exception.

Halley's father was a wealthy London soap maker, who took great pride in
the growing intellectuality of his son. Graduating at Queen's College,
Oxford, the latter began his astronomical labors at twenty by publishing
a work on planetary orbits; and the next year he voyaged to St. Helena
to catalogue the stars of the southern firmament, to measure the force
of terrestrial gravity, and observe a transit of Mercury over the disk
of the sun.

While clouds seriously interfered with his observations on that lonely
isle, what he saw of the transit led to his invention of "Halley's
method," which, as applied to the transit of Venus, though not till long
after his death, helped greatly in the accurate determination of the
sun's distance from the earth. Halley's researches on the proper motions
of the stars of both hemispheres soon made him famous, and it was said
of him, "If any star gets displaced on the globe, Halley will presently
find it out."

His return to London and election to the Royal Society (of which he was
many years secretary) added much to his fame, and he was commissioned
by the society to visit Danzig and arbitrate an astronomical controversy
between Hooke and Hevelius, both his seniors by a generation.

On the continent he associated with other great astronomers, especially
Cassini, who had already found three Saturnian moons; and it was then he
observed the great comet of 1680, which led up to the most famous event
of Halley's life.

The seerlike Seneca may almost be said to have predicted the advent of
Halley, when he wrote ("Quaestiones Naturales," vii): "Some day there
will arise a man who will demonstrate in what region of the heavens
comets pursue their way; why they travel apart from the planets; and
what their sizes and constitution are. Then posterity will be amazed
that simple things of this sort were not explained before."

To Newton it appeared probable that cometary voyagers through space
might have orbits of their own; and he proved that the comet of 1680
never swerved from such a path. As it could nowhere approach within the
moon's orbit, clearly threats of its wrecking the earth and punishing
its inhabitants ought to frighten no more.

Halley then became intensely interested in comets, and gathered whatever
data concerning the paths of all these bodies he could find. His first
great discovery was that the comets seen in 1531 by Apian, and in 1607
by Kepler, traveled round the sun in identical paths with one he had
himself observed in 1682. A still earlier appearance of Halley's comet
(1456) seems to have given rise to a popular and long-reiterated myth of
a papal bull excommunicating "the Devil, the Turk, and the Comet."

No longer room for doubt: so certain was Halley that all three were one
and the same comet, completing the round of its orbit in about
seventy-six years, that he fearlessly predicted that it would be seen
again in 1758 or 1759. And with equal confidence he might have foretold
its return in 1835 and 1910; for all three predictions have come true to
the letter.

Halley's span of existence did not permit his living to see even the
first of these now historic verifications. But we in our day may
emphatically term the epoch of the third verified return _Annus
Halleianus_.

Says Turner, Halley's successor in the Savilian chair at Oxford to-day:
"There can be no more complete or more sensational proof of a scientific
law, than to predict events by means of it. Halley was deservedly the
first to perform this great service for Newton's Law of Gravitation, and
he would have rejoiced to think how conspicuous a part England was to
play in the subsequent prediction of the existence of Neptune."

Halley rose rapidly among the chief astronomical figures of his day. But
he had little veneration for mere authority, and the significant veering
of his religious views toward heterodoxy was for years an obstacle to
his advance.

Still Halley the astronomer was great enough to question any
contemporary dicta that seemed to rest on authority alone. Everyone
called the stars "fixed" stars; but Halley doubting this, made the first
discovery of a star's individual motion--proper motion, as astronomers
say. To-day, two hundred years after, every star is considered to be in
motion, and astronomers are ascertaining their real motions in the
celestial spaces to a nicety undreamed of by even the exacting Halley.

The moon, of priceless service to the early navigator, was regarded by
all astronomers as endowed with an average rate of motion round the
earth that did not vary from age to age. But Halley questioned this too;
and on comparing with the ancient value from Chaldean eclipses, he made
another discovery--the secular acceleration of the moon's mean motion,
as it is technically termed. This was a colossal discovery in celestial
dynamics; and the reason underlying it lay hidden in Newton's law for
yet another century, till the keener mathematics of Laplace detected its
true origin.

       *       *       *       *       *

With Newton, Halley laid down the firm foundations of celestial
mechanics, and they pushed the science as far as the mathematics of
their day would permit. Halley, however, was not content with
elucidating the motion of bodies nearest the earth, and pressed to the
utmost confines of the solar system known to him. Here, too, he made a
signal discovery of that mutual disturbance of the planets in their
motion round the sun, called the great inequality of Jupiter and Saturn.

Halley's versatile genius attacked all the great problems of the day.
His observation of the sun's total eclipse in 1715 is the earliest
reliable account of such a phenomenon by a trained astronomer. He
described the corona minutely and was the first to see that other
interesting phenomenon which only an alert observer can detect, which a
great astronomer of a later day compared to the "ignition of a fine
train of gunpowder," and which has ever since borne the name of "Baily's
beads."

Besides being a great astronomer, Halley was a man of affairs as well,
which Newton, although the greater mathematician, was not. Without
Halley, Newton's superb discovery might easily have been lost to the age
and nation, for the latter was bent merely on making discoveries, and on
speculative contemplation of them, with never a thought of publishing to
the world.

Halley, more practical and businesslike, insisted on careful writing out
and publication. Newton was then only forty-two, and Halley fully
fourteen years his junior. But the philosophers of that day were keenly
alive to the mystery of Kepler's laws, and Halley was fully conscious of
the grandeur and far-reaching significance of Newton's great
generalization which embodied all three of Kepler's laws in one.

Newton at last yielded, though reluctantly, and the "Principia" was
given to the world, though wholly at Halley's private charges.

But Halley was far from being completely engrossed with the absorbing
problems of the sky; things terrestrial held for years his undivided
attention. Imagine present-day Lords Commissioners of the Admiralty
intrusting a ship of the British navy to civilian command. Yet such was
their confidence in Halley that he was commissioned as captain of H.
M.'s pink _Paramour_ in 1698, with instructions to proceed to southern
seas for geographical discoveries, and for improving knowledge of the
longitude problem, and of the variations of the compass. Trade winds and
monsoons, charts of magnetic variation, tides and surveys of the Channel
coast, and experiments with diving bells were practical activities that
occupied his attention.

Halley in 1720 became Astronomer Royal. He was the second incumbent of
this great office, but the first to supply the Royal Observatory with
instruments of its own, some of which adorn its walls even to-day. His
long series of lunar observations and his magnetic researches were of
immense practical value in navigation.

Halley lived to a ripe old age and left the world vastly better than he
found it. His rise from humblest obscurity was most remarkable, and he
lived to gratify all the ambitions of his early manhood. "Of attractive
appearance, pleasing manners, and ready wit," says one of his
biographers, "loyal, generous, and free from self-seeking, he was one of
the most personally engaging men who ever held the office of Astronomer
Royal."

He died in office at Greenwich in 1742.

"Halley was buried," says Chambers, "in the churchyard of St.
Margaret's, Lee, not far from Greenwich, and it has lately been
announced that the Admiralty have decided to repair his tomb at the
public expense, no descendants of his being known." There is no suitable
monument in England to the memory of one of her greatest scientific men.
In any event the collection and republication of his epoch-making papers
would be welcomed by astronomers of every nation.




CHAPTER XVII

BRADLEY AND ABERRATION


Living at Kew in London early in the 18th century was an enthusiastic
young astronomer, James Bradley. He is famous chiefly for his accurate
observations of star places which have been invaluable to astronomers of
later epochs in ascertaining the proper motions of stars.

The latitude of Bradley's house in Kew was very nearly the same as the
declination of the bright star Gamma Draconis, so that it passed through
his zenith once every day. Bradley had a zenith sector, and with this he
observed with the greatest care the zenith distance of Gamma Draconis at
every possible opportunity. This he did by pointing the telescope on the
star and then recording the small angle of its inclination to a fine
plumb line. So accurate were his measures that he was probably certain
of the star's position to the nearest second of arc.

What he hoped to find was the star's motion round a very slight orbit
once each year, and due to the earth's motion in its orbit round the
sun. In other words, he sought to find the star's parallax if it turned
out to be a measurable quantity.

It is just as well now that his method of observation proved
insufficiently delicate to reveal the parallax of Gamma Draconis; but
his assiduity in observation led him to an unexpected discovery of
greater moment at that time. What he really found was that the star had
a regular annual orbit; but wholly different from what he expected, and
very much larger in amount. This result was most puzzling to Bradley.
The law of relative motion would require that the star's motion in its
expected orbit should be opposite to that of the earth in its annual
orbit; instead of which the star was all the time at right angles to the
earth's motion.

Bradley was a frequent traveler by boat on the Thames, and the apparent
change in the direction of the wind when the boat was in motion is said
to have suggested to him what caused the displacement of Gamma Draconis.
The progressive motion of light had been roughly ascertained by Roemer:
let that be the velocity of the wind. And the earth's motion in its
orbit round the sun, let that be the speed of the boat. Then as the wind
(to an observer on the moving boat) always seems to come from a point in
advance of the point it actually proceeds from (to an observer at rest),
so the star should be constantly thrown forward by an angle given by the
relation of the velocity of light to the speed of the earth in orbital
revolution round the sun.

The apparent places of all stars are affected in this manner, and this
displacement is called the aberration of light. Astronomers since
Bradley's discovery of aberration in 1726 have devoted a great deal of
attention to this astronomical constant, as it is called, and the arc
value of it is very nearly 20".5. This means that light travels more
than ten thousand times as fast as the earth in its orbit (186,330 miles
per second as against the earth's 18.5). And we can ascertain the sun's
distance by aberration also because the exact values of the velocity of
light and of the constant of aberration when properly combined give the
exact orbital speed of the earth; and this furnishes directly by
geometry the radius of the earth's orbit, that is the distance of the
sun.

In fact, this is one of the more accurate modern methods of ascertaining
the distance of the sun. As early as 1880 it enabled the writer to
calculate the sun's parallax equal to 8".80, a value absolutely
identical with that adopted by the Paris Conference of 1896, and now
universally accepted as the standard.

In whatever part of the sky we observe, every star is affected by
aberration. At the poles of the ecliptic, 23-1/2 degrees from the
earth's poles, the annual aberration orbits of the stars are very small
circles, 41" in diameter. Toward the ecliptic the aberration orbits
become more and more oval, ellipses in fact of greater and greater
eccentricity, but with their major axes all of the same length, until we
reach the ecliptic itself; and then the ellipse is flattened into a
straight line 41" in length, in which the star travels forth and back
once a year. Exact correspondence of the aberration ellipses of the
stars with the annual motion of the earth round the sun affords
indisputable proof of this motion, and as every star partakes of the
movement, this proof of our motion round the sun becomes many
million-fold.

Indeed, if we were to push a little farther the refinement of our
analysis of the effect of aberration on stellar positions, we could
prove also the rotation of the earth on its axis, because that motion is
swift enough to bear an appreciable ratio to the velocity of light.
Diurnal aberration is the term applied to this slight effect, and as
every star partakes of it, demonstration of the earth's turning round on
its axis becomes many million-fold also.




CHAPTER XVIII

THE TELESCOPE


Had anyone told Ptolemy that his earth-centered system of sun, moon, and
stars would ultimately be overthrown, not by philosophy but by the
overwhelming evidence furnished by a little optical instrument which so
aided the human eye that it could actually see systems of bodies in
revolution round each other in the sky, he would no doubt have
vehemently denied that any such thing was possible. To be sure, it took
fourteen centuries to bring this about, and the discovery even then was
without much doubt due to accident.

Through all this long period when astronomy may be said to have merely
existed, practically without any forward step or development, its
devotees were unequipped with the sort of instruments which were
requisite to make the advance possible. There were astrolabes and
armillary spheres, with crudely divided circles, and the excellent work
done with them only shows the genius of many of the early astronomers
who had nothing better to work with. Regarding star-places made with
instruments fixed in the meridian, Bessel, often called the father of
practical astronomy, used to say that, even if you provided a bad
observer with the best of instruments, a genius could surpass him with a
gun barrel and a cart wheel.

Before the days of telescopes, that is, prior to the seventeenth
century, it was not known whether any of the planets except the earth
had a moon or not; consequently the masses of these planets were but
very imperfectly ascertained; the phases of Mercury and Venus were
merely conjectured; what were the actual dimensions of the planets could
only be guessed at; the approximate distances of sun, moon, and planets
were little better than guesses; the distances of the stars were wildly
inaccurate; and the positions of the stars on the celestial sphere, and
of sun, moon, and planets among them were far removed from modern
standards of precision--all because the telescope had not yet become
available as an optical adjunct to increase the power of the human eye
and enable it to see as if distances were in considerable measure
annihilated.

Galileo almost universally is said to have been the inventor of the
telescope, but intimate research into the question would appear to give
the honor of that original invention to another, in another country.
What Galileo deserves the highest praise for, however, is the
reinvention independently of an "optick tube" by which he could bring
distant objects apparently much nearer to him; and being an astronomer,
he was by universal acknowledgment first of all men to turn a telescope
on the heavenly bodies. This was in the year 1609, and his first
discovery was the phase of Venus, his second the four Medicean moons or
satellites of Jupiter, discoveries which at that epoch were of the
highest significance in establishing the truth of the Copernican system
beyond the shadow of doubt.

But the first telescopes of which we have record were made, so far as
can now be ascertained, in Holland very early in the 17th century.
Metius, a professor of mathematics, and Jansen and Lipperhey, who were
opticians in Middelburg--all three are entitled to consideration as
claimants of the original invention of the telescope. But that such an
instrument was pretty well known would appear to be shown by his
government's refusal of a patent to Lipperhey in 1608; while the
officials recognizing the value of such an instrument for purposes of
war, got him to construct several telescopes and ordered him to keep the
invention a secret.

Within a year Galileo heard that an instrument was in use in Holland by
which it was possible to see distant objects as if near at hand. Skilled
in optics as he was, the reinvention was a task neither long nor
difficult for him. One of his first instruments magnified but three
times; still it made a great sensation in Venice where he exhibited the
little tube to the authorities of that city, in which he first invented
it.

Galileo's telescope was of the simplest type, with but two lenses; the
one a double convex lens with which an image of the distant object is
formed, the other a double concave lens, much smaller which was the
eye-lens for examining the image. It is this simple form of Galilean
telescope that is still used in opera glasses and field glasses, because
of the shorter tube necessary.

Galileo carried on the construction of telescopes, all the time
improving their quality and enlarging their power until he built one
that magnified thirty times. What the diameter of the object glass was
we do not know, perhaps two inches or possibly a little more. Glass of a
quality good enough to make a telescope of cannot have been abundant or
even obtainable except with great difficulty in those early days.

Other discoveries by this first of celestial observers were the spots on
the sun, the larger mountains of the moon, the separate stars of which
the Milky Way is composed, and, greatest wonder of all, the anomalous
"handles" (_ansæ_, he called them) of Saturn, which we now know as the
planet's ring, the most wonderful of all the bodies in the sky.

Since Galileo's time, only three centuries past, the progress in size
and improvement in quality of the telescope have been marvelous. And
this advance would not have been possible except for, first, the
discoveries still kept in large part secret by the makers of optical
glass which have enabled them to make disks of the largest size; second,
the consummate skill of modern opticians in fashioning these disks into
perfect lenses; and third, the progress in the mechanical arts and
engineering, by which telescope tubes of many tons' weight are mounted
or poised so delicately that the thrust of a finger readily swerves them
from one point of the heavens to another.

As the telescope is the most important of all astronomical instruments,
it is necessary to understand its construction and adjustment and how
the astronomer uses it. Telescopes are optical instruments, and nothing
but optical parts would be requisite in making them, if only the optical
conditions of their perfect working could be obtained without other
mechanical accessories.

In original principle, all telescopes are as simple as Galileo's; first,
an object glass to form the image of the distant object; second the
eyepiece usually made of two lenses, but really a microscope, to magnify
that image, and working in the same way that any microscope magnifies an
object close at hand; and third, a tube to hold all the necessary
lenses in the true relative positions.

    [Illustration: THE 100-INCH HOOKER TELESCOPE, LARGEST REFLECTOR
       IN THE WORLD, ON MT. WILSON. (_Photo, Mt. Wilson Solar
       Observatory._)]

    [Illustration: THE LARGEST REFRACTOR, THE 40-INCH TELESCOPE AT
       YERKES OBSERVATORY. DOME 90 FT. IN DIAMETER. (_Photo, Yerkes
       Observatory._)]

    [Illustration: THE 150-FT. TOWER AT THE MT. WILSON SOLAR
       OBSERVATORY. At the left is a diagram of tower, telescope
       and pit. At the upper right is an exterior view of the
       tower; below a view looking down into the pit, 75 ft. deep.
       (_Photo, Mt. Wilson Solar Observatory._)]

The focal lengths of object glass and eyepiece will determine just what
distance apart the lenses must be in order to give perfect vision. But
it is quite as important that the axes of all the lenses be adjusted
into one and the same straight line, and then held there rigidly and
permanently. Otherwise vision with the telescope will be very imperfect
and wholly unsatisfactory. The distance from the objective, or object
glass to its focal point is called its focal length; and if we divide
this by the focal length of the eyepiece, we shall have the magnifying
power of the telescope. The eyepiece will usually be made of two lenses,
or more, and we use its focal length considered as a single lens, in
getting the magnifying power. A telescope will generally have many
eyepieces of different focal lengths, so that it will have a
corresponding range of magnifying powers. The lowest magnifying power
will be not less than four or five diameters for each inch of aperture
of the objective; otherwise the eye will fail to receive all the light
which falls upon the glass. A 4-inch telescope will therefore have no
eyepiece with a lower magnifying power than about 20 diameters. The
highest magnifying power advantageous for a glass of this size will be
about 250 to 300, the working rule being about 70 diameters to each inch
of aperture, although the theoretical limit is regarded as 100.

The reason for a variety of eyepieces with different magnifying powers
soon becomes apparent on using the telescope. Comets and nebulæ call for
very low powers, while double stars and the planetary surfaces require
the higher powers, provided the state of the atmosphere at the moment
will allow it. If there is much quivering and unsteadiness, nothing is
gained by trying the higher powers, because all the waves of
unsteadiness are magnified also in the same proportion, and sharpness of
vision, or fine definition, or "good seeing," as it is called, becomes
impossible. The vibrations and tremors of the atmosphere are the
greatest of all obstacles to astronomical observation, and the search is
always in order for regions of the world, in deserts or on high
mountains, where the quietest atmosphere is to be found.

Quite another power of the telescope is dependent on its objective
solely: its light-gathering power. Light by which we see a star or
planet is admitted to the retina of the eye through an adjustable
aperture called the pupil. In the dark or at night, the pupil expands to
an average diameter of one-fourth of an inch. But the object-glass of a
telescope, by focusing the rays from a star, pours into the eye, almost
as a funnel acts with water, all the light which falls on its larger
surface. And as geometry has settled it for us that areas of surfaces
are proportioned to the squares of their diameters, a two-inch object
glass focuses upon the retina of the eye 64 times as much light as the
unassisted eye would receive. And the great 40-inch objective of the
Yerkes telescope would, theoretically, yield 25,600 times as much light
as the eye alone. But there would be a noticeable percentage of this
lost through absorption by the glasses of the telescope and scattering
by their surfaces.

The first makers of telescopes soon encountered a most discouraging
difficulty, because it seemed to them absolutely insuperable. This is
known as chromatic aberration, or the scattering of light in a
telescope due simply to its color or wave length. When light passes
through a prism, red is refracted the least and violet the most. Through
a lens it is the same, because a lens may be regarded as an indefinite
system of prisms. The image of a star or planet, then, formed by a
single lens cannot be optically perfect; instead it will be a confused
intermingling of images of various colors. With low powers this will not
be very troublesome, but great indistinctness results from the use of
high magnifying powers.

The early makers and users of telescopes in the latter part of the
seventeenth century found that the troublesome effects of chromatic
aberration could be much reduced by increasing the focal length of the
objective. This led to what we term engineering difficulties of a very
serious nature, because the tubes of great length were very awkward in
pointing toward celestial objects, especially near the zenith, where the
air is quietest. And it was next to impossible to hold an object
steadily in the field, even after all the troubles of getting it there
had been successfully overcome.

Bianchini and Cassini, Hevelius and Huygens were among the active
observers of that epoch who built telescopes of extraordinary length, a
hundred feet and upward. One tube is said to have been built 600 feet in
length, but quite certainly it could never have been used. So-called
aerial telescopes were also constructed, in which the objective was
mounted on top of a tower or a pole, and the eyepiece moved along near
the ground. But it is difficult to see how anything but fleeting
glimpses of the heavenly bodies could have been obtained with such
contrivances, even if the lenses had been perfect. Newton indeed, who
was expert in optics, gave up the problem of improving the refracting
telescope, and turned his energies toward the reflector.

In 1733, half a century after Newton and a century and a quarter after
Galileo, Chester More Hall, an Englishman, found by experiment that
chromatic aberration could be nearly eliminated by making the objective
of two lenses instead of one, and the same invention was made
independently by Dollond, an English optician, who took out letters
patent about 1760. So the size of telescopes seemed to be limited only
by the skill of the glassmaker and the size of disks that he might find
it practicable to produce.

What Hall and Dollond did was to make the outer or crown lens of the
objective as before, and place behind it a plano-concave lens of dense
flint glass. This had the effect of neutralizing the chromatic effect,
or color aberration, while at the same time only part of the refractive
effect of the crown lens was destroyed. This ingenious but costly
combination prepared the way for the great refracting telescopes of the
present day, because it solved, or seemed to solve, the important
problem of getting the necessary refraction of light rays without
harmful dispersion or decomposition of them.

Through the 18th century and the first years of the 19th many telescopes
of a size very great for that day were built, and their success seemed
complete. With large increase in the size of the disks, however, a new
trouble arose, quite inherent in the glass itself. The two kinds of
glass, flint and crown, do not decompose white light with uniformity, so
that when the so-called achromatic objective was composed of flint and
crown, there was an effect known as irrationality of dispersion, or
secondary spectrum, which produced a very troublesome residuum of blue
light surrounding the images of bright objects. This is the most serious
defect of all the great refractors of the day, and effectively it limits
their size to about 60 inches of aperture, with present types of flint
and crown. It is expected by present experimenters, however, that
further improvements in optical glass will do much to extend this limit;
so that a refracting telescope of much greater size than any now in
existence will be practicable.

Improvements in mounting telescopes, too, are still possible. Within
recent years, Hartness, of Springfield, Vermont, has erected a new and
ingenious type of turret telescope which protects the observer from wind
and cold while his instrument is outside. It affords exceptional
facilities for rapid and convenient observing, as for variable stars,
and is adaptable to both refractors and reflectors.

The captivating study of the heavens can of course be begun with the
naked eye alone, but very moderate optical assistance is a great help
and stimulates. An opera-glass affords such assistance; a field-glass
does still better, and best of all, for certain purposes, is a modern
prism-binocular.




CHAPTER XIX

REFLECTORS--MIRROR TELESCOPES


Cherished with the utmost care in the rooms of the Royal Society of
London is a world-famous telescope, a diminutive reflector made by the
hands of Sir Isaac Newton. We have already mentioned his connection with
the refractor; and how he abandoned that type of telescope in favor of
the reflecting mirror, or reflector in which the obstacles to great size
appeared to be purely mechanical. By many, indeed, Newton is regarded as
the inventor of the reflector.

By the principles of optics, all the rays from a star that strike a
concave mirror will be reflected to the geometric focal point, provided
a section of that mirror is a parabola. Such a mirror is called a
speculum, and is an alloy of tin, copper, and bismuth. Its surface takes
a very high polish, reflecting when newly polished nearly 90 per cent of
the light that falls upon it.

But the focus where the eyepiece must be used is in front of the mirror,
and if the eye were placed there, the observer's head would intercept
all or much of the light that would otherwise reach the mirror. Gregory,
probably the real inventor of the reflector, was the first to dodge this
difficulty by perforating the mirror at the center and applying the
eyepiece there, at the back of the speculum; but it was necessary to
first send the rays to that point by reflection from a second or
smaller mirror, in the optical axis of the speculum. This reflects the
rays backward down the tube to the eyepiece, or spectroscope, or camera.

Another English optician, Cassegrain, improved on this design somewhat
by placing the secondary mirror inside the focus of the speculum, or
nearer to it, so that the tube is shorter. This form is preferable for
many kinds of astronomical work, especially photography. Herschel sought
to do away with the secondary reflector entirely and save the loss of
light by tilting the speculum slightly, so as to throw the image at one
side of the tube; but this modification introduces bad definition of the
image and has never been much used.

A better plan is that of Newton, who placed a small plane speculum at an
angle of 45 degrees in the optical axis where the secondary mirror of
the Gregory-Cassegrainian type is placed. The rays are then received by
the eyepiece at the side of the upper end of the tube, the observer
looking in at right angles to the axis. And a modern improvement first
used by Draper is a small rectangular prism in place of the little plane
speculum, effecting a saving of five to ten per cent of the light.

It is not easy to say which type of telescope, the refractor or the
reflector, is the more famous. Nor which is the better or more useful,
or the more likely to lead in the astronomy of the future. When the
successors of Dollond had carried the achromatic refractor to the limit
enforced by the size of the glass disks they were able to secure, they
found these instruments not so great an improvement after all. The
single-lens telescopes of great focal length were nearly as good
optically, though much more awkward to handle. But the quality of the
glass obtainable in that day appeared to set an arbitrary limit to that
great amplification of size and power which progress in observational
astronomy demanded.

Then came the elder Herschel, best known and perhaps the greatest of all
astronomers. At Bath, England, music was his profession, especially the
organ. But he was dissatisfied with his little Gregorian reflector, and
being a very clever mechanician he set out to build a reflector for
himself. It is said that he cast and polished nearly 200 mirrors, in the
course of experiments on the most highly reflective type of alloys, and
the sort of mechanism that would enable him to give them the highest
polish. In all his work he was ably and enthusiastically aided by his
sister, Caroline Herschel, most famous of all women astronomers.

Upward in size of his mirrors he advanced, till he had a speculum of two
feet diameter with a tube 20 feet long. Twelve to fifteen years had
elapsed when in 1781, while testing one of these reflectors on stars in
the constellation Gemini, he made the first discovery of a planet since
the invention of the telescope--the great planet now known as Uranus.

Under the patronage of King George, he advanced to telescopes of still
greater size, his largest being no less than forty feet in length, with
a speculum of four feet in diameter. Two new satellites of Saturn were
discovered with this giant reflector, which was dismantled by Sir John
Herschel with appropriate ceremonies, including the singing of an ode by
the Herschel family assembled inside of the tube, on New Year's Eve,
1839-40.

We have record of but few attempts to improve the size and definition of
great reflectors by the continental astronomers during this era. In
England and Ireland, however, great progress was made. About 1860
Lassell built a two-foot reflector, with which he discovered two new
satellites of Uranus, and which he subsequently set up in the island of
Malta. Ten years later Thomas Grubb and Son of Dublin constructed a
four-foot reflector, now at the Observatory in Melbourne, Australia.
Calver in conjunction with Common of Ealing, London, about 1880-95 built
several large reflectors, the largest of five feet diameter, now owned
by Harvard College Observatory; and, rather earlier, Martin of Paris
completed a four-foot reflector.

The mirrors of these latter instruments were not made of speculum metal,
but of solid glass, which must be very thick (one-seventh their
diameter) in order to prevent flexure or bending by their own weight. So
sensitive is the optical surface to distortion that unless a complicated
series of levers and counterpoises is supplied, to support the under
surface of the mirror, the perfection of its optical figure disappears
when the telescope is directed to objects at different altitudes in the
sky. The upper or outer surface of the glass is the one which receives
the optical polish on a heavy coat of silver chemically deposited on the
polished glass after its figure has been tested and found satisfactory.

But far and away the most famous reflecting telescope of all is the
"Leviathan" of Lord Rosse, built at Birr Castle, Parsonstown, Ireland,
about the middle of the last century. His Lordship made many ingenious
improvements in grinding the mirror, which was of speculum metal, six
feet in diameter and weighed seven tons. It was ground to a focal
length of fifty-four feet and mounted between heavy walls of masonry, so
that the motion of the great tube was restricted to a few degrees on
both sides of the meridian. The huge mechanism was very cumbersome in
operation, and photography was not available in those days; nevertheless
Lord Rosse's telescope made the epochal discovery of the spiral nebulæ,
which no other telescope of that day could have done.

In America the reflector has always kept at least even pace with the
refractor. As early as 1830, Mason and Smith, two students at Yale
College, enthused by Denison Olmsted, built a 12-inch speculum with
which they made unsurpassed observations of the nebulæ. Dr. Henry
Draper, returning from a visit to Lord Rosse, began about 1865 the
construction of two silver-on-glass reflectors, one of 15 inches
diameter, the other of 28 inches, with which he did important work for
many years in photography and spectroscopy, and his mirrors are now the
property of Harvard College Observatory. Alvan Clark and Sons have in
later years built a 40-inch mirror for the Lowell Observatory in
Arizona, and very recently a 6-foot silver-on-glass mirror has been set
up in the Dominion of Canada Astrophysical Observatory at Victoria,
British Columbia, where it is doing excellent work in the hands of
Plaskett, its designer.

The huge glass disk for the reflector weighs two tons, and it must be
cast so that there are no internal strains; otherwise it is liable to
burst in fragments in the process of grinding. It should be free from
air-bubbles, too; so the glass is cast in one melting, if possible. This
disk was made by the St. Gobain Plate Glass Company, whose works have
been ruthlessly destroyed by the enemy during the war; but fortunately
the great disk had been shipped from Antwerp only a week before
declaration of hostilities.

Brashear of Allegheny was intrusted with the optical parts, which
occupied many months of critical work. The finished mirror is 73 inches
in diameter, its focal length is 30 feet, and its thickness 12 inches. A
central hole 10 inches in diameter makes possible its use as a Gregorian
or Cassegrainian type, as well as Newtonian. The mechanical parts of
this great telescope are by Warner and Swasey of Cleveland, after the
well-known equatorial mounting of the Melbourne reflector by Grubb of
Dublin. Friction of the polar and declination axes is reduced by ball
bearings. The 66-foot dome has an opening 15 feet wide and extending six
feet beyond the zenith. All motions of the telescope, dome shutters, and
observing platform are under complete control by electric motors.
Spectroscopic binaries form one of the special fields of research with
this powerful instrument, and many new binaries have already been
detected.

The great reflectors designed and constructed by Ritchey, formerly of
Chicago and now of Pasadena, deserve especial mention. While connected
with the Yerkes Observatory he constructed a two-foot reflector for that
institution, with which he had exceptional success in photography of the
stars and nebulæ. Later he built a 5-foot reflector, now at the Carnegie
Observatory on Mount Wilson, California, with which the spiral nebulæ
and many other celestial objects have been especially well photographed.
Ritchey's later years have been spent on the construction of an even
greater mirror, no less than 100 inches in diameter, which was completed
in 1919, and has already yielded photographic results dealt with farther
on, and far surpassing anything previously obtained. Theoretically this
huge mirror, if its surface were perfectly reflective so that it would
transmit all the rays falling upon it, would gather 160,000 times as
much light as the unaided eye alone.

Whether a 72-inch refractor, should it ever be constructed, would
surpass the 100-inch reflector as an all-round engine for astronomical
research, is a question that can only be fully answered by building it
and trying the two instruments alongside.

Probably three-quarters of all the really great astronomical work in the
past has been done by refractors. They are always ready and convenient
for use, and the optical surfaces rarely require cleaning and
readjustment. With increase of size, however, the secondary spectrum
becomes very bothersome in the great lenses; and the larger they are,
the more light is lost by absorption on account of the increasing
thickness of the lenses. With the reflector on the other hand, while
there is clearly a greater range of size, the reflective surface retains
its high polish only a brief period, so that mere tarnish effectively
reduces the aperture; and the great mirror is more or less ineffective
in consequence of flexure uncompensated by the lever system that
supports the back of the mirror.

Both types of telescope still have their enthusiastic devotees; and the
next great reflector would doubtless be a gratifying success, if mounted
in some elevated region of the world, like the Andes of northern Chile,
where the air is exceptionally steady and the sky very clear a large
part of the year. The highest magnifying powers suitable for work with
such a telescope could then be employed, and new discoveries added as
well as important work done in extension of lines already begun on the
universe of stars.

On the authority of Clark, even a six-foot objective would not
necessitate a combined thickness of its glasses in excess of six inches.
Present disks are vastly superior to the early ones in transparency, and
there is reason to expect still greater improvement. The engineering
troubles incident to execution of the mechanical side of the scheme need
not stand in the way; they never have, indeed the astronomer has but
just begun to invoke the fertile resources of the modern engineer. Not
long before his death the younger Clark who had just finished the great
lenses of the 40-inch Yerkes telescope, ventured this prevision, already
in part come true: "The new astronomy, as well as the old, demands more
power. Problems wait for their solution, and theories to be
substantiated or disproved. The horizon of science has been greatly
broadened within the last few years, but out upon the borderland I see
the glimmer of new lights that await for their interpretation, and the
great telescopes of the future must be their interpreters."

Practically all the great telescopes of the world have in turn
signalized the new accession of power by some significant astronomical
discovery: to specify, one of Herschel's reflectors first revealed the
planet Uranus; Lord Rosse's "Leviathan" the spiral nebulæ; the 15-inch
Cambridge lens the crape, or dusky ring of Saturn; the 18-1/2-inch
Chicago refractor the companion of Sirius; the Washington 26-inch
telescope the satellites of Mars; the 30-inch Pulkowa glass the
nebulosities of the Pleiades; and the 36-inch Lick telescope brought to
light a fifth satellite of Jupiter. At the time these discoveries were
made, each of these great telescopes was the only instrument then in
existence with power enough to have made the discovery possible. So we
may advance to still farther accessions of power with the expectation
that greater discoveries will continue to gratify our confidence.




CHAPTER XX

THE STORY OF THE SPECTROSCOPE


Sir Isaac Newton ought really to have been the inventor of the
spectroscope, because he began by analyzing light in the rough with
prisms, was very expert in optics, and was certainly enough of a
philosopher to have laid the foundations of the science.

What Newton did was to admit sunlight into a darkened room through a
small round aperture, then pass the rays through a glass prism and
receive the band of color on a screen. He noticed the succession of
colors correctly--violet, indigo, blue, green, yellow, orange, red; also
that they were not pure colors, but overlapping bands of color.
Apparently neither he nor any other experimenter for more than a century
went any further, when the next essential step was taken by Wollaston
about 1802 in England. He saw that by receiving the light through a
narrow slit instead of a round hole, he got a purer spectrum, spectrum
being the name given to the succession of colors into which the prism
splits up or decomposes the original beam of white sunlight. This
seemingly insignificant change, a narrow slit replacing the round hole,
made Wollaston and not Newton the discoverer of the dark lines crossing
the spectrum at various irregular intervals, and these singularly
neglected lines meant the basis of a new and most important science.

Even Wollaston, however, passed them by, and it was Fraunhofer who in
1814-1815 first made a chart of them. Consequently they are known as
Fraunhofer lines, or dark absorption lines. Sending the beam of light
through a succession of prisms gives greater dispersion and increases
the power of the spectroscope. The greater the dispersion the greater
the number of absorption lines; and it is the number and intensity of
these lines, with their accurate position throughout the range of the
spectrum which becomes the basis of spectrum analysis.

The half century that saw the invention of the steam engine,
photography, the railroad and the telegraph elapsed without any farther
developments than mere mapping of the fundamental lines, A, B, C, D, E,
F, G, H of the solar spectrum. The moon, too, was examined and its
spectrum found the same, as was to be expected from sunlight simply
reflected.

Sir John Herschel and other experimenters came near guessing the
significance of the dark lines, but the problem of unraveling their
mystery was finally solved by Bunsen and Kirchhoff who ascertained that
an incandescent gas emits rays of exactly the same degree of
refrangibility which it absorbs when white light is passed through it.
This great discovery was at once received as the secure basis of
spectrum analysis, and Kirchhoff in 1858 put in compact and
comprehensive form the three following principles underlying the theory
of the science:

(1) Solid and liquid bodies, also gases under high pressure, give when
incandescent a continuous spectrum, that is one with a mere succession
of colors, and neither bright nor dark lines;

(2) Gases under low pressure give a discontinuous spectrum, crossed by
bright lines whose number and position in the spectrum differ according
to the substances vaporized;

(3) When white light passes through a gas, this medium absorbs or
quenches rays of identical wave-length with those composing its own
bright-line spectrum.

Clearly then it makes no difference where the light originates whether
it comes from sun or star. Only it must be bright enough so that we can
analyze it with the spectroscope. But our analysis of sun and star could
not proceed until the chemist had vaporized in the laboratory all the
elements, and charted their spectra with accuracy. When this had been
done, every substance became at once recognizable by the number and
position of its lines, with practical certainty.

How then can we be sure of the chemical and physical composition of sun
and stars? Only by detailed and critical comparison of their spectra
with the laboratory spectra of elements which chemical and physical
research have supplied. As in the sun, so in the stars, each of which is
encircled by a gaseous absorptive layer or atmosphere, the light rays
from the self-luminous inner sphere must pass through this reversing
layer, which absorbs light of exactly the same wave-length as the lines
that make up its own bright line spectrum. Whatever substances are here
found in gaseous condition, the same will be evident by dark lines in
the spectrum of sun or star, and the position of these dark lines will
show, by coincidence with the position of the laboratory bright lines,
all the substances that are vaporized in the atmospheres of the
self-luminous bodies of the sky.

Here then originated the science of the new astronomy: the old astronomy
had concerned itself mainly with positions of the heavenly bodies,
_where_ they are; the new astronomy deals with their chemical
composition and physical constitution, and _what_ they are. Between 1865
and 1875 the fundamental application of the basic principles was well
advanced by the researches of Sir William Huggins in England, of Father
Angelo Secchi in Rome, of Jules Janssen in Paris, and of Dr. Henry
Draper in New York.

In analyzing the spectrum of the sun, many thousands of dark absorption
lines are found, and their coincidences with the bright lines of
terrestrial elements show that iron, for instance, is most prominently
identified, with rather more than 2,000 coincidences of bright and dark
lines. Calcium, too, is indicated by peculiar intensity of its lines, as
well as their great number. Next in order are hydrogen, nickel and
sodium. By prolonged and minute comparison of the solar spectrum with
spectra of terrestrial elements, something like forty elemental
substances are now known to exist in the sun. Rowland's splendid
photographs of the solar spectrum have contributed most effectively.
About half of these elements, though not in order of certainty, are
aluminum, cadmium, calcium, carbon, chromium, cobalt, copper, hydrogen,
iron, magnesium, manganese, nickel, scandium, silicon, silver, sodium,
titanium, vanadium, yttrium, zinc, and zirconium. Oxygen, too, is pretty
surely indicated; but certain elements abundant on earth, as nitrogen
and chlorine, together with gold, mercury, phosphorus, and sulphur, are
not found in the sun.

The two brilliant red stars, Aldebaran in Taurus, and Betelgeuse in
Orion, were the first stars whose chemical constitution was revealed to
the eye of man, and Sir William Huggins of London was the astronomer
who achieved this epoch-making result. Father Secchi of the Vatican
Observatory proceeded at once with the visual examination of the spectra
of hundreds of the brighter stars, and he was the first to provide a
classification of stellar spectra. There were four types.

Secchi's type I is characterized chiefly by the breadth and intensity of
dark hydrogen lines, together with a faintness or entire absence of
metallic lines. These are bluish or white stars and they are very
abundant, nearly half of all the stars. Vega, Altair, and numerous other
bright stars belong to this type, and especially Sirius, which gives to
the type the name "Sirians."

Type II is characterized by a multitude of fine dark metallic lines,
closely resembling the lines of the solar spectrum. These stars are
somewhat yellowish in tinge like the sun, and from this similarity of
spectra they are called "solars." Arcturus and Capella are "solars," and
on the whole the solars are rather less numerous than the Sirians. Stars
nearest to the solar system are mostly of this type, and, according to
Kapteyn of Groningen, the absolute luminous power of first type stars
exceeds that of second type stars seven-fold.

Secchi's type III is characterized by many dark bands, well defined on
the side toward the blue end of the spectrum, but shading off toward the
red--a "colonnaded spectrum", as Miss Clerke aptly terms it. Alpha
Herculis, Antares, and Mira, together with orange and reddish stars and
most of the variable stars, belong in type III.

Type IV is also characterized by dark bands, often called "flutings,"
similar to those of type III, but reversed as to shading, that is, well
defined on the side toward the red, but fading out toward the blue.
Their atmospheres contain carbon; but they are not abundant, besides
being faint and nearly all blood-red in tint.

Following up the brilliant researches of Draper, who in 1872 obtained
the first successful photograph of a star's spectrum, that of Vega,
Pickering of Harvard supplemented Secchi's classification by Type V, a
spectrum characterized by bright lines. They, too, are not abundant and
are all found near the middle of the Galaxy. These are usually known as
Wolf-Rayet stars, from the two Paris astronomers who first investigated
their spectra. Type V stars are a class of objects seemingly apart from
the rest of the stellar universe, and many of the planetary nebulæ yield
the same sort of a spectrum.

The late Mrs. Anna Palmer Draper, widow of Dr. Henry Draper, established
the Henry Draper Memorial at Harvard, and investigation of the
photographic spectra of all the brighter stars of the entire heavens has
been prosecuted on a comprehensive scale, those of the northern
hemisphere at Cambridge, and of the southern at Arequipa, Peru. These
researches have led to a broad reclassification of the stars into eight
distinct groups, a work of exceptional magnitude begun by the late Mrs.
Fleming and recently completed by Miss Annie Cannon, who classified the
photographic spectra of more than 230,000 stars on the new system, as
follows:--

The letters O, B, A, F, G, K, M, N represent a continuous gradation in
the supposed order of stellar evolution, and farther subdivision is
indicated by tenths, G5K meaning a type half way between G and K, and
usually written G5 simply. B2 would indicate a type between B and A, but
nearer to B than A, and so on. On this system, the spectrum of a star in
the earliest stages of its evolution is made up of diffuse bright bands
on a faint continuous background. As these bands become fewer and
narrower, very faint absorption lines begin to appear, first the helium
lines, followed by several series of hydrogen lines. On the
disappearance of the bright bands, the spectrum becomes wholly
absorptive bands and lines. Then comes a very great increase in
intensity of the true hydrogen spectrum, with wide and much diffused
lines, and few if any other lines. Then the H and K calcium lines and
other lines peculiar to the sun become more and more intense. Then the
hydrogen lines go through their long decline. The calcium spectrum
becomes intense, and later the spectrum becomes quite like that of the
sun with a great wealth of lines. Following this stage the spectrum
shortens from the ultra violet, the hydrogen lines fade out still
farther, and bands due to metallic compounds make their appearance, the
entire spectrum finally resembling that of sun spots. To designate these
types rather more categorically:--

Type O--bright bands on a faint continuous background, with five
subdivisions, Oa, Ob, Oc, Od, Oe, according to the varying width and
intensity of the bands.

Type B--the Orion type, or helium type, with additional lines of origin
unknown as yet, but without any of the bright bands of type O.

Type A--the Sirian type, the regular Balmer series of hydrogen lines
being very intense, with a few other lines not conspicuously marked.

Type F--the calcium type, hydrogen lines less strongly marked, but with
the narrow calcium lines H and K very intense.

Type G--the solar type, with multitudes of metallic lines.

Type K--in some respects similar to G, but with the hydrogen lines
fading out, and the metallic lines relatively more prominent.

Type M--spectrum with peculiar flutings due to titanium oxide, with
subdivisions Ma and Mb, and the variable stars of long period, with a
few bright hydrogen lines additional, in a separate class Md.

Type N--similar to M, in that both are pronouncedly reddish, but with
characteristic flutings probably indicating carbon compounds.

The Draper classification being based on photographic spectra, and the
original Secchi classification being visual, the relation of the two
systems is approximately as follows:

  Secchi Type I includes Draper B & A
             II includes Draper F, G & K
            III includes Draper M
             IV includes Draper N

Pickering's marked success in organization and execution of this great
programme was due to his adoption of the "slitless spectroscope," which
made it possible to photograph stellar spectra in vast numbers on a
single plate. The first observers of stellar spectra placed the
spectroscope beyond the focus of the telescope with which it was used,
thereby limiting the examination to but one star at a time. In the
slitless spectroscope, a large prism is mounted in front of the
objective (of short focus), so that the star's rays pass through it
first, and then are brought to the same focus on the photographic
plate, for all the stars within the field of view, sometimes many
thousand in number. This arrangement provides great advantages in the
comparison and classification of stellar spectra.

When spectroscopic methods were first introduced into astronomy, there
was no expectation that the field of the old or so-called exact
astronomy would be invaded. Physicists were sometimes jocularly greeted
among astronomers as "ribbon men," and no one even dreamed that their
researches were one day to advance to equal recognition with results
derived from micrometer, meridian circle, and heliometer.

The first step in this direction was taken in 1868 by Sir William
Huggins of London, who noticed small displacements in the lines of
spectra of very bright stars. In fact the whole spectrum appeared to be
shifted; in the case of Sirius it was shifted toward the red, while the
whole spectrum of Arcturus was shifted by three times this amount toward
the violet end of the spectrum. The reason was not difficult to assign.

As early as 1842 Doppler had enunciated the principle that when we are
approaching or are approached by a body which is emitting regular
vibrations, then the number of waves we receive in a second is
increased, and their wave-length correspondingly diminished; and just
the reverse of this occurs when the distance of the vibrating body is
increasing. It is the same with light as with sound, and everyone has
noticed how the pitch of a locomotive whistle suddenly rises as it
passes, and falls as suddenly on retreating from us. So Huggins drew the
immediate inference that the distance between the earth and Sirius was
increasing at the rate of nearly twenty miles per second, while
Arcturus was nearing us with a velocity of sixty miles per second.

These pioneer observations of motions in the line of sight, or radial
velocities as they are now called, led directly to the acceptance of the
high value of spectroscopic work as an adjunct of exact astronomy in
stellar research. Nor has it been found wanting in application to a
great variety of exact problems in the solar system which would have
been wholly impossible to solve without it.

Foremost is the sun, of course, because of the overplus of light. Young
early measured the displacement of lines in the spectra of the
prominences, and found velocities sometimes exceeding 250 miles per
second. Many astronomers, Dunér among them, investigated the rotation of
the sun by the spectroscopic method. The sun's east limb is coming
toward us, while the west is going from us; and by measuring the sum of
the displacements, the rate of rotation has been calculated, not only at
the sun's equator but at many solar latitudes also, both north and
south. As was to be expected, these results agree well with the sun's
rotation as found by the transits of sun spots in the lower latitudes
where they make their appearance.

Bélopolsky has applied the same method to the rotation of the planet
Venus, and Keeler, by measuring the displacement of lines in the
spectrum of Saturn, on opposite sides of the ring, provided a brilliant
observational proof of the physical constitution of the rings; because
he showed that the inner ring traveled round more swiftly than the outer
one, thus demonstrating that the ring could not be solid, but must be
composed of multitudes of small particles traveling around the ball of
Saturn, much as if they were satellites. Indeed, Keeler ascertained the
velocity of their orbital motion and found that in each case it agreed
exactly with that required by the Keplerian law.

