[Illustration]




Curiosities of the Sky

by Garrett P. Serviss




Contents

 PREFACE
 CURIOSITIES OF THE SKY
 I. The Windows of Absolute Night
 II. Star-Clouds, Star-Clusters, and Star-Streams
 III. Stellar Migrations
 IV. The Passing of the Constellations
 V. Conflagrations in the Heavens
 VI. Explosive and Whirling Nebulæ
 VII. The Banners of the Sun
 VIII. The Zodiacal Light Mystery
 IX. Marvels of the Aurora
 X. Strange Adventures of Comets
 XI. Meteors, Fire-Balls, and Meteorites
 XII. The Wrecking of the Moon
 XIII. The Great Mars Problem
 XIV. The Riddle of the Asteroids




[Illustration: Great spiral nebula in Andromeda]




Illustrations

 Great spiral nebula in Andromeda
 The Milky Way
 Star-cluster in Hercules
 Great southern star-cluster, Omega Centauri
 The Pleiades
 The “Great Dipper”
 Cassiopeia
 The “Northern Crown”
 The “Southern Cross”
 Chart showing location of Tycho’s star, 1572,  and Nova Persei of 1901
 Nova Persei, with its nebular rings
 Lord Rosse’s nebula
 Wonderful spiral in triangulum
 Spiral in Ursa Major
 Nebula in Cetus
 The Orion nebula
 The Corona
 A solar “prominence”
 Auroral beams seen in England
 Auroral arches of an eliptic form seen in the Arctic regions
 Auroral curtain seen in Scandinavia
 Auroral arches seen in Scandinavia
 Swift’s comet
 Daniels’ comet
 Brooks’ comet
 Curious forms of meteorite trains
 Section of the atmosphere up to 100 kilometers
 A meteor photographed in flight
 Looking across Coon Butte crater from northern rim
 Trail on south side, Coon Butte crater
 The craters Clavius, Longomontanus, Tycho, etc.
 Western part of the Mare Serenitatis
 Mare Tranquilitatis and surroundings
 Lunar craters Theophilus and surrounding region
 Mare Crisium
 Schiaparelli’s chart of Mars, showing the so-called system of canals




PREFACE


What Froude says of history is true also of astronomy: it is the most
impressive where it transcends explanation. It is not the mathematics
of astronomy, but the wonder and the mystery that seize upon the
imagination. The calculation of an eclipse owes all its prestige to the
sublimity of its data; the operation, in itself, requires no more
mental effort than the preparation of a railway time-table.

The dominion which astronomy has always held over the minds of men is
akin to that of poetry; when the former becomes merely instructive and
the latter purely didactic, both lose their power over the imagination.
Astronomy is known as the oldest of the sciences, and it will be the
longest-lived because it will always have arcana that have not been
penetrated.

Some of the things described in this book are little known to the
average reader, while others are well known; but all possess the
fascination of whatever is strange, marvelous, obscure, or
mysterious—magnified, in this case, by the portentous scale of the
phenomena.

The idea of the author is to tell about these things in plain language,
but with as much scientific accuracy as plain language will permit,
showing the wonder that is in them without getting away from the facts.
Most of them have hitherto been discussed only in technical form, and
in treatises that the general public seldom sees and never reads.

Among the topics touched upon are:

The strange unfixedness of the “fixed stars,” the vast migrations of
the suns and worlds constituting the universe.

The slow passing out of existence of those collocations of stars which
for thousands of years have formed famous “constellations,” preserving
the memory of mythological heroes and heroines, and perhaps of
otherwise unrecorded history.

The tendency of stars to assemble in immense clouds, swarms, and
clusters.

The existence in some of the richest regions of the universe of
absolutely black, starless gaps, deeps, or holes, as if one were
looking out of a window into the murkiest night.

The marvelous phenomena of new, or temporary, stars, which appear as
suddenly as conflagrations, and often turn into something else as
eccentric as themselves.

The amazing forms of the “whirlpool,” “spiral,” “pinwheel,” and “lace,”
or “tress,” nebulæ.

The strange surroundings of the sun, only seen in particular
circumstances, but evidently playing a constant part in the daily
phenomena of the solar system.