Even the filmy corona of the sun was investigated in similar fashion by
Deslandres at the total eclipse of 1893, and he found that it rotates
bodily with the sun. But the complete vindication of the spectroscopic
method as an adjunct of the old astronomy came with its application to
measurement of the distance of the sun. The method is very interesting
and was first suggested by Campbell in 1892. Spectrum-line measurements
have become very accurate with the introduction of dry-plate
photography, and ecliptic stars were spectrographed, toward and from
which the earth is traveling by its orbital motion round the sun. By
accurate measurement of these displacements, the orbital velocity of the
earth is calculated; and as we know the exact length of the year, or a
complete period, the length of the orbit itself in miles becomes known,
and thus, by simple mensuration, the length of the radius of the
orbit--which is the distance of the sun.

If we pass from sun to star, the triumph of the spectroscope has been
everywhere complete and significant. As the spectroscopic survey of the
stars grew toward completeness, it became evident that the swarming
hosts of the stellar universe are in constant motion through space, not
only athwart the line of vision as their proper motions had long
disclosed, but some stars are swiftly moving toward our solar system and
others as swiftly from it.

Fixed stars, strictly speaking--there are no such. All are in relative
motion. Exact astronomy by discussion of the proper motions had
assigned a region of the sky toward which the sun and planets are
moving. Spectrography soon verified this direction not only, but gave a
determination of the velocity of our motion of twelve miles per second
in a direction approximately that of the constellation Lyra. From
corresponding radial velocities, we draw the ready conclusion that
certain groups or clusters of stars are actually connected in space and
moving as related systems, as in the Pleiades and Ursa Major.

Rather more than a quarter century ago, the spectroscope came to the
assistance of the telescope in helping to solve the intricate problem of
stellar distribution. Kapteyn, by combining the proper motions of
certain stars with their classification in the Draper catalogue of
stellar spectra, drew the conclusion that, as stars having very small
proper motions show a condensation toward the Galaxy, the stars
composing this girdle are mostly of the Sirian type, and are at vast
distances from the solar system. The proper motion of a star near to us
will ordinarily be large, and, in the case of solar stars, the larger
their proper motion the greater their number. So it would appear that
the solar stars are aggregated round the sun himself, and this
conclusion is greatly strengthened by the fact that of stars whose
distances and spectral type are both ascertained, seven of the eight
nearest to us are solar stars.

In 1889 the spectroscope achieved an unexpected triumph by enabling the
late Professor Pickering to make the first discovery of a spectroscopic
double, or binary star, a type of object now quite abundant. Unlike the
visual binary systems whose periods are years in length, the
spectroscopic binaries have short periods, reckoned in some cases in
days, or hours even. If the orbit of a very close binary is seen edge
on, the light of the two stars will coalesce twice in every revolution.
Halfway between these points there are two times when the two stars will
be moving, one toward the earth and the other from it. At all times the
light of the star, in so far as the telescope shows it, proceeds from a
single object.

Now photograph the star's spectrum at each of the four critical points
above indicated: in the first pair the lines are sharply defined and
single, because at conjunction the stars are simply moving athwart the
line of sight, while at the intermediate points the lines are double.
Doppler's principle completely accounts for this: the light from the
receding companion is giving lines displaced toward the red, while the
approaching companion yields lines displaced toward the violet. Mizar,
the double star at the bend of the handle in the Great Dipper was the
first star to yield this peculiar type of spectrum, and the period of
its invisible companion is about 52 days. The relative velocity of the
components is 100 miles a second, and applying Newton's law we find its
mass exceeds that of the sun forty-fold. Capella has been found to be a
spectroscopic binary; also the pole star. Spectroscopic binaries have
relatively short periods, one of the shortest known being only 35 hours
in length. It is in the constellation Scorpio. Beta Aurigæ is another
whose lines double on alternate nights, giving a period of four days;
and the combined mass of both stars is more than twice that of the sun.
The catalogue of spectroscopic binaries is constantly enlarging; but
thousands doubtless exist that can never be discovered by this method,
as is evident if their orbits are perpendicular to the line of sight or
nearly so. The history of the spectroscopic binaries is one of the most
interesting chapters in astronomy, and affords a marvelous confirmation
of the prediction of Bessel who first wrote of "the astronomy of the
invisible."

Find a star's distance by the spectroscope? Impossible, everyone would
have said, even a very few years ago. Now, however, the thing is done,
and with increasing accuracy.

Adams of Mount Wilson has found, after protracted investigation, that
the relative intensity of certain spectral lines varies according to the
absolute brightness of a star; indeed, so close is the correspondence
that the spectroscopic observations are employed to provide in certain
cases a good determination of the absolute magnitude, and therefore of
the distance. To test this relation, the spectroscopic parallaxes have
been compared with the measured parallaxes in numerous instances, and an
excellent agreement is shown. This new method is adding extensively to
our knowledge of stellar luminosities and distances, and even the vast
distances of globular clusters and spiral nebulæ are becoming known.

In fact, but few departments of the old astronomy are left which the new
astronomy has not invaded, and this latest triumph of the spectroscope
in determining accurately the distances of even the remotest stars is
enthusiastically welcomed by advocates of the old and new astronomy
alike.




CHAPTER XXI

THE STORY OF ASTRONOMICAL PHOTOGRAPHY


The most powerful ally of both telescope and spectroscope is
photography. Without it the marvelous researches carried on with both
these types of instrument would have been essentially impossible. Even
the great telescopes of Herschel and Lord Rosse, notwithstanding their
splendid record as optical instruments, might have achieved vastly more
had photography been developed in their time to the point where the
astronomer could have employed its wonderful capabilities as he does
to-day. And, with the spectroscope, it is hardly too much to say that no
investigator ever observes visually with that instrument any more:
practically every spectrum is made a matter of photographic record
first. The observing, or nowadays the measuring, is all done afterward.

All telescopes and cameras are alike, in that each must form or have
formed within it an image by means of a lens or mirror. In the telescope
the eye sees the fleeting image, in the camera the process of
registering the image on a plate or film is known as photography.
Daguerre first invented the process (silver film on a copper plate) in
1839. The year following it was first employed on the moon, in 1850 the
first star was photographed, in 1851 the first total eclipse of the sun;
all by the primitive daguerreotype process, which, notwithstanding its
awkwardness and the great length of exposure required, was found to
possess many advantages for astronomical work.

About the middle of the last century the wet plate process, so called
because the sensitized collodion film must be kept moist during
exposure, came into general use, and the astronomers of that period were
not slow to avail themselves of the advantages of a more sensitive
process, which in 1872, in the skillful hands of Henry Draper, produced
the first spectrum of a star. In 1880 a nebula was first photographed,
and in 1881 a comet.

Before this time, however, the new dry-plate process had been developed
to the point where astronomers began to avail of its greater convenience
and increased sensitiveness, even in spite of the coarseness of grain of
the film. Forty years of dry-plate service have brought a wealth of
advantages scarcely dreamed of in the beginning, and nearly every
department of astronomical research has been enhanced thereby, while
many entirely new photographic methods of investigation have been worked
out.

Continued improvement in photographic processes has provided the
possibility of pictures of fainter and fainter celestial objects, and
all the larger telescopes have photographed stars and nebulæ of such
exceeding faintness that the human eye, even if applied to the same
instrument, would never be able to see them. This is because the eye, in
ten or twelve seconds of keen watching, becomes fatigued and must be
rested, whereas the action of very faint light rays is cumulative on the
highly sensitive film; so that a continuous exposure of many hours'
duration becomes readily visible to the eye on development. So a
supersensitive dry plate will often record many thousand stars in a
region where the naked eye can see but one.

Perhaps the greatest amplification of photography has taken place at the
Harvard Observatory under Pickering, where a library of many hundred
thousand plates has accumulated; and at Groningen, Holland, where
Kapteyn has established an astronomical laboratory without instruments
except such as are necessary to measure photographic plates, whenever
and wherever taken. So it is possible to select the clearest of skies,
all over the world, for exposure of the plates, and bring back the
photographs for expert discussion.

Of course the sun was the celestial body first photographed, and its
surpassing brilliance necessitates reduction of exposure to a minimum.
In moments of exceptional steadiness of the atmosphere, a very high
degree of magnification of the solar surface on the photographic plate
is permitted, and the details in formation, development, and ending of
sun spots are faithfully registered. Nevertheless, it cannot be said
that photography has yet entirely replaced the eye in this work, and
careful drawings of sun spots at critical stages in their life are
capable of registering fine detail which the plate has so far been
unable to record. Janssen of Paris took photographs of the solar
photosphere so highly magnified that the granulation or willow-leaf
structure of the surface was clearly visible, and its variations
traceable from hour to hour.

The advantages of sun spot photography in ascertaining the sun's
rotation, keeping count of the spots, and in a permanent record for
measurement of position of the sun's axis and the spot zones, are
obvious. In direct portrayal of the sun's corona during total eclipses,
photography has offered superior advantages over visual sketching, in
the form and exact location of the coronal streamers; but the
extraordinary differences of intensity between the inner corona and its
outlying extensions are such that halation renders a complete picture on
a single plate practically impossible. The filamentous detail of the
inner corona, and the faintest outlying extensions or streamers, the eye
must still reveal directly.

In solar spectrum photography, research has been especially benefited;
indeed, exact registry of the multitudinous lines was quite impossible
without it. Photographic maps of the spectrum by Thollon, McClean and
Rowland are so complete and accurate that no visual charts can approach
them. Rowland's great photographic map of the solar spectrum spread out
into a band about forty feet in length; and in the infra-red, Langley's
spectrobolometer extended the invisible heat spectrum photographically
to many times that length. At the other end of the spectrum, special
photographic processes have extended the ultra-violet spectrum far
beyond the ocular limit, to a point where it is abruptly cut off by
absorption of the earth's atmosphere. On the same plate with certain
regions of the sun's spectrum, the spectra of terrestrial metals are
photographed side by side, and exact coincidences of lines show that
about forty elemental substances known to terrestrial chemistry are
vaporized in the sun.

    [Illustration: A VIEW OF THE 100-FOOT DOME IN WHICH THE LARGEST
       TELESCOPE IN THE WORLD IS HOUSED. (_Courtesy, Mt. Wilson Solar
       Observatory._)]

    [Illustration: MOUNT CHIMBORAZO, NEAR THE EQUATOR. An observatory
       located on this mountain would make it possible to study the
       phenomena of northern and southern skies from the same point.
       (_Courtesy, Pan-American Union._)]

    [Illustration: LICK OBSERVATORY, ON THE SUMMIT OF MT. HAMILTON,
       ABOUT TWENTY-FIVE MILES S. W. OF SAN JOSE, CALIFORNIA. It
       contains the famous Lick telescope, a 36-inch refractor.]

    [Illustration: NEAR VIEW OF THE EYE-END OF THE YERKES TELESCOPE.
       The eyepiece is removed and its place taken by a photographic
       plate.]

Young was the first to photograph a solar prominence in 1870, and twenty
years later Deslandres of Paris and Hale of Chicago independently
invented the spectroheliograph, by which the chromosphere and
prominences of the sun, as well as the disk of the sun itself, are all
photographed by monochromatic light on a single plate. Hale has
developed this instrument almost to the limit, first at the Yerkes
Observatory of the University of Chicago, and more recently at the Mount
Wilson Observatory of the Carnegie Institution, where spectroheliograms
of marvelous perfection are daily taken. It was with this instrument
that Hale discovered the effect of an electro-magnetic field in sun
spots which has revolutionized solar theories, a research impossible to
conceive of without the aid of photography.

When we apply Doppler's principle, photography becomes doubly
advantageous, whether we determine, as Dunér did and more recently
Adams, the sun's own rotation and find it to vary in different solar
latitudes, the equator going fastest; or apply the method to the sun's
corona at the east and west limbs of the sun, which Deslandres in 1893
proved to be rotating bodily with the sun, because of the measured
displacement of spectral lines of the corona in juxtaposition on the
photographic plate.

In the solar astronomy of measurement, too, photography has been
helpfully utilized, as in registering the transits of Mercury over the
sun's disk, for correcting the tables of the planet's orbital motion;
and most prominently in the action taken by the principal governments of
the world in sending out expeditions to observe the transits of Venus in
1874 and 1882, for the purpose of determining the parallax of Venus and
so the distance of the earth from the sun.

In our studies of the moon, photography has almost completely superseded
ocular work during the past sixty years. Rutherfurd and Draper of New
York about 1865 obtained very excellent lunar photographs with wet
plates, which were unexcelled for nearly half a century. The Harvard,
Lick, and Paris Observatories have published pretty complete
photographic atlases of the moon, and the best negatives of these series
show nearly everything that the eye can discern, except under unusual
circumstances. Later lunar photography was taken up at the Yerkes
Observatory, and exceptionally fine photographs on a large scale were
obtained with the 40-inch refractor, using a color screen. More recently
the 60-inch and 100-inch mirrors of the Mount Wilson Observatory have
taken a series of photographs of the moon far surpassing everything
previously done, as was to be expected from the unique combination of a
tranquil mountain atmosphere with the extraordinary optical power of the
instruments, and a special adaptation of photographic methods. During
lunar eclipses, Pickering has made a photographic search for a possible
satellite of the moon, occultations of stars by the moon have been
recorded by photography, and Russell of Princeton has shown how the
position of the moon among the stars can be determined by the aid of
photography with a high order of precision.

The story of planetary photography is on the whole disappointing. Much
has been done, but there is much that is within reach, or ought to be,
that remains undone. From Mercury nothing ought perhaps to be expected.
On many of the photographs of the transit of Venus, especially those
taken under the writer's direction at the Lick Observatory in 1882, we
have unmistakable evidence of the planet's atmosphere. Here again the
wet plate process, although more clumsy, demonstrated its superiority
over the dry process used by other expeditions.

In spectroscopy, Bélopolsky has sought to determine the period of
rotation of Venus on her axis. At the Lowell Observatory, Douglass
succeeded in photographing the faint zodiacal light, and very successful
photographs of Mars were taken at this institution as early as 1905 by
Slipher. Two years later these were much improved upon by the writer's
expedition to the Andes of Chile, when 12,000 exposures of Mars were
made, many of them showing the principal _canali_, and other prominent
features of the planet's disk. At subsequent oppositions of the planet,
Barnard at the Yerkes Observatory and the Mount Wilson observers have
far surpassed all these photographs.

For future oppositions a more sensitive film is highly desired, in
connection with instruments possessing greater light-gathering power, so
permitting a briefer exposure that will be less influenced by
irregularities and defects of the atmosphere. The spectrum of Mars is of
course that of sunlight, very much reduced, and modified to a slight
extent by its passing twice through the atmosphere of Mars. What amount
of aqueous vapor that atmosphere may contain is a question that can be
answered only by critical comparison of the Martian spectrum with the
spectrum of the moon, and photography affords the only method by which
this can be done.

Many are the ways in which photography has aided research on the
asteroid group. Since 1891 more than 600 of them have been discovered by
photography, and it is many times easier to find the new object on the
photographic plate than to detect it in the sky as was formerly done by
means of star charts. The planet by its motion during the exposure of
the plate produces a trail, whereas the surrounding stars are all round
dots or images. Or by moving the plate slightly during exposure, as in
Metcalf's ingenious method, we may catch the planet at that point where
it will give a nearly circular image, and thus be quite as easy to
detect, because all the stars on the same plate will then be trails.

Photographic photometry of the asteroids has revealed marked variations
in their light, due perhaps to irregularities of figure. On account of
their faint light, the asteroids are especially suited, as Mars is not,
to exact photography for ascertaining their parallax, and from this the
sun's distance when the asteroid's distance has been found. Many
asteroids have been utilized in this way, in particular Eros (433). In
1931 it approaches the earth within 13 million miles, when the
photographic method will doubtless give the sun's distance with the
utmost accuracy.

Photographs of Jupiter have been very successfully taken at the Yerkes
and Lowell Observatories and elsewhere, but the great depth of the
planet's atmosphere is highly absorptive, so that the impression is very
weak in the neighborhood of the limb, if the exposure is correctly timed
for the center of the disk. The striking detail of the belts, however,
is excellently shown. Wood of Baltimore has obtained excellent results
by monochromatic photography of Jupiter and Saturn with the 60-inch
reflector on Mount Wilson. Jupiter's satellites have not been neglected
photographically, and Pickering has observed hundreds of the eclipses
of the satellites by a sort of cinematographic method of repeated
exposures, around the time of disappearance and reappearance by eclipse.
The newest outer satellites of Jupiter were all discovered by
photography, and it is extremely doubtful if they would have been found
otherwise.

Saturn has long been a favorite object with the astronomical
photographer, and there are many fine pictures in spite of his yellowish
light, relatively weak photographically. The marvelous ring system with
the Cassini division, the oblateness of the ball, the occasional
markings on it--all are well shown in the best photographs; but the call
is for more light and a more sensitive photographic process. Pickering's
ninth satellite (Phoebe) was discovered by photography, one of the
faintest moons in the solar system. Like the faint outer moons of
Jupiter, few existing telescopes are powerful enough to show it. Its
orbit has been found from photographic observations, and its position is
checked up from time to time by photography.

But the crowning achievement of spectrum photography in the Saturnian
system is Keeler's application of Doppler's principle in determining the
rate of orbital motion of particles in different zones of the rings,
thereby establishing the Maxwellian theory of the constitution of the
rings beyond the possibility of doubt. For Uranus and Neptune
photography has availed but little, except to negative the existence of
additional satellites of these planets, which doubtless would have been
discovered by the thorough photographic search which has been made for
them by W. H. Pickering without result.

As with the asteroids, so with comets: several of these bodies have been
discovered by photography; none more spectacular than the Egyptian comet
of May 17th, 1882, which impressed itself on the plates of the corona of
that date. Withdrawal of the sun's light by total eclipse made the comet
visible, and it had never been seen before, nor is it known whether it
will ever return. In cometary photography, much the same difficulties
are present as in photographing the corona: if the plate is exposed long
enough to get the faint extensions of the tail, the fine filaments of
the coma or head are obliterated by halation and overexposure.

No one has had greater success in this work than Barnard, whose
photographs of comets, particularly at the Lick Observatory, are
numerous and unexcelled. His photographs of the Brooks Comet of 1893
revealed rapid and violent changes in the tail, as if shattered by
encounter with meteors; and the tail of Halley's comet in 1910 showed
the rapid propagation of luminous waves down the tail, similar to
phenomena sometimes seen in streamers of the aurora. Draper obtained the
first photograph of a comet's spectrum in 1881, disclosing an identity
with hydrocarbons burning in a Bunsen flame, also bands in the violet
due to carbon compounds. The photographic spectra of subsequent comets
have shown bright lines due to sodium and the vapor of iron and
magnesium.

Even the elusive meteor has been caught by photography, first by Wolf in
1891, who was exposing a plate on stars in the Milky Way. On developing
it, he found a fine, dark nearly uniform line crossing it, due to the
accidental flight across the field of a meteor of varying brightness.
Since then meteor trails have been repeatedly photographed, and even
the trail spectra of meteors have been registered on the Harvard plates.
At Yale in 1894 Elkin employed a unique apparatus for securing
photographic trails of meteors: six photographic cameras mounted at
different angles on a long polar axis driven by clockwork, the whole
arranged so as to cover a large area of the sky where meteors were
expected.

When we pass from the solar system to the stellar universe the
advantages of photography and the amplification of research due to its
employment as accessory in nearly every line of investigation are
enormous. So extensively has photography been introduced that plates,
and to a slight extent films, are now almost exclusively used in
securing original records. Regrettably so in case of the nebulæ, because
the numerous photographs of the brighter nebulæ taken since 1880 when
Draper got the first photograph of the nebula of Orion, are as a rule
not comparable with each other. Differences of instruments, of plates,
of exposure, and development--all have occasioned differences in
portrayal of a nebula which do not exist. When we consider faithful
accuracy of portrayal of the nebulæ for purposes of critical comparison
from age to age, many of our nebular photographs of the past forty
years, fine as they are and marvelous as they are, must fail to serve
the purpose of revealing progressive changes in nebular features in the
future.

Roberts and Common in England were among the first to obtain nebular
photographs with extraordinary detail, also the brothers Henry of Paris.
As early as 1888 Roberts revealed the true nature of the great nebula in
Andromeda, which had never been suspected of being spiral; and Keeler
and Perrine at the Lick Observatory pushed the photographic discovery of
spiral nebulæ so far that their estimates fill the sky with many hundred
thousands of these objects.

In the southern hemisphere the 24-inch Bruce telescope of Harvard
College Observatory has obtained many very remarkable photographs of
nebulæ, particularly in the vicinity of Eta Carinæ. But the great
reflectors of the Mount Wilson Observatory, on account of their
exceptional location and extraordinary power, have surpassed all others
in the photographic portrayal of these objects, especially of the spiral
nebulæ which appear to show all stages in transition from nebula to
star. No less remarkable are the photographs of such wonderful clusters
as Omega Centauri, a perfect visual representation of which is wholly
impossible. Intercomparison of the photographs of clusters has afforded
Bailey of Harvard, Shapley of Mount Wilson and others the opportunity of
discovery that hundreds of the component stars are variable.

What is the longest photographic exposure ever made? At the Cape of Good
Hope, under the direction of the late Sir David Gill, exposures on
nebulæ were made, utilizing the best part of several nights, and
totaling as high as seventeen, or even twenty-three hours. But the Mount
Wilson observers have far surpassed this duration. To study the rotation
and radial velocity of the central part of the nebula of Andromeda, an
exposure of no less than 79 hours' total duration was made on the
exceedingly faint spectrum, and even that record has since been
exceeded. The eye cannot be removed from the guiding star for a moment
while the exposure is in progress, and this tedious piece of work was
rewarded by determining the velocity of the center of the nucleus as a
motion of approach at the rate of 316 kilometers per second.

But when the stars, their magnitudes and their special peculiarities are
to be investigated _en masse_, photography provides the facile means for
researches that would scarcely have been dreamed of without it. The
international photographic chart of the entire heavens, in progress at
twenty observatories since 1887, the photographic charts of the northern
heavens at Harvard and of the southern sky at Cape Town, the manifold
investigations that have led up to the Harvard photometry, and the
unparalleled photographic researches of the Henry Draper Memorial,
enabling the spectra of many hundred thousand stars to be examined and
classified--all this is but a part of the astronomical work in stellar
fields that photography has rendered possible.

Then there are the stellar parallaxes, now observed for many stars at
once photographically, when formerly only one star's parallax could be
measured at a time and with the eye at the telescope. And photo-electric
photometry, measuring smaller differences of light than any other
method, and providing more accurate light-curves of the variable stars.
And perhaps most remarkable of all, the radial velocity work on both
stars and nebulæ, giving us the distance of whole classes of stars,
discovering large numbers of spectroscopic binaries and checking up the
motion of the solar system toward Lyra within a fraction of a mile per
second.

All told, photography has been the most potent adjunct in astronomical
research, and it is impossible to predict the future with more powerful
apparatus and photographic processes of higher sensitiveness. The field
of research is almost boundless, and the possibilities practically
without limit.

What would Herschel have done with £100,000--and photography!




CHAPTER XXII

MOUNTAIN OBSERVATORIES


The century that has elapsed since the time of Sir William Herschel,
known as the father of the new or descriptive astronomy, has witnessed
all the advances of the science that have been made possible by adopting
the photographic method of making the record, instead of depending upon
the human eye. Only one eye can be looking at the eyepiece at a time:
the photograph can be studied by a thousand eyes.

At mountain elevations telescopes are now extensively employed, and
there the camera is of especial and additional value, because the
photograph taken on the mountain can be brought down for the expert to
study, at ease and in the comfort of a lower elevation. We shall next
trace the movement that has led the astronomer to seek the summits of
mountains for his observatories, and the photographer to follow him.

Not only did the genius of Newton discover the law of universal
gravitation, and make the first experiments in optics essential to the
invention of the spectroscope, but he was the real originator also of
the modern movement for the occupation of mountain elevations for
astronomical observatories. His keen mind followed a ray of light all
the way from its celestial source to the eye of the observer, and
analyzed the causes of indistinct and imperfect vision.

Endeavoring to improve on the telescope as Galileo and his followers had
left it, he found such inherent difficulties in glass itself that he
abandoned the refracting type of telescope for the reflector, to the
construction of which he devoted many years. But he soon found out, what
every astronomer and optician knew to their keen regret, that a
telescope, no matter how perfectly the skill of the optician's hand may
make it, cannot perform perfectly unless it has an optically perfect
atmosphere to look through.

So Newton conceived the idea of a mountain observatory, on the summit of
which, as he thought, the air would be not only cloudless, but so steady
and equable that the rays of light from the heavenly bodies might reach
the eye undisturbed by atmospheric tremors and quiverings which are
almost always present in the lower strata of the great ocean of air that
surrounds our planet.

This is the way Newton puts the question in his treatise on
_Opticks_--he says: "The Air through which we look upon the Stars, is in
a perpetual Tremor; as may be seen by the tremulous Motion of Shadows
cast from high Towers, and by the twinkling of the fix'd stars.... The
only remedy is a most serene and quiet Air, such as may perhaps be found
on the tops of the highest Mountains above the grosser Clouds."

Newton's suggestion is that the _highest_ mountains may afford the best
conditions for tranquillity; and it is an interesting coincidence that
the summits of the highest mountains, about 30,000 feet in elevation,
are at about the same level where the turbulence of the atmosphere most
likely ceases, according to the indications of recent meteorological
research. These heights are far above any elevations permanently
occupied as yet, but a good beginning has been made and results of great
value have already been reached.

Curiously, investigation of mountain peaks and their suitability for
this purpose was not undertaken till nearly two centuries after Newton,
when Piazzi Smyth in 1856 organized his expedition to the summit of a
mountain of quite moderate elevation, and published his "Teneriffe: an
Astronomer's Experiment." Teneriffe is an accessible peak of about
10,000 feet, on an island of the Canaries off the African coast, where
Smyth fancied that conditions of equability would exist; and on reaching
the summit with his apparatus and spending a few days and nights there,
he was not disappointed. Could he have reached an elevation of 13,000
feet, he would have had fully one-third of all the atmosphere in weight
below him, and that the most turbulent portion of all. Nevertheless, the
gain in steadiness of the atmosphere, providing "better seeing," as the
astronomer's expression is, even at 10,000 feet, was most encouraging,
and led to attempts on other peaks by other astronomers, a few of whom
we shall mention.

Davidson, an observer of the United States Coast Survey, with a broad
experience of many years in mountain observing, investigated the summit
of the Sierra Nevada mountains as early as 1872, at an elevation of
7,200 feet. His especial object was to make an accurate comparison
between elevated stations at different heights. He found the seeing
excellent, especially on the sun; but the excessive snowfall at his
station, 45 feet annually, was a condition very adverse to permanent
occupation.

In the summer of 1872, Young spent several weeks at Sherman, Wyoming, at
an elevation exceeding 8,300 feet. He carried with him the 9.4-inch
telescope of Dartmouth College, where he was then professor, and this
was the first expedition on which a large glass was used by a very
skillful observer at great elevation. He found the number of good days
and nights small, but the sky was exceedingly favorable when clear. Many
7th magnitude stars could be detected with the naked eye. Young's
observations at Sherman were mainly spectroscopic, however, and they
demonstrated the immense advantage of a high-level station, far above
the dust and haze of the lower atmosphere. He pronounced the 9.4-inch
glass at 8,000 feet the full equivalent of a 12-inch at sea level.

Mont Blanc of 15,000 feet elevation was another summit where the veteran
Janssen of Paris maintained a station for many years; but the
continental conditions of atmospheric moisture and circulation were not
favorable on the whole. Janssen was mainly interested in the sun, and
the daylight seeing is rarely benefited, owing to the strong upward
currents of warm air set in motion by the sun itself.

Mountains in the beautiful climate of California were among the earliest
investigated, and when in 1874 the trustees of Mr. James Lick's estate
were charged with equipping an observatory with the most powerful
telescope in existence, they wisely located on the summit of Mount
Hamilton. It is 4,300 feet above sea level, and Burnham and other
astronomers made critical tests of the steadiness of vision there by
observing double stars, which afford perhaps the best means of comparing
the optical quality of the atmosphere of one region with another. The
writer was fortunate in having charge of the observations of the transit
of Venus in 1882 on the mountain, when the Observatory was in process of
construction, and the quality of the photographs obtained on that
occasion demonstrated anew the excellence of the site. Particularly at
night, for about nine months of the year, the seeing is exceptionally
good, especially when fog banks rolling in from the Pacific, cover the
valleys below like a blanket, preventing harmful radiation from the soil
below.

The great telescope mounted in 1888, a 36-inch refractor by Alvan Clark,
has fulfilled every expectation of its projectors, and justified the
selection of the site in every particular. The elevation, although
moderate, is still high enough to secure very marked advantage in
clearness and steadiness of the air, and at the same time not so high
that the health and activities of the observers are appreciably affected
by the thinner air of the summit. This telescope is known the world over
for the monumental contributions to science made by the able astronomers
who have worked with it: among them Barnard who discovered the fifth
satellite of Jupiter in 1892; Burnham, Hussey, and Aitken, who have
discovered and measured thousands of close double stars; Keeler, who
spent many faithful years on the summit; and Campbell, the present
director, whose spectroscopic researches on stellar movements have added
greatly to our knowledge of the structure of the universe. Among the
many lines of research now in progress at the Lick Observatory and in
the D. O. Mills Observatory at Santiago, Chile, are the discoveries of
stars whose velocities in space are not constant, but variable with the
spectral type of the star. Mr. Lick's bequest for the Observatory was
about $700,000. So ably has this scientific trust been administered that
he might well have endowed it with his entire estate, exceeding
$4,000,000.

Another California mountain that was early investigated is Mount
Whitney. Its summit elevation is nearly 15,000 feet, and in 1881 Langley
made its ascent for the purpose of measuring the solar constant. He
found conditions much more favorable than on Mount Etna,
Sicily--elevation about 10,000 feet--which he had visited the year
before. But the height of Mount Whitney was such as to occasion him much
inconvenience from mountain sickness, an ailment which is most
distressing and due partly to lack of oxygen and partly to mere
diminution of mechanical pressure. Mount Whitney was also visited many
years after by Campbell for investigating the spectrum of Mars in
comparison with that of the moon. Langley found on Mount Whitney an
excellent station lower down, at about 12,000 feet elevation; and by
equipping the two stations with like apparatus for measuring the solar
heat, he obtained very important data on the selective absorption of the
atmosphere.

Returning from the transit of Venus in 1882, Copeland of Edinburgh
visited several sites in the Andes of Peru, ascending on the railway
from Mollendo. Vincocaya was one of the highest, something over 14,000
feet elevation. His report was most enthusiastic, not only as to
clearness and transparency of the atmosphere, but also as to its
steadiness, which for planetary and double star observations is almost
as important. Copeland's investigation of this region of the Andes has
led many other astronomers to make critical tests in the same general
region. Climatic conditions are particularly favorable, and the sites
for high-level research are among the best known, the atmosphere being
not only clear a large part of the year, but in certain favored spots
exceedingly steady.

In 1887 the writer ascended the summit of Fujiyama, Japan, 12,400 feet
elevation. The early September conditions as to steadiness of atmosphere
were extraordinarily fine, but the mountain is covered by cloud many
months in each year. There is a saddle on the inside of the crater that
would form an ideal location for a high-level observatory. This
expedition was undertaken at the request of the late Professor
Pickering, director of Harvard College Observatory, which had recently
received a bequest from Uriah A. Boyden, amounting to nearly a quarter
of a million dollars, to "establish and maintain, in conjunction with
others, an astronomical observatory on some mountain peak."

Great elevations were systematically investigated in Colorado and
California, the Chilean desert of Atacama was visited, and a temporary
station established at Chosica, Peru, elevation about 5,000 feet.
Atmospheric conditions becoming unfavorable, a permanent station was
established in 1891 at Arequipa, Peru, elevation 8,000 feet, which has
been maintained as an annex to the Harvard Observatory ever since. The
cloud conditions have been on the whole less favorable than was
expected, but the steadiness of the air has been very satisfactory. In
addition to planetary researches conducted there in the earlier years by
W. H. Pickering, many large programs of stellar research have been
executed, especially relating to the magnitudes and spectra of the
stars. In conjunction with the home observatory in the northern
hemisphere, this afforded a vast advantage in embracing all the stars of
the entire heavens, on a scale not attempted elsewhere. The Bruce
photographic telescope of 24-inch aperture has been employed for many
years at Arequipa, and with it the plates were taken which enabled
Pickering to discover the ninth satellite of Saturn (Phoebe), and the
splendid photographs of southern globular clusters in which Bailey has
found numerous variable stars of very short periods--very faint objects,
but none the less interesting, and of much significance in modern study
of the evolution and structure of the stellar universe. The crowning
research of the observatory is the Henry Draper catalogue of stellar
spectra, now in process of publication, which is of the first order of
importance in statistical studies of stellar distribution with reference
to spectral type, and in studying the relation of parallax and distance,
proper motion, radial velocity and its variation to the spectral
characteristics of the stars.

Perrine of Cordova is now establishing on Sierra Chica about twenty-five
miles southwest of Cordova, a great reflecting telescope comparable in
size with the instruments of the northern hemisphere, for investigation
of the southern nebulæ and clusters, and motions of the stars. The
elevation of this new Argentine observatory will be 4,000 feet above sea
level.

Another observatory at mountain elevation and in a highly favorable
climate is the Lowell Observatory, located at about 7,000 feet elevation
at Flagstaff, Arizona. Many localities were visited and the atmosphere
tested especially for steadiness, an optical quality very essential for
research on the planetary surfaces. Mexico was one of these stations,
but local air currents and changes of temperature there were such that
good seeing was far from prevalent, as had been expected. At Flagstaff,
on the other hand, conditions have been pretty uniformly good, and an
enormous amount of work on the planet Mars has been accumulated and
published. The first successful photographs of this planet were taken
there in 1905, and Jupiter, Saturn, the zodiacal light and many other
test objects have been photographed, which demonstrates the excellence
of the site for astronomical research. Within recent years spectrum
research by Slipher, especially on the nebulæ, has been added to the
program, and the rotation and radial velocities of many nebulæ have been
determined.

On Mount Wilson, near Pasadena, California, at an elevation of nearly
6,000 feet, is the Carnegie Solar Observatory, founded and equipped
under the direction of Professor George E. Hale, as a department of the
Carnegie Institution of Washington, of which Dr. John Campbell Merriam
is President. The climatology of the region was carefully investigated
and tests of the seeing made by Hussey and others. Although equipped
primarily for study of the sun, the program of the observatory has been
widely amplified to include the stars and nebulæ. The instrumental
equipment is unique in many respects. To avoid the harmful effect of
unsteadiness of air strata close to the ground a tower 150 feet high was
erected, with a dome surmounting it and covering a coelostat with mirror
for reflecting the sun's rays vertically downward. Underneath the tower
a dry well was excavated to a depth equal to 1/2 the height of the tower
above it. In the subterranean chamber is the spectroheliograph of
exceptional size and power. The sun's original image is nearly 17 inches
in diameter on the plate, and the solar chromosphere and prominences,
together with the photosphere and faculæ, are all recorded by
monochromatic light.

Connected with the observatory on Mount Wilson are the laboratories,
offices and instrument shops in Pasadena, 16 miles distant, where the
remarkable apparatus for use on the mountain is constructed. A
reflecting telescope with silver-on-glass mirror 60 inches in diameter
was first built by Ritchey and thoroughly tested by stellar photographs.
Also the northern spiral nebulæ were photographed, exhibiting an
extraordinary wealth of detail in apparent star formation. The success
of this instrument paved the way for one similar in design, but with a
mirror 100 inches in diameter, provided by gift of the late John D.
Hooker of Los Angeles. The telescope was completed in 1919.
Notwithstanding its huge size and enormous weight, the mounting is very
successful, as well as the mirror. Mercurial bearings counterbalance the
weight of the polar axis in large part. This great telescope, by far the
largest and most powerful ever constructed, is now employed on a program
of research in which its vast light-gathering power will be utilized to
the full. Under the skillful management of Hale and his enthusiastic and
capable colleagues, the confines of the stellar heavens will be
enormously extended, and secrets of evolution of the universe and of its
structure no doubt revealed.

In all the mountain stations hitherto established, as the Lick
Observatory at 4,000 feet, the Mount Wilson Observatory at 6,000 feet,
the Lowell Observatory at 7,000 feet, the Harvard Observatory at 8,000
feet; and Teneriffe and Etna at 10,000, Fujiyama at 12,000, Pike's Peak
at 14,000, Mont Blanc and Mount Whitney at 15,000, the researches that
have been carried on have fully demonstrated the vast advantage of
increased elevation in localities where climatological conditions as
well as elevation are favorable. Nevertheless, only one-half of the
extreme altitude contemplated by Sir Isaac Newton has yet been attained.

Can the greater heights be reached and permanently occupied?
Geographically and astronomically the most favorably located mountain
for a great observatory is Mount Chimborazo in Ecuador. Its elevation is
22,000 feet, and it was ascended by Edward Whymper in 1880. Situated
very nearly on the earth's equator, almost the entire sidereal heavens
are visible from this single station, and all the planets are favored by
circumzenith conditions when passing the meridian. No other mountain in
the world approaches Chimborazo in this respect. But the summit is
perpetually snow-capped, exceedingly inaccessible, and the defect of
barometric pressure would make life impossible up there in the open.

Only one method of occupation appears to be feasible. The permanent snow
line is at about 16,000 feet, where excellent water power is available.
By tunneling into the mountain at this point, and diagonally upward to
the summit, permanent occupation could be accomplished, at a cost not to
exceed one million dollars.

The rooms of the summit observatory would need to be built as steel
caissons, and supplied with compressed air at sea-level tension. The
practicability of this plan was demonstrated by the writer in
September, 1907, at Cerro de Pasco, Peru. A steel caisson was carried up
to an elevation exceeding 14,000 feet. Patients suffering acutely with
mountain sickness were placed inside this caisson, and on restoring the
atmospheric pressure within it artificially all unfavorable
symptoms--headache, high respiration and accelerated pulse--disappeared.
There was every indication that if persons liable to this uncomfortable
complaint were brought up to this elevation, or indeed any attainable
elevation, under unreduced pressure, the symptoms of mountain sickness
would be unknown. Comfortable occupation of the highest mountain summits
was thereby assured.

The working of astronomical instruments from within air-tight
compartments does not present any insurmountable difficulties, either
mechanical or physical. Since the time these experiments were made, the
Guayaquil-Quito railway has been constructed over a saddle of
Chimborazo, at an elevation of 12,000 feet; and only six miles of
railway would need to be built from this station to the point where the
tunnel would enter the mountain.

Only by the execution of some such plan as this can astronomers hope to
overcome the baleful effects of an ever mobile atmosphere, and secure
the advantages contemplated by Sir Isaac Newton in that tranquillity of
atmosphere, which he conceived as perpetually surrounding the summits of
the highest mountains.

In Russell's theory of the progressive development of the stars, from
the giant class to the dwarf, an element of verification from
observation is lacking, because hitherto no certain method of measuring
the very minute angular diameters of the stars has been successfully
applied. The apparent surface brightness corresponding to each spectral
type is pretty well known, and by dividing it into the total apparent
brightness, we have the angular area subtended by the star, quite
independent of the star's distance. This makes it easy to estimate the
angular diameter of a star, and Betelgeuse is the one which has the
greatest angular diameter of all whose distances we know. Antares is
next in order of angular diameter, 0".043, Aldebaran 0".022, Arcturus
0".020, Pollux 0".013, and Sirius only 0".007.

Can these theoretical estimates be verified by observation? Clearly it
is of the utmost importance and the exceedingly difficult inquiry has
been undertaken with the 100-inch reflector on Mount Wilson, employing
the method of the interferometer developed by Michelson and described
later on, an instrument undoubtedly capable of measuring much smaller
angles than can be measured by any other known method. Unquestionably
the interference of atmospheric waves, or in other words what
astronomers call "poor seeing," will ultimately set the limit to what
can be accomplished. "But even if," says Eddington, "we have to send
special expeditions to the top of one of the highest mountains in the
world, the attack on this far-reaching problem must not be allowed to
languish."




CHAPTER XXIII

THE PROGRAM OF A GREAT OBSERVATORY


The Mount Wilson Observatory has now been in operation about fifteen
years. The novelty in construction of its instruments, the
investigations undertaken with them and the discoveries made, the
interpretation of celestial phenomena by laboratory experiment, and the
recent addition to its equipment of a telescope 100 inches in diameter,
surpassing all others in power, directs especial attention to the
extensive activities of this institution, whose budget now exceeds a
million dollars annually. Results are only achieved by a carefully
elaborated program, such as the following, for which the reader is
mainly indebted to Dr. Hale, the director of the observatory, who gives
a very clear idea of the trend of present-day research on the magnetic
nature of the sun, and the structure and evolution of the sidereal
universe.

The purpose of the observatory, as defined at its inception, was to
undertake a general study of stellar evolution, laying especial emphasis
upon the study of the sun, considered as a typical star; physical
researches on stars and nebulæ; and the interpretation of solar and
stellar phenomena by laboratory experiments. Recognizing that the
development of new instruments and methods afforded the most promising
means of progress, well-equipped machine shops and optical shops were
provided with this end in view.

The original program of the observatory has been much modified and
extended by the independent and striking discovery by Campbell and
Kapteyn of an important relationship between stellar speed and spectral
type; the demonstration by Hertzsprung and Russell of the existence of
giant and dwarf stars; the successful application of the 60-inch
reflector by Van Maanen to the measurement of minute parallaxes of stars
and nebulæ; the important developments of Shapley's investigation of
globular star clusters; the possibilities of research resulting from
Seares's studies in stellar photometry; and the remarkable means of
attack developed by Adams through the method of spectroscopic
parallaxes.

By this method the absolute magnitude, and hence the distance of a star
is accurately determined from estimates of the relative intensities of
certain lines in stellar spectra. Attention was first directed toward
lines of this character in 1906, when it was inferred that the weakening
of some lines in the spectra of sun spots and the strengthening of
others was the result of reduced temperature of the spot vapors. On
testing this hypothesis by laboratory experiments, it was fully
verified.

Subsequently Adams, who had thus become familiar with these lines and
their variability, studied them extensively in the spectra of other
stars. In this way was discovered the dependence of their relative
intensities on the star's absolute magnitude, so providing the powerful
method of spectroscopic parallaxes.

This method, giving the absolute magnitude as well as the distance of
every star (excepting those of the earliest type) whose spectrum is
photographed, is no less important from the evolutional than from the
structural point of view.

Investigations in solar physics which formerly held chief place in the
research program have developed along unexpected lines. It could not be
foreseen at the outset that solar magnetic phenomena might become a
subject of inquiry, demanding special instrumental facilities, and
throwing light on the complex question of the nature of the sun spots
and other solar problems of long standing. It is obvious that these
researches, together with those on the solar rotation and the motions of
the solar atmosphere, developed by Adams and St. John, must be carried
to their logical conclusion, if they are to be utilized to the fullest
in interpreting stellar and nebular phenomena.

The discovery of solar magnetism, like many other Mount Wilson results,
was the direct outcome of a long series of instrumental developments.
The progressive improvement and advance in size of the tools of research
was absolutely necessary. Hale's first spectroheliograph at Kenwood in
1890 was attached to a 12-inch refractor, and the solar image was but
two inches in diameter. It was soon found that a larger solar image was
essential, and a spectrograph of much greater linear dispersion; in
fact, the spectrograph must be made the prime element in the
combination, and the telescope so designed as to serve as a necessary
auxiliary.

Accordingly, successive steps have led through spectrographs of 18 and
30 feet dimension to a vertical spectrograph 75 feet in focal length.
The telescope is the 150 feet tower telescope, giving a solar image of
16.5 inches in diameter. Its spectrograph is massive in construction,
and by extending deep into the earth, it enjoys the stability and
constancy of temperature required for the most exacting work.

Another direct outgrowth of the work of sun-spot spectra is a study of
the spectra of red stars, where the chemistry of these coolest regions
of the sun is partially duplicated. The combination of titanium and
oxygen, and the significant changes of line intensity already observed
in both instances, and also in the electric furnace at reduced
temperatures, give indication of what may be expected to result from an
attack on the spectra of the red stars with more powerful instrumental
means, which is now provided by the 100-inch telescope and its large
stellar spectrograph.

Other elements in the design of the 100-inch Hooker telescope have the
same general object in view--that of developing and applying in
astronomical practice the effective research methods suggested by recent
advances in physics. Fresh possibilities of progress are constantly
arising, and these are utilized as rapidly as circumstances permit.

The policy of undertaking the interpretations of celestial phenomena by
laboratory experiments, an important element in the initial organization
of Mount Wilson, has certainly been justified by its results. Indeed,
the development of many of the chief solar investigations would have
been impossible without the aid of special laboratory studies, going
hand in hand with the astronomical observations. So indispensable are
such researches, and so great is the promise of their extension, that
the time has now come for advancing the laboratory work from an
accessory feature to full equality with the major factors in the work of
the observatory. Accordingly a new instrument now under installation is
an extremely powerful electro-magnet, designed by Anderson for the
extension of researches on the Zeeman effect, and for other related
investigations. Within the large and uniform field of this magnet, which
is built in the form of a solenoid, a special electric furnace, designed
for this purpose by King, is used for the study of the inverse Zeeman
effect at various angles with the lines of force. This will provide the
means of interpreting certain remarkable anomalies in the magnetic
phenomena of sun spots.

The 100-inch telescope is now in regular use. All the tests so far
applied show that it greatly surpasses the 60-inch telescope in every
class of work. For many months most of the observations and photographs
have been made with the Cassegrain combination of mirrors, giving an
equivalent focal length of 134 feet and involving three reflections of
light. The 100-inch telescope is found to give nearly 2.8 times as much
light as the 60-inch telescope, and therefore extends the scope of the
instrument to all the stars an entire magnitude fainter. This is a very
important gain for research on the faint globular clusters, as well as
the small and faint spiral and planetary nebulæ, providing a much larger
scale for these objects and sufficient light at the same time.
Photographs of the moon and many other less critical tests have been
made with very satisfactory results. Those of the moon appear to be
decidedly superior in definition to any previously taken with other
instruments.

Another investigation is of great importance in the light of recent
advances in theoretical dynamics. Darwin, in his fundamental researches
on the dynamics of rotating masses, dealt with incompressible matter,
which assumes the well-known pear-shaped figure, and may ultimately
separate into two bodies. Roche on the other hand discussed the
evolution of a highly compressible mass, which finally acquires a
lens-shaped form and ejects matter at its periphery. Both of these are
extreme cases. Jeans has recently dealt with intermediate cases, such as
are actually encountered in stars and nebulæ. He finds that when the
density is less than about one-fourth that of water, a lens-shaped
figure will be produced with sharp edges, as depicted by Roche. Matter
thrown off at opposite points on the periphery, under the influence of
small tidal forces from neighboring masses, may take the form of two
symmetric filaments, though it is not yet entirely clear how these may
attain the characteristic configuration of spiral nebulæ. The
preliminary results of Van Maanen indicate motion outward along the
arms, in harmony with Jeans's views.

Jeans further discusses the evolution of the arms, which will break up
into nuclei (of the order of mass of the sun) if they are sufficiently
massive, but will diffuse away if their gravitational attraction is
small. The mass of our solar system is apparently not great enough,
according to Jeans, to account for its formation in this way. As is
apparent, these investigations lead to conclusions very different from
those derived by Chamberlin and Moulton from the planetesimal
hypothesis.

This is a critical study of spiral nebulæ for which the 100-inch
telescope is of all instruments in existence the best suited. The
spectra of the spirals must be studied, as well as the motions of the
matter composing the arms. Their parallaxes, too, must be ascertained.
A photographic campaign including spiral nebulæ of various types will
settle the question of internal motions. The large scale of the spiral
nebulæ at the principal focus of the Hooker telescope, and the
experience gained in the measurement of nebular nuclei for parallax
determination, will help greatly in this research. A multiple-slit
spectrograph, already applied at Mount Wilson, will be employed, not
only on spiral nebulæ whose plane is directed toward us, but also on
those whose plane lies at an angle sufficient to permit both components
of motion to be measured by the two methods.

In dealing with problems of structure and motion in the Galactic system,
the 100-inch telescope offers especial advantages, because of its vast
light-gathering power. Studies of radial velocities of the stars have
hitherto been necessarily confined to the brighter stars, for the most
part even to those visible to the naked eye. While some of these are
very distant, most of the stars whose radial velocities are known belong
to a very limited group, perhaps constituting a distinct cluster of
which the sun is a member, but in any event of insignificant proportions
when contrasted with the Galaxy. Current spectrographic work with the
60-inch telescope includes stars of the eighth magnitude, and some even
fainter. But while the 60-inch has enabled Adams to measure the
distances of many remote stars by his new spectroscopic method, and to
double the known extent (so far as spectroscopic evidence is concerned)
of the star streams of Kapteyn, a much greater advance into space is
necessary to find out the community of motion among the stars comprising
the Galactic system. The Hooker telescope will enable us to determine
accurate radial velocities to stars of the eleventh magnitude, which
doubtless truly represent the Galaxy.

In order to secure a maximum return within a reasonable period of time,
the stars in the selected areas of Kapteyn will be given the preference,
because of the vast amount of work already done, relating to their
positions, proper motions, and visual and photographic magnitudes. Such
consideration as spectral type, the known directions of star-streaming,
and the position of the chosen regions with reference to the plane of
the Galaxy are given adequate weight, and it is of fundamental
importance that the method of spectroscopic parallaxes will permit dwarf
stars to be distinguished from stars that are in the giant class, but
rendered faint by their much greater distance. In addition to these
problems, the stellar spectrograms will provide rich material for study
of the relationship between stellar mass and speed, and the nature of
giant stars and dwarf stars.

Shapley's recent studies of globular clusters have indicated the
significance of these objects in both evolutional and structural
problems, and the possibility of determining their parallaxes by a
number of independent methods is of prime importance, both in its
bearing on the structure of the universe and because it permits a host
of apparent magnitudes to be at once transformed into absolute
magnitudes. Here the advantage of the Hooker telescope is two-fold: at
its 134-foot focus the increased scale of the crowded clusters makes it
possible to select separate stars for spectrum photography (which could
not be done with the 60-inch where the images were commingled); and the
great gain in light is such that the spectra of stars to the 14th
magnitude have been photographed in less than an hour.

Faint globular clusters, then, will comprise a large part of the early
program with the 100-inch telescope: the faintest possible stars in them
must be detected and their magnitudes and colors measured; spectral
types must be determined, and the radial velocities of individual stars
and of clusters as a whole; spectroscopic evidence of possible axial
rotation of globular clusters must be searched for; and the method of
spectroscopic parallaxes, as well as other methods, must be applied to
ascertaining the distances of these clusters.

The possibility of dealing with many problems relating to the
distribution and evolution of the faintest stars depends upon the
establishment of photographic and photovisual magnitude scales. Below
the twelfth magnitude, the only existing scale of standard visual or
photovisual magnitudes is the Mount Wilson sequence, already extended by
Seares to magnitude 17.5 with the 60-inch telescope.

Extension of this scale to even fainter magnitudes, and its application
to the faintest stars within its range is an important task for this
great telescope, as it will doubtless bring within range hundreds of
millions of stars that are beyond the reach of the 60-inch. The giants
among them will form for us the outer boundary of the Galactic system,
while the dwarfs will be of almost equal interest from the evolutional
standpoint. The photometric program of the 100-inch, then, will deal
with such questions as the condensation of the fainter stars toward the
Galactic plane, the color of the most distant stars, and the final
settlement of the long inquiry regarding the possible absorption of
light in space.

    [Illustration: GREAT SUN-SPOT GROUP, AUGUST 8, 1917. The disk in the
       lower left corner represents the comparative size of the earth.
       (_Photo, Mt. Wilson Solar Observatory._)]

    [Illustration: THE SUN'S DISK. The view shows the "rice grain"
       structure of the photosphere and brilliant calcium flocculi.
       (_Photo, Yerkes Observatory._)]

    [Illustration: THE LUNAR SURFACE VISIBLE DURING A TOTAL ECLIPSE OF
       THE MOON, FEBRUARY 8, 1906. (_Photo, Yerkes Observatory._)]

Another research of exceptional promise will be undertaken, which is of
great importance in a general study of stellar evolution; and that is
the determination of the spectral-energy curves of stars of various
classes, for the purpose of measuring their surface temperatures. A very
few of the nebulæ are found to be variable, and their peculiarities need
investigation, also special problems of variable stars and temporary
stars, and the spectra of the components of close double stars which are
beyond the power of all other instruments to photograph.

Such a program of research conveys an excellent idea of many of the
great problems that are under investigation by astronomers to-day, and
gives some notion of the instrumental means requisite in executing
comprehensive plans of this character. It will not escape notice that
the climax of instrumental development attained at Mount Wilson has only
been made possible by an unbroken chain of progress, link by link, each
antecedent link being necessary to the successful forging of its
following one. In very large part, and certainly indispensable to these
instrumental advances, has the art of working in glass and metals been
the mainstay of research. As we review the history of astronomical
progress, from Galileo's time to our own, the consummate genius of the
artisan and his deft handiwork compel our admiration almost equally with
the keen intelligence of the astronomer who uses these powerful engines
of his own devising to wrest the secrets of nature from the heavens.




CHAPTER XXIV

OUR SOLAR SYSTEM


Now let us go upward in imagination, far, far beyond the tops of the
highest mountains, beyond the moon and sun, and outward in space until
we reach a point in the northern heavens millions and millions of miles
away, directly above and equally distant from all points in the
ecliptic, or path in which our earth travels yearly round the sun. Then
we should have that sort of comprehensive view of the solar system which
is necessary if we are to visualize as a whole the working of the vast
machine, and the motions, sizes, and distances of all the bodies that
comprise it. Of such stupendous mechanism our earth is part.

Or in lieu of this, let us attempt to get in mind a picture of the solar
system by means of Sir William Herschel's apt illustration: "Choose any
well-leveled field. On it place a globe two feet in diameter. This will
represent the sun; Mercury will be represented by a grain of mustard
seed on the circumference of a circle 164 feet in diameter for its
orbit; Venus, a pea on a circle of 284 feet in diameter; the Earth also
a pea, on a circle of 430 feet; Mars a rather larger pin's head on a
circle of 654 feet; the asteroids, grains of sand in orbits of 1,000 to
1,200 feet; Jupiter, a moderate sized orange in a circle of nearly half
a mile across; Saturn, a small orange on a circle of four-fifths of a
mile; Uranus, a full-sized cherry or small plum upon the circumference
of a circle more than a mile and a half; and finally Neptune, a
good-sized plum on a circle about two miles and a half in diameter....
To imitate the motions of the planets in the above mentioned orbits,
Mercury must describe its own diameter in 41 seconds; Venus in 4
minutes, 14 seconds; the Earth in 7 minutes; Mars in 4 minutes 48
seconds; Jupiter in 2 minutes 56 seconds; Saturn in 3 minutes 13
seconds; Uranus in 2 minutes 16 seconds; and Neptune in 3 minutes 30
seconds."

Now, let us look earthward from our imaginary station near the north
pole of the ecliptic. All these planetary bodies would be seen to be
traveling eastward round the sun, that is, in a counter-clockwise
direction, or contrary to the motions of the hands of a timepiece. Their
orbits or paths of motion are very nearly circular, and the sun is
practically at the center of all of them except Mercury and Mars; of
Venus and Neptune, almost at the absolute center. The planes of all
their orbits are very nearly the same as that of the ecliptic, or plane
in which the earth moves. These and many other resemblances and
characteristics suggest a uniformity of origin which comports with the
idea of a family, and so the whole is spoken of as the solar system, or
the sun and his family of planets.

In addition to the nine bodies already specified, the solar system
comprises a great variety of other and lesser bodies; no less than
twenty-six moons or satellites tributary to the planets and traveling
round them in various periods as the moon does round our earth. Then
between the orbits of Mars and Jupiter are many thousands of asteroids,
so called, or minor planets (about 1,000 of them have actually been
discovered, and their paths accurately calculated). And at all sorts of
angles with the planetary orbits are the paths of hundreds of comets,
delicate filmy bodies of a wholly different constitution from the
planets, and which now and then blaze forth in the sky, their tails
appearing much like the beam of a searchlight, and compelling for the
time the attention of everybody. Connected with the comets and doubtless
originally parts of them are uncounted millions of millions of meteors,
which for the time become a part of the solar system, their minute
masses being attracted to the planets, upon which they fall, those
hitting the earth being visible to us as familiar shooting stars.

We next follow the story of astronomy through the solar system,
beginning with the sun itself and proceeding outward through his family
of planets, now much more numerous and vastly more extended than it was
to the ancient world, or indeed till within a century and a half of our
own day.




CHAPTER XXV

THE SUN AND OBSERVING IT


As lord of day, king of the heavens, mankind in the ancient world adored
the sun. By their researches into the epoch of the Assyrians, Hittites,
Phoenicians and other early peoples now passed from earth, archæologists
have unearthed many monuments that evidence the veneration in which the
early peoples who inhabited Egypt and Asia Minor many thousand years ago
held the sun. A striking example is found in the architecture of early
Egyptian temples, on the lintels of which are carved representations of
the winged globe or the winged solar disk, and there is a bare
possibility that the wings of the globe were suggested by a type of the
solar corona as glimpsed by the ancients.

Little knew they about the distance and size of the sun; but the effects
of his light and heat upon all vegetal and animal life were obvious to
them. Doubtless this formed the basis for their worship of the sun.
Occasional huge spots must have been visible to the naked eye, and the
sun's corona was seen at rare intervals. Plutarch and Philostratus
describe it very much as we see it to-day.

How completely dependent mankind is upon the sun and its powerful
radiations, only the science of the present day can tell us. By means of
the sun's heat the forests of early geologic ages were enabled to wrest
carbon from the atmosphere and store it in forms later converted by
nature's chemistry into peat and coal. Through processes but imperfectly
understood, the varying forms of vegetable life are empowered to
conserve, from air and soil, nitrogen and other substances suitable for
and essential to the life maintenance of animal creatures. Breezes that
bring rain and purify the air; the energy of water held under storage in
stream and dam and fall; trade winds facilitating commerce between the
continents; oceanic currents modifying coastal climates; the violence of
tornado, typhoon and water-spout, together with other manifestations of
natural forces--all can be traced back to their origin in the tremendous
heating power of the solar rays. In everything material the sun is our
constant and bountiful benefactor. If his light and heat were withdrawn,
practically every form of human activity on this planet would come to an
early end.

How far away is the sun? What is the size of the sun? These are
questions that astronomers of the present day can answer with accuracy.

So closely do they know the sun's distance that it is employed as their
yardstick of the sky, or unit of celestial measurement. Many methods
have been utilized in ascertaining the distance of the sun, and the
remarkable agreement among them all is very extraordinary. Some of them
depend upon pure geometry, and the basic measure which we make from the
earth is not the distance of the sun directly; but we find out how far
away Venus is during a transit of Venus, for example, or how far away
Mars is or some of the asteroids are at their closer oppositions. Then
it is possible to calculate how far away the sun is, because one
measurement of distance in the solar system affords us the scale on
which the whole structure is built. But perhaps the simplest method of
getting the sun's distance is by the velocity of light, 186,300 miles a
second. From eclipses of Jupiter's moons we know that light takes 8
minutes 20 seconds to pass from sun to earth. So that the sun's distance
is the simple product of the two, or 93 millions of miles.

Once this fundamental unit is established, we have a firm basis on which
to build up our knowledge of the distances, the sizes and motions of the
heavenly bodies, especially those that comprise the solar system. We can
at once ascertain the size of the sun, which we do by measuring the
angle which it fills, that is, the sun's apparent diameter. Finding this
to be something over a half a degree in arc, the processes of elementary
trigonometry tell us that the sun's globe is 865,000 miles in diameter.
For nearly a century this has been accurately measured with the greatest
care, and diameters taken in every direction are found to be equal and
invariably the same. So we conclude that the sun is a perfect sphere,
and so far as our instruments can inform us, its actual diameter is not
subject to appreciable change.

The vastness of the sun's volume commands our attention. As his diameter
is 110 times that of the earth, his mere size or volume is 110×110×110
or 1,300 thousand times that of the earth, because the volumes of
spheres are in proportion as the cubes of their diameters. If the
materials that compose the sun were as heavy as those that make up the
earth, it would take 1,300 thousand earths to weigh as much as the sun
does. But by a method which we need not detail here, the sun's actual
weight or mass is found to be only 300 thousand (more nearly 330,000),
times greater than the earth's. So we must infer that, bulk for bulk,
the component materials of the sun are about one-fourth lighter than
those of the earth, that is, about one and one-half times as dense as
water.

To look at this in another way: it is known that a body falling freely
toward the earth from outer space would acquire a speed of seven miles a
second, whereas if it were to fall toward the sun instead, the velocity
would be 383 miles a second on reaching his surface. If all the other
bodies of the solar system, that is, the earth and moon, all the planets
and their satellites, the comets and all were to be fused together in a
single globe, it would weigh only one-seven hundred and fiftieth as much
as the sun does.

At the surface, however, the disproportion of gravity is not so great,
because of the sun's vast size: it is only about twenty-eight times
greater on the sun than on the earth; and instead of a body falling 16
feet the first second as here, it would fall 444 feet there. Pendulums
of clocks on the sun would swing five times for every tick here, and an
athlete's running high jump would be scaled down to three inches.

Let us next inquire into the amount of the sun's light and heat, and the
enormously high temperature of a body whose heat is so intense even at
the vast distance at which we are from it. The intensity of its
brightness is such that we have no artificial source of light that we
can readily compare it with. In the sky the next object in brightness is
the full moon, but that gives less than the half-millionth part as much
light as the sun. The standard candle used in physics gives so little
light in comparison that we have to use an enormous number to express
the quantity of light that the sun gives.

A sperm candle burning 120 grains hourly is the standard, and if we
compare this with the sun when overhead, and allow for the light
absorbed by the atmosphere, we get the number 1575 with twenty-four
ciphers following it, to express the candlepower of the sun's light. If
we interpose the intense calcium light or an electric arc light between
the eye and the sun, these artificial sources will look like black spots
on the disk. Indeed, the sun is nearly four times brighter than the
"crater," or brightest part of the electric arc. The late Professor
Langley at a steel works in Pennsylvania once compared direct sunlight
with the dazzling stream of molten metal from a Bessemer converter; but
bright as it was, sunlight was found to be five thousand times brighter.

Equally enormous is the heat of the sun. Our intensest sources of
artificial heat do not exceed 4,000 degrees Fahrenheit, but the
temperature at the sun's surface is probably not less than 16,000
degrees F. One square meter of his surface radiates enough heat to
generate 100,000 horsepower continuously. At our vast distance of 93
millions of miles, the sun's heat received by the earth is still
powerful enough to melt annually a layer of ice on the earth more than a
hundred feet in thickness. If the solar heat that strikes the deck of a
tropical steamship could be fully utilized in propelling it, the speed
would reach at least ten knots.

Many attempts have been made in tropical and sub-tropical climates to
utilize the sun's heat directly for power, and Ericsson in Sweden,
Mouchot in France, and Shuman in Egypt have built successful and
efficient solar engines. Necessary intermission of their power at
night, as well as on cloudy days, will preclude their industrial
introduction until present fuels have advanced very greatly in cost. All
regions of the sun's disk radiate heat uniformly, and the sun's own
atmosphere absorbs so much that we should receive 1.7 times more heat if
it were removed. So far as is known, solar light and heat are radiated
equally in all directions, so that only a very minute fraction of the
total amount ever reaches the earth, that is, 1 2200 millionth part of
the whole. Indeed all the planets and other bodies of the solar system
together receive only one one hundred millionth part; the vast remainder
is, so far as we know, effectively wasted. It is transformed, but what
becomes of it, and whether it ever reappears in any other form, we
cannot say.

How is this inconceivably vast output of energy maintained practically
invariable throughout the centuries? Many theories have been advanced,
but only one has received nearly universal assent, that of secular
contraction of the sun's huge mass upon itself. Shrinkage means
evolution of heat; and it is found by calculation that if the sun were
to contract its diameter by shrinking only two-hundred and fifty feet
per year, the entire output of solar heat might thus be accounted for.
So distant is the sun and so slow this rate of contraction that
centuries must elapse before we could verify the theory by actual
measurements. Meanwhile, the progress of physical research on the
structure and elemental properties of matter has brought to light the
existence of highly active internal forces which are doubtless
intimately concerned in the enormous output of radiant energy, though
the mechanism of its maintenance is as yet known only in part.

Abbot, from many years' observations of the solar constant, at
Washington, on Mount Wilson, and in Algeria, finds certain evidence of
fluctuation in the solar heat received by the earth. It cannot be a
local phenomenon due to disturbances in our atmosphere, but must
originate in causes entirely extraneous to the earth. Interposition of
meteoric dust might conceivably account for it, but there is sufficient
evidence to show that the changes must be attributed to the sun itself.
The sun, then, is a variable star; and it has not only a period
connected with the periodicity of the sun spots, but also an irregular,
nonperiodic variation during a cycle of a week or ten days, though
sometimes longer, and occasioning irregular fluctuations of two to ten
per cent of the total radiation. Radiation is found to increase with the
spottedness.

Attempts have been made on the basis of the contraction theory to find
out the past history of the sun and to predict its future. Probably 20
to 50 millions of years in the past represents the life of the sun much
as it is at present; and if solar radiation in the future is maintained
substantially as now, the sun will have shrunk to one-half its present
diameter in the next five million years.

So far then as heat and light from the sun are concerned, the sun may
continue to support life on the earth not to exceed ten million years in
the future. But the sun's own existence, independently of the orbs of
the system dependent upon it, might continue for indefinite millions of
aeons before it would ever become a cold dead globe; indeed, in the
present state of science, we cannot be sure that it is destined to reach
that condition within calculable time.

A few words on observing the sun, an object much neglected by amateurs.
On account of the intense light, a very slight degree of optical power
is sufficient. Indeed a piece of window glass, smoked in a candle flame
with uniform graduation from end to end, will be found worth while in a
beginner's daily observation of the sun. The glass should be smoked
densely enough at one end so that the sunlight as seen through it will
not dazzle the eye on the clearest days. At the other end of the glass,
the degree of smoke film should not be quite so dense, so that the sun
can be examined on hazy, foggy or partly cloudy days. An occasional
naked-eye spot will reward the patient observer.

If a small spyglass, opera glass or field glass is at hand, excellent
views of the sun may be had by mounting the glass so that it can be held
steadily pointed on the sun, and then viewing the disk by projection on
a white card or sheet of paper. Care must be taken to get a good focus
on the projected image, and then the faculæ, or whitish spots, or
mottling nearer the sun's edge will usually be well seen. By moving the
card farther away from the eyepiece, a larger disk may be obtained, in
effect a higher degree of magnification. But care must be used not to
increase it too much. Keep direct sunlight outside the tube from falling
on the card where the image is being examined. This is conveniently done
by cutting a large hole, the size of the brass cell of the object glass,
through a sheet of corrugated strawboard, and slipping this on over the
cell. In this way the spots on the sun can be examined with ease and
safety to the eye.

For large instruments a special type of eyepiece is provided known as a
helioscope, which disposes of the intense heat rays that are harmful to
the eye. Frequent examination of the eyepiece should be made and the
eyepiece cooled if necessary. That part of the sun's surface under
observation is known as the photosphere, that is, the part which
radiates light. If the atmosphere admits the use of high magnifying
powers, the structure of the photosphere will be found more and more
interesting the higher the power employed. It is an irregularly mottled
surface showing a species of rice-grain structure under fairly high
magnification. These grains are grouped irregularly and are about 500
miles across. Under fine conditions of vision they may be subdivided
into granules. The faculæ, or white spots, are sometimes elevations
above the general solar level; they have occasionally been seen
projecting outside the limb, or edge of the disk.




CHAPTER XXVI

SUN SPOTS AND PROMINENCES


Dark spots of a deep bluish black will often be seen on the photosphere
of the sun. Sometimes single, though generally in groups, the larger
ones will have a dark center, called the umbra, surrounded by the very
irregular penumbra which is darker near its outer edge and much brighter
apparently on its inner edge where it joins on the umbra. The penumbra
often shows a species of thatch-work structure, and systematic sketches
of sun spots by observers skilled in drawing are greatly to be desired,
because photography has not yet reached the stage where it is possible
to compete with visual observation in the matter of fine detail. The
spots themselves nearly always appear like depressions in the
photosphere, and on repeated occasions they have been seen as actual
notches when on the edge of the sun.

Many spots, however, are not depressions: some appear to be actual
elevations, with the umbra perhaps a central depression, like the crater
in the general elevation of a volcano. Spots are sometimes of enormous
size. The largest on record was seen in 1858; it was nearly 150,000
miles in breadth, and covered a considerable proportion of the whole
visible hemisphere of the sun. A spot must be nearly 30,000 miles across
in order to be seen with the naked eye.

In their beginning, development, and end, each spot or group of spots
appears to be a law unto itself. Sometimes in a few hours they will
form, though generally it is a question of days and even weeks. Very
soon after their formation is complete, tonguelike encroachments of the
penumbra appear to force their way across the umbra, and this splitting
up of the central spot usually goes on quite rapidly. Sun spots in
violent disturbance are rarely observed. As the sun turns round on his
axis, the spots will often be carried across the disk from the center to
the edge, when they become very much foreshortened. The sun's period of
rotation is 28 days, so that if a spot lasts more than two weeks without
breaking up, it may reappear on the eastern limb of the sun after having
disappeared at the western edge. Two or three months is an average
duration for a spot; the longest on record lasted through 18 months in
1840-41.

The position of the sun's axis is well known, its equator being tilted
about 7 degrees to the ecliptic, and the spots are distributed in zones
north and south of the equator, extending as far as 30 degrees of solar
latitude. In very high latitudes spots are never seen; they are most
abundant in about latitude 15 degrees both north and south, and rather
more numerous in the northern than in the southern hemisphere of the
sun. Recent research at Mount Wilson makes the sun a great magnet; and
its magnetic axis is inclined at an angle of 6 degrees to the axis of
rotation, around which it revolves in 32 days.

There is a most interesting periodicity of the spots on the sun, for
months will sometimes elapse with spots in abundance and visible every
day, while at other periods, days and even weeks will elapse without a
single spot being seen. There is a well recognized period of eleven and
one-tenth years, the reason underlying which is not, however, known.
After passing through the minimum of spottedness, they begin to break
out again first in latitudes of 25 degrees-30 degrees, rather suddenly,
and on both sides of the equator, and they move toward the equator as
their number and individual size decrease.

The last observed epoch of maximum spot activity on the sun was passed
in 1917.

Many attempts have been made to ascertain the cause of the periodicity
of sun spots, but the real cause is not yet known. If the spots are
eruptional in character, the forces held in check during seasons of few
spots may well break out in period. The brighter streaks and mottlings
known as faculæ are probably elevations above the general photosphere,
and seem to be crusts of luminous matter, often incandescent calcium,
protruding through from the lower levels. Generally the faculæ are
numerous around the dark spots, and absorption of the sun's light by his
own atmosphere affords a darker background for them, with better
visibility nearer the rim of the solar disk. The spectroheliograph
reveals vast zones of faculæ otherwise invisible, related to the
sun-spot zones proper on both sides of the equator.

In some intimate way the magnetism of sun and earth are so related that
outbreaks of solar spots are accompanied with disturbances of electrical
and other instruments on the earth; also the aurora borealis is seen
with greater frequency during periods when many spots are visible.

Within very recent years the discovery of a magnetic field in sun spots
has been made by Hale with powerful instruments of his own design. Sun
spots had never been investigated before with adequate instrumental
means. He recognized the necessity of having a spectroscope that would
record the widened lines of sun-spot spectra, and the strengthened and
weakened lines on a large scale. Certain changes in relative intensity
were traced to a reduced temperature of the spot vapors by comparison
with photographs of the spectrum of iron and other metallic vapors in an
electric arc at different temperatures. Here the work of the laboratory
was essential. Sun spots were thus found to be regions of reduced
temperature in the solar atmosphere. Chemical unions were thus possible,
and thousands of faint lines in spot-spectra were measured and
identified as band lines due to chemical compounds. Thus the chemical
changes at work in sun-spot vapors were recognized.

Then followed the highly significant investigations of solar vortices
and magnetic fields. Improvements in photographic methods had revealed
immense vortices surrounding sun spots in the higher part of the
hydrogen atmosphere; and this led to the hypothesis that a sun spot is a
solar storm, resembling a terrestrial tornado, and in which the hot
vapors whirling at high velocity are cooled by expansion. This would
account for the observed intensity changes of the spectrum lines and the
presence of chemical compounds. The vortex hypothesis suggested an
explanation of the widening of many spot lines, and the doubling or
trebling of some of them. As it is known that electrons are emitted by
hot bodies, they must be present in vast numbers in the sun; and
positive or negative electrons, if caught and whirled in a vortex, would
produce a magnetic field.

Zeeman in 1896 had discovered that the lines in the spectrum of a
luminous vapor in a magnetic field are widened, or even split into
several components if the field is strong enough. Characteristic effects
of polarization appear also. The new apparatus of the observatory in
conjunction with experiments in the laboratory immediately provided
evidence that proved the existence of magnetic fields in sun spots, and
strengthened the view that the spots are caused by electric vortices.

Extended investigations have led Hale to the conclusion that the sun
itself is a magnet, with its poles situated at or near the poles of
rotation. In this respect the sun resembles the earth, which has long
been known to be a magnet. The sun's axial rotation permits
investigation of the magnetic phenomena of all parts of its surface, so
that ultimately the exact position of the sun's magnetic poles and the
intensity of the field at different levels in the solar atmosphere will
be ascertained. Schuster is of the opinion that not only the sun and
earth, but every star, and perhaps every rotating body, becomes a magnet
by virtue of its rotation. Hale is confident that the 100-inch reflector
will permit the test for magnetism to be applied to a few of the stars.

The sun can be observed at Mount Wilson on at least nine-tenths of all
the days in the year, and a daily record of the polarities of all spots
with the 150-foot tower telescope is a part of the routine. A method has
been devised for classifying sun spots on the basis of their magnetic
properties, and more than a thousand spots have already been so
classified. About 60 per cent of all sun spots are found to be binary
groups, the single or multiple members of which are of opposite magnetic
polarity. Unipolar spots are very seldom observed without some
indication of the characteristics of bipolar groups. These are usually
exhibited in the form of flocculi following the spot. The bipolar spot
seems to be the dominant type, and the unipolar type a variant of it.

Although devised for quite another purpose, that of photographing the
hydrogen prominences on the limb of the sun, the spectroheliograph has
contributed very effectively to many departments of solar research. The
prominences are dull reddish cloudlets that were first seen during total
eclipses of the sun. Probably Vassenius, a Swedish astronomer, during
the total eclipse of 1733, made the earliest record of them, as pinkish
clouds quite detached from the edge of the moon; and in that day, when
it had not yet been proved that the moon was without atmosphere, he
naturally thought they belonged to the moon, not the sun. Undoubtedly
Ulloa, a Spanish admiral, also saw the prominences in observing the
total eclipse of 1778; but they seem to have attracted little attention
till 1842, when a very important total eclipse was central throughout
Europe, and observed with great care by many of the eminent astronomers
of all countries.

So different did the prominences appear to different eyes, and so many
were the theories as to what they were, that no general consensus of
opinion was reached, and some thought them no part of either sun or
moon, but a mere mirage or optical illusion. But at the return of this
eclipse in 1860, photography was employed so as to demonstrate beyond a
shadow of doubt the real existence and true solar character of the
prominences. By the slow progress of the moon across the sun and the
prominences on the edge, a unique series of photographs by De la Rue
showed the moon's edge gradually cutting off the prominences piecemeal
on one side of the sun, and equally gradually uncovering them on the
opposite side.

The prominences, then, were known to be real phenomena of the sun, some
of them disconnectedly floating in his atmosphere, as if clouds. Their
forms did not vary rapidly, they were very abundant, and their light was
so rich in rays of great photographic intensity that many were caught on
the plate which the eye failed to see; they appeared at every part of
the sun's limb and their height above it indicated that they must be
many thousand miles in actual dimension. What they were, however,
remained an entire mystery, and no one even thought it possible to find
out what their chemical constitution might be or to measure the speed
with which they moved.

A few years later came the great Indian eclipse (August 28, 1868), at
that date the longest total eclipse ever observed. Janssen of France and
many others went out to India to witness it. Fortunately the prominences
were very brilliant and this led Janssen to believe it would be possible
for him to see them the day after the eclipse was over. By modifying the
adjustment of his apparatus suitably and changing its relation to the
sun's edge, he found that hydrogen is the main constituent in the light
of the prominences. In addition to this he was able to trace out the
shapes of the prominences, and even measure their dimensions. His
station in India was at Guntoor, many weeks by post from home; so that
his account of this important discovery reached the Paris Academy of
Sciences for communication with another from the late Sir Norman Lockyer
of England, announcing a like discovery, wholly independently.

The principle is simply this, and admirably stated by Young: "Under
ordinary circumstances the prominences are invisible, for the same
reason as the stars in the daytime: they are hidden by the intense light
reflected from the particles of our own atmosphere near the sun's place
in the sky; and if we could only sufficiently weaken this aerial
illumination, without at the same time weakening _their_ light, the end
would be gained. And the spectroscope accomplishes this very thing.
Since the air-light is reflected sunshine, it of course presents the
same spectrum as sunlight, a continuous band of color crossed by dark
lines. Now, this sort of spectrum is greatly weakened by every increase
of dispersive power, because the light is spread out into a longer
ribbon and made to cover a more extended area. On the other hand, a
spectrum of bright lines undergoes no such weakening by an increase in
the dispersive power of the spectroscope. The bright lines are only more
widely separated--not in the least diffused or shorn of their
brightness."

Simultaneous announcement of this great discovery, by astronomers of
different nations, working in widely separate regions of the earth, led
to the striking of a gold medal by the French Government in honor of
both astronomers and bearing their united effigies. Ever since the
famous Indian eclipse of 1868, it has not been necessary to wait for a
total eclipse in order to observe the solar prominences, but every
observer provided with suitable apparatus has been able to observe them
in full sunlight whenever desired, and the charting of them is part of
the daily routine at several observatories in different parts of the
world. So vast has been the accumulation of data about them that we know
their numbers to fluctuate with the spots on the sun; and their
distribution over the sun's surface resembles in a way that of the
spots.

While the spots and protuberances are most numerous around solar
latitude 20 degrees both north and south, the prominences do not
disappear above latitude 35 to 40 degrees, as the spots do, but from
latitude 60 degrees they increase in number to about 75 degrees, and are
occasionally observed even at the sun's poles. Faculæ and prominences
are more closely related than the sun spots and prominences. There are
wide variations in both magnitude and type of the prominences. Heights
above the sun's limb of a few thousand miles are very common, and they
rarely reach elevations as great as 100,000 miles, though a very
occasional one reaches even greater heights.

Classification of the prominences divides them into two broad types, the
quiescent and the eruptive. The former are for the most part hydrogen,
and the latter metallic. The quiescent prominences resemble closely the
stratus and cirrus type of terrestrial clouds, and are frequently of
enormous extent along the sun's edge. They are relatively long-lived,
persisting sometimes for days without much change. The eruptive
prominences are more brilliant, changing their form and brightness
rapidly. Often they appear as brilliant spikes or jets, reaching
altitudes that average about 25,000 miles. Rarely seen near the sun's
poles, they are much more numerous nearer the sun spots. Speed of
motion of their filaments sometimes exceeds one hundred miles a second,
and the changing variety of shapes of the eruptive prominences is most
interesting. Oftentimes they change so rapidly that only photography can
do them justice.

Prominence photography began with Young a half century ago, who obtained
the first successful impression on a microscope slide with a sensitized
film of collodion; as was necessary in the earlier wet-plate process of
photography, which required exposures so long that little progress was
effected for about twenty years. Then it was taken up by Deslandres of
Paris and Hale of Chicago independently, both of whom succeeded in
devising a complex type of apparatus known as the spectroheliograph, by
which all the prominences surrounding the entire limb of the sun can be
photographed at any time by light of a single wave-length, together with
the disk of the sun on the same negative.

The prominences appear to be intimately connected with a gaseous
envelope surrounding the solar photosphere, in which sodium and
magnesium are present as well as hydrogen. The depth of the chromosphere
is usually between 5,000 and 10,000 miles, and its existence was first
made out during the total solar eclipses of 1605 and 1706, when it
appeared as an irregular rose-tinted fringe, though not at the time
recognized as belonging to the sun.

The constitution of the sun and its envelopes are still under
discussion, and no complete theory of the sun has yet been advanced
which commands the widest acceptance. Of the interior of the sun we can
only surmise that it is composed of gases which, because of intense heat
and compression, are in a state unfamiliar on earth and impossible to
reproduce in our laboratories. Their consistency may be that of melted
pitch or tar.

Surrounding the main body of the sun are a series of layers, shells, or
atmospheres. Outside of all and very irregular in structure, indeed
probably not a solar atmosphere at all, is the solar corona, parts of
which behave much as if it were an atmosphere, but it appears to be
bound up in some way with the sun's radiation. It has streamers that
vary with the sun-spot period, but its constitution and function are
very imperfectly known, because it has never been seen or photographed
except at rare intervals on occasion of total eclipses of the sun.

Beneath the corona we meet the projecting prominences, to which parts of
the corona are certainly related, and beneath them the first true layer
or atmosphere of the sun known as the chromosphere, its average depth
being about one-hundredth part of the sun's diameter. Beneath the
chromosphere is the layer of the sun from which emanates the light by
which we see it, called the photosphere. It appears to be composed of
filaments due to the condensation of metallic vapors, and it is the
outer extremities of these filaments which are seen as the granular
structures everywhere covering the disk of the sun. Their light shines
through the chromosphere and the spots are ruptures in this envelope.

Between photosphere and chromosphere is a very thin envelope, probably
not over 700 miles in thickness, called the reversing layer. It is this
relatively thin shell that is responsible for the absorption which
produces the dark lines in the spectrum of the sun. Under normal
conditions the filaments of the photosphere are radial, that is vertical
on the sun; but whenever eruptions take place, as during the occurrence
of spots, the adjacent filaments are violently swept out of their normal
vertical lines and these displaced columns then form what we view as the
spot's penumbra. From the outer surface of the sun's chromosphere rise
in eruptive columns vapors of hydrogen and the various metals of which
the sun is composed. These and the spots would naturally occur in
periods just as we see them.

We have said that the sun is composed of a mass of highly heated or
incandescent vapors or gases, whose compression on account of gravity
must render their physical condition quite different from any gaseous
forms known on the earth or which we can reproduce here. As the result
of more than half a century of studious observation of the sun and
mapping of its spectrum in every part, and diligent comparison with the
spectra of all known chemical elements on the earth, we find that the
sun contains no elements not already found here, but that a great
preponderance of elements known to earth are found in the sun.

The intensity of their spectral lines is one prominent indication of the
presence of elements in the sun, and the number of coincidences of
spectral lines is another. Iron, nickel, calcium, manganese, sodium,
cobalt, and carbon are among the elements most strongly identified. A
few of the rarer terrestrial elements are of doubtful existence in the
sun, and a very few, as gold, bismuth, antimony, and sulphur are not
found there, and the existence of oxygen in the sun is regarded by some
experts as doubtful. But if the whole earth were vaporized by heat,
probably its spectrum would resemble that of the sun very closely.

What are the effects of the sun, and sun spots in particular, on our
weather? Is the influence of their periodicity potent or negligible? If
we investigate conditions pertaining to terrestrial magnetism, as
fluctuations of the magnetic needle, and the frequency of auroræ, there
is no occasion for doubt of the sun's direct influence, although we are
not able to say just how that influence becomes potent. If, however, we
look into questions of temperature, barometric pressure, rainfall,
cyclones, crops, and consequent financial conditions, we find fully as
much evidence against solar influence as for it. The slight variations
of the sun's light and heat due to the presence or absence of sun spots
can scarcely be sensible, and much longer periods of closer observation
are necessary before such questions can be finally decided. The slighter
such influences are, if they actually exist, and the more veiled they
are by other influences more or less powerful, the more difficult it is
to discover their effects with certainty.

The importance of solar radiation in the prediction of terrestrial
weather has long been recognized, but until very recently no practical
application has been made. The Smithsonian Astrophysical Observatory at
Washington, under the direction of Dr. Abbot, has for many years carried
on at a number of stations a series of determinations of the constant of
solar radiation by the spectro-bolometric method originated by Langley.
A new station in Calama, Chile, has recently been inaugurated, at which
the solar constant is worked out each day, and telegraphed to the
Argentine weather service, where it is employed in forecasting for the
day.

Abbot's new method of solar constant determination is based on the fact
that atmospheric transparency varies oppositely to the variations of
brightness of the sky. Increase of haziness presents more reflecting
surface to scatter the solar rays indirectly to the earth. Of course it
presents also additional surface to obstruct the direct rays from the
sun. By measuring the brightness of the sky near the sun, it becomes
possible to infer the coefficients of atmospheric transmission at all
wave lengths. The direct observations and the complete deduction of the
solar constant for the day can all be completed within two or three
hours.

Clayton of Buenos Aires has now employed these results in the Argentine
weather predictions for two years, and the introduction of this new
element in forecasting has brought about a pronounced gain in the value
of the predictions. Its adoption by the weather bureaus of other nations
will doubtless come in due time, and the new method take a firmly
established rank in practical meteorology.

Abbot's observations many years ago first called attention to the
variability of the solar constant through a range of several per cent
both from year to year, and in irregular short periods of weeks or even
days. Abbot considers this the more likely explanation than that
atmospheric changes should take place simultaneously all over the earth.
The sun is but a star, the stars that are irregularly variable in light
and heat are numerous, and the sun itself appears to be one of these.

Especially important to the agricultural and vineyard interests of
Argentina is the question of precipitation, and Clayton finds this very
dependent on solar radiation. At epochs of practically stationary solar
intensity, there is little or no precipitation; but quite generally he
finds that great decrease of solar radiation is followed in from three
to five days by heavy precipitation. Direct temperature effects are also
traced in Buenos Aires and other South American cities, lagging from two
to three days behind the observed solar fluctuations.

The station at Calama yields about 250 determinations of the solar
constant each year, and the Mount Wilson station about half that number.
They are the only stations of this character at present in existence,
and others should be established in widely separated and cloudless
regions, as Egypt, southern California and Australia. Uniformity in the
methods of observing would be highly desirable, and the Smithsonian
Institution has perfected the details of common control of such stations
which it is expected may be established at an early day.




CHAPTER XXVII

THE INNER PLANETS


VULCAN

About the middle of the last century, Le Verrier, a great French
astronomer, having added the planet Neptune beyond the outside confines
of the solar system, sought evidence of a lesser planet traveling round
the sun within the orbit of Mercury. For many years close watch was kept
on the sun in the hope of discovering such a body in the act of passing
across the disk, or in transit, as it is technically termed.
Lescarbault, a French physician, announced that he had actually seen
such a planet, Vulcan it was called, passing over the sun in 1859. Total
eclipses of the sun would afford the best opportunity for seeing such a
body, and on several such occasions astronomers thought they had found
it. But the signal advantages of photography have been applied so often
to this search, and always unsuccessfully, that the existence of Vulcan,
or the intramercurian planet, is now regarded as mythical.


MERCURY

This planet is an elusive body that very few, even astronomers, have
ever seen. It is not very bright, has a rapid motion and never retreats
far from the sun, so that it was a puzzle to the ancients who saw it,
sometimes in the twilight after sunset and again in the twilight of
dawn. When following the sun down in the west, in March or April,
Mercury is likely to be best seen; twinkling rather violently and nearly
as bright as a star of the first magnitude.

Very little is to be seen on the minute disk of this planet, except that
it goes through all the phases of the moon--crescent, gibbous, full,
gibbous, crescent. Whether Mercury turns round on its axis or not,
cannot be said to be known, because the markings that are suspected on
its surface are too indefinite to permit exact observation. More than
likely the planet presents always the same side or face to the sun, so
that it turns round on its axis once, while traveling once around the
sun in its orbit. Mercury's day and year would therefore be equal in
length. Nor have we much evidence on the question of an atmosphere
surrounding Mercury; probably it is very thin, if indeed there is any at
all. When Mercury comes directly between us and the sun, crossing in
transit, the edge of the planet as projected against the sun is very
sharply defined, and this would indicate an absence of atmosphere on
Mercury.

Transits of Mercury can occur in May and November only: there was one on
November 7, 1914, and there will be one on May 7, 1924. The latter will
be nearly eight hours in length, which is almost the limit. Mercury's
distance from the sun averages 36 million miles, the diameter of the
planet is 3,000 miles, and his orbital speed is 30 miles per second, the
swiftest of all the planets. No moon of Mercury is known to exist,
although many times diligently searched for, especially during transits
of the planet.


VENUS

Brightest of all the planets, and the most beautiful of all is Venus.
Its path is next outside the orbit of Mercury, but within that of the
earth, so that it partakes of all the phases of the moon. Like Mercury
it sometimes passes exactly between us and the sun, a rare phenomenon
which is known as a transit of Venus.

Being without telescopes, the ancients knew nothing about these
occurrences, but they were puzzled for centuries over the appearance of
the planet in the west after sunset, when they called it Hesperus, and
in early dawn in the east when they gave it the name Phosphorus.

Venus is known to be girdled with an atmosphere denser than ours, and it
seems to be always filled with dense clouds. It is the reflection of
sunlight from this perpetually cloudy exterior which gives Venus her
singular radiance. So brilliant is she that even full daylight is not
strong enough to overpower her rays; and she may often be seen
glistening in the clear blue daytime sky, if one knows pretty nearly in
what direction to look for her.

Venus is 67 million miles from the sun, and as our own distance is 93
million miles, this planet can come within 26 million miles of the
earth. It is therefore at times our nearest known neighbor in space,
excepting only the Moon and Eros, one of the erratic little planets that
travel round the sun between Mars and Jupiter. Also possibly a comet
might come much nearer.

Astronomers always take advantage of this nearness of Venus to us, if a
transit across the sun takes place; because it affords an excellent
method of finding out what the distance of the sun is from the Earth. A
pair of these transits happens about once a century, there were transits
in 1874 and 1882, and the next pair occur in 2004 and 2012. In actual
size, Venus is almost as large a planet as our own, being 7,700 miles
in diameter, as compared with 7,920 for the earth. Her velocity in her
orbit is twenty-two miles per second, and she travels all the way round
the sun in seven and one half months or 225 days.

Venus from her striking brilliancy always leads the novice to expect to
see great things on applying the telescope. But aside from a brilliant
disk, now a slender crescent, now half full like the moon at quarter,
and again gibbous as the moon is between quarter and full, the telescope
reveals but little. There is pretty good evidence that the markings
thought to have been seen on the planet's surface are illusory, and so
it is wholly uncertain in what direction the planet's axis lies; also
there is great uncertainty about the length of the day on Venus, or the
period of turning round on its axis. Probably it is the same in length
as the planet's year.

Once when Venus passed very close to the sun, just barely escaping a
transit, Lyman of Yale University caught sight of it by hiding the sun
behind a tall building or church spire. The dark side of Venus was
turned toward us and he could not of course see that. But the planet was
clearly there, completely encircled by a narrow delicate luminous ring,
which was due to sunlight shining through the atmosphere that surrounds
the planet. Similar ring effects were seen by observers of the transits
of Venus in 1874 and 1882; and from all their observations it is
concluded that Venus has an atmosphere probably at least twice as dense
and extensive as that which encircles the earth. Spurious satellites of
Venus are many, but no real moon is known to attend this planet.

    [Illustration: THE SURFACE OF THE MOON IN THE REGION OF COPERNICUS.
       Photograph made with the Hooker 100-inch reflecting telescope.
       (_Photo, Mt. Wilson Solar Observatory._)]

    [Illustration: A VIEW OF THE SOUTH CENTRAL PORTION OF THE MOON AT
       LAST QUARTER. (_Photo, Mt. Wilson Solar Observatory._)]




CHAPTER XXVIII

THE MOON AND HER SURFACE


As the sun has always reigned as king of day, so is the moon queen of
night. Observation of her phases, now waxing, now waning, with her
stately motion always eastward among the stars, began with the earliest
ages. Often when near the full she must have been seen herself eclipsed,
and much more rarely the occurrence of total eclipses of the sun are
certain to have suggested the moon's intervention between earth and sun,
shutting off the sunlight completely, because these eclipses never took
place except when the moon was in the same part of the sky with the sun.

If we watch the nightly march of the moon, we shall find that she
travels over her own breadth in about an hour's time. By using a
telescope on the stars just eastward or to the left of her, she will now
and then be seen to pass between us and a star--on very rare occasions a
planet--extinguishing its light with great suddenness, the most nearly
instantaneous of all phenomena in nature. Draw a line connecting the
cusps, or horns of the lunar crescent, and then a line eastward at right
angles to this, and it will show the direction of the moon's own motion
in its orbit round the earth quite accurately.

As the phase advances, note the inside edge of the advancing crescent:
this will be quite rough and jagged, compared to the outside edge which
is the moon's real contour and relatively very smooth. The position of
the inside curve will change from night to night, and it marks the line
of sunrise on the moon during the fortnight elapsing between new moon
and full; while from full through last quarter and back to new moon,
this advancing line marks the region of sunset on the moon. The general
shape of this line is never a circle but always elliptical, and
astronomers call it the terminator. All along the terminator, sunlight
strikes the lunar surface at a small angle, whether near sunrise or
sunset; so that owing to the mountains and other high masses of the
moon's surface, the terminator is always a more or less jagged and
irregular line.

Onward from new moon toward full the horns of the crescent are always
turned upward or eastward. When the general line of the terminator
becomes a straight line from cusp to cusp, the moon is said to have
reached first quarter or quadrature. Onward toward full the terminator
will be seen to bend the other way, and in about a week's time it will
have merged itself with the moon's limb. The moon is then said to be
full. Afterward the phase phenomena recur in the reverse order, with
third quarter midway between full and new moon again; the phase of the
moon being called gibbous all the way from first quarter to third
quarter, except when exactly full.

As we know that the moon is, like the earth, a nonluminous body, and
shines only by virtue of the sunlight falling upon it, clearly an entire
half of the moon's globe must be perpetually illumined by sunlight. The
varying phases then are due simply to that part of the illuminated
hemisphere which is turned toward us. New moon is entirely invisible
because the sunward hemisphere is turned wholly away from us, while at
full moon we see the lunar disk complete because we are on the same side
of the moon that the sun is and practically in line with both sun and
moon.

If we could visit the moon, we should see the earth in exactly
complementary phase. At new moon here we should be enjoying full earth
there, and full moon here would be coincident with new or dark earth
there. The narrow crescent of new moon here would be the period of
gibbous earth there; and it is the reflection of sunlight from this
gibbous earth which illuminates the part of the moon but faintly seen at
this time, popularly known as the "old moon in the new moon's arms." Its
greater visibility at some times than at others is due to greater
prevalence of clouded area in the reflecting regions of the earth turned
toward the moon, and the higher reflective power of clouds than that
possessed by mere land and water.

As the moon goes all the way round the sky every month, the same as the
sun does in a year, and travels in nearly the same path, clearly it must
also go north and south every month as the sun does. So in midsummer
when the sun runs high upon the meridian, we expect to find full moons
running low, and likewise in midwinter the full moon always runs high,
as almost everyone has sometimes or other noticed.

This eastward or true orbital motion of the moon is responsible for
another relation which soon comes to light when we begin to observe the
moon; and that is the later hour of rising or setting each night. Our
clock time is regulated by the sun, which also is moving eastward about
1° daily, or twice its own breadth. So the moon's eastward gain on the
sun amounts to about 12 degrees daily, and one degree being equal to 4
minutes, the retarded time of moonrise or moonset each day amounts to
very nearly 50 minutes on the average; though sometimes the delay will
be less than a half hour and at other times it will exceed an hour and a
quarter. The season of least retardation of rising of the full moon is
in the autumn, and so the moon that falls in late September or October
is known as the Harvest moon, and the next succeeding full moon is
called the Hunter's moon.

Lunation is a term sometimes given to the moon's period from any
definite phase round to the same phase again. Its length is the true
period of the moon's revolution once around the earth, from the sun all
the way round till it overtakes the sun again. The synodic period is
another name for lunation, and its true length is 29 and one-half days,
or very accurately 29 d. 12 h. 44 m. 2.7 s. as calculated by astronomers
with great exactness from many thousand revolutions of the moon. But if
we want the true period of the moon round the earth as referred to a
star, it is much shorter than this, amounting to only 27 days and nearly
one-third. This is called the moon's sidereal period of revolution,
because it is the time elapsed while she is traveling eastward from a
given star around to coincidence with the same star again.

If we study the moon's path in the sky more critically, we shall find
that it does not quite follow the ecliptic, or the sun's path, but that
twice each month she deviates from the ecliptic, once to the north and
once to the south of it, by roughly ten times her own breadth. More
accurately this angle is 5°8'40", an almost invariable quantity, and it
is therefore known as an astronomical constant, or the inclination of
the moon's orbit to the ecliptic. So the moon's orbit must intersect the
ecliptic, and as both are great circles in the sky, the points of
intersection are known as the moon's nodes, one ascending and the other
descending, and the nodes are 180 degrees apart.

The figure of the moon's orbit is not circular, although it deviates
only slightly from that form. But like the paths of all other satellites
round their primary planets, and of the planets themselves round the
sun, the moon's orbit is also an ellipse. The distance of the moon's
center from the earth's center is therefore perpetually changing; the
point of nearest approach is called perigee, and that of farthest
recession, apogee.

The moon's distance from the earth is easier and simpler to be
ascertained than that of any other heavenly body, because it is the
nearest. An outline of the method of finding this distance is not
difficult to present; and it resembles in every particular the method a
surveyor uses to find the distance of some inaccessible point which he
cannot measure directly. Up and down a stream, for example, he measures
the length of a line, and from each end of it he measures the angle
between the other end of the line and the object on the opposite side of
the stream whose distance he wishes to find out. Then he applies the
science of trigonometry to these three measures, two of angles and one
the length of the side or base included between them, and a few minutes'
calculation gives the distance of the inaccessible object from either
end of the base line.

Now in like manner, to transfer the process to the sky, let the two ends
of the base be represented by two astronomical observatories, for
example, Greenwich in the northern hemisphere and Cape Town in the
southern. The base line is the chord or straight line through the earth
connecting the two observatories, and we know the length of this line
pretty accurately, because we know the size of the earth. The angles
measured are somewhat different from those in the terrestrial example,
but the process amounts to the same thing because the astronomers at the
two observatories measure the angular distance of the center of the moon
from the zenith, each using his own zenith at the same time; and the
same science of trigonometry enables them to figure out the length of
any side of the triangles involved. The side which belongs to both
triangles is the distance from the center of the earth to the center of
the moon, and the average of many hundred measures of this gives 238,800
miles, or about ten times the distance round the equator of the earth.

We have said that the orbit in which the moon travels round the earth is
practically a circle, but the earth's center is found not at the center
of this orbit, but set to one side, or eccentrically, so that the
distance spanning the centers of the two bodies is sometimes as small as
221,610 miles at perigee, and 252,970 miles at apogee. The moon's speed
in this orbit averages rather more than half a mile every second of
time--more accurately 3,350 feet a second, or 2,290 miles per hour.

Once the moon's distance is known, its size or diameter is easy to
ascertain. An angular measure is necessary, of course, that of the angle
which the disk of the moon fills as seen from the earth. There are many
types of astronomical instruments with which this angle can be measured,
and its value is something more than half a degree (31'7"). The moon's
actual diameter figures out from this 2,163 miles; and it would
therefore require nearly fifty moons merged in one to make a ball the
size of the earth.

Still, no other planet has a satellite as large in proportion to its
primary as the moon is in relation to the earth. But the materials that
compose the moon have less than two-thirds the average density of those
that make up the earth, so that eighty-one moons fused together would be
necessary to equal the mass or weight of the earth. If we figure out the
force of attraction of the moon for bodies on its surface, we find it
equals about one-sixth that of the earth. Athletes could perform some
astounding feats there--miracles of high jump and hammer-throw.

Our interest in the moon's physical characteristics never wanes. Her
nearness to us has always fascinated astronomer and layman alike. Early
users of the telescope were readily led into error regarding the general
characteristics of the lunar surface; and it is easy to see why they
thought the smooth level planes must be seas, and gave them names to
that effect which persist to-day, as Mare Crisium, Mare Serenitatis and
so on. We may be sure that no water exists on the moon's surface,
although some astronomers think that solid water, as ice or snow, may
still exist there at a temperature too low for appreciable evaporation.

Perhaps water, seas, and oceans were once there, but their secular
dissemination and loss as vapor have gone on through the millions of
millions of years till even the moon's atmosphere appears to have
vanished completely. At least there is much better evidence of absence
of atmosphere on the moon than of its presence--not enough at any rate
to equal a thousandth part of the barometric pressure that we have at
the earth's surface. Frequent observations of stars passing behind the
moon in occultation have satisfied astronomers on this point.

We often say of the brilliant full moon, it is as bright as day. The
photometer or instrument for accurate comparison of lights, their amount
and intensity, tells a different story. Indeed, if the entire dome of
the sky were filled with full moons, we should be receiving only
one-eighth of the light the sun gives us, and it would require more than
600,000 average full moons to equal the light radiation of the sun. Heat
from the moon, however, is quite different. Early attempts to measure it
detected none at all, but with modern instruments there is little
trouble in detecting heat from the moon, though measurement of it is not
easy.

Much of the moon's heat is sun heat, directly reflected from the moon,
as sunlight is, but most of it is due to radiation of solar heat
previously absorbed by the materials of the lunar surface. The actual
temperature of the moon's surface suffers great variation. A fortnight's
perpetual shining of the sun upon the lunar rocks would certainly heat
them above the temperature of boiling water, if the moon had an
atmosphere to conserve and store this heat; but the entire absence of
such an air blanket probably permits the sun's heat to be radiated away
nearly as fast as it is received, leaving the temperature at the surface
always very low.

What physical influences the moon really has upon the earth must be very
slight, barring the tides. But there is little hope of getting people
generally to take that view, because the moon appears to be the planet
of the people, and opinion that the moon controls the weather, for
instance, amounts with them to practical certainty. More than likely all
these notions are but legitimate survivals of superstition and
astrology. In addition to the tides, our magnetic observatories reveal
slight disturbances with the swinging of the moon from apogee to perigee
and back; but long series of weather observations have been faithfully
interrogated, with negative or contradictory results. If one believes
that the moon's changes affect the weather, it is easy to remember
coincidences, and pass over the many times when no change has taken
place. The moon changes pretty frequently anyhow. As Young well puts it:
"A change of the moon necessarily occurs about once a week.... _All_
changes, of the weather for instance, must therefore occur within three
and three-fourth days of a change of the moon, and fifty per cent of
them ought to occur within forty-six hours of a change, even if there
were no causal connection whatever."

When we turn to the strongly diversified surface of the moon itself, we
find much to rivet the attention, even with slender optical aid.
Everyone wants to know how near the telescope, the biggest possible
telescope, brings the moon to us. That will depend on many things, first
of all on the magnifying power of the eyepiece employed on the
telescope, and eyepieces are changed on telescopes just as they are on
microscopes, though not for the same reasons. The theoretical limit of
the power of a telescope is usually considered as 100 for each inch of
diameter or aperture of the object glass.

A 40-inch telescope, as that of the Yerkes Observatory, the largest
refracting telescope in existence, should bear a magnifying power not to
exceed 4,000. But this limit is practically never reached, one-half of
it or fifty to the inch of aperture being a good working limit of power,
even under exceptional conditions of steadiness of atmosphere. If we
reduce the effective distance of the moon from 240,000 miles to 100
miles, that is about the utmost that can be expected. But even at that
distance we can make out only landscape details, nothing whatever like
buildings or the works of intelligence.

The larger relations of light and shade, so obvious to the naked eye on
the moon, vanish on looking at it with the telescope, but we are at once
captivated by the novel character of the surface and the seemingly great
variety of detail that is clearly visible. As soon as the new moon comes
out in the west, one may begin to gaze with interest and watch the
terminator or sunrise line gradually steal over the roughened surface,
bringing new and striking craters into view each night. Around the time
of quarter moon, or a little past it, is one of the best times for
telescopic views of the moon, because the huge craters, Tycho and
Copernicus, are then in fine illumination. Close to the phase of full
moon is never a good time, because there are no shadows of the rough
surface then, and its entire structure seems to be quite flat and
uninteresting, except for the streaks or rills which radiate from Tycho
in every direction, and are the only lunar features that are best seen
near full.

In a broad, general way, the moon's surface, if compared with the
earth's, differs in having no water. Our extensive oceans are replaced
there by smooth, level plains which were at first thought to be seas and
so named. There are ten or twelve of them in all. Then we find mountain
ranges, so numerous on the earth, relatively few on the moon. Those that
exist are named, in part, for terrestrial mountain ranges, as the Alps,
Caucasus, and the Apennines.

But the nearly circular crater, a relatively rare formation on the
earth, is seen dotted all over the moon in every size, from a fraction
of a mile in diameter up to sixty, seventy, and in extreme cases a
hundred miles. No mere description of plains and mountains and craters
affords an adequate idea of the moon's surface as it actually is; a
telescopic view is necessary, or some of the modern photographs which
give an even better notion of the moon than any telescopic view. Many of
the lunar craters are without doubt volcanic in origin, others seem to
be ruins of molten lakes. Many thousands of the smaller ones appear as
if formed by a violent pelting of the surface when semi-plastic, perhaps
by enormous showers of meteoric matter. More than 30,000 craters cover
the half of the lunar surface visible from the earth, and hundreds of
them are named for philosophers and astronomers.

Measurement of the height of lunar mountains has been made in numerous
instances, especially when their shadows fall on plains or surfaces that
are nearly level, so that the length of the shadow can be measured. In
general, the height of lunar peaks is greater than that of terrestrial
peaks, owing probably to the lesser surface gravity on the moon. About
forty lunar peaks are higher than Mont Blanc.

Most astronomers regard it as certain that no changes ever take place on
the moon; probably no very conspicuous changes ever do. Some, however,
have made out a fair case for comparatively recent changes in surface
detail. Extreme caution is necessary in drawing conclusions, because the
varying changes of illumination from one phase to another are themselves
sufficient to cause the appearance of change. At intervals of a double
lunation, equal to fifty-nine days, one and one-half hours, the
terminator goes very nearly through the same objects, so that the
circumstances of illumination are comparable. In Mare Serenitatis the
little crater named Linné was announced to have disappeared about a half
century ago; subsequently it became visible again and other minor
changes were reported, perhaps due to falling in of the walls of the
crater.

If one were to visit the moon, he must needs take air and water along
with him, as well as other sustenance. No atmosphere means no diffused
light; we could see nothing unless the sun's direct rays were shining
upon it. Anyone stepping into the shadow of a lunar crag would become
wholly invisible. No sound, however loud, could be heard; sound in fact
would become impossible. A rock might roll down the wall of a lunar
crater, but there would be no noise; though we should know what had
happened by the tremor produced. So slight is gravity there that a good
ball player might bat a baseball half a mile or more. Looking upward,
all the stars would be appreciably brighter than here, and visible
perpetually in the daytime as well as at night.

If one were to go to the opposite side of the moon, he would lose sight
of the earth until he came back to the side which is always turned
toward the earth. Even then the earth would never rise and set at any
given place, as the moon does to us, but would remain all the time at
about the same height above the lunar horizon. The earth would go
through all the phases that the moon shows to us here, full earth
occurring there when it is new moon here. Our globe would appear to be
nearly four times broader than the moon seems to us. Its white polar
caps of ice and snow, its dark oceans, and the vast cloud areas would be
very conspicuous. Faint stars, the zodiacal light, and the filmy solar
corona would be visible, probably even close up to the sun's edge; but
although his rays might shine upon the lunar rocks without intermission
for a fortnight, probably they would still be too cold to touch with
safety. On the side of the moon turned away from the sun, the
temperature of the moon's surface would fall to that of space, or many
hundred degrees below zero.




CHAPTER XXIX

ECLIPSES OF THE MOON


Of all the weird happenings of the nighttime sky, eclipses of the moon
are the most impressive. Rarely is there a year without one. What is the
cause? Simply the earth getting in between sun and moon, and thereby
shutting off the sunlight which at all other times enables us to see the
moon. As the earth is a dark body it must cast a black shadow on the
side away from the sun, and it is the moon's passing into this shadow or
some part of it that causes a lunar eclipse.

Sun and earth being so different in size, the earth's shadow must
stretch away from it into space, growing smaller and smaller, until at
length it comes to an end--the apex of a cone 857,000 miles long. If we
cut off this shadow at the moon's distance from the earth, we find it
about 6,000 miles in diameter at that point; and this accounts for the
fact that the curvature on the side of the moon, when the eclipse is
coming on and where it is dropping into the shadow, is always much less
rapid than the curvature of the moon's own disk is.

When an eclipse is approaching, the eastern limb will be duskily
darkened for half an hour or more, because the moon must first pass
through the outer penumbra, or half-shadow which everywhere surrounds
the true shadow itself. If the moon hits only the upper or lower part of
the shadow, the eclipse will be only partial, and during the progress
of the eclipse it will seem as if the uneclipsed part had swung or
twisted around in the sky, from the western limb of the moon to the
eastern. But when the moon passes through the middle regions of the
shadow, the eclipse is always total, and direct sunlight is wholly cut
off from every part of the moon's face, for a greater or less length of
time, according to the part of the shadow through which it passes. When
passing centrally through the shadow, the total eclipse will last about
two hours, as the moon's diameter is about one-third of the breadth of
the shadow; and the eclipse will be partial about two hours longer, an
hour at beginning and an hour at the end, because the moon moves over
her own breadth in about an hour.

While the moon is wholly immersed in the shadow, her body is
nevertheless visible, as a dull tarnished copper disk; and this is
caused by the reddish sunlight which grazes the earth all around and is
refracted or bent by our atmosphere into the shadow itself. If this belt
or ring of terrestrial atmosphere happens to be everywhere filled with
dense clouds, as was the case in 1886, even the familiar copper moon of
a total lunar eclipse disappears completely in the black sky.

Quite different from a solar eclipse, all the phases of a lunar eclipse
are visible at the same time on the earth wherever the moon is above the
horizon. Eclipses of the moon are therefore seen with great frequency at
any given place as compared with solar eclipses, which are restricted to
relatively narrow areas of the earth's surface. Nor are lunar eclipses
of very much significance to the astronomer, mainly because of the
slowness and indefiniteness of the phenomena. It is a good time to
observe occultations of faint stars at the moon's edge or limb, and
several such programs have been carried out by cooperation of
observatories in widely separate regions of the world: the object being
improvement in our knowledge of the distance of the moon, and in the
accuracy of the mathematical tables of her motion. Search by photography
for a possible satellite, or moon of the moon, has been made on several
occasions, though without success.

A lunar eclipse was first observed and photographed from an aeroplane,
May 2, 1920. At the request of the writer, two aviators of the United
States navy ascended to a height of 15,000 feet above Rockaway, and
secured many advantages accruing from great elevation in viewing a
celestial phenomenon of this character.




CHAPTER XXX

TOTAL ECLIPSES OF THE SUN


Primitive peoples indulged in every variety of explanation of mysterious
happenings in the sky. To the Chinese and all through India, a total
eclipse of the sun is caused by "a certain dragon with very black
claws," who, except for their frightening him away by every conceivable
sort of hideous noise, would most certainly "eat up the sun." The
eclipse always goes off, the sun has never been eaten yet. Can you
convince a Chinaman that Rahu, the Dragon, wouldn't have eaten up the
sun, if his unearthly din hadn't frightened him away?

In Japan the eclipse drops poison from the sky into wells, so the
Japanese cover them up. Fontenelle relates that in the middle of the
seventeenth century a multitude of people shut themselves up in cellars
in Paris during a total eclipse.

In the Shu-king, an ancient Chinese work, occurs the earliest record of
a total eclipse of the sun, in the year B. C. 2158. The Nineveh eclipse
of B. C. 763 is perhaps the first of the ancient eclipses of which we
possess a really clear description on the Assyrian eponym tablets in the
British Museum. It is the eclipse possibly referred to in the Book of
Amos, viii.

But of all the ancient eclipses none perhaps exceeds in interest the
famous eclipse of Thales, B. C. 585, May 28. It is the first eclipse to
have been predicted, probably by means of the saros, or 18-year period
of eclipses, which is useful as an approximate method even at the
present day. But the accident of a war between the Lydians and the Medes
has added greatly to the historic interest, because the combatants were
so terrified by the sudden turning of day into night that they at once
concluded a peace cemented by two marriages.

Very many of the ancient eclipses have been of great use to the
historian in verifying dates, and mathematical astronomers have employed
them in correcting the lunar tables, or intricate mathematical data by
which the motion of the moon is predicted.

Coming down to the middle of the sixth century, we find the first
eclipse recorded in England, in the "Saxon Chronicle," A. D. 538. During
the epoch of the Arabian Nights several eclipses were witnessed at
Bagdad, A. D. 829 to 928, and many a century later by Ibu-Jounis, court
astronomer of Hakem, the Caliph of Egypt. Nothing is more interesting
than to search the quaint records of these ancient eclipses. One
occurring in 1560, when Tycho Brahe was but fourteen, had much to do
with turning his permanent interest toward mathematics and astronomy.
The eclipse of 1612 was the first "seen through a tube," the telescope
having been invented only a few years before. "Paradise Lost" was
completed about 1665, and the censorship was still in existence; and it
is matter of record that the oft-quoted passage,

        "As when the Sun, new risen,
    Looks through the horizontal misty air,
    Shorn of his beams; or from behind the Moon,
    In dim eclipse, disastrous twilight sheds
    On half the nations, and with fear of change
    Perplexes monarchs."
                                      _P. L._, i. 594

was strongly urged as sufficient reason for suppressing the entire epic.

London was favored with the outflashing corona, May 3, 1715, and a
pamphlet was issued in prediction, entitled "The Black Day, or a
Prospect of Doomsday."

The first American eclipse expedition was on occasion of the totality of
Oct. 27, 1780, sent out by Harvard College and the American Academy of
Arts and Sciences under Professor Samuel Williams to Penobscot. There
was a fine total eclipse from Albany to Boston on June 16, 1806, and
many important observations of it were made in this country.

But it was not till the European eclipse of 1842 that research got fully
under way, because the germ of the new astronomy, particularly as
applied to the sun, had begun its development; and the significance of
the corona was obvious, if it could be proved a true appendage of the
sun. Photography had not long been discovered, and the corona of 1851
was the first to be automatically registered on a daguerreotype. In 1860
it was proved that prominences and corona both belong to the sun and not
to the moon.

The great Indian eclipse of 1868 brought the important discovery that
the prominences can be observed at any time without an eclipse by means
of the spectroscope. In 1869 bright lines were found in the spectrum of
the corona, one line in the green indicating the presence of an element
not then known on the earth and hence called coronium. In 1870 the
reversing layer or stratum of the sun was discovered. In 1878 a vast
ecliptic extension of the streams of the corona many millions of miles
both east and west of the sun was first seen. This is now known to be
the type of corona characteristic of minimum spots on the sun. In 1882
the spectrum of the corona was first photographed and in 1889 excellent
detail photographs of the corona were taken. In 1893 it was shown that
the corona quite certainly rotates bodily with the sun. In 1896 actual
spectrum photographs of the reversing layer established its existence
beyond doubt--"flash spectrum" it is often called. In 1898 the long
ecliptic streamers of the corona were successfully photographed for the
first time. In 1900 the depth of the reversing layer was found to
average 500 miles, the heat of the corona was first measured by the
bolometer, and many observations showed that the coronal streamers, in
part at least, partake of the nature of electric discharges.

All subsequent total eclipses have been carefully observed, in whatever
part of the world they may happen, and each has added new results of
significance to our theories of the corona and its relation to the
radiant energy of the sun. In very recent eclipses the cinematograph has
been brought into action as an efficient adjunct of observation; in 1914
the first successful "movie" of the eclipse was secured in Sweden, and
in 1918 Frost of the Yerkes Observatory first applied the cinematograph
to registry of the "flash spectrum," and Stebbins tested out his
photo-electric cell on the corona, making the brightness 0.5 that of the
full moon. In 1914 (Russia) and again in 1919 (on the Atlantic) the
obvious advantages of the aeroplane in ecliptic observation and
photography were sought by the writer, though unsuccessfully. The
photographic tests, however, conducted in preparation for these
expeditions proved the entire practicability of securing eclipse results
of much value, independently of clouds below.

Eclipses in the near future will be total in Australia about six minutes
on September 21, 1922; in California and Mexico about four minutes on
September 10, 1923; and along a line from Toronto to Nantucket about two
minutes on the morning of January 24, 1925.

To all spectators, savage or civilized, scientist or layman, a total
eclipse is wonderful and impressive. Langley said: "The spectacle is one
of which, though the man of science may prosaically state the facts,
perhaps only the poet could render the impression." Very gradually the
moon steals its way across the face of the sun, the lessened light is
hardly noticed. If one is near a tree through whose foliage the sunlight
filters, an extraordinary sight is seen; the ground all about is covered
with luminous crescents, instead of the overlapping disks which were
there before the eclipse came on; in both cases they are images of the
disk of the sun at the time, and the narrowing crescents will be watched
with interest as totality approaches. Then the shadow bands may be seen
flitting across the landscape, like "visible wind." They are probably
related to our atmosphere and the very slender crescent from which true
sunlight still comes.

Then for a few seconds the moon's actual shadow may be caught in its
approach, very suddenly the darkness steals over the landscape
and--totality is on. How lucky if there are no clouds! Every eye is
riveted on "the incomparable corona, a silvery, soft, unearthly light,
with radiant streamers, stretching at times millions of uncomprehended
miles into space, while the rosy flaming protuberances skirt the black
rim of the moon in ethereal splendor."

Then it is now or never with observer and photographer. Months of
diligent preparations at home followed by weeks of tedious journey
abroad, with days of strenuous preparation and rehearsals at the
station--all go for naught unless the whole is tuned up to perfect
operation the instant totality begins. It may last but a minute, or even
less; in 1937, however, total eclipse will last 7 minutes 20 seconds,
the longest ever observed, and within half a minute of the longest
possible. All is over as suddenly as it came on. The first thing is to
complete records, develop plates, and see if everything worked
perfectly.

There is great utility back of all eclipse research, on account of its
wide bearing on meteorology and terrestrial physics, and possibly the
direct use of solar energy for industrial purposes. With this purpose in
view the astronomer devotes himself unsparingly to the acquisition of
every possible fact about the sun and his corona.

Considering the earth as a whole, the number of total eclipses will
average nearly seventy to the century. But at any given place, one may
count himself very fortunate if he sees a single total eclipse, although
he may see several partial ones without going from home. Then, too,
there are annular or ring eclipses, averaging seven in eight years. But
had one been born in Boston or New York in the latter part of the
eighteenth century, he might have lived through the entire nineteenth
century and a long way into the twentieth without seeing more than one
total eclipse of the sun. In London in 1715 no total eclipse had been
visible for six centuries. However, taking general averages, and
recalling the comparatively narrow belt of total eclipse, every part of
the earth is likely to come within range of the moon's shadow once in
about three and a half centuries.

The longest total eclipses always occur near the equator; this is
because an observer on the equator is carried eastward by the earth's
rotation at a velocity of about 1,000 miles per hour, so that he remains
longer in the moon's shadow which is passing over him in the same
direction with a velocity about twice as great.

The general circumstances of total eclipses are readily foretold by
means of the ancient Chaldean period of eclipses known as the saros. It
is 18 years and 10 or 11 days in length (according to the number of leap
years intervening). In one complete saros, forty-one solar eclipses will
generally happen, but only about one-fourth of them will be total. The
saros is a period at the end of which the centers of sun and moon return
very nearly to their relative positions at the beginning of the cycle.
So, in general, the eclipse of any year will be a repetition of one
which took place 18 years before, and another very similar in
circumstances will happen 18 years in the future. Three periods of the
saros, or 54 years and 1 month, will usually bring about a return of any
given eclipse to any particular part of the earth, so far as longitude
is concerned, though the returning track will lie about 600 miles to the
north or south of the one 54 years earlier.

Paths of total eclipses frequently intersect, if large areas like an
entire country are considered; Spain, for instance, where total eclipses
have occurred in 1842, 1860, 1870, 1900 and 1905. Besides crossing
Spain, the tracks of totality on May 28, 1900, and August 30, 1905, were
unique in intersecting exactly over a large city--Tripoli in Barbary, on
both of which occasions the writer's expeditions to that city were
rewarded with perfect observing conditions in that now Italian province
on the edge of the great desert.

Kepler was the first astronomer to calculate eclipses with some approach
to scientific form, as exemplified in his Rudolphine Tables. His method
was of course geometrical. But La Grange, who applied the methods of
more refined analysis to the problem, was the first to develop a method
by which an eclipse and all its circumstances could be accurately
predicted for any part of the earth. To many minds, the prediction of an
eclipse affords the best illustration of the superior knowledge of the
astronomer: it seems little short of the marvelous. But recalling that
the motion of the moon follows the law of gravitation, and that its
position in the sky is predictable for years in advance with a high
degree of precision, it will readily be seen how the arrival of the
moon's shadow, and hence the total eclipses of the sun, can be foretold
for any place over which the shadow passes.

All these data derived by the mathematician are known as the elements of
the eclipse, and they are prepared many years in advance and published
in the nautical almanacs and astronomical ephemerides issued by the
leading nations. Buchanan's "Treatise on Eclipses" will supply all the
technical information regarding the prediction of eclipses that anyone
desirous of inquiring into this phase of the problem may desire.

So important are total eclipses in the scheme of modern solar research,
and so necessary are clear skies in order that expeditions may be
favored with success, that every effort is now made to ascertain the
weather chances at particular stations along the line of eclipse many
years in advance. This method of securing preliminary cloud observations
for a series of years has proved especially useful for the eclipses of
1893, 1896, 1900, and 1918; and had it been employed in Russia for
totality of 1914, many well-equipped expeditions might have been spared
disaster. The California and Mexico totality of 1923 does not require
this forethought, as the regions visited are quite likely to be free
from cloud; but observations are now in process of accumulation for the
total eclipse of 1925. The out-look for clear skies on that occasion,
the total eclipse nearest New York for more than a century, is not very
promising. The path of totality passes over Marquette, Michigan,
Rochester and Poughkeepsie, New York, Newport, Rhode Island, and
Nantucket about nine in the morning.

Everyone who saw it will remember the last total eclipse in this part of
the world--on June 8, 1918, visible from Oregon to Florida. Many will
recall the last total eclipse that was visible before that in the
eastern part of the United States, on May 28, 1900, visible in a narrow
path from New Orleans to Norfolk. One's father or grandfather will
perhaps remember the total eclipse of July 29, 1878, which passed over
the United States from Pike's Peak to Texas (it was the writer's maiden
eclipse), and another on August 7, 1869, which passed southeasterly over
Iowa and Kentucky. On all these occasions the paths of total eclipse
were dotted with numerous observing parties, many of them equipped with
elaborate apparatus for studying and photographing the solar corona and
prominences, together with a multitude of other phenomena which are seen
only when total eclipses take place.

Looking forward rather than backward, a striking series, or family, of
eclipses happens in the future: it is the series of May, 1901 and 1919,
recurring again on June 8, 1937 (over the Pacific Ocean), June 20, 1955
(through India, Siam, and Luzon), and June 30, 1973 (visible in Sahara,
Abyssinia, and Somali). Already in 1919 this totality was 6 minutes 50
seconds in duration; in 1937, as already mentioned, it will be 7 minutes
20 seconds, and at the subsequent returns even longer yet, approaching
the estimated maximum of 7 minutes 58 seconds which has never been
observed. This remarkable series of total eclipses is longer in duration
than any others during a thousand years. Its next subsequent return is
in 1991, occurring with the eclipsed sun practically at noon in the
zenith of Mount Popocatepetl in Mexico.

Whatever may be the progress of solar research during the intervening
years, it is impossible to imagine the alert astronomer of that remote
day without incentive for further investigation of the sun's corona, in
which are concealed no doubt many secrets of the sun's evolution from
nebula to star.




CHAPTER XXXI

THE SOLAR CORONA


"And what is the sun's corona?" mildly asked a college professor of a
student who might better have answered "Not prepared."

"I did know, Professor, but I have forgotten," was his reply.

"What an incalculable loss to science," returned the professor with a
twinkle. "The only man who ever knew what the sun's corona is, and he
has forgotten!"

Only in part has the mystery of the corona been cleared by the research
of the present day. Our knowledge proceeds but slowly, because the
corona has never been seen except during total eclipses of the sun; and
astronomers, as a matter of fact, have never had a fair chance at it.
Two total eclipses happen on the average of every three years; their
average duration is only two or three minutes; totality can be seen only
in a narrow path about a hundred miles wide, though it may be several
thousand miles long; there is usually about equal chance of cloud with
clear skies; and fully three-fourths of the totality areas of the globe
are unavailable because covered by water. So that even if we imagine the
tracks of eclipses quite thickly populated with astronomers and
telescopes, at least one every hundred miles, how much solid watching of
the corona would this permit? Only a little more than one week's time in
a whole century.

The true corona is at least a triple phenomenon and a very complex one.
The photographs reveal it much as the eye sees it, with all its
complexity of interlacing streamers projected into a flat, or plane,
surrounding the disk of the dark moon which hides the true sun
completely. But we must keep in mind the fact that the sun is a globe,
not a disk, and that the streamers of the corona radiate more or less
from all parts of the surface of the solar sphere, much as quills from a
porcupine.

From the sun's magnetic poles branch out the polar rays, nearly straight
throughout their visible extent. Gradually as the coronal rays originate
at points around the solar disk farther and farther removed from the
poles, they are more and more curved. Very probably they extend into the
equatorial regions, but it is not easy to trace them there because they
are projected upon and confused with the filaments having their origin
remote from the poles. Then there is the inner equatorial corona,
apparently connected intimately with truly solar phenomena, quite as the
polar rays are. The third element in the composite is the outer ecliptic
corona, for the most part made up of long streamers. This is most fully
developed at the time of the fewest spots on the sun. It is traceable
much farther against the black sky with the naked eye than by
photography. Without any doubt it is a solar appendage and possibly it
may merge into the zodiacal light.

Naturally this superb spectacle must have been an amazing sight to the
beholders of antiquity who were fortunate enough to see it. Historical
references are rare: perhaps the earliest was by Plutarch about A. D.
100, who wrote of it, "A radiance shone round the rim, and would not
suffer darkness to become deep and intense." Philostratus a century
later mentions the death of the emperor Domitian at Ephesus as
"announced" by a total eclipse.

Kepler thought the corona was evidence of a lunar atmosphere; indeed, it
was not until the middle of the 19th century that its lack of relation
to the moon was finally demonstrated. Later observers, Wyberd in 1652
and Ulloa, got the impression that the corona turned round the disk
catherine-wheel fashion, "like an ignited wheel in fireworks, turning on
its center." But no later observer has reported anything of the sort.
Quite the contrary, there it stands against the black sky in motionless
magnificence a colorless pearly mass of wisps and streamers for the most
part nebulous and ill-defined, fading out very irregularly into the
black sky beyond, but with a complex interlacing of filaments, sometimes
very sharply defined near the solar poles. It defies the skill of artist
and draughtsman to sketch it before it is gone.

Photograph it? Yes, but there are troubles. Of course the camera work is
superior to sketches by hand. As Langley used to say, "The camera has no
nerves, and what it sets down we may rely on." Foremost among the
photographic difficulties is the wide variation in intensity of the
coronal light in different regions of the corona. If a plate is exposed
long enough to get the outer corona, the exceeding brightness of the
inner corona overexposes and burns out that part of the plate or film.
If the exposure is short, we get certain regions of the inner corona
excellently, but the outer regions are a blank because they can be
caught only by a long exposure.

So the only way is to take a series of pictures with a wide range of
exposures, and then by careful and artistic handwork, combine them all
into a single drawing. Wesley of London has succeeded eminently in work
of this character, and his drawings of the sun's corona, visible at
total eclipses from 1871 onward, in possession of the Royal Astronomical
Society, are the finest in existence. They give a vastly better idea of
the corona, as the eye sees it, than any single photograph possibly can.

The early observers apparently never thought of the corona as being
connected with the sun. It was a halo merely, and so drawn. Its real
structure was neither known, depicted, or investigated. Sketches were
structureless, as any aureola formed by stray sunlight grazing the moon
might naturally be. That the rays are curved and far from radial round
the sun was shown for the first time in the sketches of 1842, and in
1860 Sir Francis Galton observed that the long arms or streamers "do not
radiate strictly from the center."

The inner corona had first been recorded photographically on a
daguerreotype plate during the eclipse of 1851, but the lens belonged to
a heliometer, and was of course uncorrected for the photographic rays.
The wet collodion plates of the eclipse of 1860, by De la Rue, proved
that not only the prominences but the corona were truly solar, because
his series of technically perfect pictures revealed the steady and
unchanged character of these phenomena while the moon's disk was passing
over them as totality progressed. And at the eclipse of 1869, Young put
the solar theory of the corona beyond the shadow of any further doubt
by examination of its light with the spectroscope and discovering a
green line in the spectrum due to incandescent vapor of a substance not
then identified with anything terrestrial, and therefore called
coronium.

The total brilliance of the corona was very differently estimated by the
earlier observers, though pretty carefully measured at later eclipses.
The standard full moon is used for reference, and at one eclipse the
corona falls short of, while at another it will exceed the full moon in
brightness. Variations in brilliancy are quite marked: at one eclipse it
was nearly four times as bright as the full moon. Much evidence has
already accumulated on this question; but whether the observed
variations are real, or due mainly to the varying relative sizes of sun
and moon at different eclipses, is not yet known. The coronal light is
largely bluish in tint, and this is the region of the spectrum most
powerfully absorbed by our atmosphere. Eclipses are observed by
different expeditions located at stations where the eclipsed sun stands
at very different altitudes above the horizon; besides this the
localities of observation are at varied elevations above sea level; so
that the varying amount of absorption of the coronal light renders the
problem one of much difficulty.

The long ecliptic streamers of the corona were first seen by Newcomb and
Langley during the totality of 1878. On one side of the sun there was a
stupendous extension of at least twelve solar diameters, or nearly 11
millions of miles. Langley observed from the summit of Pike's Peak, over
14,000 feet high, and was sure that he was witnessing a "real phenomenon
heretofore undescribed." The vast advantage of elevation was apparent
also from the fact that he held the corona for more than four minutes
after true totality had ended. These streamers are characteristic of the
epoch of minimum spots on the sun, as Ranyard first suggested. It was
found that this type of corona had been recorded also in 1867; and it
has reappeared in 1889, 1900 and 1911, and will doubtless be visible
again in 1922.

How rapidly the streamers of the corona vary is not known. Occasionally
an observer reports having seen the filaments vibrate rapidly as in the
aurora borealis, but this is not verified by others who saw the same
corona perfectly unmoving. Comparisons of photographs taken at widely
separate stations during the same eclipse have shown that at least the
corona remained stationary for hours at a time. Whether it may be
unchanged at the end of a day, or a week, or a month, is not known;
because no two total eclipses can ever happen nearer each other than
within an interval of 173 days, or one-half of the eclipse year. And
usually the interval between total eclipses is twice or three times this
period.

Theories of what the solar corona may be are very numerous. The extreme
inner corona is perhaps in part a sort of gaseous atmosphere of the sun,
due to matter ejected from the sun, and kept in motion by forces of
ejection, gravity, and repulsion of some sort. Meteoric matter is likely
concerned in it, and Huggins suggested the débris of disintegrating
comets. Schuster was in agreement with Huggins that the brighter
filaments of the corona might be due to electric discharges, but it
seems very unlikely that any single hypothesis can completely account
for the intricate tracery of so complex a phenomenon.

    [Illustration: SOLAR CORONA AND PROMINENCES. Photographed during a
       total eclipse of the sun, June 8, 1918. (_Courtesy, American
       Museum of Natural History._)]

    [Illustration: VENUS, SHOWING CRESCENT PHASE OF THE PLANET. Venus
       is the earth's nearest neighbor on the side toward the sun.
       (_Photo, Yerkes Observatory._)]

    [Illustration: MARS, THE PLANET NEXT BEYOND THE EARTH. The
       photograph shows one of the white polar caps. The caps are
       thought to be snow or ice and may indicate the existence of
       atmosphere. (_Photo, Yerkes Observatory._)]

Elaborate spectroscopic programs have been carried out at recent
eclipses, affording evidence that certain regions are due to
incandescent matter of lower temperature than the sun's surface. A small
part of the light of the corona is sunlight reflected from dark
particles possibly meteoric, but more likely dust particles or fog of
some sort. This accounts for the weakened solar spectrum with Fraunhofer
absorption lines, and this part of the light is polarized.

Many have been the attempts to see, or photograph, the corona without an
eclipse. None of them has, however, succeeded as yet. Huggins got very
promising results nearly forty years ago, and success was thought to
have been reached; but subsequent experiments on the Riffelberg in 1884
and later convinced him that his results related only to a spurious
corona. In 1887 the writer made an unsuccessful attempt to visualize the
corona from the summit of Fujiyama, and Hale tried both optical and
photographic methods on Pike's Peak in 1893 without success. He devised
later a promising method by which the heat of the corona in different
regions can be measured by the bolometer, and an outline corona
afterward sketched from these results.

Still another method of attacking the problem occurred to the writer in
1919, which has not yet been carried out. It would take advantage of
recent advances in aeronautics, and contemplates an artificial eclipse
in the upper air by means of a black spherical balloon. This would be
sent up to an altitude of perhaps 40,000 feet, where it would partake
of the motion of the air current in which it came to equilibrium. Then a
snapshot camera would be mounted on an aeroplane, in which the aviator
would ascend to such a height that the balloon just covered the sun, as
the moon does in a total eclipse. With the center of the balloon in line
with the sun's center, he would photograph the regions of the sky
immediately surrounding the sun, against which the corona is projected.
As the entire apparatus would be above more than an entire half of the
earth's atmosphere, the experiment would be well worth the attempt, as
pretty much everything else has been tried and found wanting. Needless
to say, the importance of seeing the corona at regular intervals
whenever desired, without waiting for eclipses of the sun, remains as
insistent as ever.




CHAPTER XXXII

THE RUDDY PLANET


Mars is a planet next in order beyond the earth, and its distance from
the sun averages 141-1/2 million miles. It has a relatively rapid motion
among the stars, its color is reddish, and, when nearest to us, it is
perhaps the most conspicuous object in the sky.

Mars appeared to the ancients just as it does to us to-day. Aristotle
recorded an observation of Mars, 356 B. C., when the moon passed over
the planet, or occulted it, as our expression is. Galileo made the first
observations of Mars with a telescope in 1610, and his little instrument
was powerful enough to enable him to discover that the planet had
phases, though it did not pass through all the phases that Mercury and
Venus do. This was obvious from the fact that Mars is always at a
greater distance from the sun than we are, and the phase can only be
gibbous, or about like the moon when midway between full and quarter.

Many observers in the seventeenth century followed up the planet with
such feeble optical power as the telescopes of that epoch provided:
Fontana (who made the first sketch), Riccioli and Bianchini in Italy,
Cassini in France, Huygens in Holland, and later Sir William Herschel in
England.

It was Cassini who first made out the whitish spots or polar caps of
Mars in 1666, but not until after Huygens had noted the fact that Mars
turned round on an axis in a period but little longer than the earth's.
Cassini followed it up later with a more accurate value; and
observations in our own day, when combined with these early ones, enable
us to say that the Martian day is equal to 24 hours 37 minutes 22.67
seconds, accurate probably to the hundredth part of a second.

When we know that a planet turns round on an axis, we know that it has a
day. When we know the direction of the axis in space or in relation to
the plane of its path round the sun, we know that it has seasons: we can
tell their length and when they begin and end. It did not take many
years of observation to prove that the axis round which Mars turns is
tilted to the plane of its path round the sun by an angle practically
the same as that at which the earth's axis is tilted. So there is the
immediate inference that on Mars the order and perhaps the character of
the seasons is much the same as here on the earth.

At least two things, however, tend to modify them. First, the year of
Mars is not 365 days like ours, but 687 days. Each of the four seasons
on Mars, therefore, is proportionally longer than our seasons are. Then
comes the question of atmosphere--how much of an atmosphere does Mars
really possess in proportion to ours, and how would its lesser amount
modify the blending of the seasons into one another?

All discussion of Mars and the problems of existence of life upon that
planet hinge upon the character and extent of Martian atmosphere. The
planet seems never to be covered, as the earth usually is, with
extensive areas of cloud which to an observer in space would completely
mask its oceans and continents. Nearly all the time Mars in his
equatorial and temperate zones is quite clear of clouds. A few whitish
spots are occasionally seen to change their form and position in both
northern and southern latitudes, and they vary with the progress of the
day on Mars, as clouds naturally would. But Schiaparelli, perhaps the
best of all observers, thought them to be not low-lying clouds of the
nimbus type that would produce rains, but rather a veil of fog, or
perhaps a temporary condensation of vapor, as dew or hoar frost. But the
strongest argument for an atmosphere is based on the temporary darkening
or obscuration of well known and permanent markings on the surface of
Mars. These are more or less frequently observed and clouds afford the
best explanation of their occurrence.

So much for evidence supplied by the telescope alone. When, however, we
employ the spectroscope in conjunction with the telescope, another sort
of evidence is at hand. Several astronomers have reached the conclusion
that watery vapor exists in the atmosphere of Mars, while other
astronomers equipped with equal or superior apparatus, and under equally
favorable or even better conditions, have reached the remarkable
conclusion that the spectra of Mars and the moon are identical in every
particular. From this we should be led to infer that Mars has perhaps no
more atmosphere than the moon has, that is to say, none whatever that
present instruments and methods of investigation have enabled us to
detect.

What then, shall we conclude? Simply that the atmosphere of Mars is
neither very dense nor extensive. Probably its lower strata close to
the planet's surface are about as dense as the earth's atmosphere is at
the summits of our highest mountains.

This conclusion is not unwelcome, if we keep a few fundamental facts in
clear and constant view. Mars is a planet of intermediate size between
the earth and the moon: twice the moon's diameter (2,160 miles) very
nearly equals the diameter of Mars (4,200 miles), and twice the diameter
of Mars does not greatly exceed the earth's diameter (7,920 miles). As
to the weights or masses of these bodies, Mars is about one-ninth, and
the moon one-eightieth of the earth. The atmospheric envelope of the
earth is abundant, the moon has none as far as we can ascertain; so it
seems safe to infer that Mars has an atmosphere of slight density: not
dense enough to be detected by spectroscopic methods, but yet dense
enough to enable us to explain the varying telescopic phenomena of the
planet's disk which we should not know how to account for, if there were
no atmosphere whatever. One astronomer has, indeed, gone so far as to
calculate that in comparison with our planet Mars is entitled to
one-twentieth as much atmosphere as we have, and that the mercurial
barometer at "sea level" would run about five and a half inches, as
against thirty inches on the earth.

In general, then, the climate of Mars is probably very much like that of
a clear season on a very high terrestrial table land or mountain--a
climate of wide extremes, with great changes of temperature from day to
night. The inequality of Martian seasons is such that in his northern
hemisphere the winter lasts 381 days and the summer only 306 days.

Now, the polar caps of Mars, which are reasonably assumed to be due to
snow or hoar frost, attain their maximum three or four months after the
winter solstice, and their minimum about the same length of time after
the summer solstice. This lagging should be interpreted as an argument
for a Martian atmosphere with heat-storing qualities, similar to that
possessed by the earth.

Upon this characteristic, indeed, depends the climate at the surface of
Mars: whether it is at all similar to our own, and whether fluid water
is a possibility on Mars or not. While the cosmic relations of the
planet in its orbit are quite the same as ours, nevertheless the greater
distance of Mars diminishes his supply of direct solar heat to about
half what we receive. On the other hand, his distance from the sun
during his year of motion around it varies much more widely than ours,
so that he receives when nearest the sun about one-half more of solar
heat than he does when farthest away.

Southern summers on Mars, therefore, must be much hotter, and southern
winters colder than the corresponding seasons of his northern
hemisphere. Indeed, the length of the southern summer, nearly twice that
of the terrestrial season, sometimes amply suffices to melt all the
polar ice and snow, as in October, 1894, when the southern polar cap of
Mars dwindled rapidly and finally vanished completely.

Very interesting in this connection are the researches of Stoney on the
general conditions affecting planetary atmospheres and their
composition. According to the kinetic theory, if the molecules of gases
which are continually in motion travel outward from the center of a
planet, as they frequently must, and with velocities surpassing the
limit that a planet's gravity is capable of controlling, these molecules
will effect a permanent escape from the planet, and travel through space
in orbits of their own.

So the moon is wholly without atmosphere because the moon's gravity is
not powerful enough to retain the molecules of its component gases. So
also the earth's atmosphere contains no helium or free hydrogen. So,
too, Mars is possessed of insufficient force of gravity to retain water
vapor, and the Martian atmosphere may therefore consist mainly of
nitrogen, argon, and carbon dioxide.

As everyone knows, the axis of the earth if extended to the northern
heavens would pass very near the north polar star, which on that account
is known as Polaris. In a similar manner the axis of Mars pierces the
northern heavens about midway between the two bright stars Alpha Cephei
and Alpha Cygni (Deneb). The direction of this axis is pretty accurately
known, because the measurement of the polar caps of the planet as they
turn round from night to night, year in and year out, has enabled
astronomers to assign the inclination of the axis with great precision.

These caps are a brilliant white, and they are generally supposed to be
snow and ice. They wax and wane alternately with the seasons on Mars,
being largest at the end of the Martian winter and smallest near the end
of summer. The existence of the polar caps together with their seasonal
fluctuations afford a most convincing argument for the reality of a
Martian atmosphere, sufficiently dense to be capable of diffusing and
transporting vapor.

The northern cap is centered on the pole almost with geometric
exactness, and as far as the 85th parallel of latitude. On the other
hand, the south polar cap is centered about 200 miles from the true
pole, and this distance has been observed to vary from one season to
another. No suggestion has been made to account for this singular
variation. On one occasion it stretched down to Martian latitude 70
degrees and was over 1,200 miles in diameter.

Pickering watched the changing conditions of shrinking of the south
polar cap in 1892 with a large telescope located in the Andes of Peru.
Mars was faithfully followed on every night but one from July 13 to
September 9, and the apparent alterations in this cap were very marked,
even from night to night. As the snows began to decrease, a long dark
line made its appearance near the middle of the cap, and gradually grew
until it cut the cap in two. This white polar area (and probably also
the northern one in similar fashion) becomes notched on the edge with
the progress of its summer season; dark interior spots and fissures
form, isolated patches separate from the principal mass, and later seem
to dissolve and disappear. Possibly if one were located on Mars and
viewing our earth with a big telescope, the seasonal variation of our
north and south polar caps might present somewhat similar phenomena. All
the recent oppositions of Mars have been critically observed by
Pickering from an excellent station in Jamaica.

Quite obviously the fluctuations of the polar caps are the key to the
physiographic situation on Mars, and they are made the subject of the
closest scrutiny at every recurring opposition of the planet. Several
observers, Lowell in particular, record a bluish line or a sort of
retreating polar sea, following up the diminishing polar cap as it
shrinks with the advance of summer. It is said that no such line is
visible during the formation of the polar cap with the approach of
winter. All such results of critical observation, just on the limit of
visibility, have to be repeated over and over again before they become
part of the body of accepted scientific fact. And in many instances the
only sure way is to fall back on the photographic record, which all
astronomers, whether prejudiced or not, may have the opportunity to
examine and draw their individual conclusions.

Already the approaching opposition of 1924, the most favorable since the
invention of the telescope, is beginning to attract attention, and
preparations are in progress, of new and more powerful instruments, with
new and more sensitive photographic processes, by means of which many of
the present riddles of Mars may be solved.




CHAPTER XXXIII

THE CANALS OF MARS


Then there are the so-called canals of Mars, about which so much is
written and relatively little known. Faint markings which resemble them
in character were first drawn in 1840 and later in 1864, but
Schiaparelli, the famous Italian astronomer, is probably their original
discoverer, when Mars was at its least distance from the earth in 1877.
He made the first accurate detailed map of Mars at this time, and most
of the important or more conspicuous canals (_canali_, he called them in
Italian, that is, channels merely, without any reference whatever to
their being watercourses) were accurately charted by him.

At all the subsequent close approaches of Mars, the canals have been
critically studied by a wide range of astronomical observers, and their
conclusions as to the nature and visibility of the canals have been
equally wide and varied. The most favorable oppositions have occurred in
1892 and 1894, also in 1907 and 1909. On these occasions a close minimum
distance of Mars was reached, that is, about 35 millions of miles; but
in 1924 the planet makes the closest approach in a period of nearly a
thousand years. Its distance will not much exceed 34 millions of miles.

But although this is a minimum distance for Mars, it must not be
forgotten that it is a really vast distance, absolutely speaking; it is
something like 150 times greater than the distance of the moon. With no
telescopic power at our command could we possibly see anything on the
moon of the size of the largest buildings or other works of human
intelligence; so that we seem forever barred from detecting anything of
the sort on Mars.

Nevertheless, the closest scrutiny of the ruddy planet by observers of
great enthusiasm and intelligence, coupled with imagination and
persistence, have built up a system of canals on Mars, covering the
surface of the planet like spider webs over a printed page, crossing
each other at intersecting spots known as "lakes," and embodying a
wealth of detail which challenges criticism and explanation.

To see the canals at all requires a favorable presentation of Mars, a
steady atmosphere and a perfect telescope, with a trained eye behind it.
Not even then are they sure to be visible. The training of the eye has
no doubt much to do with it. So photography has been called in, and very
excellent pictures of Mars have already been taken, some nearly half as
large as a dime, showing plainly the lights and shades of the grander
divisions of the Martian surface, but only in a few instances revealing
the actual canals more unmistakably than they are seen at the eyepiece.

The appearance and degree of visibility of the canals are variable:
possibly clouds temporarily obscure them. But there is a certain
capriciousness about their visibility that is little understood. In
consequence of the changing physical aspects, as to season, on Mars and
his orbital position with reference to the earth, some of the canals
remain for a long time invisible, adding to the intricacy of the
puzzle.

For the most part the canals are straight in their course and do not
swerve much from a great circle on the planet. But their lengths are
very different, some as short as 250 miles, some as long as 4,000 miles;
and they often join one another like spokes in the hub of a wheel,
though at various angles. As depicted by Lowell and his corps of
observers at Flagstaff, Arizona, the canal system is a truly marvelous
network of fine darkish stripes. Their color is represented as a bluish
green.

Each marking maintains its own breadth throughout its entire length, but
the breadth of all the canals is by no means the same: the narrowest are
perhaps fifteen to twenty miles wide, and the broadest probably ten
times that. At least that must be the breadth of the Nilosyrtis, which
is generally regarded as the most conspicuous of all the canals. The
Lowell Observatory has outstripped all others in the number of canals
seen and charted, now about 500.

What may be the true significance of this remarkable system of markings
it is impossible to conclude at present. Schiaparelli from his long and
critical study of them, their changes of width and color, was led to
think that they may be a veritable hydrographic system for distributing
the liquid from the melting polar snows. In this case it would be
difficult to escape the conviction that the canals have, at least in
part, been designed and executed with a definite end in view.

Lowell went even farther and built upon their behavior an elaborate
theory of life on the planet, with intelligent beings constructing and
opening new canals on Mars at the present epoch. Pickering propounded
the theory that the canals are not water-bearing channels at all, but
that they are due to vegetation, starting in the spring when first seen
and vitalized by the progress of the season poleward, the intensity of
color of the vegetation coinciding with the progress of the season as we
observe it.

Extensive irrigation schemes for conducting agricultural operations on a
large scale seem a very plausible explanation of the canals, especially
if we regard Mars as a world farther advanced in its life history than
our own. Erosion may have worn the continents down to their minimum
elevation, rendering artificial waterways not difficult to build; while
with the vanishing Martian atmosphere and absence of rains, the
necessity of water for the support of animal and vegetal life could only
be met by conducting it in artificial channels from one region of the
planet to another.

Interesting as this speculative interpretation is, however, we cannot
pass by the fact that many competent astronomers with excellent
instruments finely located have been unable to see the canals, and
therefore think the astronomers who do see them are deceived in some
way. Also many other astronomers, perhaps on insufficient grounds, deny
their existence _in toto_.

Many patient years of labor would be required to consult all the
literature of investigation of the planet Mars, but much of the detail
has been critically embodied in maps at different epochs, by Kayser,
Proctor, Green, and Dreyer. And Flammarion in two classic volumes on
Mars has presented all the observations from the earliest time, together
with his own interpretation of them. Areography is a term sometimes
applied to a description of the surface of Mars, and it is scarcely an
exaggeration to say that areography is now better known than the
geography of immense tracts of the earth.

For some reason well recognized, though not at all well understood, Mars
although the nearest of all the planets, Venus alone excepted, is an
object by no means easy to observe with the telescope. Possibly its
unusual tint has something to do with this. With an ordinary opera glass
examine the moon very closely, and try to settle precise markings,
colors, and the nature of objects on her surface; Mars under the best
conditions, scrutinized with our largest and best telescope, presents a
problem of about the same order of difficulty. There are delicate and
changing local colors that add much uncertainty. Nevertheless, the
planet's leading features are well made out, and their stability since
the time of the earliest observers leaves no room to doubt their reality
as parts of a permanent planetary crust.

The border of the Martian disk is brighter than the interior, but this
brightness is far from uniform. Variations in the color of the markings
often depend on the planet's turning round on its axis, and the relation
of the surface to our angle of vision. If we keep in mind these
obstacles to perfect vision in our own day, it is easy to see why the
early users of very imperfect telescopes failed to see very much, and
were misled by much that they thought they saw. Then, too, they had to
contend, as we do, with unsteadiness of atmosphere, which is least
troublesome near the zenith.

As their telescopes were all located in the northern hemisphere, the
northern hemisphere of Mars is the one best circumstanced for their
investigation; because at the remote oppositions of Mars, which always
happen in our northern winter with the planet in high north declination,
it is always the north pole of Mars which is presented to our view.
Whereas the close oppositions of the planet always come in our northern
midsummer, with Mars in south declination and therefore passing through
the zenith of places in corresponding south latitude.

With Mars near opposition, high up from the horizon, a fairly steady
atmosphere, and a magnifying power of at least 200 diameters, even the
most casual observer could not fail to notice the striking difference in
brightness of the two hemispheres: the northern chiefly bright and the
southern markedly dark. Formerly this was thought to indicate that the
southern hemisphere of Mars was chiefly water and the northern land,
much as is the case on the earth: with this difference, however, that
water and land on the earth are proportioned about as eleven to four.

But Mars in its general topography presents no analogy with the present
relation of land and water on the earth. There seems no reason to doubt
that the northern regions with their prevailing orange tint, in some
places a dark red and in others fading to yellow and white, are really
continental in character. Other vast regions of the Martian surface are
possibly marshy, the varying depth of water causing the diversity of
color. If we could ever catch a reflection of sunlight from any part of
the surface of Mars, we might conclude that deep water exists on the
planet; but the farther research progresses, the more complete becomes
the evidence that permanent water areas on Mars, if they exist at all,
are extremely limited.

Since 1877 Mars has been known to possess two satellites, which were
discovered in August of that year by Hall at Washington. Moons of this
planet had long been suspected to exist and on one or two previous
occasions critically looked for, though without success. In the writings
of Dean Swift there is a fanciful allusion to the two moons of Mars; and
if astronomers had chanced to give serious attention to this, Phobos and
Deimos, as Hall named them, might have been discovered long before.

They are very small bodies, not only faint in the telescope, but
actually of only ten or twenty miles diameter; and from the strange
relation that Phobos, the inner moon, moves round Mars three times while
the planet itself is turning round only once on its axis, some
astronomers incline to the hypothesis that this moon at least was never
part of Mars itself, but that it was originally an inner or very
eccentric member of the asteroid group, which ventured within the sphere
of gravitation of Mars, was captured by that planet, and has ever since
been tributary to it as a secondary body or satellite.




CHAPTER XXXIV

LIFE IN OTHER WORLDS


Popular interest in astronomy is exceedingly wide, but it is very
largely confined to the idea of resemblances and differences between our
earth and the bodies of the sky. The question most frequently asked the
astronomer is, "Have any of the stars got people on them?" Or more
specifically, "Is Mars inhabited?" The average questioner will not
readily be turned off with yes or no for an answer. He may or may not
know that it is quite impossible for astronomers to ascertain anything
definite in this matter, most interesting as it is. What he wants to
find out is the view of the individual astronomer on this absorbing and
ever recurring inquiry.

We ought first to understand what is meant by the manifestation here on
the earth called life, and agree concerning the conditions that render
it possible. Apparently they are very simple. We may or may not agree
that a counterpart of life, or life of a wholly different type from
ours, may exist on other planets under conditions wholly diverse from
those recognized as essential to its existence here. The problem of the
origin of life is, in the present state of knowledge, highly speculative
and hardly within the domain of science. Here on earth, life is
intimately associated with certain chemical compounds, in which carbon
is the common element without which life would not exist. Also
hydrogen, oxygen, and nitrogen are present, with iron, sulphur,
phosphorus, magnesium and a few less important elements besides. But
carbon is the only substance absolutely essential. Protoplasm cannot be
built without it, and protoplasm makes up the most of the living cell.
Closely related to carbon is silica also, as a substitution in certain
organic compounds. Protoplasm is able to stand very low temperatures,
but its properties as a living cell cease when the temperature reaches
150 Fahrenheit.

Animal life as it exists on the earth to-day appears to have been here
many million years. The palæontologists agree that all life originated
in the waters of the earth. It has passed through evolutionary stages
from the lowest to the highest. Throughout this vast period the
astronomer is able to say that the conditions of the earth which appear
to be essential to the maintenance of life have been pretty constantly
what they are to-day. The higher the type of life, the narrower the
range of conditions under which it thrives. Man can exist at the frigid
poles even if the temperature is 75 degrees below Fahrenheit zero; and
in the deserts and the tropics, he swelters under temperatures of 115
degrees, but he still lives. At these extremes, however, he can scarcely
be said to thrive.

We have, then, a relatively narrow range of temperatures which seems to
be essential to his comfortable existence and development: we may call
it 150 degrees in extent. Had not the surface temperature of the earth
been maintained within this range for indefinite ages, in the regions
where the human race has developed, quite certainly man would not be
here. How this equability of temperature has been maintained does not
now matter. Clearly the earth must have existed through indefinite ages
in the process of cooling down from temperatures of at least 6,000
degrees.

During this stage the temperature of the surface was earth-controlled.
Then this period merged very gradually into the stage where life became
possible, and the temperature of the surface became, as it now is,
sun-controlled. How many years are embraced in this span of periods, or
ages, we have no means of knowing. But of the sequence of periods and
the secular diminution of temperature, we may be certain.

Then there is the equally important consideration of water necessary for
the origination, support, and development of life. We cannot conceive of
life existing without it. On the earth water is superabundant, and has
been for indefinite ages in the past. There is little evidence that the
oceans are drying up; although the commonly accepted view is that the
waters of the earth will very gradually disappear. Water can exist in
the fluid state, which is essential to life, at all temperatures between
32 degrees and 680 degrees F.

Air to breathe is essential to life also. The atmosphere which envelops
the earth is at least 100 miles in depth, and its own weight compresses
it to a tension of nearly 15 pounds to the square inch at sea level.
This atmosphere and its physical properties have had everything to do
with the development of animal life on the planet. Without it and its
remarkable property of selective absorption, which imprisons and
diffuses the solar heat, it is inconceivable that the necessary
equability of surface temperature could be maintained. This appears to
be quite independent of the chemical constituents of the atmosphere, and
is perhaps the most important single consideration affecting the
existence of life on a planet. If the surface of a planet is partly
covered with water, it will possess also an atmosphere containing
aqueous vapor.

Heat, water, and air: these three essentials determine whether there is
life on a planet or not. Of course there must be nutrition suitable to
the organism; mineral for the vegetal, and vegetal for the animal. But
the narrow range of variation appears to be the striking thing:
relatively but a few degrees of temperature, and a narrow margin of
atmospheric pressure. If this pressure is doubled or trebled, as in
submarine caissons, life becomes insupportable. If, on the other hand,
it is reduced even one-third, as on mountains even 13,000 feet high, the
human mechanism fails to function, partly from lack of oxygen necessary
in vitalizing the blood, but mainly because of simple reduction of
mechanical pressure.

If, then, we conceive of life in other worlds and it is agreed that life
there must manifest itself much as it does here, our answer to the
question of habitability of the planets must follow upon an
investigation of what we know, or can reasonably surmise, about the
surface temperatures of these bodies, whether they have water, and what
are the probable physical characteristics of their atmospheres.

We may inquire about each planet, then, concerning each of these
details.

The case of Mercury is not difficult. At an average distance of only 36
million miles from the sun, and with a large eccentricity of orbit which
brings it a fifth part nearer, conditions of temperature alone must be
such as to forbid the existence of life. The solar heat received is
seven times greater than at the earth, and this is perhaps sufficient
reason for a minimum of atmosphere, as indicated by observation. If no
air, then quite certainly no water, as evaporation would supply a slight
atmosphere. But according to the kinetic theory of gases, the mass of
Mercury, only a very small fraction of that of the sun, is inadequate to
retain an atmospheric envelope. If, however, the planet's day and year
are equal, so that it turns a constant face to the sun, surface
conditions would be greatly complicated, so that we cannot regard the
planet as absolutely uninhabitable on the hemisphere that is always
turned away from the sun.

Venus at 67 millions of miles from the sun presents conditions that are
quite different. She receives double the solar heat that we do, but
possessing an atmosphere perhaps threefold denser than ours, as reliably
indicated by observations of transits of Venus, the intensity of the
heat and its diffusion may be greatly modified. What the selective
absorption of the atmosphere of Venus may be, we do not know. Nor is the
rotation time of the planet definitely ascertained: if equal to her
year, as many observations show and as indicated by the theory of tidal
evolution, there may well be certain regions on the hemisphere
perpetually turned away from the sun where temperature conditions are
identical with those on the tropical earth, and where every condition
for the origin and development of life is more fully met than anywhere
else in the solar system. Whether Venus has water distributed as on the
earth we do not know, as her surface is never seen, owing to dense
clouds under which she is always enshrouded. Her cloudy condition
possibly indicates an overplus of water.

Is the moon inhabited? Quite certainly not: no appreciable air, no
water, and a surface temperature unmodified by atmosphere--rising
perhaps to 100 degrees F. during the day, which is a fortnight in
length, and falling at night to 300 degrees below zero, if not lower.

Is Mars inhabited? The probable surface temperature is much lower than
the earth's, because Mars receives only half as much solar heat as we
do; and more important still, the atmosphere of Mars is neither so dense
nor so extensive as our own. Seasons on Mars are established, much the
same as here, except that they are nearly twice as long as ours; and
alternate shrinking and enlarging of the polar caps keeps even pace with
the seasons, thereby indicating a certainty of atmosphere whose
equatorial and polar circulation transports the moisture poleward to
form the snow and ice of which the polar caps no doubt consist.

There is a variety of evidence pointing to an atmosphere on Mars of
one-third to one-half the density of our own: an atmosphere in which
free hydrogen could not exist, although other gases might. The
spectroscopic evidence of water vapor in the Martian atmosphere is not
very strong. It is very doubtful whether water exists on Mars in large
bodies: quite certainly not as oceans, though the evidence of many small
"lakes" is pretty well made out. With very little water, a thin
atmosphere and a zero temperature, is Mars likely to be inhabited at the
present time? The chances are rather against it. If, however, the past
development of the planet has progressed in the way usually considered
as probable, we may be practically certain that Mars has been inhabited
in the past, when water was more abundant, and the atmosphere more dense
so as to retain and diffuse the solar heat.

Biologists tell me that they hardly know enough regarding the extreme
adaptability of organisms to environment to enable them to say whether
life on such a planet as Mars would or would not keep on functioning
with secular changes of moisture and temperature. The survival of a race
might be insured against extremely low temperatures by dwelling in
sub-Martian caves, and sufficient water might be preserved by
conceivable engineering and mechanical schemes; but the secular
reduction of the quantity and pressure of atmosphere--it is not easy to
see how a race even more advanced than ourselves could maintain itself
alive under serious lack of an element so vital to existence. Both
Wallace, the great biologist, and Arrhenius, the eminent chemist (but
biologist, astronomer, and physicist as well), both reject the
habitation theory of Mars, regarding the so-called canals as quite like
the luminous streaks on the moon; that is, cracks in the volcanic crust
caused by internal strains due to the heated interior. Wallace, indeed,
argues that the planet is absolutely uninhabitable.

The asteroids, or minor planets? We may dismiss them with the simple
consideration that their individual masses are so insignificant and
their gravity so slight that no atmosphere can possibly surround them.
Their temperatures must be exceedingly low, and water, if present at
all, can only exist in the form of ice.

Jupiter, the giant planet, presents the opposite extreme. His mass is
nearly a thousandth part of the sun's, and is sufficient to retain a
very high temperature, probably approximating to the condition we call
red-hot. This precludes the possibility of life at the outset, although
the indications of a very dense atmosphere many thousand miles in depth
are unmistakable.

Of Saturn, one thirty-five hundredth the mass of the sun, practically
the same may be said. Proctor thought it quite likely that Saturn might
be habitable for living creatures of some sort, but he regarded the
planet as on many accounts unsuitable as a habitation for beings
constituted like ourselves. Mere consideration of surface temperature
precludes the possibility of life in the present stage of Saturn's
development; but the consensus of opinion is to the effect that life may
make its appearance on these great planets at some inconceivably remote
epoch in the future when the surface temperature is sufficiently reduced
for life processes to begin. Discoveries of algæ flourishing in hot
springs approaching 200 degrees Fahrenheit make it possible that these
beginnings may take place earlier and at much higher temperatures than
have hitherto been thought possible.

A century ago, when the ring of Saturn was believed to be a continuous
plane, this was a favorite corner of the solar system for speculation as
to habitability; but now that we know the true constitution of the
rings, no one would for a moment consider any such possibility.
Conditions may, however, be quite different with Saturn's huge satellite
Titan, the giant moon of the solar system. Its diameter makes it
approximately the size of the planet Mars; and although it is much
farther removed from the sun, its relative nearness to the highly
heated globe of Saturn may provide that equability of temperature which
is essential to life processes.

Also the three inner Galilean moons of Jupiter, especially III which is
about the size of Titan, are excellently placed for life possibilities,
as far as probable temperature is concerned, but we have of course no
basis for surmising what their conditions may be as to air and water,
except that their small mass would indicate a probable deficiency of
those elements.

Uranus and Neptune are planets so remote, and their apparent disks are
so small, that very little is known about their physical condition. They
are each about one-third the diameter of Jupiter, and the spectrum of
Uranus shows broad diffused bands, indicating strong absorption by a
dense atmosphere very different from that of the earth. Indications are
that Neptune has a similar atmosphere.

It is possible that the denser atmospheres of these remote planets may
be so conditioned as to selective absorption that the relatively slender
supply of solar heat may be conserved, and thus insure a relatively high
surface temperature when the sun comes into control. If our theories of
origin of the planets are to be trusted, we may rather suppose that
Uranus and Neptune are still in a highly heated condition; that life has
not yet made its appearance on them, but that it will begin its
development ages before Saturn and Jupiter have cooled to the requisite
temperature.

Comets? In his _Lettres Cosmologiques_ (1765) Lambert considers the
question of habitability of the comets, naturally enough in his day,
because he thought them solid bodies surrounded by atmosphere, and
related to the planets. The extremes of temperature at perihelia and
aphelia to which comets are subjected did not bother him particularly.

After calculating that the comet of 1680, "being 160 times nearer to the
sun than we are ourselves, must have been subjected to a degree of heat
25,600 times as great as we are," Lambert goes on to say: "Whether this
comet was of a more compact substance than our globe, or was protected
in some other way, it made its perihelion passage in safety, and we may
suppose all its inhabitants also passed safely. No doubt they would have
to be of a more vigorous temperament and of a constitution very
different from our own. But why should all living beings necessarily be
constituted like ourselves? Is it not infinitely more probable that
amongst the different globes of the universe a variety of organizations
exist, adapted to the wants of the people who inhabit them, and fitting
them for the places in which they dwell, and the temperatures to which
they will be subjected? Is man the only inhabitant of the earth itself?
And if we had never seen either bird or fish, should we not believe that
the air and water were uninhabitable? Are we sure that fire has not its
invisible inhabitants, whose bodies, made of asbestos, are impenetrable
to flame? Let us admit that the nature of the beings who inhabit comets
is unknown to us; but let us not deny their existence, and still less
the possibility of it."

Little enough is really known about the physical nature of comets even
now, but what we do know indicates incessant transformation and
instability of conditions that would render life of any type exceedingly
difficult of maintenance.

A word about Sir William Herschel's theory of the sun and its
habitability. He thought the core of the sun a dark, solid body, quite
cold, and surrounded by a double layer, the inner one of which he
conceived to act as a sort of fire screen to shield the sun proper
against the intense heat of the outer layer, or photosphere by which we
see it. Viewed in this light, the sun, he says, "appears to be nothing
else than a very eminent, large and lucid planet, evidently the first,
or, in strictness of speaking, the only primary one of our system.... It
is most probably also inhabited, like the rest of the planets, by beings
whose organs are adapted to the peculiar circumstances of that vast
globe." But physics and biology were undeveloped sciences in Herschel's
days.

Herschel knew, however, that the stars are all suns, so that he must
have conceived that they are inhabited also, quite independently of the
question whether they possess retinues of planets, after the manner of
our solar system.

This again is a question to which the astronomer of the present day can
give no certain answer. So immensely distant are even the nearest of
these multitudinous bodies that no telescope can ever be built large
enough or powerful enough to reveal a dark planet as large as Jupiter,
alongside even the nearest fixed star. Whatever may be the process of
stellar evolution, there doubtless is an era of many hundreds of
millions of years in the life of a star when it is passing through a
planet-maintaining stage. This would likely depend upon spectral type,
or to be indicated by it; and as about half of the stars are of the
solar type, it would be a reasonable inference that at least half of the
stars may have planets tributary to them.

In such a case, the chances must be overwhelmingly in favor of vast
numbers of the planets of other stellar systems being favorably
circumstanced as to heat and moisture for the maintenance of life at the
present time. That is, they are habitable, and if habitable, then
thousands of them are no doubt inhabited now. But astronomers know
absolutely nothing about this question, nor are they able to conceive at
present any way that may lead them to any definite knowledge of it.
There is, indeed, one piece of quasi-evidence which might reasonably be
interpreted as implying that it is more likely that the stars are not
attended by families of planets than that they are.




CHAPTER XXXV

THE LITTLE PLANETS


Along toward the end of the eighteenth century and the beginning of the
nineteenth, astronomers were leading a quiet unexcited life. Sir William
Herschel had been knighted by King George for his discovery of the outer
planet Uranus, and practically everything seemed to be known and
discovered in the solar system with a single exception. Between Mars and
Jupiter there existed an obvious gap in the planetary brotherhood.

Could it be possible that some time in the remote cosmic past a planet
had actually existed there, and that some celestial cataclysm had blown
it to fragments? If so, would they still be traveling round the sun as
individual small planets? And might it not be possible to discover some
of them among the faint stars that make up the belt of the zodiac in
which all the other planets travel?

So interesting was this question that the first international
association of astronomers banded themselves together to carry on a
systematic search round the entire zodiacal heavens in the faint hope of
detecting possible fragments of the original planet of mere hypothesis.

The astronomers of that day placed much reliance on what is known as
Bode's law--not a law at all, but a mere arithmetical succession of
numbers which represented very well the relative distances of all the
planets from the sun. And the distance of the newly found Uranus fitted
in so well with this law that the utter absence of a planet in the gap
between Mars and Jupiter became very strongly marked.

Quite by accident a discovery of one of the guessed-at small planetary
bodies was made, on January 1, 1801, in Palermo, Sicily, by Piazzi, who
was regularly occupied in making an extensive catalogue of the stars.
His observations soon showed that the new object he had seen could not
be a fixed star, because it moved from night to night among the stars.
He concluded that it was a planet, and named it Ceres (1), for the
tutelary goddess of Sicily.

Other astronomers kept up the search, and another companion planet,
Pallas (2) was found in the following year. Juno (3) was found in 1804,
and Vesta (4), the largest and brightest of all the minor planets, in
1807. Vesta is sometimes bright enough when nearest the earth to be seen
with the naked eye; but it was the last of the brighter ones, and no
more discoveries of the kind were made till the fifth was found in 1845.
Since then discoveries have been made in great abundance, more and more
with every year till the number of little planets at present known is
very near 1,000.

The early asteroid hunters found the search rather tedious, and the
labor increased as it became necessary to examine the increasing
thousands of fainter and fainter stars that must be observed in order to
detect the undiscovered planets, which naturally grow fainter and
fainter as the chase is prolonged. First a chart of the ecliptic sky had
to be prepared containing all the stars that the telescope employed in
the search would show. Some of the most detailed charts of the sky in
existence were prepared in connection with this work, particularly by
the late Dr. Peters of Hamilton College. Once such charts are complete,
they are compared with the sky, night after night when the moon is
absent. Thousands upon thousands of tedious hours are spent in this
comparison, with no result whatever except that chart and sky are found
to correspond exactly.

But now and then the planet hunter is rewarded by finding a new object
in the sky that does not appear on his chart. Almost certainly this is a
small planet, and only a few night's observation will be necessary to
enable the discoverer to find out approximately the orbit it is
traveling in, and whether it is out-and-out a new planet or only one
that had been previously recognized, and then lost track of.

Nearly all the minor planets so far found have had names assigned to
them principally legendary and mythological, and a nearly complete
catalogue of them, containing the elements of their orbits (that is, all
the mathematical data that tell us about their distance from the sun and
the circumstances of their motion around him) is published each year in
the "Annuaire du Bureau des Longitudes" at Paris. But these little
planets require a great deal of care and attention, for some astronomers
must accurately observe them every few years, and other astronomers must
conduct intricate mathematical computations based on these observations;
otherwise they get lost and have to be discovered all over again.
Professor Watson, of the University of Michigan and later of the
University of Wisconsin, endowed the 22 asteroids of his own discovery,
leaving to the National Academy of Sciences a fund for prosecuting this
work perpetually, and Leuschner is now ably conducting it.

    [Illustration: JUPITER, LARGEST OF THE PLANETS. The irregular belts
       change their mutual relation and shapes because they do not
       represent land, but are part of the atmosphere. (_Photo, Yerkes
       Observatory._)]

    [Illustration: THE PLANET NEPTUNE AND ITS SATELLITE. The photograph
       required an exposure of the plate for one hour. (_Photo, Yerkes
       Observatory._)]

    [Illustration: SATURN, AS SEEN THROUGH THE 40-INCH REFRACTOR, at
       the time when only the edge of the rings is visible, showing
       condensations. (_Photo, Yerkes Observatory._)]

    [Illustration: SATURN, PHOTOGRAPHED THROUGH THE 40-INCH REFRACTOR.
       The rings appear opened to the fullest extent they can be seen
       from the earth. The picture was made July 7, 1898. (_Photo,
       Yerkes Observatory._)]

While the number of the asteroids is gratifyingly large, their
individual size is so small and their total mass so slight that, even if
there are a hundred thousand of them (as is wholly possible), they would
not be comparable in magnitude with any one of the great planets. Vesta,
the largest, is perhaps 400 miles in diameter, and if composed of
substances similar to those which make up the earth, its mass may be
perhaps one twenty-thousandth of the earth's mass. If we calculate the
surface gravity on such a body, we find it about one-thirtieth of what
it is here; so that a rifle ball, if fired on Vesta with a muzzle
velocity of only 2,000 feet a second, might overmaster the gravity of
the little planet entirely and be projected in space never to return.

If, as is likely, some of the smallest asteroids are not more than ten
miles in diameter, their gravity must be so feeble a force that it might
be overcome by a stone thrown from the hand. There is no reliable
evidence that any of the asteroids are surrounded by atmospheric gases
of any sort. Probably they are for the most part spherical in form,
although there is very reliable evidence that a few of the asteroids,
being variable in the amount of sunlight that they reflect, are
irregular in form, mere angular masses perhaps.

The network of orbits of the asteroids is inconceivable complicated.
Nevertheless, there is a wide variation in their average distance from
the sun, and their periods of traveling round him vary in a similar
manner, the shortest being only about three years. While the longest is
nearly nine years in duration, the average of all their periods is a
little over four years. The gap in the zone of asteroids, at a distance
from the sun equal to about five-eighths that of Jupiter, is due to the
excessive disturbing action of Jupiter, whose periodic time is just
twice as long as that of a theoretical planet at this distance.

The average inclination of their orbits to the plane of the ecliptic is
not far from 8 degrees. But the orbit of Pallas, for example, is
inclined 35 degrees, and the eccentricities of the asteroid orbits are
equally erratic and excessive. Both eccentricity and inclination of
orbit at times suggest a possible relation to cometary orbits, but
nothing has ever been definitely made out connecting asteroids and
comets in a related origin.

No comprehensive theory of the origin of the asteroid group has yet been
propounded that has met with universal acceptance. According to the
nebular hypothesis the original gaseous material, which should have been
so concentrated as to form a planet of ordinary type, has in the case of
the asteroids collected into a multitude of small masses instead of
simply one. That there is a sound physical reason for this can hardly be
denied. According to the Laplacian hypothesis, the nearness of the huge
planetary mass of Jupiter just beyond their orbits produced violent
perturbations which caused the original ring of gaseous material to
collect into fragmentary masses instead of one considerable planet. The
theory of a century ago that an original great planet was shattered by
internal explosive forces is no longer regarded as tenable.

To astronomers engaged upon investigation of distances in the solar
system, the asteroid group has proved very useful. The late Sir David
Gill employed a number of them in a geometrical research for finding the
sun's distance, and more recently the discovery of Eros (433) has made
it possible to apply a similar method for a like purpose when it
approaches nearest to the earth in 1924 and 1931. Then the distance of
Eros will be less than half that of Mars or even Venus at their nearest.

When the total number of asteroids discovered has reached 1,000, with
accurate determination of all their orbits, we shall have sufficient
material for a statistical investigation of the group which ought to
elucidate the question of its origin, and bear on other problems of the
cosmogony yet unsolved. Present methods of discovery of the asteroids by
photography replace entirely the old method by visual observation alone,
with the result that discoveries are made with relatively great ease and
rapidity.




CHAPTER XXXVI

THE GIANT PLANET


I can never forget as a young boy my first glimpse of the planet Jupiter
and his moons; it was through a bit of a telescope that I had put
together with my own hands; a tube of pasteboard, and a pair of old
spectacle lenses that chanced to be lying about the house.

In the field of view I saw five objects; four of them looking quite
alike, and as if they were stars merely (they were Jupiter's moons),
while the fifth was vastly larger and brighter. It was circular in
shape, and I thought I could see a faint darkish line across the middle
of it.

This experience encouraged me immensely, and I availed myself eagerly of
the first chance to see Jupiter through a bigger and better glass. Then
I saw at once that I had observed nothing wrongly, but that I had seen
only the merest fraction of what there was to see.

In the first place, the planet's disk was not perfectly circular, but
slightly oval. Inquiring into the cause of this, we must remember that
Jupiter is actually not a flat disk but a huge ball or globe, more than
ten times the diameter of the earth, which turns swiftly round on its
axis once every ten hours as against the earth's turning round in
twenty-four hours. Then it is easy to see how the centrifugal force
bulges outward the equatorial regions of Jupiter, so that the polar
regions are correspondingly drawn inward, thereby making the polar
diameter shorter than the equatorial one, which is in line with the
moons or satellites. The difference between the two diameters is very
marked, as much as one part in fifteen. All the planets are slightly
flattened in this way, but Jupiter is the most so of all except Saturn.

The little darkish line across the planet's middle region or equator was
found to be replaced by several such lines or irregular belts and spots,
often seen highly colored, especially with reflecting telescopes; and
they are perpetually changing their mutual relation and shapes, because
they are not solid territory or land on Jupiter, but merely the outer
shapes of atmospheric strata, blown and torn and twisted by atmospheric
circulation on this planet, quite the same as clouds in the atmosphere
on the earth are.

Besides this the axial turning of Jupiter brings an entirely different
part of the planet into view every two or three hours; so that in making
a map or chart of the planet, an arbitrary meridian must be selected.
Even then the process is not an easy one, and it is found that spots on
Jupiter's equator turn round in 9 hours 50 minutes, while other regions
take a few minutes longer, the nearer the poles are approached. The
Great Red Spot, about 30,000 miles long and a quarter as much in breadth
has been visible for about half a century. Bolton, an English observer,
has made interesting studies of it very recently.

The four moons, or satellites, which a small telescope reveals, are
exceedingly interesting on many accounts. They were the first heavenly
bodies seen by the aid of the telescope, Galileo having discovered them
in 1610. They travel round Jupiter much the same as the moon does round
the earth, but faster, the innermost moon about four times per week, the
second moon about twice a week, the third or largest moon (larger than
the planet Mercury) once a week, and the outermost in about sixteen
days. The innermost is about 260,000 miles from Jupiter, and the
outermost more than a million miles. From their nearness to the huge and
excessively hot globe of Jupiter, some astronomers, Proctor especially,
have inclined to the view that these little bodies may be inhabited.

Jupiter has other moons; a very small one, close to the planet, which
goes round in less than twelve hours, discovered by Barnard in 1892.
Four others are known, very small and faint and remote from the planet,
which travel slowly round it in orbits of great magnitude. The ninth, or
outermost, is at a distance of fifteen and one-half million miles from
Jupiter, and requires nearly three years in going round the planet. It
was discovered by Nicholson at the Lick Observatory in 1914. The eighth
was discovered by Melotte at Greenwich in 1908, and is peculiar in the
great angle of 28 degrees, at which its orbit is inclined to the equator
of Jupiter. The sixth and seventh satellites revolve round Jupiter
inside the eighth satellite, but outside the orbit of IV; and they were
discovered by photography at the Lick Observatory in 1905 by Perrine,
now director of the Argentine National Observatory at Cordoba.

The ever-changing positions of the Medicean moons, as Galileo called the
four satellites that he discovered--their passing into the shadow in
eclipse, their transit in front of the disk, and their occultation
behind it--form a succession of phenomena which the telescopist always
views with delight. The times when all these events take place are
predicted in the "Nautical Almanac," many thousand of them each year,
and the predictions cover two or three years in advance.

Jupiter, as the naked eye sees him high up in the midnight sky, is the
brightest of all the planets except Venus; indeed, he is five times
brighter than Sirius, the brightest of all the fixed stars. His stately
motion among the stars will usually be visible by close observation from
day to day, and his distance from the earth, at times when he is best
seen, is usually about 400 million miles. Jupiter travels all the way
round the sun in twelve years; his motion in orbit is about eight miles
a second.

The eclipses of Jupiter's moons, caused by passing into the shadow of
the planet, would take place at almost perfectly regular intervals, if
our distance from Jupiter were invariable. But it was early found out
that while the earth is approaching Jupiter the eclipses take place
earlier and earlier, but later and later when the earth is moving away.
The acceleration of the earliest eclipse added to the retardation of the
latest makes 1,000 seconds, which is the time that light takes in
crossing a diameter of the earth's orbit round the sun. Now the velocity
of light is well known to be 186,300 miles per second, so we calculate
at once and very simply that the sun's distance from the earth, which is
half the diameter of the orbit, equals 500 times 186,300, or 93,000,000
miles.




CHAPTER XXXVII

THE RINGED PLANET


Saturn is the most remote of all the planets that the ancient peoples
knew anything about. These anciently known planets are sometimes called
the lucid or naked-eye planets--five in number: Mercury, Venus, Mars,
Jupiter, and Saturn. Saturn shines as a first-magnitude star, with a
steady straw-colored light, and is at a distance of about 800 million
miles from the earth when best seen. Saturn travels completely round the
sun in a little short of thirty years, and the telescope, when turned to
Saturn, reveals a unique and astonishing object; a vast globe somewhat
similar to Jupiter, but surrounded by a system of rings wholly unlike
anything else in the universe, as far as at present known; the whole
encircled by a family of ten moons or satellites. The Saturnian system,
therefore, is regarded by many as the most wonderful and most
interesting of all the objects that the telescope reveals.

At first the flattening of the disk of Saturn is not easily made out,
but every fifteen years (as 1921 and 1936) the earth comes into a
position where we look directly at the thin edge of the rings, causing
them to completely disappear. Then the remarkable flattening of the
poles of Saturn is strikingly visible, amounting to as much as one-tenth
of the entire diameter. The atmospheric belt system is also best seen at
these times.

But the rings of Saturn are easily the most fascinating features of the
system. They can never be seen as if we were directly above or beneath
the planet so they never appear circular, as they really are in space,
but always oval or elliptical in shape. The minor axis or greatest
breadth is about one-half the major axis or length. The latter is the
outer ring's actual diameter, and it amounts to 170,000 miles, or two
and one-half times the diameter of Saturn's globe.

There are in fact no less than four rings; an outer ring, sometimes seen
to be divided near its middle; an inner, broader and brighter ring; and
an innermost dusky, or crape ring, as it is often called. This comes
within about 10,000 miles of the planet itself. After the form and size
of the rings were well made out, their thickness, or rather lack of
thickness, was a great puzzle.

If a model about a foot in diameter were cut out of tissue paper, the
relative proportion of size and thickness would be about right. In space
the thickness is very nearly 100 miles, so that, when we look at the
ring system edge-on, it becomes all but invisible except in very large
telescopes. Clearly a ring so thin cannot be a continuous solid object
and recent observations have proved beyond a doubt that Saturn's rings
are made up of millions of separate particles moving round the planet,
each as if it were an individual satellite.

Ever since 1857 the true theory of the constitution of the Saturnian
ring has been recognized on theoretic grounds, because Clerke-Maxwell
founded the dynamical demonstration that the rings could be neither
fluid nor solid, so that they must be made up of a vast multitude of
particles traveling round the planet independently. But the physical
demonstration that absolutely verified this conclusion did not come
until 1895, when, as we have said in a preceding chapter, Keeler, by
radial velocity measures on different regions of the ring by means of
the spectroscope, proved that the inner parts of the ring travel more
swiftly round the planet than the outer regions do. And he further
showed that the rates of revolution in different parts of the ring
exactly correspond to the periods of revolution which satellites of
Saturn would have, if at the same distance from the center of the
planet. The innermost particles of the dusky ring, for example, travel
round Saturn in about five hours, while the outermost particles of the
outer bright ring take 137 hours to make their revolution. For many
years it was thought that the Saturnian ring system was a new satellite
in process of formation, but this view is no longer entertained; and the
system is regarded as a permanent feature of the planet, although
astronomers are not in entire agreement as to the evolutionary process
by which it came into existence--whether by some cosmic cataclysm, or by
gradual development throughout indefinite aeons, as the rest of the
solar system is thought to have come to its present state of existence.
Possibly the planetesimal hypothesis of Chamberlin and Moulton affords
the true explanation, as the result of a rupture due to excessive tidal
strain.




CHAPTER XXXVIII

THE FARTHEST PLANETS


On the 13th of March, 1781, between 10 and 11 P. M., as Sir William
Herschel was sweeping the constellation Gemini with one of his great
reflecting telescopes, one star among all that passed through the field
of view attracted his attention. Removing the eyepiece and applying
another with a higher magnifying power, he found that, unlike all the
other stars, this one had a small disk and was not a mere point of
light, as all the fixed stars seem to be.

A few nights' observation showed that the stranger was moving among the
stars, so he thought it must be a comet; but a week's observation
following showed that he had discovered a new member of the planetary
system, far out beyond Saturn, which from time immemorial had been
assumed to be the outermost planet of all. This, then, was the first
real discovery of a planet, as the finding of the satellites of Jupiter
had been the first of all astronomical discoveries. Herschel's discovery
occasioned great excitement, and he named the new planet Georgium Sidus
or the Georgian, after his King. The King created him a knight and gave
him a pension, besides providing the means for building a huge
telescope, 40 feet long, with which he subsequently made many other
astronomical discoveries. The planet that Herschel discovered is now
called Uranus.

Uranus is an object not wholly impossible to see with the naked eye, if
the sky background is clear and black, and one knows exactly where to
look for it. Its brightness is about that of a sixth magnitude star or a
little fainter. Its average distance from the sun is about 1,800 million
miles and it takes eighty-four years to complete its journey round the
sun, traveling only a little more than four miles a second. When we
examine Uranus closely with a large telescope, we find a small disk
slightly greenish in tint, very slightly flattened, and at times faint
bands or belts are apparently seen. Uranus is about 30,000 miles in
diameter, and is probably surrounded by a dense atmosphere. Its rotation
time is 10 h. 50 m.

Uranus is attended by four moons or satellites, named Ariel, Umbriel,
Titania, and Oberon, the last being the most remote from the planet.
This system of satellites has a remarkable peculiarity: the plane of the
orbits in which they travel round Uranus is inclined about 80 degrees to
the plane of the ecliptic, so that the satellites travel backward, or in
a retrograde direction; or we might regard their motion as forward, or
direct, if we considered the planes of their orbits inclined at 100
degrees.

For many years after the discovery of Uranus it was thought that all the
great bodies of the solar system had surely been found. Least of all was
any planet suspected beyond Uranus until the mathematical tables of the
motion of Uranus, although built up and revised with the greatest care
and thoroughness, began to show that some outside influence was
disturbing it in accordance with Newton's law of gravitation. The
attraction of a still more distant planet would account for the
disturbance, and since no such planet was visible anywhere a
mathematical search for it was begun.


NEPTUNE

Wholly independently of each other, two young astronomers, Adams of
England and Le Verrier of France, undertook to solve the unique problem
of finding out the position in the sky where a planet might be found
that would exactly account for the irregular motion of Uranus. Both
reached practically identical results. Adams was first in point of time,
and his announcement led to the earliest observation, without
recognition of the new planet (July 30, 1846), although it was Le
Verrier's work that led directly to the new planet's being first seen
and recognized as such (September 23, 1846). Figuring backward, it was
found that the planet had been accidentally observed in Paris in 1795,
but its planetary character had been overlooked.

Neptune is the name finally assigned to this historical planet. It is
thirty times farther from the sun than the earth, or 2,800 million
miles; its velocity in orbit is a little over three miles per second,
and it consumes 164 years in going once completely round the sun. So
faint is it that a telescope of large size is necessary to show it
plainly. The brightness equals that of a star of the eighth magnitude,
and with a telescope of sufficient magnifying power, the tiny disk can
be seen and measured. The planet is about 30,000 miles in diameter, and
is not known to possess more than one moon or satellite. If there are
others, they are probably too faint to be seen by any telescope at
present in existence.




CHAPTER XXXIX

THE TRANS-NEPTUNIAN PLANET


Investigation of the question of a possible trans-Neptunian planet was
undertaken by the writer in 1877. As Neptune requires 164 years to
travel completely round the sun, and the period during which it has been
carefully observed embraces only half that interval, clearly its orbit
cannot be regarded as very well known. Any possible deviations from the
mathematical orbit could not therefore be traced to the action of a
possible unknown planet outside. But the case was different with Uranus,
which showed very slight disturbances, and these were assumed to be due
to a possible planet exterior to both Uranus and Neptune. As a position
for this body in the heavens was indicated by the writer's
investigation, that region of the sky was searched by him with great
care in 1877-1878 with the twenty-six-inch telescope at Washington; and
photographs of the same region were afterward taken by others, though
only with negative results.

In 1880, Forbes of Edinburgh published his investigation of the problem
from an entirely independent angle. Families of comets have long been
recognized whose aphelion distances correspond so nearly with the
distances of the planets that these comet families are now recognized as
having been created by the several planets, which have reduced the high
original velocities possessed by the comets on first entering the solar
system.

Their orbits have ever since been ellipses with their aphelia in groups
corresponding to the distances of the planets concerned. Jupiter has a
large group of such comets, also Saturn. Uranus and Neptune likewise
have their families of comets, and Forbes found two groups with average
distances far outside of Neptune; from which he drew the inference that
there are two trans-Neptunian planets. The position he assigned to the
inner one agreed fairly well with the writer's planet as indicated by
unexplained deviations of Uranus.

The theoretical problem of a trans-Neptunian planet has since been taken
up by Gaillot and Lau of Paris, the late Percival Lowell, and W. H.
Pickering of Harvard. The photographic method of search will, it is
expected, ultimately lead to its discovery. On account of the probable
faintness of the planet, at least the twelfth or thirteenth magnitude,
Metcalf's method of search is well adapted to this practical problem.
When near its opposition the motion of Neptune retrograding among the
stars amounts to five seconds of arc in an hour; while the
trans-Neptunian planet would move but three seconds. By shifting the
plate this amount hourly during exposure, the suspected object would
readily be detected on the photographic plate as a minute and nearly
circular disk, all the adjacent stars being represented by short trails.

Interest in a possible planet or planets outside the orbit of Neptune is
likely to increase rather than diminish. To the ancients seven was the
perfect number, there were seven heavenly bodies already known, so there
could be no use whatever in looking for an eighth. The discovery of
Uranus in 1781 proved the futility of such logic, and Neptune followed
in 1846 with further demonstration, if need be. The cosmogony of the
present day sets no outer limit to the solar system, and some
astronomers advocate the existence of many trans-Neptunian planets.




CHAPTER XL

COMETS--THE HAIRY STARS


Comets--hairy stars, as the origin of the name would indicate--are the
freaks of the heavens. Of great variety in shape, some with heads and
some without, some with tails and some without, moving very slowly at
one time and with exceedingly high velocity at another, in orbits at all
possible angles of inclination to the general plane of the planetary
paths round the sun, their antics and irregularities were the wonder and
terror of the ancient world, and they are keenly dreaded by
superstitious people even to the present day.

Down through the Middle Ages the advent of a comet was regarded as:

    Threatening the world with famine, plague and war;
    To princes, death; to kingdoms, many curses;
    To all estates, inevitable losses;
    To herdsmen, rot; to plowmen, hapless seasons;
    To sailors, storms; to cities, civil treasons.

Comets appeared to be marvelous objects, as well as sinister, chiefly
because they bid apparent defiance to all law. Kepler had shown that the
moon and the planets travel in regular paths--slightly elliptical to be
sure, but nevertheless unvarying. None of the comets were known to
follow regular paths till the time of Halley late in the seventeenth
century, when, as we have before told, a fine comet made its
appearance, and Halley calculated its orbit with much precision.
Comparing this with the orbits of comets that had previously been seen,
he found its path about the sun practically identical with that of at
least two comets previously observed in 1531 and 1607.

So Halley ventured to think all these comets were one and the same body,
and that it traveled round the sun in a long ellipse in a period of
about seventy-five or seventy-six years. We have seen how his prediction
of its return in 1758 was verified in every particular. On the comet's
return in 1910, Crowell and Crommelin of Greenwich made a thorough
mathematical investigation of the orbit, indicating that the year 1986
will witness its next return to the sun.

There is a class of astronomers known as comet-hunters, and they pass
hours upon hours of clear, sparkling, moonless nights in search for
comets. They are equipped with a peculiar sort of telescope called a
comet-seeker, which has an object glass usually about four or five
inches in diameter, and a relatively short length of focus, so that a
larger field of view may be included. Regions near the poles of the
heavens are perhaps the most fruitful fields for search, and thence
toward the sun till its light renders the sky too bright for the finding
of such a faint object as a new comet usually is at the time of
discovery. Generally when first seen it resembles a small circular patch
of faint luminous cloud.

When a suspect is found, the first thing to do is to observe its
position accurately with relation to the surrounding stars. Then, if on
the next occasion when it is seen the object has moved, the chances are
that it is a comet; and a few days' observation will provide material
from which the path of the comet in space can be calculated. By
comparing this with the complete lists of comets, now about 700 in
number, it is possible to tell whether the comet is a new one, or an old
one returning. The total number of comets in the heavens must be very
great, and thousands are doubtless passing continually undetected,
because their light is wholly overpowered by that of the sun. Of those
that are known, perhaps one in twelve develops into a naked-eye comet,
and in some years six or seven will be discovered. With sufficiently
powerful telescopes, there are as a rule not many weeks in the year when
no comet is visible. Brilliant naked-eye comets are, however,
infrequent.

Comets, except Halley's, generally bear the name of their discoverer, as
Donati (1858), and Pons-Brooks (1893). Pons was a very active discoverer
of comets in France early in the nineteenth century: he was a doorkeeper
at the observatory of Marseilles, and his name is now more famous in
astronomy than that of Thulis, then the director of the Observatory, who
taught and encouraged him. Messier was another very successful
discoverer of comets in France, and in America we have had many: Swift,
Brooks, and Barnard the most successful.

How bright a comet will be and how long it will be visible depends upon
many conditions. So the comets vary much in these respects. The first
comet of 1811 was under observation for nearly a year and a half, the
longest on record till Halley's in 1910. In case a comet eludes
discovery and observation until it has passed its perihelion, or nearest
point to the sun, its period of visibility may be reduced to a few
weeks only. The brightest comets on record were visible in 1843 and
1882: so brilliant were they that even the effulgence of full daylight
did not overpower them. In particular the comet of 1843 was not only
excessively bright, but at its nearest approach to the earth its tail
swept all the way across the sky from one horizon to the other. It must
have looked very much like the straight beam of an enormous searchlight,
though very much brighter.

The tails of comets are to the naked eye the most compelling thing about
them, and to the ancient peoples they were naturally most terrifying.
Their tails are not only curved, but sometimes curved with varying
degrees of curvature, and this circumstance adds to their weirdness of
appearance. If we examine the tail of a comet with a telescope, it
vanishes as if there were nothing to it: as indeed one may almost say
there is not. Ordinarily, only the head of the comet is of interest in
the telescope. When first seen there is usually nothing but the head
visible, and that is made up of portions which develop more or less
rapidly, presenting a succession of phenomena quite different in
different comets.

When first discovered a comet is usually at a great distance from the
sun, about the distance of Jupiter; and we see it, not as we do the
planets, by sunlight reflected from them, but by the comet's own light.
This is at that time very faint, and nearly all comets at such a
distance look alike: small roundish hazy patches of faint, cloudlike
light, with very often a concentration toward the center called the
nucleus, on the average about 4,000 miles in diameter. Approach toward
the sun brightens up the comet more and more, and the nucleus usually
becomes very much brighter and more starlike. Then on the sunward side
of the nucleus, jetlike streamers or envelopes appear to be thrown off,
often as if in parallel curved strata, or concentrically. As they expand
and move outward from the nucleus, these envelopes grow fainter and are
finally merged in the general nebulosity known as the comet's head,
which is anywhere from 30,000 to 100,000 miles in diameter. As a rule,
this is an orderly development which can be watched in the telescope
from hour to hour and from night to night; but occasionally a cometary
visitor is quite a law to itself in development, presenting a
fascinating succession of unpredictable surprises.

Then follows the development of the comet's tail, perhaps more striking
than anything that has preceded it. Here a genuine repulsion from the
sun appears to come into play. It may be an electrical repulsion. Much
of the material projected from the comet's nucleus, seems to be driven
backward or repelled by the sun, and it is this that goes to form the
tail. The particles which form the tail then travel in modified paths
which nevertheless can be calculated. The tail is made up of these
luminous particles and it expands in space much in the form of a hollow,
horn-shaped cone, the nucleus being near the tip of the horn.

Some comets possess multiple tails with different degrees of curvature,
Donati's for example. Usually there is a nearly straight central dark
space, marking the axis of the comet, and following the nucleus. But
occasionally this is replaced by a thin light streak very much less in
breadth than the diameter of the head. Cometary tails are sometimes 100
million miles in length.

Three different types of cometary tails are recognized. First, the long
straight ones, apparently made up of matter repelled by the sun twelve
to fifteen times more powerfully than gravitation attracts it. Such
particles must be brushed away from the comet's head with a velocity of
perhaps five miles a second, and their speed is continually increasing.
Probably these straight tails are due to hydrogen. The second type tails
are somewhat curved, or plume-like, and they form the most common type
of cometary tail. In them the sun's repulsion is perhaps twice its
gravitational attraction, and hydrocarbons in some form appear to be
responsible for tails of this character. Then there is a third type,
much less often seen, short and quickly curving, probably due to heavier
vapors, as of chlorine, or iron, or sodium, in which the repulsive force
is only a small fraction of that of gravitation.

Many features of this theory of cometary tails are borne out by
examination of their light with the spectroscope, although the
investigation is as yet fragmentary. It is evident that the tail of a
comet is formed at the expense of the substance of the nucleus and head;
so that the matter repelled is forever dissipated through the regions of
space which the comet has traveled. Comets must lose much of their
original substance every time they return to perihelion. Comets actually
age, therefore, and grow less and less in magnitude of material as well
as brightness, until they are at last opaque, nonluminous bodies which
it becomes impossible to follow with the telescope.




CHAPTER XLI

WHERE DO COMETS COME FROM?


Where do comets come from? The answer to this question is not yet fully
made out. Most likely they have not all had a similar origin, and
theories are abundant. Apparently they come into the solar system from
outer space, from any direction whatsoever. The depths of interstellar
space seem to be responsible for most, if not all, of the new ones.
Whether they have come from other stars or stellar systems we cannot
say.

While comets are tremendous in size or volume, their mass or the amount
of real substance in them is relatively very slight. We know this by the
effect they produce on planets that they pass near, or rather by the
effect that they fail to produce. The earth's atmosphere weighs about
one two hundred and fifty thousandth as much as the earth itself, but a
comet's entire mass must be vastly less than this. Even if a comet were
to collide with the earth head on, there is little reason to believe
that dire catastrophe would ensue. At least twice the earth is known to
have passed through the tail of a comet, and the only effect noticed was
upon the comet itself; its orbit had been modified somewhat by the
attraction of the earth. If the comet were a small one, collision with
any of the planets would result in absorption and dissipation of the
comet into vapor.

The whole of a large comet has perhaps as much mass or weight as a
sphere of iron a hundred miles in diameter. Even this could not wreck
the earth, but the effect would depend upon what part of the earth was
hit. A comet is very thin and tenuous, because its relatively small mass
is distributed through a volume so enormous. So it is probable that the
earth's atmosphere could scatter and burn up the invading comet, and we
should have only a shower of meteors on an unprecedented scale.
Diffusion of noxious gases through the atmosphere might vitiate it to
some extent, though probably not enough to cause the extinction of
animal life.

Every comet has an interesting history of its own, almost indeed unique.
One of the smallest comets and the briefest in its period round the sun
is known as Encke's comet. It is a telescopic comet with a very short
tail, its time of revolution is about three and a half years, and it
exhibits a remarkable contraction of volume on approach to the sun.

Biela's comet has a period about twice as long. At one time it passed
within about 15 million miles of the earth, and somewhere about the year
1840 this comet divided into two distinct comets, which traveled for
months side by side, but later separated and both have since completely
disappeared. Perhaps the most beautiful of all comets is that discovered
by Donati of Florence in 1858. Its coma presented the development of
jets and envelopes in remarkable perfection, and its tail was of the
secondary or hydrocarbon type, but accompanied by two faint streamer
tails, nearly tangential to the main tail and of the hydrogen type.
Donati's comet moves in an ellipse of extraordinary length, and it will
not return to the sun for nearly 2,000 years.

The most brilliant comet of the last half century is known as the great
comet of 1882. In a clear sky it could readily be seen at midday. On
September 17 it passed across the disk of the sun and was practically as
bright as the surface of the sun itself. The comet had a multiple
nucleus and a hydrocarbon tail of the second type, nearly a hundred
million miles in length. Doubtless this great comet is a member of what
is known as a cometary group, which consists of comets having the same
orbit and traveling tandem round the sun. The comets of 1668, 1843,
1880, 1882 and 1887 belong to this particular group, and they all pass
within 300,000 miles of the sun's surface, at a maximum velocity
exceeding 300 miles a second. They must therefore invade the regions of
the solar corona, the inference being that the corona as well as the
comet is composed of exceedingly rare matter.

Photography of comets has developed remarkably within recent years,
especially under the deft manipulation of Barnard, whose plates, in
particular during his residence at the Lick Observatory on Mount
Hamilton, California, show the features of cometary heads and tails in
excellent definition. Halley's comet, at the 1910 apparition, was
particularly well photographed at many observatories.

The question is often asked, When will the next comet come? If a large
bright comet is meant, astronomers cannot tell. At almost any time one
may blaze into prominence within only a few days. During the latter half
of the last century, bright comets appeared at perihelion at intervals
of eight years on the average. Several of the lesser and fainter
periodic comets return nearly every year, but they are mostly
telescopic, and are rarely seen except by astronomers who are
particularly interested in observing them.




CHAPTER XLII

METEORS AND SHOOTING STARS


"Falling stars," or "shooting stars," have been familiar sights in all
ages of the world, but the ancient philosophers thought them scarcely
worthy of notice. According to Aristotle they were mere nothings of the
upper atmosphere, of no more account than the general happenings of the
weather. But about the end of the eighteenth century and the beginning
of the nineteenth the insufficiency of this view began to be fully
recognized, and interplanetary space was conceived as tenanted by shoals
of moving bodies exceedingly small in mass and dimension as compared
with the planets.

Millions of these bodies are all the time in collision with the outlying
regions of our atmosphere; and by their impact upon it and their
friction in passing swiftly through it, they become heated to
incandescence, thus creating the luminous appearances commonly known as
shooting stars. For the most part they are consumed or dissipated in
vapor before reaching the solid surface of the earth; but occasionally a
luminous cloud or streak is left glowing in the wake of a large meteor,
which sometimes remains visible for half an hour after the passage of
the meteor itself. These mistlike clouds projected upon the dark sky
have been especially studied by Trowbridge of Columbia University.

Many more meteors are seen during the morning hours, say from four to
six, than at any other nightly period of equal length, because the
visible sky is at that time nearly centered around the general direction
toward which the earth is moving in its orbit round the sun; so that the
number of meteors that would fall upon the earth if at rest is increased
by those which the earth overtakes by its own motion. Also from January
to July while the earth is traveling from perihelion to aphelion, fewer
meteors are seen than in the last half of the year; but this is chiefly
because of the rich showers encountered in August and November.

Although the descent of meteoric bodies from the sky was pretty
generally discredited until early in the nineteenth century, such falls
had nevertheless been recorded from very early times. They were usually
regarded as prodigies or miracles, and such stones were commonly objects
of worship among ancient peoples. For example, the Phrygian Stone, known
as the "Diana of the Ephesians which fell down from Jupiter," was a
famous stone built into the Kaaba at Mecca, and even to-day it is
revered by Mohammedans as a holy relic. Perhaps the earliest known
meteoric fall is that historically recorded in the Parian Chronicle as
having occurred in the island of Crete, B. C. 1478. Also in the imperial
museum of Petrograd is the Pallas or Krasnoiarsk iron, perhaps
three-quarters of a ton in weight, found in 1772 by Pallas, the famous
traveler, at Krasnoiarsk, Siberia.

But a fall of meteoric stones that chanced upon the department of Orne,
France, in 1805, led to a critical investigation by Biot, the
distinguished physicist and academician. According to his report a
violent explosion in the neighborhood of L'Aigle had been heard for a
distance of seventy-five miles around, and lasting five or six minutes,
about 1 P. M. on Tuesday, April 26. From several adjoining towns a
rapidly moving fireball had been seen in a sky generally clear, and
there was absolutely no room for doubt that on the same day many stones
fell in the neighborhood of L'Aigle. Biot estimated their number between
two and three thousand, and they were scattered over an elliptical area
more than six miles long, and two and a half miles broad. Thenceforward
the descent of meteoric matter from outer space upon the earth has been
recognized as an unquestioned fact.

The origin of these bodies being cosmic, meteors may be expected to fall
upon the earth without reference to latitude, or season, or day and
night, or weather. On entering our upper atmosphere their temperature
must be that of space, many hundred degrees below zero; and their
velocities range from ten miles per second upward. But atmospheric
resistance to their flight is so great that their velocity is quickly
reduced: at ground impact it does not exceed a few hundred feet per
second. On January 1, 1869, several meteoric stones fell on ice only a
few inches thick in Sweden, rebounding without either breaking through
the ice or being themselves fractured.

Naturally the flight of a meteor through the atmosphere will be only a
few seconds in duration, and owing to the sudden reduction of velocity,
it will continue to be luminous throughout only the upper part of its
course. Visibility generally begins at an elevation of about seventy
miles, and ends at perhaps half that altitude.

What is the origin of meteors? Theories there are in great abundance:
that they come from the sun, that they come from the moon, that they
come from the earth in past ages as a result of volcanic action, and so
on. But there are many difficulties in the way of acceptance of these
and several other theories. That all meteors were originally parts of
cometary masses is however a theory that may be accepted without much
hesitation.

Comets have been known to disintegrate. Biela's comet even disappeared
entirely, so that during a shower of Biela meteors in November, 1885, an
actual fragment of the lost comet fell upon the earth, at Mazapil,
Mexico. And as the Bielid meteors encounter the earth with the
relatively low velocity of ten miles a second, we may expect to capture
other fragments in the future. Numerous observers saw the weird
disintegration of the nucleus of the great comet of 1882, well
recognized as a member of the family of the comet of 1843. As these
comets are fellow voyagers through space along the same orbit, probably
all five members of the family, with perhaps others, were originally a
single comet of unparalleled magnitude.

The Brooks comet of 1890 affords another instance of fragmentary
nucleus. The oft-repeated action of solar forces tending to disrupt the
mass of a comet more and more, and scatter its material throughout
space, the secular dismemberment of all comets becomes an obvious
conclusion. During the hundreds of millions of years that these forces
are known to have been operant, the original comets have been broken up
in great numbers, so that elliptical rings of opaque meteoric bodies now
travel round the sun in place of the comets.

These bodies in vast numbers are everywhere through space, each too
small to reflect an appreciable amount of sunlight, and becoming
visible only when they come into collision with our outer atmosphere.
The practical identity of several such meteor streams and cometary
orbits has already been established, and there is every reason for
assigning a similar origin to all meteoric bodies. Meteors, then, were
originally parts of comets, which have trailed themselves out to such
extent that particles of the primal masses are liable to be picked up
anywhere along the original cometary paths. The historic records of all
countries contain trustworthy accounts of meteoric showers. Making due
allowances for the flowery imagery of the oriental, it is evident that
all have at one time or another seen much the same thing. In A. D. 472,
for instance, the Constantinople sky was reported alive with flying
stars. In October, 1202, "stars appeared like waves upon the sky; and
they flew about like grasshoppers." During the reign of King William II
occurred a very remarkable shower in which "stars seemed to fall like
rain from heaven."

But the showers of November, 1799 and 1833, are easily the most striking
of all. The sky was filled with innumerable fiery trails and there was
not a space in the heavens a few times the size of the moon that was not
ablaze with celestial fireworks. Frequently huge meteors blended their
dazzling brilliancy with the long and seemingly phosphorescent trails of
the shooting stars.

The interval of thirty-four years between 1799 and 1833 appeared to
indicate the possibility of a return of the shower in November of 1866
or 1867, and all the people of that day were aroused on this subject and
made every preparation to witness the spectacle. Extemporized
observatories were established, watchmen were everywhere on the
lookout, and bells were to be rung the minute the shower began. The
newspapers of the day did little to allay the fears of the multitude,
but the critical days of November, 1866, passed with disappointment in
America. In Europe, however, a fine shower was seen, though it was not
equal to that of 1833. The astronomers at Greenwich counted many
thousand meteors. In November of 1867, however, American astronomers
were gratified by a grand display, which, although failing to match the
general expectation, nevertheless was a most striking spectacle, and the
careful preparation for observing it afforded data of observation which
were of the greatest scientific value. The actual orbits of these bodies
in space became known with great exactitude, and it was found that their
general path was identical with that of the first comet of 1866, which
travels outward somewhat beyond the planet Uranus. When the visible
paths of these meteors are traced backward, all appear as if they
originated from the constellation Leo. So they are known as Leonids, and
a return of the shower was confidently predicted for November,
1900-1901, which for unknown reasons failed to appear.

    [Illustration: TWO VIEWS OF HALLEY'S COMET. Taken with the same
       camera from the same position, one on May 12, and the other on
       May 15, 1910. (_Photo, Mt. Wilson Solar Observatory._)]

    [Illustration: SWIFT'S COMET OF 1892. This comet showed
       extraordinary and rapid transformations, one day having a dozen
       streamers in its tail, another only two. (_Photo by Prof. E. E.
       Barnard._)]

    [Illustration: A LARGE METEOR TRAIL IN THE FIELD WITH FINE NEBULÆ.
       (_Photo, Yerkes Observatory._)]

During the last half century meteors have been pretty systematically
observed, especially by the astronomers of Italy and Denning of England,
so that several hundred distinct showers are now known, their radiant
points fall in every part of the heavens, and there is scarcely a clear
moonless night when careful watching for meteors will be unrewarded.
Besides November, the months of August (Perseids), April (Lyrids), and
December (Geminids) are favorable. Following in tabular form is a fairly
comprehensive list of the meteoric showers of the year, with the
positions of the radiant points and the epochs of the showers according
to Denning:

RADIANT POINT

  ============================================================
  Name of Shower         |  R. A.  | Decl.  | Date of Shower
  -----------------------+---------+--------+-----------------
  Quadrantids            |   230°  |  +53°  |  Jan. 2-4
  Zeta Cepheids          |   331°  |  +56°  |  Jan. 25
  Alpha Leonids          |   155°  |  +14°  |  Feb. 19-March 1
  Tau Leonids            |   166°  |   +4°  |  March 1-4
  Beta Ursids            |   161°  |  +58°  |  March 13-24
  Lyrids                 |   271°  |  +33°  |  April 20-22
  Gamma Aquarids         |   338°  |   -2°  |  May 1-6
  Zeta Herculids         |   246°  |  +29°  |  May 18-26
  Eta Pegasids           |   330°  |  +28°  |  May 30-June 4
  Theta Boötids          |   213°  |  +53°  |  June 27-28
  Alpha Capricornids     |   304°  |  -12°  |  July 15-28
  Delta Aquarids         |   339°  |  -11°  |  July 25-30
  Perseids               |    45°  |  +57°  |  Aug. 10-12
  Omicron Draconids      |   291°  |  +60°  |  Aug. 15-25
  Zeta Draconids         |   262°  |  +63°  |  Aug. 21-Sept. 2
  Piscids                |   348°  |   +2°  |  Sept. 4-14
  Alpha Andromedids      |     4°  |  +28°  |  Sept. 27
  Epsilon Arietids       |    40°  |  +20°  |  Oct. 11-24
  Orionids               |    92°  |  +15°  |  Oct. 17-24
  Epsilon Perseids       |    61°  |  +35°  |  Nov. 5
  Leonids                |   150°  |  +23°  |  Nov. 13-15
  Epsilon Taurids        |    64°  |  +22°  |  Nov. 14-25
  Andromedids            |    25°  |  +43°  |  Nov. 17-23
  Beta Geminids          |   119°  |  +31°  |  Dec. 1-12
  Geminids               |   108°  |  +33°  |  Dec. 1-14
  Alpha Ursæ Majorids    |   161°  |  +58°  |  Dec. 18-21
  Kappa Draconids        |   194°  |  +68°  |  Dec. 18-28
  ------------------------------------------------------------

The year 1916 was exceptional in providing an abundant and previously
unknown shower on June 28, and its stream has nearly the same orbit as
that of the Pons-Winnecke periodic comet. Useful observations of meteors
are not difficult to make, and they are of service to professional
astronomers investigating the orbits of these bodies, among whom are
Mitchell and Olivier of the University of Virginia.




CHAPTER XLIII

METEORITES


Meteorites, the name for meteors which have actually gone all the way
through our atmosphere, are never regular in form or spherical. As a
rule the iron meteorites are covered with pittings or thumb marks, due
probably to the resistance and impact of the little columns of air which
impede its progress, together with the unequal condition and fusibility
of their surface material. The work done by the atmosphere in suddenly
checking the meteor's velocity appears in considerable part as heat,
fusing the exterior to incandescence. This thin liquid shell is quickly
brushed off, making oftentimes a luminous train.

But notwithstanding the exceedingly high temperature of the exterior,
enforced upon it for the brief time of transit through the atmosphere,
it is probable that all large meteorites, if they could be reached at
once on striking the earth, would be found to be cold, because the
smooth, black, varnishlike crust which always incases them as a result
of intense heat is never thick. On one occasion a meteor which was seen
to fall in India was dug out of the ground as quickly as possible, and
found to be, not hot as was expected, but coated thickly over with ice
frozen on it from the moisture in the surrounding soil.

As to the composition of shooting stars, and their probable mass, and
its effect upon the earth, our data are quite insufficient. The lines of
sodium and magnesium have been hurriedly caught in the spectroscope,
and, estimating on the basis of the light emitted by them, the largest
meteors must weigh ounces rather than pounds. Nevertheless, it is
interesting to inquire what addition the continual fall of many millions
daily upon the earth makes to its weight: somewhere between thirty and
fifty thousand tons annually is perhaps a conservative estimate, but
even this would not accumulate a layer one inch in thickness over the
entire surface of the earth in less than a thousand million years.

Many hundreds of the meteors actually seen to fall, together with those
picked up accidentally, are recovered and prized as specimens of great
value in our collections, the richest of which are now in New York,
Paris, and London. The detailed investigation of them is rather the
province of the chemist, the crystallographer and the mineralogist than
of the astronomer whose interest is more keen in their life history
before they reach the earth. To distinguish a stony meteorite from
terrestrial rock substances is not always easy, but there is usually
little difficulty in pronouncing upon an iron meteorite. These are most
frequently found in deserts, because the dryness of the climate renders
their oxidation and gradual disappearance very slow.

The surface of a suspected iron meteorite is polished to a high luster
and nitric acid is poured upon it. If it quickly becomes etched with a
characteristic series of lines, or a sort of cross-hatching, it is
almost certain to be a meteorite. Occasionally carbon has been found in
meteorites, and the existence of diamond has been suspected. The
minerals composing meteorites are not unlike terrestrial materials of
volcanic origin, though many of them are peculiar to meteorites only.
More than one-third of all the known chemical elements have been found
by analysis in meteorites, but not any new ones.

Meteoric iron is a rich alloy containing about ten per cent of nickel,
also cobalt, tin, and copper in much smaller amount. Calcium, chlorine,
sodium, and sulphur likewise are found in meteoric irons. At very high
temperatures iron will absorb gases and retain them until again heated
to red heat. Carbonic oxide, helium, hydrogen, and nitrogen are thus
imprisoned, or occluded, in meteoric irons in very small quantities; and
in 1867, during a London lecture by Graham, a room in the Royal
Institution was for a brief space illuminated by gas brought to earth in
a meteorite from interplanetary space. Meteorites, too, have been most
critically investigated by the biologist, but no trace of germs of
organic life of any type has so far been found. Farrington of Chicago
has published a full descriptive catalogue of all the North American
meteorites.

Recent investigations of the radioactivity of meteorites show that the
average stone meteorite is much less radioactive than the average rock,
and probably less than one-fourth as radioactive as in average granite.
The metallic meteorites examined were found about wholly free from
radioactivity.

From shooting stars, perhaps the chips of the celestial workshop, or
more possibly related to the planetesimals which the processes of growth
of the universe have swept up into the vastly greater bodies of the
universe, transition is natural to the stars themselves, the most
numerous of the heavenly bodies, all shining by their own light, and all
inconceivably remote from the solar system, which nevertheless appears
to be not far removed from the center of the stellar universe.




CHAPTER XLIV

THE UNIVERSE OF STARS


Our consideration of the solar system hitherto has kept us quite at home
in the universe. The outer known planets, Uranus and Neptune, are indeed
far removed from the sun, and a few of the comets that belong to our
family travel to even greater distances before they begin to retrace
their steps sunward. When we come to consider the vast majority of the
glistening points on the celestial sphere--all in fact except the five
great planets, Mercury, Venus, Mars, Jupiter, and Saturn--we are dealing
with bodies that are self-luminous like the sun, but that vary in size
quite as the bodies of the solar system do, some stars being smaller
than the sun and others many hundred fold larger than he is; some being
"giants," and others "dwarfs." But the overwhelming remoteness of all
these bodies arrests our attention and even taxes our credulity
regarding the methods that astronomers have depended on to ascertain
their distances from us.

Their seeming countlessness, too, is as bewildering as are the
distances; though, if we make actual counts of those visible to the
naked eye within a certain area, in the body of the "Great Bear," for
example, the great surprise will be that there are so few. And if the
entire dome of the sky is counted, at any one time, a clear, moonless
sky would reveal perhaps 2,500, so that in the entire sky, northern and
southern, we might expect to find 5,000 to 6,000 lucid stars, or stars
visible to the naked eye.

But when the telescope is applied, every accession of power increases
the myriads of fainter and fainter stars, until the number within
optical reach of present instruments is somewhere between 400 and 500
millions. But if we were to push the 100-inch reflector on Mount Wilson
to its limit by photography with plates of the highest sensitiveness,
millions upon millions of excessively faint stars would be plainly
visible on the plates which the human eye can never hope to see directly
with any telescope present or future, and which would doubtless swell
the total number of stars to a thousand millions. Recent counts of stars
by Chapman and Melotte of Greenwich tend to substantiate this estimate.

What have astronomers done to classify or catalogue this vast array of
bodies in the sky? Even before making any attempt to estimate their
number, there is a system of classification simply by the amount of
light they send us, or by their apparent stellar magnitudes--not their
actual magnitudes, for of those we know as yet very little. We speak of
stars of the "first magnitude," of which there are about 20, Sirius
being the brightest and Regulus the faintest. Then there are about 65 of
the second, or next fainter, magnitude, stars like Polaris, for example,
which give an amount of light two and a half times less than the average
first magnitude star. Stars of the third magnitude are fainter than
those of the second in the same ratio, but their number increases to
200; fourth magnitude, 500; fifth magnitude, 1,400; sixth magnitude,
5,000, and these are so faint that they are just visible on the best
nights without telescopic aid.

Decimals express all intermediate graduations of magnitude. Astronomers
carry the telescopic magnitudes much farther, till a magnitude beyond
the twentieth is reached, preserving in every case the ratio of two and
one-half for each magnitude in relation to that numerically next to it.
Even Jupiter and Venus, and the sun and moon, are sometimes calculated
on this scale of stellar magnitude, numerically negative, of course,
Venus sometimes being as bright as magnitude -4.3, and the sun -26.7.

Knowing thus the relation of sun, moon, and stars, and the number of the
stars of different magnitudes, it is possible to estimate the total
light from the stars. This interesting relation comes out this way: that
the stars we cannot see with the naked eye give a greater total of light
than those we can because of their vastly greater numbers. And if we
calculate the total light of all the brighter stars down to magnitude
nine and one-half, we find it equal to 1/80th of the light of the
average full moon.

Many stars show marked differences in color, and strictly speaking the
stars are now classified by their colors. The atmosphere affects star
colors very considerably, low altitudes, or greater thickness of air,
absorbing the bluish rays more strongly and making the stars appear
redder than they really are. Aldebaran, Betelgeuse and Antares are
well-known red stars, Capella and Alpha Ceti yellowish, Vega and Sirius
blue, and Procyon and Polaris white. Among the telescopic stars are many
of a deep blood-red tint, variable stars being numerous among them.
Double stars, too, are often complementary in color. There is evidence
indicating change of color of a very few stars in long periods of time;
Sirius, for example, two thousand years ago was a red star, now it is
blue or bluish white. But the meaning of color, or change of color in a
star is as yet only incompletely ascertained. It may be connected with
the radiative intensity of the star, or its age, or both.

The late Professor Edward C. Pickering was famous for his life-long
study and determination of the magnitudes of the stars. Standards of
comparison have been many, and have led to much unnecessary work.
Pickering chose Polaris as a standard and devised the meridian
photometer, an ingenious instrument of high accuracy, in which the light
of a star is compared directly with that of the pole star by reflection.
All the bright stars of both the northern and the southern skies are
worked into a standard system of magnitudes known as HP, or the Harvard
Photometry.

Astronomers make use of several different kinds of magnitude for the
stars: the apparent magnitude, as the eye sees it, often called the
visual magnitude; the photographic magnitude, as the photographic plate
records it, and these are now determined with the highest accuracy; the
photovisual magnitude, quite the same as the visual, but determined
photographically on an isochromatic plate with a yellow screen or
filter, so that the intensity is nearly the same as it appears to the
eye. The difference between the star's visual or photovisual magnitude
and its photographic magnitude is called its color-index, and is often
used as a measure of the star's color. Light of the shorter wave
lengths, as blue and violet, affects the photographic plate more rapidly
than the reds and yellows of longer wave length by which the eye mainly
sees; so that red stars will appear much fainter and blue stars much
brighter on the ordinary photographic plate than the eye sees them.

So great are the differences of color in the stars that well-known
asterisms, with which the eye is perfectly familiar, are sometimes quite
unrecognizable on the photographic plate, except by relative positions
of the stars composing them. White stars affect the eye and the plate
about equally, so that their visual or photovisual and photographic
magnitudes are about equal. The studies of the colors of the stars, the
different methods of determining them, and the relations of color to
constitution have been made the subject of especial investigation by
Seares of Mount Wilson and many other astronomers.

Centuries of the work of astronomers have been faithfully devoted to
mapping or charting the stars and cataloguing them. Just as we have
geographical maps of countries, so the heavens are parceled out in
sections, and the stars set down in their true relative positions just
as cities are on the map. Recent years have added photographic charts,
especially of detailed regions of the sky; but owing to spectral
differences of the stars, their photographic magnitudes are often quite
different from their visual magnitudes. From these maps and charts the
positions of the stars can be found with much precision; but if we want
the utmost accuracy, we must go to the star catalogues--huge volumes
oftentimes, with stellar positions set down therein with the last degree
of precision.

First there will be the star's name, and in the next column its
magnitude, and in a third the star's right ascension. This is its
angular distance eastward around the celestial sphere starting from the
vernal equinox, and it corresponds quite closely to the longitude of a
place which we should get from a gazetteer, if we wished to locate it on
the earth. Then another column of the catalogue will give the star's
declination, north or south of the equator, just as the gazetteer will
locate a city by its north or south latitude.




CHAPTER XLV

STAR CHARTS AND CATALOGUES


Who made the first star chart or catalogue? There is little doubt that
Eudoxus (B. C. 200) was the first to set down the positions of all the
brighter stars on a celestial globe, and he did this from observations
with a gnomon and an armillary sphere. Later Hipparchus (B. C. 130)
constructed the first known catalogue of stars, so that astronomers of a
later day might discover what changes are in progress among the stars,
either in their relative positions or caused by old stars disappearing
or new stars appearing at times in the heavens. Hipparchus was an
accurate observer, and he discovered an apparent and perpetual shifting
of the vernal equinox westward, by which the right ascensions of the
stars are all the time increasing. He determined the amount of it pretty
accurately, too. His catalogue contained 1,080 stars, and is printed in
the "Almagest" of Ptolemy.

Centuries elapsed before a second star catalogue was made, by Ulugh-Beg,
an Arabian astronomer, A. D. 1420, who was a son of Tamerlane, the
Tartar monarch of Samarcand, where the observations for the catalogue
were made. The stars were mainly those of Ptolemy, and much the same
stars were reobserved by Tycho Brahe (A. D. 1580) with his greatly
improved instruments, thus forming the third and last star catalogue of
importance before the invention of the telescope.

From the end of the seventeenth century onward, the application of the
telescope to all the types of instruments for making observations of
star places has increased the accuracy many-fold. The entire heavens has
been covered by Argelander in the northern hemisphere, and Gould in the
southern--over 700,000 stars in all. Many government observatories are
still at work cataloguing the stars. The Carnegie Institution of
Washington maintains a department of astrometry under Boss of Albany,
which has already issued a preliminary catalogue of more than 6,000
stars, and has a great general catalogue in progress, together with
investigations of stellar motions and parallaxes. This catalogue of star
positions will include proper motions of stars to the seventh magnitude.

In 1887 on proposal of the late Sir David Gill, an international
congress of astronomers met at Paris and arranged for the construction
of a photographic chart of the entire heavens, allotting the work to
eighteen observatories, equipped with photographic telescopes
essentially alike. The total number of plates exceeds 25,000. Stars of
the fourteenth magnitude are recorded, but only those including the
eleventh magnitude will be catalogued, perhaps 2,000,000 in all. The
expense of this comprehensive map of the stars has already exceeded
$2,000,000, and the work is now nearly complete. Turner of Oxford has
conducted many special investigations that have greatly enhanced the
progress of this international enterprise.

Other great photographic star charts have been carried through by the
Harvard Observatory, with the annex at Arequipa, Peru, employing the
Bruce photographic telescope, a doublet with 24-inch lenses; also
Kapteyn of Groningen has catalogued about 300,000 stars on plates taken
at Cape Town. Charting and cataloguing the stars, both visually and
photographically, is a work that will never be entirely finished.
Improvements in processes will be such that it can be better done in the
future than it is now, and the detection of changes in the fainter stars
and investigation of their motions will necessitate repetition of the
entire work from century to century.

The origin of the names of individual stars is a question of much
interest. The constellation figures form the basis of the method, and
the earliest names were given according to location in the especial
figure; as for instance, Cor Scorpii, the heart of the Scorpion, later
known as Antares or Alpha Scorpii. The Arabians adopted many star names
from the Greeks, and gave about a hundred special names to other stars.
Some of these are in common use to-day, by navigators, observers of
meteors and of variable stars. Sirius, Vega, Arcturus, and a few other
first magnitude stars, are instances.

But this method is quite insufficient for the fainter stars whose
numbers increase so rapidly. Bayer, a contemporary of Galileo,
originated our present system, which also employs the names of the
constellations, the Latin genitive in each case, prefixed by the small
letters of the Greek alphabet, from alpha to omega, in order of
decreasing brightness; and followed by the Roman letters when the Greek
alphabet is exhausted.

If there were still stars left in a constellation unnamed, numbers were
used, first by Flamsteed, Astronomer Royal; and numbers in the order of
right ascension in various catalogues are used to designate hundreds of
other stars. The vast bulk of the stars are, however, nameless; but
about one million are identifiable by their positions (right ascension
and declination) on the celestial sphere.




CHAPTER XLVI

THE SUN'S MOTION TOWARD LYRA


If Hipparchus or Galileo should return to earth to-night and look at the
stars and constellations as we see them, there would be no change
whatever discernible in either the brightness of the stars or in their
relative positions. So the name fixed stars would appear to have been
well chosen. Halley in the seventeenth century was the first to detect
that slow relative change of position of a few stars which is known as
proper motion, and all the modern catalogues give the proper motions in
both right ascension and declination. These are simply the small annual
changes in position athwart the line of vision; and, as a whole, the
proper motions of the brighter stars exceed the corresponding motions of
the fainter ones because they are nearer to us. The average proper
motion of the brightest stars is 0".25, and of stars of the sixth
magnitude only one-sixth as great.

A few extreme cases of proper motion have been detected, one as large as
9", of an orange yellow star of the eighth magnitude in the southern
constellation Pictor, and Barnard has recently discovered a star with a
proper motion exceeding 10"; several determinations of its parallax give
0".52, corresponding to a distance of 6.27 light years. Nevertheless,
two centuries would elapse before these stars would be displaced as much
as the breadth of the moon among their neighbors in the sky. The proper
motions of stars are along perfectly straight lines, so far as yet
observed. Ultimately we may find a few moving in curved paths or orbits,
but this is hardly likely.

As for a central sun hypothesis, that pointing out Alcyone in
particular, there is no reliable evidence whatever. Analysis of the
proper motions of stars in considerable numbers, first by Sir William
Herschel, showed that they were moving radially from the constellation
Hercules, and in great numbers also toward the opposite side of the
stellar sphere. Later investigation places this point, called the sun's
goal, or apex of the sun's way, over in the adjacent constellation Lyra;
and the opposite point, or the sun's quit, is about halfway between
Sirius and Canopus. By means of the radial velocities of stars in these
antipodal regions of the sky, it is found that the sun's motion toward
Lyra, carrying all his planetary family along with him, is taking place
at the rate of about 12 miles in every second.

While the right ascensions of the solar apex as given by the different
investigations have been pretty uniform, the declination of this point
has shown a rather wide variation not yet explained. For example, there
is a difference of nearly ten degrees between the declination (+34°.3)
of the apex as determined by Boss from the proper motions of more than
6,000 stars, and the declination (+25°.3) found by Campbell from the
radial velocities of nearly 1,200 stars. Several investigations tend to
show that the fainter the stars are, the greater is the declination of
the solar apex. More remarkable is the evidence that this declination
varies with the spectral type of the stars, the later types, especially
G and K, giving much more northerly values. On the whole the great
amount of research that has been devoted to the solar motion relative to
the system of the stars for the past hundred years may be said to
indicate a point in right ascension 18h. (270°) and declination 34° N.
as the direction toward which the sun is moving. This is not very far
from the bright star Alpha Lyræ, and the antipodal point from which the
sun is traveling is quite near to Beta Columbæ.

So swift is this motion (nearly twenty kilometers per second) that it
has provided a base line of exceptional length, and very great service
in determining the average distance of stars in groups or classes. After
thousands of years the sun's own motion combined with the proper motions
of the stars will displace many stars appreciably from their familiar
places. The constellations as we know them will suffer slight
distortions, particularly Orion, Cassiopeia and Ursa Major. Identity or
otherwise of spectra often indicates what stars are associated together
in groups, and their community of motion is known as star drift. Recent
investigation of vast numbers of stars by both these methods have led to
the epochal discovery of star streaming, which indicates that the stars
of our system are drifting by, or rather through, each other, in two
stately and interpenetrating streams. The grand primary cause underlying
this motion is as yet only surmised.




CHAPTER XLVII

STARS AND THEIR SPECTRAL TYPE


When in 1872 Dr. Henry Draper placed a very small wet plate in the
camera of his spectroscope and, by careful following, on account of the
necessarily long exposure, secured the first photographic spectrum of a
star ever taken, he could hardly have anticipated the wealth of the new
field of research which he was opening. His wife, Anna Palmer Draper,
was his enthusiastic assistant in both laboratory and observatory, and
on his death in 1882, she began to devote her resources very
considerably to the amplification of stellar spectrum photography. At
first with the cooperation of Professor Young of Princeton, and later
through extension of the facilities of Harvard College Observatory,
whose director, the late Professor Edward C. Pickering, devoted his
energies in very large part to this matter, all the preliminaries of the
great enterprise were worked out, and a comprehensive program was
embarked upon, which culminated in the "Henry Draper Memorial," a
catalogue and classification of the spectra of all the stars brighter
than the ninth magnitude, in both the northern and southern hemispheres.

One very remarkable result from the investigation of large numbers of
stars according to their type is the close correlation between a star's
luminosity and its spectral type. But even more remarkable is the
connection between spectral type and speed of motion. As early as 1892
Monck of Dublin, later Kapteyn, and still later Dyson, directed
attention to the fact that stars of the Secchi type II had on the
average larger proper motions than those of type I. In 1903 Frost and
Adams brought out the exceptional character of the Orion stars, the
radial velocities of twenty of which averaged only seven kilometers per
second.

Soon after, with the introduction of the two-stream hypothesis, a wider
generalization was reached by Campbell and Kapteyn, whose radial
velocities showed that the average linear velocity increases continually
through the entire series B, A, F, G, K, M, from the earliest types of
evolution to the latest. The younger stars of early type have velocities
of perhaps five or six kilometers per second, while the older stars of
later type have velocities nearly fourfold greater.

The great question that occurs at once is: How do the individual stars
get their motions? The farther back we go in a star's life history, the
smaller we find its velocity to be. When a star reaches the Orion stage
of development, its velocity is only one-third of what it may be
expected to have finally. Apparently, then, the stars at birth have no
motion, but gradually acquire it in passing through their several types
or stages of development.

More striking still is the motion of the planetary nebulæ, in excess of
25 kilometers per second, while type A stars move 11 kilometers, type G
15 kilometers, and type M 17 kilometers per second. Can the law
connecting speed of motion and spectral type be so general that the
planetary nebula is to be regarded as the final evolutionary stage?
Stars have been seen to become nebulæ, and one astronomer at least is
strongly of the opinion that a single such instance ought to outweigh
all speculation to the contrary, as that stars originate from nebulæ.

In his discussion of stellar proper motions, Boss has reached a striking
confirmation of the relation of speed to type, finding for the cross
linear motion of the different types a series of velocities closely
paralleling those of Kapteyn and Campbell.

Concerning the marked relation of the luminosities of the stars to their
spectral types, there is a pronounced tendency toward equality of
brightness among stars of a given type; also the brightness diminishes
very markedly with advance in the stage of evolution. There has been
much discussion as to the order of evolution as related to the type of
spectrum, and Russell of Princeton has put forward the hypothesis of
giant stars and dwarf stars, each spectral type having these two
divisions, though not closely related. One class embraces intensely
luminous stars, the other stars only feebly luminous. When a star is in
process of contraction from a diffused gaseous mass, its temperature
rises, according to Lane's law, until that density is reached where the
loss of heat by radiation exceeds the rise in temperature due to
conversion of gravitational energy into heat. Then the star begins to
cool again. So that if the spectrum of a star depends mainly on the
effective temperature of the body, clearly the classification of the
Draper catalogue would group stars together which are nearly alike in
temperature, taking no note as to whether their present temperature is
rising or falling.

Another classification of stars by Lockyer divides them according to
ascending and descending temperatures. Russell's theory would assign
the succession of evolutionary types in the order, M_{1}, K_{1}, G_{1},
F_{1}, A_{1}, B, A_{2}, F_{2}, G_{2}, K_{2}, M_{2}, the subscript 1
referring to the "giants," and 2 to the dwarf stars. In large part the
weight of evidence would appear to favor the order of the Harvard
classification, independently confirmed as it is by studies of stellar
velocities, Galactic distribution, and periods of binary stars both
spectroscopic and visual, where Campbell and Aiken find a marked
increase in length of period with advance in spectral type. At the same
time, a vast amount of evidence is accumulating in support of Russell's
theory. Investigations in progress will doubtless reveal the ground on
which both may be harmonized.

The publication of the new Henry Draper Catalogue of Stellar Spectra is
in progress, a work of vast magnitude. The great catalogue of thirty
years ago embraced the spectra of more than ten thousand stars, and was
a huge work for that day; but the new catalogue utterly dwarfs it, with
a classification much more detailed than in the earlier work, and with
the number of stars increased more than twenty-fold. This work,
projected by the late director of the Harvard Observatory, has been
brought to a conclusion by the energy and enthusiasm of Miss Annie J.
Cannon through six years of close application, aided by many assistants.
The catalogue ranges over the stars of both hemispheres, and is a
monument to masterly organization and completed execution which will be
of the highest importance and usefulness in all future researches on the
bodies of the stellar universe.




CHAPTER XLVIII

STAR DISTANCES


So vast are the distances of the stars that all attempts of the early
astronomers to ascertain them necessarily proved futile. This led many
astronomers after Copernicus to reject his doctrine of the earth's
motion round the sun, so that they clung rather to the Ptolemaic view
that the earth was without motion and was the center about which all the
celestial motions took place. The geometry of stellar distances was
perfectly understood, and many were the attempts made to find the
parallaxes and distances of the stars; but the art of instrument making
had not yet advanced to a stage where astronomers had the mechanisms
that were absolutely necessary to measure very small angles.

About 1835, Bessel undertook the work of determining stellar parallax in
earnest. His instrument was the heliometer, originally designed for
measuring the sun's diameter; but as modified for parallax work it is
the most accurate of all angle-measuring instruments that the
astronomers employ. The star that he selected was 61 Cygni, not a bright
star, of the sixth magnitude only, but its large proper motion suggested
that it might be one of those nearest to us. He measured with the
heliometer, at opposite seasons of the year, the distance of 61 Cygni
from another and very small star in the same field of view, and thus
determined the relative parallax of the two stars. The assumption was
made that the very faint star was very much more distant than the bright
one, and this assumption will usually turn out to be sound. Bessel got
0".35 for his parallax of 61 Cygni, and Struve by applying the same
method to Alpha Lyræ, about the same time, got 0".25 for the parallax of
that star.

These classic researches of Bessel and Struve are the most important in
the history of star distances, because they were the first to prove that
stellar parallax, although minute, could nevertheless be actually
measured. About the same time success was achieved in another quarter,
and Henderson, the British astronomer at the Cape of Good Hope, found a
parallax of nearly a whole second for the bright star Alpha Centauri.

Although the parallaxes of many hundreds of stars have been measured
since, and the parallaxes of other thousands of stars estimated, the
measured parallax of Alpha Centauri, as later investigated by Elkin and
Sir David Gill, and found to be 0".75, is the largest known parallax,
and therefore Alpha Centauri is our nearest neighbor among the stars, so
far as we yet know. This star is a binary system and the light of the
two components together is about the same as that of Capella (Alpha
Aurigæ). But it is never visible from this part of the world, being in
60 degrees of south declination: one might just glimpse it near the
southern horizon from Key West.

How the distances of the stars are found is not difficult to explain,
although the method of doing it involves a good deal of complication,
interesting to the practical astronomer only. Recall the method of
getting the moon's distance from the earth: it was done by measuring
her displacement among the stars as seen from two widely separated
observatories, as near the ends of a diameter of the earth as
convenient. This is the base line, and the angle which a radius of the
earth as seen from the center of the moon fills, or subtends, is the
moon's parallax.

So near is the moon that this angle is almost an entire degree, and
therefore not at all difficult to measure. But if we go to the distance
of even Alpha Centauri, the nearest of the stars, our earth shrinks to
invisibility; so that we must seek a longer base line. Fortunately there
is one, but although its length is 25,000 times the earth's diameter, it
is only just long enough to make the star distances measurable. We found
that the sun's distance from the earth was 93 million miles; the
diameter of the earth's orbit is therefore double that amount. Now
conceive the diameter of the earth replaced by the diameter of the
earth's orbit: by our motion round the sun we are transported from one
extremity of this diameter to the opposite one in six month's time; so
we may measure the displacement of a star from these two extremities,
and half this displacement will be the star's parallax, often called the
annual parallax because a year is consumed in traversing its period. And
it is this very minute angle which Bessel and Struve were the first to
measure with certainty, and which Henderson found to be in the case of
Alpha Centauri the largest yet known.

Evidently the earth by its motion round the sun makes every star
describe, a little parallactic ellipse; the nearer the star is the
larger this ellipse will be, and the farther the star the smaller: if
the star were at an infinite distance, its ellipse would become a
point, that is, if we imagine ourselves occupying the position of the
star, even the vast orbit of the earth, 186 million miles across, would
shrink to invisibility or become a mathematical point.

Measurement of stellar parallax is one of many problems of exceeding
difficulty that confront the practical astronomer. But the actual
research nowadays is greatly simplified by photography, which enables
the astronomer to select times when the air is not only clear, but very
steady for making the exposures. Development and measurement of the
plates can then be done at any time. Pritchard of Oxford, England, was
among the earliest to appreciate the advantages of photography in
parallax work, and Schlesinger, Mitchell, Miller, Slocum and Van Maanen,
with many others in this country, have zealously prosecuted it.

How shall we intelligently express the vast distances at which the stars
are removed from us? Of course we can use miles, and pile up the
millions upon millions by adding on ciphers, but that fails to give much
notion of the star's distance. Let us try with Alpha Centauri: its
parallax of 0".75 means that it is 275,000 times farther from the sun
than the earth is. Multiplying this out, we get 25 trillion miles, that
is, 25 millions of million miles--an inconceivable number, and an
unthinkable distance.

Suppose the entire solar system to shrink so that the orbit of Neptune,
sixty times 93 million miles in diameter, would be a circle the size of
the dot over this letter i. On the same scale the sun itself, although
nearly a million miles in diameter, could not be seen with the most
powerful microscope in existence; and on the same scale also we should
have to have a circle ten feet in diameter, if the solar system were
imagined at its center and Alpha Centauri in its circumference.

So astronomers do not often use the mile as a yardstick of stellar
distance, any more than we state the distance from London to San
Francisco in feet or inches. By convention of astronomers, the average
distance between the centers of sun and earth, or 93 million miles, is
the accepted unit of measure in the solar system. So the adopted unit of
stellar distance is the distance traveled by a wave of light in a year's
time: and this unit is technically called the light-year. This unit of
distance, or stellar yardstick, as we may call it, is nearly 6 millions
of million miles in length. Alpha Centauri, then, is four and one-third
light-years distant, and 61 Cygni seven and one-fifth light-years away.

For convenience in their calculations most astronomers now use a longer
unit called the parsec, first suggested by Turner. Its length is equal
to the distance of a star whose parallax is one second of arc; that is,
one parsec is equal to about three and a quarter light-years. Or the
light-year is equal to 0.31 parsec. Also the parsec is equal to 206,000
astronomical units, or about 19 millions of million miles.

We have, then four distinct methods of stating the distance of a star:
Sirius, for example, has a parallax of 0".38 or its distance is two and
two-thirds parsecs, or eight and a half light-years, or 50 millions of
million miles. It is the angle of parallax which is always found first
by actual measurement and from this the three other estimates of
distance are calculated.

So difficult and delicate is the determination of a stellar distance
that only a few hundred parallaxes have been ascertained in the past
century. The distance of the same star has been many times measured by
different astronomers, with much seeming duplication of effort.
Comprehensive campaigns for determining star parallaxes in large numbers
have been undertaken in a few instances, particularly at the suggestion
of Kapteyn, the eminent astronomer of Groningen, Holland. His catalogue
of star parallaxes is the most complete and accurate yet published, and
is the standard in all statistical investigations of the stars.

That we find relatively large parallaxes for some of the fainter stars,
and almost no measurable parallax for some of the very bright stars is
one of the riddles of the stellar universe. We may instance Arcturus, in
the northern hemisphere and Canopus in the southern; the latter almost
as bright as Sirius. Dr. Elkin and the late Sir David Gill determined
exhaustively the parallax of Canopus, and found it very minute, only
0".03, making its distance in excess of a hundred light-years. The
stupendous brilliancy of this star is apparent if we remember that the
intensity of its light must vary inversely as the square of the
distance; so that if Canopus were to be brought as near us as even 61
Cygni is, it would be a hundredfold brighter than Sirius, the brightest
of all the stars of the firmament.

In researches upon the distribution of the more distant stars, the
method of measuring parallaxes of individual stars fails completely, and
the secular parallax, or parallactic motion of the stars is employed
instead. By parallactic motion is meant the apparent displacement in
consequence of the solar motion which is now known with great accuracy,
and amounts to 19.5 kilometers per second. Even in a single year, then,
the sun's motion is twice the diameter of the earth's orbit, so that in
a hundred or more years, a much longer base line is available than in
the usual type of observations for stellar parallax. If we ascertain the
parallactic motion of a group of stars, then we can find their average
distance. It is found, for example, that the mean parallax of stars of
the sixth magnitude is 0".014. Also the mean distances of stars thrown
into classes according to their spectral type have been investigated by
Boss, Kapteyn, Campbell and others. The complete intermingling of the
two great star streams has been proved, too, by using the magnitude of
the proper motions to measure the average distances of both streams.
These come out essentially the same, so that the streaming cannot be due
to mere chance relation in the line of sight.

Most unexpected and highly important is the discovery that the peculiar
behavior of certain lines in the spectrum leads to a fixed relation
between a star's spectrum and its absolute magnitude, which provides a
new and very effective method of ascertaining stellar distances. By
absolute magnitudes are meant the magnitudes the stars would appear to
have if they were all at the same standard distance from the earth.

Very satisfactory estimates of the distance of exceedingly remote
objects have been made within recent years by this indirect method,
which is especially applicable to spiral nebulæ and globular clusters.
The absolute magnitude of a star is inferred from the relative
intensities of certain lines in its spectrum, so that the observed
apparent magnitude at once enables us to calculate the distance of the
star. Adams and Joy have recently determined the luminosities and
parallaxes of 500 stars by this spectroscopic method. Of these stars 360
have had their parallaxes previously measured; and the average
difference between the spectroscopic and the trigonometric values of the
parallax is only the very small angle 0".0037, a highly satisfactory
verification.

An indirect method, but a very simple one, and of the greatest value
because it provides the key to stellar distances with the least possible
calculation, and we can ascertain also the distances of whole classes of
stars too remote to be ascertained in any other way at present known.

The problem of spectroscopic determinations of luminosity and parallax
has been investigated at Mount Wilson with great thoroughness from all
sides, the separate investigations checking each other. A definitive
scale for the spectroscopic determination of absolute magnitudes has now
been established, and the parallaxes and absolute magnitudes have
already been derived for about 1,800 stars.




CHAPTER XLIX

THE NEAREST STARS


Of especial interest are the few stars that we know are the nearest to
us, and the following table includes all those whose parallax is 0".20
or greater. There are nineteen in all and nearly half of them are binary
systems. The radial motions given are relative to the sun. The
transverse velocities are formed by using the measured parallaxes to
transform proper motions into linear measures. They are given by
Eddington in his "Stellar Movements":

  Column Key
  ====================
  A) Magnitude
  B) Parallax in Seconds of Arc
  C) Proper Motion in Seconds of Arc
  D) Linear Velocity Km. per sec.
  E) Radial Velocity Km. per sec.
  F) Spectral Type
  G) Luminosity (Sun=1)
  H) Star Stream

  ====================================================================
  Star's Name    |  A  |  B   |  C   |  D  |  E  |  F   |   G   |  H
  ---------------+-----+------+------+-----+-----+------+-------+-----
  Groombridge 34 | 8.2 | 0.28 | 2.85 |  48 |  .. | Ma   | 0.010 | I
  Eta Cassiop    | 3.6 | 0.20 | 1.25 |  30 | +10 | F8   | 1.4   | I
  Tau Ceti       | 3.6 | 0.33 | 1.93 |  28 | -16 | K    | 0.50  | II
  Epsilon Erid   | 3.3 | 0.31 | 1.00 |  15 | +16 | K    | 0.79  | II
  CZ 5h 243      | 8.3 | 0.32 | 8.70 | 129 |+242 | G-K  | 0.007 | II
  Sirius         |-1.6 | 0.38 | 1.32 |  16 |  -7 | A    |48.0   | II
  Procyon        | 0.5 | 0.32 | 1.25 |  19 |  -3 | F5   | 9.7   | I ?
  Lal. 21185     | 7.6 | 0.40 | 4.77 |  57 |  .. | Ma   | 0.009 | II
  Lal. 21258     | 8.9 | 0.20 | 4.46 | 106 |  .. | Ma   | 0.011 | I
  OA (N) 11677   | 9.2 | 0.20 | 3.03 |  72 |  .. | ..   | 0.008 | I
  Alpha Centauri | 0.3 | 0.76 | 3.66 |  23 | -22 | G,K5 |{2.0   | I
                 |     |      |      |     |     |      |{0.6   |
  OA (N) 17415   | 9.3 | 0.27 | 1.31 |  23 |  .. | F    | 0.004 | II
  Pos. Med. 2164 | 8.8 | 0.29 | 2.28 |  37 |  .. | K    | 0.006 | I
  Sigma Draco    | 4.8 | 0.20 | 1.84 |  43 | +25 | K    | 0.5   | II
  Alpha Aquilæ   | 0.9 | 0.24 | 0.65 |  13 | -33 | A5   |12.3   | I
  61 Cygni       | 5.6 | 0.31 | 5.25 |  80 | -39 | K5   | 0.10  | I
  Epsilon Indi   | 4.7 | 0.28 | 4.67 |  79 | -62 | K5   | 0.25  | I
  Krüger 60      | 9.2 | 0.26 | 0.92 |  17 |  .. | ..   | 0.005 | II
  Lacaille 9352  | 7.4 | 0.29 | 7.02 | 115 | +12 | Ma   | 0.019 | I
  --------------------------------------------------------------------

These stars are distant less than five parsecs (about 16 light-years)
from the sun, so they make up the closest fringe of the stellar universe
immediately surrounding our system. The large number of binary systems
is quite remarkable. Why some stars are single and others double is not
yet known. By the spectroscopic method the proportion is not so large;
Campbell finding that about one quarter of 1,600 stars examined are
spectroscopic binaries, and Frost two-fifths to a half. The exceptional
number of large velocities is very remarkable; the average transverse
motion of the nineteen stars is fifty kilometers per second, whereas
thirty is about what would have been expected.

As to star streams to which these nearest stars belong, eleven are in
Stream I and eight in Stream II, in close accord with the ratio 3:2
given by the 6,000 stars of Boss's catalogue. "We are not able," says
Eddington, "to detect any significant difference between the
luminosities, spectra, or speeds of the stars constituting the two
streams. The thorough interpenetration of the two star streams is well
illustrated, since we find even in this small volume of space that
members of both streams are mingled together in just about the average
proportion."

    [Illustration: THE RING NEBULA IN _Lyra_. This is the best example
       of the annular and elliptic nebulæ, which are not very abundant.
       (_Photo, Mt. Wilson Solar Observatory._)]

    [Illustration: THE DUMB-BELL NEBULA OF _Vulpecula_. To take the
       photograph required an exposure of five hours. (_Photo, Mt.
       Wilson Solar Observatory._)]




CHAPTER L

ACTUAL DIMENSIONS OF THE STARS


We have seen that the distances of the stars from the solar system are
immense beyond conception, and millions upon millions of them are
probably forever beyond our power of ascertaining by direct measurement
what their distance really is. After we had found the sun's distance and
measured the angle filled by his disk, it was easy to calculate his
actual size. This direct method, however, fails when we try to apply it
to the stars, because their distances are so vast that no star's disk
fills an angle of any appreciable size; and even if we try to get a disk
with the highest magnifying powers of a great telescope our efforts end
only in failure. There is, indeed, no instrumentally appreciable angle
to measure.

How then shall we ascertain the actual dimensions of the vast spheres
which we know the stars actually are, as they exist in the remotest
regions of space? Clearly by indirect methods only, and it must be said
that astronomers have as yet no general method that yields very
satisfactory results for stellar dimensions. The actual magnitude of the
variable system of Algol, Beta Persei, is among the best known of all
the stars, because the spectroscope measures the rate of approach and
recession of Algol when its invisible satellite is in opposite parts of
the orbit; the law of gravitation gives the mass of the star and the
size of its orbit, and so the length of the eclipse gives the actual
size of the dark, eclipsing body. This figures out to be practically the
same size as that of our sun, while Algol's own diameter is rather
larger, exceeding a million miles.

If we try to estimate sizes of stars by their brightness merely, we are
soon astray. Differences of brightness are due to difference of
dimensions, of course, or of light-giving area; but differences of
distance also affect the brightness, inversely as the squares of the
distances, while differences of temperature and constitution affect, in
very marked degree, the intrinsic brilliance of the light-emitting
surface of the star. There are big stars and little stars, stars
relatively near to us and stars exceedingly remote, and stars highly
incandescent as well as others feebly glowing.

We have already shown how the angular diameters subtended by many of the
stars have been estimated, through the relation of surface brightness
and spectral type. Antares and Betelgeuse appear to be the most inviting
for investigation, because their estimated angular diameters are about
one-twentieth of a second of arc. This is the way in which their direct
measurement is being attempted.

As early as 1890, Michelson of Chicago suggested the application of
interference methods to the accurate measurement of very small angles,
such as the diameters of the minor planets, and the satellites of
Jupiter and Saturn, as well as the arc distance between the components
of double stars. Two portions of the object glass are used, as far apart
as possible on the same diameter, and the interference fringes produced
at the focus of the objective are then the subject of observation. These
fringes form a series of equidistant interference bands, and are most
distinct when the light comes from a source subtending an infinitesimal
angle. If the object presents an appreciable angle, the visibility is
less and may even become zero.

Michelson tested this method on the satellites of Jupiter at the Lick
Observatory in 1891, and showed its accuracy and practicability.
Nevertheless, the method has not been taken up by astronomers, until
very recently at the Mount Wilson Observatory, where Anderson has
applied it to the measurement of close double stars. It is found that,
contrary to general expectation, the method gives excellent results,
even if the "seeing" is not the best--2 on a scale of 10, for instance.

To simplify the manipulation of the interferometer, a small plate with
two apertures in it is placed in the converging beam of light coming
from the telescope objective or mirror. The interference fringes formed
in the focal plane are then viewed with an eyepiece of very high power,
many thousand diameters. The resolving power of the interferometer is
found to be somewhat more than double that of a telescope of the same
aperture. By applying the interferometer method to Capella, arc
distances of much less than one-twentieth of a second of arc were
measured. More recently the method has been applied to the great star
Betelgeuse in Orion, whose angular diameter was found to be 0".46,
corresponding to an actual diameter of 260,000,000 miles, if the star's
parallax is as small as it appears to be.




CHAPTER LI

THE VARIABLE STARS


Spectacular as they are to the layman, novæ, or temporary stars, are to
the astronomers simply a class among many thousands of stars which they
call variables, or variable stars. There are a few objects classified as
irregular variables, one of which is very remarkable. We refer to Eta
Argus, an erratic variable in the southern constellation Argo and
surrounded by a well-known nebula. There is a pretty complete record of
this star. Halley in 1677 when observing at Saint Helena recorded Eta
Argus as of the fourth magnitude. During the 18th century, it fluctuated
between the fourth magnitude and the second. Early in the 19th it
rapidly waxed in brightness, fluctuating between the first and second
magnitudes from 1822 to 1836. But two years later its light tripled,
rivaling all the fixed stars except Canopus and Sirius. In 1843 it was
even brighter for a few months, but since then it has declined fairly
steadily, reaching a minimum at magnitude seven and a half in 1886, with
a slight increase in brightness more recently. A period of half a
century has been suggested, but it is very doubtful if Eta Argus has any
regular period of variation.

Another very interesting class of variables is known as the Omicron Ceti
type. Nearly all the time they are very faint, but quite suddenly they
brighten through several magnitudes, and then fade away, more or less
slowly, to their normal condition of faintness. But the extraordinary
thing is that most of these variables go through their fluctuations in
regular periods: from six months to two years in length. The type star,
Omicron Ceti, or Mira, is the oldest known variable, having been
discovered by Fabricius in 1596. Most of the time it is a relatively
faint star of the 12th magnitude; but once in rather less than a year
its brightness runs up to the fourth, third and sometimes even the
second magnitude, where it remains for a week or ten days, and afterward
it recedes more slowly to its usual faintness, the entire rise and
decline in brightness usually requiring about 100 days. The spectrum of
Omicron Ceti contains many very bright lines, and a large proportion of
the variable stars are of this type.

Another class of variables is designated as the Beta Lyræ type. Their
periods are quite regular, but there are two or more maxima and minima
of light in each period, as if the variation were caused by superposed
relations in some way. Their spectra show a complexity of helium and
hydrogen bands. No wholly satisfactory explanation has yet been offered.
Probably they are double stars revolving in very small orbits compared
with their dimensions, their plane of motion passing nearly through the
earth.

But the most interesting of all the variables are those of the Algol
type, their light curves being just the reverse of the Omicron Ceti
type; that is, they are at their maximum brightness most of the time,
and then suffer a partial eclipse for a relatively brief interval. Algol
goes through its variations so frequently that its period is very
accurately known; it is 2d. 20h. 48m. 55.4s. For most of this period
Algol is an easy second magnitude star; then in about four and a half
hours it loses nearly five-sixths of its light, receding to the fourth
magnitude. Here at minimum it remains for fifteen or twenty minutes, and
then in the next three and a half hours it regains its full normal
brilliancy of the second magnitude. During these fluctuations the star's
spectrum undergoes no marked changes. The spectra of all the Algol
variables are of the first or Sirian type.

To explain the variation of the Algol type of variables is easy: a dark,
eclipsing body, somewhat smaller than the primary is supposed to be
traveling round it in an orbit lying nearly edgewise to our line of
sight. The gravitation of this dark companion displaces Algol itself
alternately toward and from the earth, because the two bodies revolve
round their common center of gravity. With the spectroscope this
alternate motion of Algol, now advancing and now receding at the rate of
26 miles per second, has been demonstrated; and the period of this
motion synchronizes exactly with the period of the star's variability.

Russell and Shapley have made extended studies of the eclipsing
binaries, and developed the formulæ by which the investigations of their
orbits are conducted. Heretofore, visual binaries and spectroscopic
binaries afforded the only means of deriving data regarding double
systems, but it is now possible to obtain from the orbits of eclipsing
variables fully as much information relating to binary systems in
general and their bearing on stellar evolution. After an orbit has been
determined from the photometric data of the light curve, the addition
of spectroscopic data often permits the calculation of the masses,
dimensions and densities in terms of the sun. Shapley's original
investigation included the orbits of ninety eclipsing variables, and
with the aid of hypothetical parallaxes, he computed the approximate
position of each system in space. The relation to the Milky Way is
interesting, the condensation into the Galactic plane being very marked;
only thirteen of the ninety systems being found at Galactic latitudes
exceeding 30 degrees.

If we can suppose the variable stars covered with vast areas of spots,
perhaps similar to the spots on the sun, and then combine the variation
of these spot areas with rotation of the star on its axis, there is a
possibility of explanation of many of the observed phenomena, especially
where the range of variation is small. But for the Omicron Ceti type, no
better explanation offers than that afforded by Sir Norman Lockyer's
collision theory. First he assumes that these stars are not condensed
bodies, but still in the condition of meteoric swarms, and the
revolution of lesser swarms around larger aggregations, in elliptic
orbits of greater or less eccentricity, must produce vast multitudes of
collisions; and these collisions, taking place at pretty regular
periods, produce the variable maximum light by raising hosts of meteoric
particles to a state of incandescence simultaneously.

The catalogues of variable stars now contain many thousands of these
objects. They are often designated by the letters R, S, T, and so on,
followed by the genitive form of the name of the constellation wherein
they are found. Most of the recently found variables have a range of
less than one magnitude. They are so distributed as to be most numerous
in a zone inclined about 18 degrees to the celestial equator, and split
in two near where the cleft in the Galaxy is located. Nearly all the
temporary stars are in this duplex region. Bailey of Harvard a quarter
century ago began the investigation of variables in close star clusters,
where they are very abundant, with marked changes of magnitude within
only a few hours.

Many amateur astronomers afford very great assistance to the
professional investigator of variable stars by their cooperation in
observing these interesting bodies, in particular the American
Association of Observers of Variable Stars, organized and directed by
William Tyler Olcott.

For a high degree of accuracy in determining stellar magnitudes the
photo-electric cell is unsurpassed. Stebbins of Urbana has been very
successful in its application and he discovered the secondary minimum of
Algol with the selenium cell. His most recent work was done with a
potassium cell with walls of fused quartz, perfected after many trial
attempts. The stars he has recently investigated are Lambda Tauri,
and Pi Five Orionis. Combining results with those reached by the
spectroscope, the masses of the two component stars of the former are
2.5 and 1.0 that of the sun, and the radii are 4.8 and 3.6 times the
sun's.

Russell of Princeton thinks it probable that similar causes are at work
in all these variables. In the case of the typical Novæ there is
evidence that when the outburst takes place a shell of incandescent gas
is actually ejected by the star at a very high velocity. What may be the
forces that cause such an explosion can only be guessed. Repeated
outbursts have not, in the case of T Pyxidis, destroyed the star,
because it has gone through this process three times in the past thirty
years. Russell inclines to regard it as a standard process occurring
somewhere in the stellar universe probably as often as once a year.

Novæ, then, cannot be due to collisions between two stars, for even if
we suppose the stars to be a thousand millions in number, no two should
collide except at average intervals of many million years. The idea is
gaining ground that the stars are vast storehouses of energy which they
are gradually transforming into heat and radiating into space. "Under
ordinary circumstances, it is probable that the rate of generation of
heat is automatically regulated to balance the loss by radiation. But it
is quite conceivable that some sudden disturbance in the substance of
the star, near the surface, might cause an abrupt liberation of a great
amount of energy, sufficient to heat the surface excessively, and drive
the hot material off into infinite space, in much the form of a shell of
gas, as seems to have been observed in the case of Nova Aquilæ.... With
the rapid advance of our knowledge of the properties of the stars on one
hand, and of the very nuclei of atoms on the other, we may, perhaps
before many years have passed, find ourselves nearer a solution of the
problem."

The Cepheid variables increase very rapidly in brightness from their
least light to their maximum, and then fade out much more slowly, with
certain irregularities or roughnesses of their light-curves when
declining. Their spectral lines also shift in period with their
variations of light. In the case of these variables, whose regular
fluctuation of light cannot be due to eclipse, and is as a rule embraced
within a few days, there is a fluctuation in color also between maximum
and minimum, as if there were a periodic change in the star's physical
condition. Eddington and Shapley advocate the theory of a mechanical
pulsation of the star as most plausible. Knowledge of the internal
conditions of the stars make it possible to predict the period of
pulsation within narrow limits; and for Delta Cephei this theoretical
period is between four and ten days. Its observed period is five and
one-third days, and corresponding agreement is found in all the Cepheids
so far tested.

Shapley of Mount Wilson finds that the Cepheid variables with periods
exceeding a day in length all lie close to the Galactic lane. So greatly
have the studies of these objects progressed that, as before remarked,
when we know the star's period, we can get its absolute magnitude, and
from this the star's distance. On all sides of the sun, the distances of
the Cepheids range up to 4,000 parsecs. So they indicate the existence
of a Galactic system far greater in extent than any previously dealt
with.




CHAPTER LII

THE NOVÆ, OR NEW STARS


New stars, or temporary stars, we have already mentioned in connection
with variables. They are, next to comets, the most dramatic objects in
the heavens. They may be variable stars which, in a brief period,
increase enormously in brightness, and then slowly wane and disappear
entirely, or remain of a very faint stellar magnitude.

In the ancient historical records are found accounts of several such
stars. For instance, in the Chinese annals there is an allusion to such
a stellar outburst in the constellation of Scorpio, B. C. 134. This was
observed also by Hipparchus and, no doubt, it was the immediate
incentive which led to his construction of the first known catalogue of
stars, so that similar happenings might be detected in the future. In
November, 1572, Tycho Brahe observed the most famous of all new stars,
which blazed out in the constellation Cassiopeia. In something over a
year it had completely disappeared.

In 1604-1605 a new star of equal brightness was seen in Ophiuchus by
Kepler; it also faded out to invisibility in 1606. Kepler and Tycho
printed very complete records of these remarkable objects. The
eighteenth century passed without any new stars being seen or recorded.
There was one of the fifth magnitude in 1848, and another of the
seventh magnitude in 1860; and in May, 1866, a star of the second
magnitude suddenly made its appearance in Corona Borealis; and one of
the third magnitude in Cygnus in November, 1876. The latter was fully
observed by Schmidt of Athens and became a faint telescopic star within
a few weeks. It is now of the fifteenth magnitude.

In 1885 astronomers were surprised to find suddenly a new star of the
sixth magnitude very close to the brightest part of the great nebula in
Andromeda; it ran its course in about six months, fading with many
fluctuations in brightness, and no star is now visible in its position
even with the telescope. Stars of this class are known to astronomers as
Novæ, usually with the genitive of the constellation name, as Nova
Andromedæ.

In 1891-1892 Nova Aurigæ made its spectacular appearance and yielded a
distinctly double and complex spectrum for more than a month. Many pairs
of lines indicated a community of origin as to substance, and accurate
measurement showed a large displacement with a relative velocity of more
than 500 miles per second. For each bright hydrogen line displaced
toward the red there was a dark companion line or band about equally
displaced toward the violet much as if the weird light of Nova Aurigæ
originated in a solid globe moving swiftly away from us and plunging
into an irregular nebulous mass as swiftly approaching us. Parallax
observations of Nova Aurigæ made it immensely remote, perhaps within the
Galaxy, and it still exists as a faint nebulous star.

In February, 1901, in the constellation Perseus appeared the most
brilliant nova of recent years. It was first discovered by Dr. Anderson,
an amateur of Glasgow, and at maximum on February 23 it outshone
Capella. There were many unusual fluctuations in its waning brightness.
Its spectrum closely resembled that of Nova Aurigæ, with calcium,
helium, and hydrogen lines. In August, 1901, an enveloping nebula was
discovered, and a month later certain wisps of this nebulosity appeared
to have moved bodily, at a speed seventy-fold greater than ever
previously observed in the stellar universe.

According to Sir Norman Lockyer's meteoritic hypothesis, a vast nebulous
region was invaded, not by one but by many meteor swarms, under
conditions such that the effects of collision varied greatly in
intensity. The most violent of these collisions gave birth to Nova
Persei itself, and the least violent occurred subsequently in other
parts of the disturbed nebula, perhaps immeasurably removed. This
explanation would avoid the necessity of supposing actual motion of
matter through space at velocities heretofore unobserved and
inconceivably high. A recent photograph of Nova Persei, by Ritchey,
reveals a nebulous ring of regular structure surrounding the star. The
great power of the 60-inch has made it possible to photograph even the
spectra of many of the novæ of years ago which are now very faint. After
the lapse of years the characteristic lines of the nebular spectrum
generally vanish, as if the star had passed out of the nebula--a plunge
into which is generally thought to be the cause of the great and sudden
outburst of light.

Many novæ have recently been found in the spiral nebulæ, especially in
the great nebula of Andromeda.




CHAPTER LIII

THE DOUBLE STARS


Examining individual stars of the heavens more in detail, thousands of
them are found to be double; not the stars that appear double to the
naked eye, as Theta Tauri, Mizar, Epsilon Lyræ, and others; but pairs of
stars much closer together, and requiring the power of the telescope to
divide or separate them. Only a very few seconds apart they are or, in
many cases, only the merest fraction of a second of arc. Some of them,
called binaries, are found to be revolving around a common center,
sometimes in only a few years, sometimes in stately periods of hundreds
of years. Many such binary systems are now known, and the number is
constantly increasing. Castor is one, Gamma Virginis another, Sirius
also is one of these binaries, and a most interesting one, having a
period of revolution of about 52 years.

Aitken, of the Lick Observatory, in his work on binary stars, directs
special attention to the correlation between the elements of known
binary orbits and the star's spectral type, and presents a statistical
study of the distribution of 54,000 visual double stars, of which the
spectra of 3919 are known. That the masses of binary systems average
about twice that of the sun's mass has long been known, and this fact
can be employed with confidence in estimates of the probable parallax
of these systems. Aitken applies the test to fourteen visual systems
for which the necessary data are available, and deduces for them a mean
mass of 1.76 times that of the sun. For the spectroscopic binaries the
masses are much greater.

Triple, quadruple and multiple stars are less frequent; but many
exceedingly interesting objects of this class exist. Epsilon Lyræ is
one, a double-double, or four stars as seen with slender telescopic
power, and six or seven stars with larger instruments. Sigma Orionis and
12 Lyncis, also Theta Cancri and Mu Bootis are good examples of triple
stars.




CHAPTER LIV

THE STAR CLUSTERS


From multiple stars the transition is natural to star clusters although
the gap between these types of stellar objects is very broad. The
familiar group of the winter sky known as the Pleiades is a loose
cluster, showing relatively very few stars even in telescopes or on
photographic plates. The "Beehive," or cluster known as Praesepe in
Cancer, and a double group in the sword-handle of Perseus, both just
visible to the naked eye, are excellent examples of star clusters of the
average type. When the moon is absent, they are easily recognized
without a telescope as little patches of nebulous light; but every
increase of optical power adds to their magnificence.

Then we come in regular succession to the truly marvelous globular
clusters, that for instance in Hercules. Messier 13, a recent photograph
of which, taken by Ritchey with the 60-inch reflector on Mount Wilson,
reveals an aggregation of more than 50,000 stars. But the finest
specimens are in the southern hemisphere. Sir John Herschel spent much
time investigating them nearly a century ago at the Cape of Good Hope.
His description of the cluster in the constellation of Centaurus is as
follows: "The noble globular cluster Omega Centauri is beyond all
comparison the richest and largest object of the kind in the heavens.
The stars are literally innumerable, and as their total light when
received by the naked eye affects it hardly more than a star of the
fifth or fourth to fifth magnitude, the minuteness of each star may be
imagined."

Others of these clusters are so remote that the separate stars are not
distinguishable, especially at the center, and their distances are
entirely beyond our present powers of direct measurement, although
methods of estimating them are in process of development. If gravitation
is regnant among the uncounted components of stellar clusters, as
doubtless it is, these stars must be in rapid motion, although our
photographs of measurements have been made too recently for us to detect
even the slightest motion in any of the component stars of a cluster.
The only variations are changes of apparent magnitude, of a type first
detected in a large number of stars in Omega Centauri, by Bailey of
Harvard, who by comparison of photographs of the globular clusters was
the first to find variable stars quite numerous in these objects. Their
unexplained variations of magnitude take place with great rapidity,
often within a few hours.

There are about a hundred of these globular clusters, and the radial
velocities of ten of them have been measured by Slipher and found to
range from a recession of 410 to an approach of 225 kilometers per
second. These excessive velocities are comparable with those found for
the spiral nebulæ. Shapley has estimated the distances of many of these
bodies, which contain a large number of variable stars of the Cepheid
type. By assuming their absolute magnitudes equal to those of similar
Cepheids at known distances, he finds their distance represented by the
inconceivably minute parallax of 0".00012, corresponding to 30,000
light-years. This research also places the globular clusters far
outside and independent of our Galactic system of stars. The
distribution of the globular clusters has also been investigated, and
these interesting objects are found almost exclusively in but one
hemisphere of the sky. Its center lies in the rich star clouds of
Scorpio and Sagittarius. Success in finding the distances of these
objects has made it possible to form a general idea of their
distribution in three-dimensional space.

The numerous variable stars in any one cluster are remarkable for their
uniformity. Accepting variables of this type as a constant standard of
absolute brightness, and assuming that the differences of average
magnitude of the variables in different clusters are entirely due to
differences of distance, the relative distances of many clusters were
ascertained with considerable accuracy. Then it was found that the
average absolute magnitude of the twenty-five brightest stars in a
cluster is also a uniform standard, or about 1.3 magnitudes brighter
than the mean magnitude of the variables. This new standard was employed
in ascertaining the distances of other clusters not containing many
variables.

Shapley further shows that the linear dimensions of the clusters are
nearly uniform, and the proper relative positions in space are charted
for sixty-nine of these objects. We can determine the scale of the
charts, if we know the absolute brightness of our primary standard--the
variable stars; and this is deduced from a knowledge of the distances of
variables of the same type in our immediate stellar system.

The most striking of all the globular clusters, Omega Centauri, comes
out the nearest; nevertheless it is distant 6.5 kiloparsecs. A
kiloparsec is a thousand parsecs, and is the equivalent of 3,256
light-years. At the inconceivable distance of sixty-seven kiloparsecs,
or more than 200,000 light-years, is the most remote of the globular
clusters, known to astronomers as N.G.C. 7006, from its number in the
catalogue which records its position in the sky, the New General
Catalogue of nebulæ by Dreyer of Armagh.

The clusters are widely scattered, and their center of diffusion is
about twenty kiloparsecs on the Galactic plane toward the region of
Scorpio-Sagittarius. Marked symmetry with reference to this plane makes
it evident that the entire system of globular clusters is associated
with the Galaxy itself. But to conceive of this it is necessary to
extend our ideas of the actual dimensions of the Galactic system. Almost
on the circumference of the great system of globular clusters our local
stellar system is found, and it contains probably all the naked-eye
stars, with millions of fainter ones. Its size seems almost diminutive,
only about one kiloparsec in diameter. The relative location of our
local stellar system shows why the globular clusters appear to be
crowded into one hemisphere only.

Shapley suggests that globular clusters can exist only in empty space,
and that when they enter the regions of space tenanted by stars, they
dissolve into the well-known loose clusters and the star clouds of the
Milky Way. Strangely the radial velocities of the clusters already
observed show that most of them are traveling toward this region, and
that some will enter the stellar regions within a period of the order of
a hundred million years.

The actual dimensions of globular clusters are not easy to determine,
because the outer stars are much scattered. To a typical cluster,
Messier 3, Shapley assigns a diameter of 150 parsecs, which makes it
comparable with the size of the stellar cluster to which the sun
belongs. Also on certain likely assumptions, he finds that the diameter
of the great cluster in Hercules, the finest one in our northern sky, is
about 350 parsecs, and its distance no less than 30,000 parsecs; in
other words, the staggering distance that light would require 9,750,000
years to travel over. While these distances can never be verified by
direct measurement, it lends great weight to the three methods of
indirect measurement, or estimation, (1) from the diameter of the image
of the clusters, (2) from the mean magnitude of the twenty-five
brightest stars, and (3) from the mean magnitude of the short period
variables, that they are in excellent agreement.




CHAPTER LV

MOVING CLUSTERS


Recent researches on the proper motions of stars have brought to light
many groups of stars whose individual members have equal and parallel
velocities. Eddington calls these moving clusters. The component stars
are not exceptionally near to each other, and it often happens that
other stars not belonging to the group are actually interspersed among
them. They may be likened to double stars which are permanent neighbors,
with some orbital motion, though exceedingly slow.

The connection is rather one of origin; occurring in the same region of
space, perhaps, from a single nebula. They set out with the same motion,
and have "shared all the accidents of the journey together." Their
equality of motion is intact because any possible deflections by the
gravitative pull of the stellar system is the same for both. Mutual
attraction may tend to keep the stars together, but their community of
motion persists chiefly because no forces tend to interfere with it. In
this way physically connected pairs may be separated by very great
distances.

So with the moving clusters: their component stars may be widely
separate on the celestial sphere, but equality of their motions affords
a clue to their association in groups. The Hyades, a loose cluster in
Taurus, is a group of thirty-nine stars, within an area of about 15
degrees square, which has been pretty fully investigated, especially by
the late Professor Lewis Boss; and no doubt many fainter stars in the
same region will ultimately be found to belong to the same group.

If we draw arrows on a chart representing the amount and direction of
the proper motions of these stars, these arrows must all converge toward
a point. This shows that their motions are parallel in space. It is a
relatively compact group, and the close convergence shows that their
individual velocities must agree within a small fraction of a kilometer
per second. Radial velocity measures of six of the component stars are
in very satisfactory accord, giving 45.6 kilometers per second for the
entire group.

We can get the transverse velocity, and therefrom the distances of the
stars, which are among the best known in the heavens, because the proper
motions are very accurately known. The mean parallax of the group by
this indirect method comes out 0".025, agreeing almost exactly with the
direct determination by photography, 0".023, by Kapteyn, De Sitter, and
others.

Eddington concludes that this Taurus group is a globular cluster with a
slight central condensation. Its entire diameter is about ten parsecs,
and its known motion enables us to trace its past and future history. It
was nearest the sun 800,000 years ago, when it was at about half its
present distance. Boss calculated that in 65 million years, if the
present motion is maintained, this group will have receded so far as to
appear like an ordinary globular cluster 20' in diameter, its stars
ranging from the ninth to the twelfth apparent magnitude. We may infer
that the motion will likely continue undisturbed, because there are
interspersed among the group many stars not belonging to it, and these
have neither scattered its members nor sensibly interfered with the
parallelism of their motion.

Another moving cluster, the similarity of proper motion of whose
component stars was first pointed out by Proctor, is known as the Ursa
Major system, which embraces primarily Beta, Gamma, Delta, Epsilon, and
Zeta Ursæ Majoris, or five of the seven stars that mark the familiar
Dipper. But as many as eight other stars widely scattered are thought to
belong to the same system, including Sirius and Alpha Coronæ Borealis.
The absolute motion amounts to 28.8 kilometers per second, and is
approximately parallel to the Galaxy. Turner has made a model of the
cluster, which has the form of a flat disk.

Among stars of the Orion type of spectrum are several examples of moving
clusters. The Pleiades together with many fainter stars form another
moving cluster; as also do the brighter stars of Orion, together with
the faint cloudlike extensions of the great nebula in Orion, whose
radial velocity agrees with that of the stars in the constellation.
Still another very remarkable moving cluster is in Perseus, first
detected by Eddington, and embracing eighteen stars, the brightest of
which is Alpha Persei.

The further discovery of moving clusters is most important in the future
development of stellar astronomy, because with their aid we can find out
the relative distribution, luminosity, and distance of very remote
stars. So far the stars found associated in groups are of early types of
spectrum; but the Taurus cluster embraces several members equally
advanced in evolution with the sun, and in the more scattered system of
Ursæ Major there are three stars of Type F.

"Some of these systems," Eddington concludes, "would thus appear to have
existed for a time comparable with the lifetime of an average star. They
are wandering through a part of space in which are scattered stars not
belonging to their system--interlopers penetrating right among the
cluster stars. Nevertheless, the equality of motion has not been
seriously disturbed. It is scarcely possible to avoid the conclusion
that the chance attractions of stars passing in the vicinity have no
appreciable effect on stellar motions; and that if the motions change in
course of time (as it appears they must do) this change is due, not to
the passage of individual stars, but to the central attraction of the
whole stellar universe, which is sensibly constant over the volume of
space occupied by a moving cluster."




CHAPTER LVI

THE TWO STAR STREAMS


Consider the ships on the Atlantic voyaging between Europe and America:
at any one time there may be a hundred or more, all bound either east or
west, some moving in interpenetrating groups, individuals frequently
passing each other, but rarely or never colliding. We might say, there
are two great streams of ships, one moving east and the other west.

Now in place of each ship, imagine a hundred ships, and magnify their
distances from each other to the vast distances that the stars are from
each other, and all in motion in two great streams as before. This will
convey some idea of the relatively recent discovery, called by
astronomers "star-streaming."

Early in this century the investigation of moving clusters began to
reveal the fact that the motions of the stars were not at random
throughout the universe, and about 1904 Kapteyn was the first to show
that the stellar motions considered in great groups are very far from
being haphazard, but that the stars tend to travel in two great streams,
or favored directions. This was ascertained by analyzing the proper
motions of stars in the sky, many thousands of them, and correcting all
for the effect which the known motion of the sun would have upon them.
The corrected motion, or part that is left over, is known as the star's
own motion, or _motus peculiaris_.

This important investigation was very greatly facilitated by the general
catalogue of 6,188 stars well distributed over the entire sky, the work
of the late Professor Boss. It was published by the Carnegie Institution
of Washington, and includes all stars down to the sixth magnitude. Boss
was very critical in the matter of stellar positions and proper motions
and his work is the most accurate at present available. Excluding stars
of the Orion type and the known members of moving clusters, Kapteyn's
investigation was based on 5,322 stars, which he divided into seventeen
regions of the sky, each northern region having an antipodal one in the
southern hemisphere.

Mathematical analysis of these regions showed them all in substantial
agreement, with one exception, and enabled Kapteyn to draw the
conclusion that the stars of one stream, called Drift I, move with a
speed of thirty-two kilometers per second, while those of the other,
Drift II, travel with a speed of eighteen kilometers per second. Their
directions are not, like those of east and west bound ships, 180 degrees
from each other, but are inclined at an angle of 100 degrees. Drift I
embraces about three-fifths of the stars, and Drift II the remaining
two-fifths. Quite as remarkable as the drifts themselves is the fact
that the relative motion of the two is very closely parallel to the
plane of the Milky Way.

This epochal research has very great significance in all investigations
of stellar motions, and it has been verified in various ways,
particularly by the Astronomer Royal, Sir Frank Dyson, who limited the
stars under consideration to 1,924 in number, but all having very large
proper motions. In this way the two streams are even more
characteristically marked. But radial velocity determinations afford the
ultimate and most satisfactory test, and Campbell has this investigation
in hand, classifying the stars in their streaming according to the type.

Type A stars are so far found to be confirmatory. Turning to the
question of physical differences between the stars of the two streams,
Eddington inquires into the average magnitude of the stars in both
drifts, and their spectral type. Also whether they are distributed at
the same distance from the sun, and in the same proportion in all parts
of the sky. His conclusion is that there is no important difference in
the magnitudes of the stars constituting the two drifts. Regarding their
spectra, stars of early and late types are found in both streams, with a
somewhat higher proportion of late types among the stars of Drift II
than those of Drift I. Campbell and Moore of the Lick Observatory have
investigated seventy-three planetary nebulæ which exhibit the phenomena
of star-streaming, and have motions which are characteristic of the
stars.

Dealing with the very important question whether the two streams are
actually intermingled in space, Eddington finds them nearly at the same
mean distance and thoroughly intermingled, and there is no possible
hypothesis of Drifts I and II passing one behind the other in the same
line of sight. A third drift, to which all the Orion stars belong, is
under investigation, together with comprehensive analysis of the drifts
according to the spectral type of all the stars included.

The farther research on star-streaming is pushed, the more it becomes
evident that a third stream, called Drift O, is necessary, especially
to include B-type stars. The farther we recede from the sun, the more
this drift is in evidence. At the average distances of B-type stars, the
observed motions are almost completely represented by Drift O alone.
Halm of Cape Town concludes from recent investigations that the
double-drift phenomena (Drifts I and II) is of a distinctly _local_
character, and concerns chiefly the stars in the vicinity of the solar
system; while stars at the greatest distances from the sun belong
preeminently to Drift O.

The 60-inch reflector on Mount Wilson gathers sufficient light so that
the spectra of very faint stars can be photographed, and a discussion of
velocities derived in this manner has shown that Kapteyn's two star
streams extend into space much farther than it was possible to trace
them with the nearer stars. Star-streaming, then, may be a phenomenon of
the widest significance in reference to the entire universe.

As to the fundamental causes for the two opposite and nearly equal star
streams, it is early perhaps to even theorize upon the subject.
Eddington, however, finds a possible explanation in the spiral nebulæ,
which are so numerous as to indicate the certainty of an almost
universal law compelling matter to flow in these forms. Why it does so,
we cannot be said to know; but obviously matter is either flowing into
the nucleus from the branches of the spiral, or it is flowing out from
the nucleus into the branches. Which of the two directions does not
matter, because in either case there would be currents of matter in
opposite directions at the points where the arms merge in the central
aggregation. The currents continue through the center, because the
stars do not interfere with one another's paths. As Eddington concludes:
"There then we have an explanation of the prevalence of motions to and
fro in a particular straight line; it is the line from which the spiral
branches start out. The two star streams and the double-branched spirals
arise from the same cause."




CHAPTER LVII

THE GALAXY OR MILKY WAY


Grandest of all the problems that have occupied the mind of man is the
distribution of the stars throughout space. To the earliest astronomers
who knew nothing about the distances of the stars, it was not much of a
problem because they thought all the fixed stars were attached to a
revolving sphere, and therefore all at essentially the same distance; a
very moderate distance, too. Even Kepler held the idea that the
distances of individual stars from each other are much less than their
distances from our sun.

Thomas Wright, of Durham, England, seems to have been the first to
suggest the modern theory of the structure of the stellar universe,
about the middle of the eighteenth century. His idea was taken up by
Kant who elaborated it more fully. It is founded on the Galaxy, the
basal plane of stellar distribution, just as the ecliptic is the
fundamental circle of reference in the solar system.

What is the Galaxy or Milky Way?

Here is a great poet's view of the most poetic object in all nature:

    A broad and ample road, whose dust is gold,
    And pavement stars, as stars to thee appear
    Seen in the Galaxy, that Milky Way
    Which nightly as a circling zone thou seest
    Powder'd with stars.
                                     _Milton, P. L._ vii, 580.

Were the earth transparent as crystal, so that we could see downward
through it and outward in all directions to the celestial sphere, the
Galaxy or Milky Way would appear as a belt or zone of cloudlike
luminosity extending all the way round the heavens. As the horizon cuts
the celestial sphere in two, we see at anyone time only one-half of the
Milky Way, spanning the dome of the sky as a cloudlike arch.

As the general plane of the Galaxy makes a large angle with our equator,
the Milky Way is continually changing its angle with the horizon, so
that it rises at different elevations. One-half of the Milky Way will
always be below our horizon, and a small region of it lies so near the
south pole of the heavens that it can never be seen from medium northern
latitudes.

Galileo was the first to explain the fundamental mystery of this belt,
when he turned his telescope upon it and found that it was not a
continuous sheet of faint light, as it seemed to be, but was made up of
countless numbers of stars, individually too faint to be visible to the
naked eye, but whose vast number, taken in the aggregate, gave the
well-known effect which we see in the sky. In some regions, as Perseus,
the stars are more numerous than in others, and they are gathered in
close clusters. The larger the telescope we employ, the greater the
number of stars that are seen as we approach the Galaxy on either side;
and the farther we recede from the Galaxy and approach either of its
poles fewer and fewer stars are found. Indeed, if all the stars visible
in a 12-inch telescope could be conceived as blotted out, nearly all the
stars that are left would be found in the Galaxy itself.

The naked eye readily notes the variations in breadth and brightness of
the galactic zone. Nearly a third of it, from Scorpio to Cygnus, is
split into two divisions nearly parallel. In many regions its light is
interrupted, especially in Centaurus, where a dark starless region
exists, known as the "coal sack." Sir John Herschel, who followed up the
stellar researches of his father, Sir William, in great detail, places
the north pole of the Galactic plane in declination 37 degrees N., and
right ascension 12 h. 47 m. This makes the plane of the Milky Way lie at
an angle of about 60 degrees with the ecliptic, which it intersects not
far from the solstices.

Now Kant, in view of the two great facts about the Galaxy known in his
time, (1) that it wholly encircles the heavens, and (2) that it is
composed of countless stars too faint to be individually visible to the
naked eye, drew the safe conclusions that the system of the stars must
extend much farther in the direction of the Milky Way than in other
directions.

This theory of Kant was next investigated from an observational
standpoint by Sir William Herschel, the ultimate goal of whose
researches was always a knowledge of the construction of the heavens.
The present conclusion is that we may regard the stellar bodies of the
sidereal universe as scattered, without much regard to uniformity,
throughout a vast space having in general the shape of a thick watch,
its thickness being perhaps one-tenth its diameter. On both sides of
this disk of stars, and clustered about the poles of the sidereal system
are the regions occupied by vast numbers of nebulæ. The entire visible
universe, then, would be spheroidal in general shape. The plane of the
Milky Way passes through the middle of this aggregation of stars and
nebulæ, and the solar system is near the center of the Milky Way.
Throughout the watch-form space the stars are clustered irregularly, in
varied and sometimes fantastic forms, but without approach to order or
system. If we except some of the star groups and star clusters and
consider only the naked-eye stars, we find them scattered with fair
approach to uniformity.

    [Illustration: STAR CLOUDS AND BLACK HOLES IN SAGITTARIUS. The dark
       rifts and lanes resemble those in the nearby Milky Way. (_Photo,
       Yerkes Observatory._)]

    [Illustration: THE GREAT NEBULA OF ANDROMEDA, LARGEST (APPARENTLY)
       OF ALL THE SPIRAL NEBULÆ. This nebula can be seen very faintly
       with the naked eye, but no telescope has yet resolved it into
       separate stars. (_Photo, Yerkes Observatory._)]

The watch-shaped disk is not to be understood as representing the actual
form of the stellar system, but only in general the limits within which
it is for the most part contained.

A vigorous attack on the problem of the evolution and structure of the
stellar universe as a whole is now being conducted by cooperation of
many observatories in both hemispheres. It is known as the Kapteyn "Plan
of Selected Areas," embracing 206 regions which are distributed
regularly over the entire sky. Besides this a special plan includes
forty-six additional regions, either very rich or extremely poor in
stars, or to which other interest attaches.

Of all investigators Kapteyn has gone into the question of our precise
location in the Milky Way most thoroughly, concluding that the solar
system lies, not at the center in the exact plane, but somewhat to the
north of the Galaxy. Discussing the Sirian stars he finds that if stars
of equal brightness are compared, the Sirians average nearly three times
more distance from the sun than those of the solar type. So, probably,
the Sirians far exceed the Solars in intrinsic brightness. Farther,
Kapteyn concludes that the Galaxy has no connection with our solar
system, and is composed of a vast encircling annulus or ring of stars,
far exceeding in number the stars of the great central solar cluster,
and everywhere exceedingly remote from these stars, as well as differing
from them in physical type and constitution. So it would be mainly the
mere element of distance that makes them appear so faint and crowded
thickly together into that gauzy girdle which we call the Galaxy.

The Milky Way reveals irregularities of stellar density and star
clustering on a large scale, with deep rifts between great clouds of
stars. Modern photographs, particularly those of Barnard in Sagittarius,
make this very apparent. Within the Milky Way, nearly in its plane and
almost central, is what Eddington terms the inner stellar system, near
the center of which is the sun. Surrounding it and near its plane are
the masses of star clouds which make up the Milky Way. Whether these
star clouds are isolated from the inner system or continuous with it, is
not yet ascertained.

The vast masses of the Milky Way stars are very faint, and we know
nothing yet as to their proper motions, their radial motions, or their
spectra. Probably a few stars as bright as the sixth magnitude are
actually located in the midst of the Milky Way clusters, the fainter
ninth magnitude stars certainly begin the Milky Way proper, while the
stars of the twelfth or thirteenth magnitude carry us into the very
depths of the Galaxy.

It is now pretty generally believed that many of the dark regions of the
Milky Way are due not to actual absence of stars so much as to the
absorption of light by intervening tracts of nebulous matter on the
hither side of the Galactic aggregations and, probably in fact, within
the oblate inner stellar system itself. Easton has made many hundred
counts of stars in galactic regions of Cygnus and Aquila where the range
of intensity of the light is very marked; in fact, the star density of
the bright patches of the Galaxy is so far in excess of the density
adjacent and just outside the Milky Way, that the conclusion is
inevitable that this excess is due to the star clouds.

Of the distance of the Milky Way we have very little knowledge. It is
certainly not less than 1,000 parsecs, and more likely 5,000 parsecs, a
distance over which light would travel in about 16,000 years. Quite
certainly all parts of the Galaxy are not at the same distance, and
probably there are branches in some regions that lie behind one another.
While the general regions of the nebulæ are remote from the Galactic
plane, the large irregular nebulæ, as the Trifid, the Keyhole, and the
Omega nebulæ, are found chiefly in the Milky Way.

In addition to the irregular nebulæ many types of stellar objects appear
to be strongly condensed toward the Milky Way, but this may be due to
the inner stellar system, rather than a real relation to Galactic
formations. Quite different are the Magellanic clouds, which contain
many gaseous nebulæ and are unique objects of the sky, having no
resemblance to the true spiral nebulæ which, as a rule, avoid the
Galactic regions. Worthy of note also is the theory of Easton that the
Milky Way has itself the form of a double-branched spiral, which
explains the visible features quite well, but is incapable of either
disproof or verification. The central nucleus he locates in the rich
Galactic region of Cygnus, with the sun well outside the nucleus itself.
By combining the available photographs of the Galaxy, he has produced a
chart which indicates in a general way how the stellar aggregations
might all be arrayed so as to give the effect of the Galaxy as we see
it.

Shapley, at Mount Wilson, has studied the structure of the Galactic
system, in which he has been aided by Mrs. Shapley. An interesting part
of this work relates to the distribution of the spiral nebulæ, and to
certain properties of their systematic recessional motion, suggesting
that the entire Galactic system may be rapidly moving through space.
Apparently the spiral nebulæ are not distant stellar organizations or
"island universes," but truly nebular structures of vast volume which in
general are actively repelled from stellar systems. A tentative
cosmogonic hypothesis has been formulated to account for the motions,
distribution, and observed structure of clusters and spiral nebulæ.

An additional great problem of the Galaxy is a purely dynamical one.
Doubtless it is in some sort of equilibrium, according to Eddington,
that is to say, the individual stars do not oscillate to and fro across
the stellar system in a period of 300 million years, but remain
concentrated in clusters as at present. Poincaré has considered the
entire Milky Way as in stately rotation, and on the assumption that the
total mass of the inner stellar system is 1,000,000,000 times the sun's
mass, and that the distance of the Milky Way is 2,000 parsecs, the
angular velocity for equilibrium comes out 0".5 per century. That is to
say, a complete revolution would take place in about 250 million years.




CHAPTER LVIII

STAR CLOUDS AND NEBULÆ


From star clusters to nebulæ, only a century ago, the transition was
thought to be easy and immediate. Accuracy in determining the distances
of stars was just beginning to be reached, the clusters were obviously
of all degrees of closeness following to the verge of irresolvability,
and it was but natural to jump to the conclusion that the mystery of the
nebulæ consisted in nothing but their vaster distance than that of
clusters, and it was believed that all nebulæ would prove resolvable
into stars whenever telescopes of sufficiently great power could be
constructed.

But the development of the spectroscope soon showed the error of this
hypothesis, by revealing bright lines in the nebular spectra showing
that many nebulæ emit light that comes from glowing incandescent gas,
not from an infinitude of small stars.

In pre-telescope days nothing was known about the nebulæ. The great
nebula in Andromeda, and possibly the great nebula in Orion, are alone
visible to the naked eye, but as thus seen they are the merest wisps of
light, the same as the larger clusters are. Galileo, Huygens and other
early users of the telescope made observations of nebulæ, but long-focus
telescopes were not well adapted to this work. Simon Mayer has left us
the first drawing of a nebula, the Orion nebula as he saw it in 1612.
The vast light-gathering power of the reflectors built by Sir William
Herschel first afforded glimpses of the structure of the nebulæ, and if
his drawings are critically compared with modern ones, no case of motion
with reference to the stars or of change in the filaments of the nebulæ
themselves has been satisfactorily made out.

Only very recently has the distance of a nebula been determined, and the
few that have been measured seem to indicate that the nebulæ are at
distances comparable with the stars. Of all celestial objects the nebulæ
fill the greatest angles, so that we are forced to conclude, with regard
to the actual size of the greater nebulæ as they exist in space, that
they far surpass all other objects in bulk.

Photography invaded the realm of the nebulæ in 1880, when Dr. Henry
Draper secured the first photograph of the nebula of Orion.
Theoretically photography ought to help greatly in the study of the
nebulæ, and enable us in the lapse of centuries to ascertain the exact
nature of the changes which must be going on. The differences of
photographic processes, of plates, of exposure and development produce
in the finished photograph vastly greater differences than any actual
changes that might be going on, so that we must rely rather on optical
drawings made with the telescope, or on drawings made by expert artists
from photographs with many lengths of exposure on the same object.

The great work on nebulæ and star clusters recently concluded by
Bigourdan of the Paris Observatory and published in five volumes
received the award of the gold medal of the Royal Astronomical Society.
While D'Arrest measured about 2,000 nebulæ, and Sir John Herschel about
double that number in both hemispheres, Bigourdan has measured about
7,000. His work forms an invaluable lexicon of information concerning
the nebulæ.

Classification of the nebulæ is not very satisfactory, if made by their
shapes alone. There are perhaps fifteen thousand nebulæ in all that have
been catalogued, described, and photographed. Dreyer's new general
catalogue (N.G.C.) is the best and most useful. Many of the nebulæ,
especially the large ones, can only be classified as irregular nebulæ.
The Orion nebula is the principal one of this class, revealing an
enormous amount of complicated detail, with exceptional brilliancy of
many regions and filaments. An extraordinary multiple star, Theta
Orionis, occupies a very prominent position in the nebula, and
photographs by Pickering have brought to light curved filaments, very
faint and optically invisible, in the outlying regions which give the
Orion nebula in part a spiral character. But the delicate optical wisps
of this nebula are well seen, even in very small telescopes. Its
spectrum yields hydrogen, helium and nitrogen. The Orion nebula is
receding from the earth about eleven miles in every second. Keeler and
Campbell have shown that nearly every line of the nebular spectrum is a
counterpart of a prominent dark line in the spectrum of the brighter
stars of the constellation of Orion. A recent investigator of the
distribution of luminosity in the great nebula of Orion finds that
radiations from nebulium are confined chiefly to the Huygenian region of
the nebula and its immediate neighborhood.

Photography has revealed another extraordinary nebula or group of nebulæ
surrounding the stars in the Pleiades, which the deft manipulation of
Barnard has brought to light. All the stars and the nebula are so
interrelated that they are obviously bound together physically, as the
common proper motion of the stars also appears to show. Also in the
constellation Cygnus, Barnard has discovered very extensive nebulosities
of a delicate filmy cloudlike nature which are wholly invisible with
telescopes, but very obvious on highly sensitive plates with long
exposures.

Another class of these objects are the annular and elliptic nebulæ which
are not very abundant. The southern constellation Grus, the crane,
contains a fine one, but by far the best example is in the constellation
Lyra. It is a nearly perfect ring, elliptic in figure, exceedingly faint
in small telescopes; but large instruments reveal many stars within the
annulus, one near the center which, although very faint to the eye, is
always an easy object on the photographic plate, because it is rich in
blue and violet rays. The parallax of the ring nebula in Lyra comes out
only one-sixth of that of the planetary nebulæ, and the least greatest
diameters of this huge continuous ring are 250 and 330 times the orbit
of Neptune.

Planetary nebulæ and nebulous stars are yet another class of nebulæ, for
the most part faint and small, resembling in some measure a planetary
disk or a star with nebulous outline. Practically all are gaseous in
composition, and have large radial velocities. Probably they are located
within our own stellar system. The parallaxes of several of them have
been measured by Van Maanen: one of the very small angle 0".023, which
enables us to calculate the diameter of this faint but interesting
object as equal to nineteen times the orbit of Neptune.




CHAPTER LIX

THE SPIRAL NEBULÆ


Last and most important of all are the spiral nebulæ. The finest example
is in the constellation Canes Venatici, and its spiral configuration was
first noted by Lord Rosse, an epoch-making discovery. The convolutions
of its spiral are filled with numerous starlike condensations,
themselves engulfed in nebulosity. Photography possesses a vast
advantage over the eye in revealing the marvelous character of this
object, an inconceivably vast celestial whirlpool. Naturally the central
regions of the whorl would revolve most swiftly, but no comparison of
drawings and photographs, separated by intervals of many years, has yet
revealed even a trace of any such motion.

The number of large spiral nebulæ is not very great; the largest of all
is the great nebula of Andromeda, whose length stretches over an arc of
seven times the breadth of the moon, and its width about half as great.
This nebula is a naked-eye object near Eta Andromedæ, and it is often
mistaken for a comet. Optically it was always a puzzle, but photographs
by Roberts of England first revealed the true spiral, with ringlike
formations partially distinct, and knots of condensing nebulosity as of
companion stars in the making. While its spectrum shows the nongaseous
constitution of this nebula, no telescope has yet resolved it into
component stars.

Systematic search for spiral nebulæ by Keeler, and later continued by
Perrine, at the Lick Observatory, with the 36-inch Crossley reflector,
disclosed the existence of vast numbers of these objects, in fact many
hundreds of thousands by estimation; so that, next to the stars, the
spiral nebulæ are by far the most abundant of all objects in the sky.
They present every phase according to the angle of their plane with the
line of sight, and the convolutions of the open ones are very perfectly
marked. Many are filled with stars in all degrees of condensation, and
the appearance is strongly as if stars are here caught in every step of
the process of making.

The vast multitude of the spiral nebulæ indicates clearly their
importance in the theory of the cosmogony, or science of the development
of the material universe. Curtis of the Lick Observatory has lately
extended the estimated number of these objects to 700,000. He has also
photographed with the Crossley reflector many nebulæ with lanes or dark
streaks crossing them longitudinally through or near the center. These
remarkable streaks appear as if due to opaque matter between us and the
luminous matter of the nebula beyond. Perhaps a dark ring of absorptive
or occulting matter encircles the nebula in nearly the same plane with
the luminous whorls. Duncan has employed the 60-inch Mount Wilson
reflector in photographing bright nebulæ and star clusters in the very
interesting regions of Sagittarius. One of these shows unmistakable dark
rifts or lanes in all parts of the nebula, resembling the dark regions
of the neighboring Milky Way.

Pease of Mount Wilson has recently employed the 60-inch and the 100-inch
reflectors of the Mount Wilson Observatory to good advantage in
photographing several hundred of the fainter nebulæ. Many of these are
spirals, and others present very intricate and irregular forms. A search
was made for additional spirals among the smaller nebulæ along the
Galaxy, but without success. Several of the supposedly variable nebulæ
are found to be unchanging. Many nights in each month when the moon is
absent are devoted to a systematic survey of the smaller nebulæ and
their spectra by photography. The visible spiral figure of all these
objects is a double-branched curve, its two arms joining on the nucleus
in opposing points, and coiling round in the same geometrical direction.
The spiral nebulæ, as to their distribution, are remote from the Galaxy,
and the north Galactic polar region contains a greater aggregation than
the south. The distances of the spiral nebulæ are exceedingly great.
They lie far beyond the planetary and irregular gaseous nebulæ, like
that of Orion, which are closely related to the stars forming part of
our own system. Possibly the spiral nebulæ are exterior or separate
"island universes." If so, they must be inconceivably vast in size, and
would develop, not into solar systems, but into stellar clusters. The
enormous radial velocities of the spiral nebulæ, averaging 300 to 400
kilometers per second, or twenty-fold that of the stars, tend to sustain
the view that they may be "island universes," each comparable in extent
with the universe of stars to which our sun belongs.

Recent spectroscopic observations of the nebulæ applying the principle
of Doppler have revealed high velocities of rotation. Slipher of the
Lowell Observatory made the first discovery of this sort and Van Maanen
of Mount Wilson has detected in the great Ursa Major spiral, No. 101 in
Messier's catalogue, a speed of rotation at five minutes of arc from
the center that would correspond to a complete period in 85,000 years.
As was to be expected, the nebula does not rotate as a rigid body, but
the nearer the center the greater the angular velocity, and Van Maanen
finds evidence of motion along the arms and away from the center.

These great velocities appear to belong to the spiral nebulæ as a class,
and not to other nebulæ. Thirteen nebulæ investigated by Keeler are as a
whole almost at rest relatively to our system, as are the large
irregular objects in Orion, and the Trifid nebula. This would seem to
indicate that the spiral nebulæ form systems outside our own and
independent of it.

Quite different from the spirals in their distribution through space are
the planetary nebulæ. The spirals follow the early general law of nebulæ
arrangement, that is, they are concentrated toward the poles of the
Galaxy; but the planetary nebulæ, on the other hand, are very few near
the poles and show a marked frequency toward the Galactic plane.
Campbell and Moore have found spectroscopic evidence of internal
rotatory motion in a large proportion of the planetary nebulæ.

The distribution of the nebulæ throughout space, like that of the stars,
is still under critical investigation, but the location of vast numbers
of the more compact nebulæ on the celestial sphere is very
extraordinary. The Milky Way appears to be the determining plane in both
cases; the nearer we approach it the more numerous the stars become,
whereas this is the general region of fewest nebulæ and they increase in
number outward in both directions from the Galaxy, and toward both poles
of the Galactic circle. Obviously this relation, or contra-relation of
stars and nebulæ on such a vast scale is not accidental, and it also
must be duly accounted for in the true theory of the cosmogony. The
nebulæ which are found principally in and near the Milky Way are the
large irregular nebulæ, and vast nebulous backgrounds, like those
photographed by Barnard in Scorpio, Taurus and elsewhere, as well as the
Keyhole, Omega, and Trifid nebulæ. Allied to these backgrounds are
doubtless some of the dark Galactic spaces, radiating little or no
intrinsic light, and absorbing the light of the fainter stars beyond
them. A peculiar veiled or tinted appearance has been remarked in some
cases visually, and examination of the photographs strongly confirms the
existence of absorbing nebulosity.

The spiral nebulæ are so abundant, and so much attention is now being
given to them, both by observers and mathematicians, that their precise
relation to the stellar systems must soon be known; that is, whether
they are comparatively small objects belonging to the stellar system, or
independent systems on the borders of the stellar system, or as seems
more likely, vast and exceedingly remote galaxies comparable with that
of the Milky Way itself. Our knowledge of the motions of the spirals,
both radial and angular, is increasing rapidly, and must soon permit
accurate general conclusions to be drawn.




CHAPTER LX

COSMOGONY


Down to the middle of the last century and later, it was commonly
believed that in the beginning the cosmos came into being by divine fiat
substantially as it is. Previously the earth had been "without form and
void," as in the Scripture. Had it not been for the growth and gradual
acceptance of the doctrine of evolution, and its reactionary effect upon
human thought, it is conceivable that the early view might have
persisted to the present day; but now it is universally held that
everything in the heavens above and the earth beneath is subject more or
less to secular change, and is the result of an orderly development
throughout indefinite past ages, a progressive evolution which will
continue through indefinite aeons of the future.

In the writings of the Greek philosophers, and down through the Middle
Ages we find the idea of an original "chaos" prevailing, with no
indication whatever of the modern view of the process by which the
cosmos came to be what they saw it and as it is to-day. If we go still
farther back, there is no glimmer of any ideas that will bear
investigation by scientific method, however interesting they may be as
purely philosophical conceptions. Many ancient philosophers, among them
Anaxagoras, Democritus, and Anaximenes, regarded the earth as the
product of diffused matter in a state of the original chaos having
fallen together haphazard, and they even presumed to predict its future
career and ultimate destiny.

In Anaximander and Anaximenes alone do we find any conception of
possible progress; their thought was that as the world had taken time to
become what it is, so in time it would pass, and as the entire universe
had undergone alternate renewal and destruction in the past, that would
be its history in the future. Aristotle, Ptolemy, and others appear to
have held the curious notion that although everything terrestrial is
evanescent, nevertheless the cosmos beyond the orbit of the moon is
imperishable and eternal.

By tracing the history of the intellectual development of Europe we may
find why it was that scientific speculation on the cosmogony was delayed
until the 18th century, and then undertaken quite independently by three
philosophers in three different countries. Swedenborg, the theologian,
set down in due form many of the principles that underlie the modern
nebular hypothesis. Thomas Wright of Durham whose early theory of the
arrangement of stars in the Galaxy we have already mentioned, speculated
also on the origin and development of the universe, and his writings
were known to Kant, who is now regarded as the author of the modern
nebular hypothesis. This presents a definite mechanical explanation of
the development and formation of the heavenly bodies, and in particular
those composing the solar system.

Kant was illustrious as a metaphysician, but he was a great physicist or
natural philosopher as well, and he set down his ideas regarding the
cosmogony with precision. Learned in the philosophy of the ancients, he
did not follow their speculative conceptions, but merely assumed that
all the materials from which the bodies of the solar system have been
fashioned were resolved into their original elements at the beginning,
and filled all that part of space in which they now move. True, this is
pretty near the chaos of the Greeks, but Kant knew of the operation of
the Newtonian law of gravitation, which the Greeks did not.

As a natural result of gravitative processes, Kant inferred that the
denser portions of the original mass would draw upon themselves the less
dense portions, whirling motions would be everywhere set up, and the
process would continue until many spherical bodies, each with a gaseous
exterior in process of condensation, had taken the place of the original
elements which filled space. In this manner Kant would explain the
sameness in direction of motion, both orbital and axial, of all the
planets and satellites of our system. But many philosophers are of the
opinion that Kant's hypothesis would result, not in the formation of
such a collection of bodies as the solar system is, but rather in a
single central sun formed by common gravitation toward a single center.

From quite another viewpoint the work of the elder Herschel is important
here. No one knew the nebulæ from actual observation better than he did;
but, while his ideas about their composition were wrong, he nevertheless
conceived of them as gradually condensing into stars or clusters of
stars. And it was this speculative aspect of the nebulæ, not as a
possible means of accounting for the birth and development of the solar
system, which constitutes Herschel's chief contribution to the nebular
hypothesis. Classifying the nebulæ which he had carefully studied with
his great telescopes, it seemed obvious to him that they were actually
in all the different stages of condensation, and subsequent research has
strongly tended to substantiate the Herschelian view.

Then came Laplace, who took up the great hypothesis where Kant and
Herschel had left it, added new and important conceptions in the light
of his mature labors as mathematician and astronomer, and put the theory
in definitive form, such that it has ever since been known under the
name of Laplacian nebular hypothesis. For reasons like those that
prevailed with Kant, he began the evolution of the solar system with the
sun already formed as the center, but surrounded by a vast incandescent
atmosphere that filled all the space which the sun's family of planets
now occupy. This entire mass, sun, atmosphere, and all, he conceived to
have a stately rotation about its axis. With rotation of the mass and
slow reduction of temperature in its outer regions, there would be
contraction toward the solar center, and an increase in velocity of
rotation until the whole mass had been much reduced in diameter at its
poles and proportionately expanded at its equator.

When the centrifugal force of the outer equatorial masses finally became
equal to the gravitational forces of the central mass, then these
conjoined outer portions would be left behind as a ring, still revolving
at the velocity it had acquired when detached. The revolution of the
entire inner mass goes on, its velocity accelerating until a similar
equilibration of forces is again reached, when a second rotating ring
is left behind. Laplace conceived the process as repeated until as many
rings had been detached as there are individual planets, all central
about the sun, or nearly so.

In all, then, we should have nine gaseous rings; the outer ones
preceding the inner in formation, but not all existing as rings at the
same time. Radiation from the ring on all sides would lead to rapid
contraction of its mass, so that many nuclei of condensation would form,
of various sizes, all revolving round the central sun in practically the
same period. Laplace conceived the evolution of the ring to proceed
still farther till the largest aggregation in it had drawn to itself all
the other separate nuclei in the ring.

This, then, was the planet in embryo, in effect a diminutive sun, a
secondary incandescent mass endowed with axial rotation in the same
direction as the parent nebula. With reduction of temperature by
radiation, polar contraction and equatorial expansion go on, and
planetary rings are detached from this secondary mass in exactly the
same way as from the original sun nebula. And these planetary rings are,
in the Laplacian hypothesis, the embryo moons or planetary satellites,
all revolving round their several planets in the same direction that the
planets revolve about the sun.

In the case of one of the planetary rings, its formation was so nearly
homogeneous throughout that no aggregation into a single satellite was
possible; all portions of the ring being of equal density, there was no
denser region to attract the less dense regions, and in this manner the
rings of Saturn were formed, in lieu of condensation into a separate
satellite. Similarly in the case of the primal solar ring that was
detached next after the Jovian ring; there was such a nice balancing of
masses and densities that, instead of a single major planet, we have the
well-known asteroidal ring, composed of innumerable discrete minor
planets.

This, then, in bare outline, is the Laplacian nebular hypothesis, and it
accounted very well for the solar system as known in his day; the fairly
regular progression of planetary distances; their orbits round the sun
all nearly circular and approximately in a single plane; the planetary
and satellite revolutions in orbit all in the same direction; the axial
rotations of planets in the same direction as their orbital revolutions;
and the plane of orbital revolution of the satellites practically
coinciding with the plane of the planet's axial rotation. But the
principle of conservation of energy was, of course, unknown to Laplace,
nor had the mechanical equivalence of heat with other forms of energy
been established in his day.

In 1870, Lane of Washington first demonstrated the remarkable law that a
gaseous sphere, in process of losing heat by radiation and contraction
because of its own gravity, actually grows hotter instead of cooler, as
long as it continues to be gaseous, and not liquid or solid. So there is
no need of postulating with Laplace an excessively high temperature of
the original nebula. The chief objection to Laplace's hypothesis by
modern theorists is that the detachment of rings, though possible, would
likely be a rare occurrence; protuberances or lumps on the equatorial
exterior of a swiftly revolving mass would be more likely, and it is
much easier to see how such masses would ultimately become planets than
it is to follow the disruption of a possible ring and the necessary
steps of the process by which it would condense into a final planet. The
continued progress of research in many departments of astronomy has had
important bearing on the nebular hypothesis, and we may rest assured
that this hypothesis in somewhat modified form can hardly fail of
ultimate acceptance, though not in every essential as its great
originator left it.

Lord Rosse's discovery of spiral nebulæ, followed up by Keeler's
photographic search for these bodies, revealing their actual existence
in the heavens by the hundreds of thousands, has led to another
criticism of the Laplacian theory. Could Laplace have known of the
existence of these objects in such vast numbers, his hypothesis would no
doubt have been suitably modified to account for their formation and
development. It is generally considered that the ring of Saturn
suggested to Laplace the ring feature in his scheme of origin of planets
and satellites; so far as we know, the Saturnian ring is unique, the
only object of its kind in the heavens. Whereas, next to the star
itself, the spiral nebula is the type object which occurs most
frequently. A theory, therefore, which will satisfactorily account for
the origin and development of spiral nebulæ must command recognition as
of great importance in the cosmogony.

Such a theory has been set forth by Chamberlin and Moulton in their
planetesimal hypothesis, according to which the genesis of spiral nebulæ
happens when two giant suns approach each other so closely that
tide-producing effects take place on a vast scale. These suns need not
be luminous; they may perhaps belong to the class of dark or
extinguished suns. The evidences of the existence of such in vast
numbers throughout the universe is thought to be well established.

Now, on close approach, what happens? There will be huge tides, and the
nearer the bodies come to each other, the vaster the scale on which
tides will be formed. If the bodies are liquid or gaseous, they will be
distorted by the force of gravitation, and the figure of both bodies
will become ellipsoidal; and at last under greater stress, the
restraining shell of both bodies will burst asunder on opposite sides in
streams of matter from the interior. In this manner the arms of the
spiral are formed.

As Chamberlin puts it: "If, with these potent forces thus nearly
balanced, the sun closely approaches another sun, or body of like
magnitude ... the gravity which restrains this enormous elastic power
will be reduced along the line of mutual attraction. At the same time
the pressure transverse to this line of relief will be increased. Such
localized relief and intensified pressure must bring into action
corresponding portions of the sun's elastic potency, resulting in
protuberances of corresponding mass and high velocity."

Only a fraction of one per cent of the sun's mass ejected in this
fashion would be sufficient to generate the entire planetary system.
Nuclei or knots in the arms of the spiral gradually grew by accretion,
the four interior knots forming Mercury, Venus, the Earth, and Mars. The
earth knot was a double one, which developed into the earth-moon system.
The absence of a dominating nucleus beyond Mars accounts for the zone of
the asteroids remaining in some sense in the original planetesimal
condition. The vaster nuclei beyond Mars gradually condensed into
Jupiter, Saturn, Uranus, and Neptune; and lesser nuclei related to the
larger ones form the systems of moons or satellites.

The orbits of the planetesimals and the planetary and satellite nuclei
would be very eccentric, forming a confusion of ellipses with frequently
crossing paths. Collisions would occur, and the nuclei would inevitably
grow by accretion. Each planet, then, would clear up the planetesimals
of its zone; and Moulton shows that this process would give rise to
axial revolution of the planet in the same direction as its orbital
revolution. The eccentricities would finally disappear, and the entire
mass would revolve in a nearly circular orbit.

Rotation twists the streams into the spiral form, and the huge amounts
of wreckage from the near-collision are thrown into eddies. The
fragments or particles (planetesimals) which have given the name to the
theory, begin their motion round their central sun in elliptical paths
as required by gravitation. The form of the spiral is preserved by the
orbital motion of its particles. There is a gradual gathering together
of the planetesimals at points or nodes of intersection, and these
become aggregations of matter, nuclei that will perhaps become planets,
though more likely other stars. The appulse or near approach is but one
of the methods by which the spiral nebulæ may have come into existence.
The planetesimal hypothesis would seem to account for the formation of
many of these objects as we see them in the sky, though perhaps it is
hardly competent to replace entirely the Laplacian hypothesis of the
formation of the solar system, which would appear to be a special case
by itself.

It will be observed that while the Laplacian hypothesis is concerned in
the main with the progressive development of the solar system, and
systems of a like order surrounding other stellar centers, whose
existence is highly probable, the origin and development of the stellar
universe is a vaster problem which can only be undertaken and completed
in its broadest bearings when the structure of the stellar universe has
been ascertained.

Darwin's important investigations in 1877-1878 on tidal friction may be
here related. Before his day acceptance of the ring-theory of
development of the moon from the earth had scarcely been questioned; but
his recondite mathematical researches on the tidal reaction between a
central yielding mass and a body revolving round it brought to light the
unsuspected effect of tides raised upon both bodies by their mutual
attraction. The type of tides here meant is not the usual rise and fall
of the waters of the ocean, but primeval tides in the plastic material
of which the earth in its early history was composed. The Newtonian law
of gravitation afforded a complete explanation of the rise and fall of
the waters of the oceans, but as applied to the motions of planets and
satellites by the Lagrangian formulæ, it presupposed that all these
bodies are rigid and unyielding. However, mutual tides of phenomenal
height in their early plastic substances must have been a necessary
consequence of the action of the Newtonian law, and they gradually drew
upon the earth's rotational moment of momentum.

In its very early history, before there was any moon to produce tides,
the earth rotated much more rapidly, that is, the day was very much
shorter than now, probably about five or six hours long. And with the
rapid whirling, it was not a Laplacian ring that was detached, but a
huge globular mass was separated from the plastic earth's equator.
Darwin shows that the gravitative interaction of the two bodies
immediately began to raise tides of extraordinary height in both,
therefore tending to slow down the rotational periods of both bodies.
Action and reaction being equal, the reaction at once began driving the
moon away from the earth and thereby lengthening its period of
revolution. So small was the mass of the moon and so near was it to the
earth, that its relative rotational energy was in time completely used
up, and the moon has ever since turned her constant face toward us.
Tides of sun and moon in the plastic earth, acting through the ages,
slowed down the earth's rotation to its present period, or the length of
the day.

Moulton, however, has investigated the tidal theory of the origin of the
moon in the light of the planetesimal hypothesis, concluding that the
moon never was part of the earth and separated therefrom by too rapid
rotation of the earth, but that the distance of the two bodies has
always been the same as now. The more massive earth has in its
development throughout time robbed the less massive moon in the gradual
process of accretion. So the moon has never acquired either an ocean or
atmosphere, and this view is acceptable to geologists who have studied
the sheer lunar surface, Shaler of Harvard among the first, and laid the
foundations for a separate science of selenology.

Tidal friction has also been operant in producing sun-raised tides upon
the early plastic substances which composed the planets: more powerfully
in the case of planets nearer the sun; less rapidly if the planet's mass
is large; also less completely if the planet has solidified earlier on
account of its small dimensions. So Darwin would account for the
present rotation periods of all the planets: both Mercury and Venus
powerfully acted on by the sun on account of their nearness to him, and
their rotational energy completely exhausted, so that they now and for
all time turn a constant face toward him, as the moon does to the earth;
earth and possibly Mars even yet undergoing a very slight lengthening of
their day; Jupiter and Saturn, also Uranus and probably Neptune, still
exhibiting relatively swift axial rotation, because of their great mass
and great original moment of momentum, and also by reason of their vast
distances from the central tide-raising body, the sun.

By applying to stellar systems the principles developed by Darwin, See
accounted for the fact, to which he was the first to direct attention,
that the great eccentricity of the binary orbits is a necessary result
of the secular action of tidal friction. The double stars, then, were
double nebulæ, originally single, but separated by a process allied to
that known as "fission" in protozoans. Indeed, Poincaré proved
mathematically that a swiftly revolving nebula, in consequence of
contraction, first undergoes distortion into a pear-shaped or hour-glass
figure, the two masses ultimately separating entirely; and the
observations of the Herschels, Lord Rosse and others, with the recent
photographic plates at the Lick and Mount Wilson observatories, afford
immediate confirmation in a multitude of double nebulæ, widely scattered
throughout the nebular regions of the heavens.

Jeans of Cambridge, England, among the most recent of mathematical
investigators of the cosmogony, balances the advantages and
disadvantages of the differing cosmogonic systems as follows, in his
"Problems of Cosmogony and Stellar Dynamics": "Some hundreds of millions
of years ago all the stars within our Galactic universe formed a single
mass of excessively tenuous gas in slow rotation. As imagined by
Laplace, this mass contracted owing to loss of energy by radiation, and
so increased its angular velocity until it assumed a lenticular
shape.... After this, further contraction was a sheer mathematical
impossibility and the system had to expand. The mechanism of expansion
was provided by matter being thrown off from the sharp edge of the
lenticular figure, the lenticular center now forming the nucleus, and
the thrown-off matter forming the arms, of a spiral nebula of the normal
type. The long filaments of matter which constituted the arms, being
gravitationally unstable, first formed into chains of condensation about
nuclei, and ultimately formed detached masses of gas. With continued
shrinkage, the temperature of these masses increased until they attained
to incandescence, and shone as luminous stars. At the same time their
velocity of rotation increased until a large proportion of them broke up
by fission into binary systems. The majority of the stars broke away
from their neighbors and so formed a cluster of irregularly moving
stars--our present Galactic universe, in which the flattened shape of
the original nebula may still be traced in the concentration about the
Galactic plane, while the original motion along the nebular arms still
persists in the form of 'star-streaming.' In some cases a pair or small
group of stars failed to get clear of one another's gravitational
attractions and remain describing orbits about one another as wide
binaries or multiple stars. The stars which were formed last, the
present B-type stars, have been unusually immune from disturbance by
their neighbors, partly because they were born when adjacent stars had
almost ceased to interfere with one another, partly because their
exceptionally large mass minimized the effect of such interference as
may have occurred; consequently they remain moving in the plane in which
they were formed, many of them still constituting closely associated
groups of stars--the moving star clusters.

"At intervals it must have happened that two stars passed relatively
near to one another in their motion through the universe. We conjecture
that something like 300 million years ago our sun experienced an
encounter of this kind, a large star passing within a distance of about
the sun's diameter from its surface. The effect of this, as we have
seen, would be the ejection of a stream of gas toward the passing star.
At this epoch the sun is supposed to have been dark and cold, its
density being so low that its radius was perhaps comparable with the
present radius of Neptune's orbit. The ejected stream of matter,
becoming still colder by radiation, may have condensed into liquid near
its ends and perhaps partially also near its middle. Such a jet of
matter would be longitudinally unstable and would condense into detached
nuclei which would ultimately form planets."




CHAPTER LXI

COSMOGONY IN TRANSITION


We have seen how Wright in 1750 initiated a theory of evolution, not
only of the solar system, but of all the stars and nebulæ as well; how
Kant in 1752 by elaborating this theory sought to develop the details of
evolution of the solar system on the basis of the Newtonian law, though
weakened, as we know, by serious errors in applying physical laws; how
Laplace in 1796 put forward his nebular hypothesis of origin and
development of the solar system, by contraction from an original gaseous
nebula in accord with the Newtonian law; how Sir William Herschel in
1810 saw in all nebulæ merely the stuff that stars are made of; how Lord
Rosse in 1845 discovered spiral nebulæ; how Helmholtz in 1854 put
forward his contraction theory of maintenance of the solar heat,
seemingly reinforcing the Laplacian theory; how Lane in 1870 proved that
a contracting gaseous star might rise in temperature; how Roche in 1873
in attempting to modify the Laplacian hypothesis, pointed out the
conditions under which a satellite would be broken up by tidal strains;
how Darwin in 1879 showed that the theory of tidal evolution of
non-rigid bodies might account for the formation of the moon, and binary
stars might originate by fission; how Keeler in 1900 discovered the vast
numbers of spiral nebulæ; how Chamberlin and Moulton in 1903 put
forward the planetesimal hypothesis of formation of the spiral nebulæ,
showing also how that hypothesis might account for the evolution of the
solar system; and how Jeans in 1916 advocated the median ground in
evolution of the arms of the spiral nebulæ, showing that they will break
up into nuclei, if sufficiently massive.

In all these theories, truth and error, or lack of complete knowledge,
appear to be intermingled in varying proportions. Is it not early yet to
say, either that any one of them must be abandoned as totally wrong, or
on the other hand that any one of them, or indeed any single hypothesis,
can explain all the evolutionary processes of the universe?

Clearly the great problems cannot all be solved by the kinetic theory of
gases and the law of gravitation alone. Recent physical researches into
sub-atomic energy and the structure and properties of matter, appear to
point in the direction where we must next look for more light on such
questions as the origin and maintenance of the sun's heat, the complex
phenomena of variable stars and the progressive evolution of the myriad
bodies of the stellar universe. Because we have actually seen one star
turn into a nebula we should not jump to the conclusion that all nebulæ
are formed from stars, even if this might seem a direct inference from
the high radial velocities of planetary nebulæ.

Quite as obviously many of the spiral nebulæ are in a stage of
transition into local universes of stars--even more obvious from the
marvelous photographs in our day than the evolution of stars from nebulæ
of all types was to Herschel in his day.

The physicist must further investigate such questions as the building up
of heavy atomic elements by gravitative condensation of such lighter
ones as compose the nebulæ; and laboratory investigation must elucidate
further the process of development of energy from atomic disintegration
under very high pressures. This leads to a reclassification of the stars
on a temperature basis.

Equally important is the inquiry into the mechanism of radiative
equilibrium in sun and stars. Not impossibly the process of the earth's
upper atmosphere in maintaining a terrestrial equilibrium may afford
some clue. What this physical mechanism may be is very incompletely
known, but it is now open to further research through recent progress of
aeronautics, which will afford the investigator a "ceiling" of 50,000
feet and probably more. Beneath this level, perhaps even below 40,000
feet, lie all the strata, including the inversion layer, where the sun's
heat is conserved and an equilibrium maintained.

Even ten years ago, had an astronomer been asked about the physical
condition of the interior of the stars, he would have replied that
information of this character could only be had on visiting the stars
themselves--and perhaps not even then. But at the Cardiff meeting of the
British Association in 1920, Eddington, the president of Section A,
delivered an address on the internal constitution of the stars. He cites
the recent investigations of Russell and others on truly gaseous stars,
like Aldebaran, Arcturus, Antares and Canopus, which are in a diffuse
state and are the most powerful light-givers, and thus are to be
distinguished from the denser stars like our Sun. The term _giants_ is
applied to the former, and _dwarfs_ to the latter, in accord with
Russell's theory.

As density increases through contraction, these terms represent the
progressive stages, from earlier to later, in a star's history. A red or
M-type star begins its history as a giant of comparatively low
temperature. Contracting, according to Lane's law, its temperature must
rise until its density becomes such that it no longer behaves as a
perfect gas. Much depends on the star's mass; but after its maximum
temperature is attained, the star, which has shrunk to the proportions
of a dwarf, goes on cooling and contracts still further.

Each temperature-level is reached and passed twice, once during the
ascending stage and once again in descending--once as a giant, and once
as a dwarf. Thus there are vast differences in luminosity: the huge
giant, having a far larger surface than the shrunken dwarf, radiates an
amount of light correspondingly greater.

The physicist recognizes heat in two forms--the energy of motion of
material atoms, and the energy of ether waves. In hot bodies with which
we are familiar, the second form is quite insignificant; but in the
giant stars, the two forms are present in about equal proportions. The
super-heated conditions of the interior of the stars can only be
estimated in millions of degrees; and the problem is not one of
convection currents, as formerly thought, bringing hot masses to the
surface from the highly heated interior, but how can the heat of the
interior be barred against leakage and reduced to the relatively small
radiation emitted by the stars. "Smaller stars have to manufacture the
radiant heat which they emit, living from hand to mouth; the giant stars
merely leak radiant heat from their store."

So a radioactive type of equilibrium must be established, rather than a
convective one. Laboratory investigations of the very short waves are
now in progress, bearing on the transparency of stellar material to the
radiation traversing it; and the penetrating power of the star's
radiation is much like that of X-rays. The opacity is remarkably high,
explaining why the star is so nearly "heat-tight."

Opacity being constant, the total radiation of a giant star depends on
its mass only, and is quite independent of its temperature or state of
diffuseness. So that the total radiation of a star which is measured
roughly by its luminosity, may readily remain constant during the entire
'giant' stage of its history. As Russell originally pointed out, giant
stars of every spectral type have nearly the same luminosity. From the
range of luminosity of the giant stars, then, we may infer their range
of masses: they come out much alike, agreeing well with results obtained
by double-star investigation.

These studies of radiation and internal condition of the stars again
bring up the question of the original source of that supply of radiant
energy continually squandered by all self-luminous bodies. The giant
stars are especially prodigal, and radiate at least a hundredfold faster
than the sun.

"A star is drawing on some vast reservoir of energy," says Eddington,
"by means unknown to us. This reservoir can scarcely be other than the
sub-atomic energy which, it is known, exists abundantly in all matter;
we sometimes dream that man will one day learn how to release it and use
it for his service. The store is well-nigh inexhaustible, if only it
could be tapped. There is sufficient in the sun to maintain its output
of heat for fifteen billion years."


       *       *       *       *       *


Transcriber's Notes:


Obvious punctuation errors repaired. Hyphenation and spelling was
standardized by using the most prevalent form. The oe ligature was
converted to the letters "oe". Whole and fractional parts of numbers
are displayed as follows: 365-1/4.


  Page Correction
  ==== ===================
    20 Aa => Aya
    39 Ulugh Begh => Ulugh Beg
    46 Instaurata Mecanica => Instauratæ Mechanica
    58 Oscillatorium Horologium => Horologium Oscillatorium
   225 seceded => succeeded
   226 areoplane => aeroplane
   320 Plate 2 - Vulpeculæ => Vulpecula


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  _Text_ - Italic

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