The mystery of the Zodiacal Light and the Gegenschein.

The extraordinary transformations undergone by comets and their tails.

The prodigies of meteorites and masses of stone and metal fallen from
the sky.

The cataclysms that have wrecked the moon.

The problem of life and intelligence on the planet Mars.

The problematical origin and fate of the asteroids.

The strange phenomena of the auroral lights.


An attempt has been made to develop these topics in an orderly way,
showing their connection, so that the reader may obtain a broad general
view of the chief mysteries and problems of astronomy, and an idea of
the immense field of discovery which still lies, almost unexplored,
before it.




CURIOSITIES OF THE SKY




I
The Windows of Absolute Night


To most minds mystery is more fascinating than science. But when
science itself leads straight up to the borders of mystery and there
comes to a dead stop, saying, “At present I can no longer see my way,”
the force of the charm is redoubled. On the other hand, the illimitable
is no less potent in mystery than the invisible, whence the dramatic
effect of Keats’ “stout Cortez” staring at the boundless Pacific while
all his men look at each other with a wild surmise, “silent upon a peak
in Darien.” It is with similar feelings that the astronomer regards
certain places where from the peaks of the universe his vision seems to
range out into endless empty space. He sees there the shore of his
little isthmus, and, beyond, unexplored immensity.

The name, “coal-sacks,” given to these strange voids is hardly
descriptive. Rather they produce upon the mind the effect of blank
windows in a lonely house on a pitch-dark night, which, when looked at
from the brilliant interior, become appalling in their rayless murk.
Infinity seems to acquire a new meaning in the presence of these black
openings in the sky, for as one continues to gaze it loses its purely
metaphysical quality and becomes a kind of entity, like the ocean. The
observer is conscious that he can actually _see_ the beginning of its
ebon depths, in which the visible universe appears to float like an
enchanted island, resplendent within with lights and life and gorgeous
spectacles, and encircled with screens of crowded stars, but with its
dazzling vistas ending at the fathomless sea of pure darkness which
encloses all.

The Galaxy, or Milky Way, surrounds the borders of our island in space
like a stellar garland, and when openings appear in it they are, by
contrast, far more impressive than the general darkness of the
interstellar expanse seen in other directions. Yet even that expanse is
not everywhere equally dark, for it contains gloomy deeps discernable
with careful watching. Here, too, contrast plays an important part,
though less striking than within the galactic region. Some of Sir
William Herschel’s observations appear to indicate an association
between these tenebrious spots and neighboring star clouds and nebulæ.
It is an illuminating bit of astronomical history that when he was
sweeping the then virgin heavens with his great telescopes he was
accustomed to say to his sister who, note-book in hand, waited at his
side to take down his words, fresh with the inspiration of discovery:
“Prepare to write; the nebulæ are coming; here space is vacant.”

The most famous of the “coal-sacks,” and the first to be brought to
general attention before astronomers had awakened to the significance
of such things, lies adjacent to the “Southern Cross,” and is truly an
amazing phenomenon. It is not alone the conspicuousness of this
celestial vacancy, opening suddenly in the midst of one of the richest
parts of the Galaxy, that has given it its fame, but quite as much the
superstitious awe with which it was regarded by the early explorers of
the South Seas. To them, as well as to those who listened in rapt
wonder to their tales, the “Coal-sack” seemed to possess some occult
connection with the mystic “Cross.” In the eyes of the sailors it was
not a vacancy so much as a sable reality in the sky, and as,
shuddering, they stared at it, they piously crossed themselves. It was
another of the magical wonders of the unknown South, and as such it
formed the basis of many a “wild surmise” and many a sea-dog’s yarn.
Scientific investigation has not diminished its prestige, and today no
traveler in the southern hemisphere is indifferent to its fascinating
strangeness, while some find it the most impressive spectacle of the
antarctic heavens.

All around, up to the very edge of the yawning gap, the sheen of the
Milky Way is surpassingly glorious; but there, as if in obedience to an
almighty edict, everything vanishes. A single faint star is visible
within the opening, producing a curious effect upon the sensitive
spectator, like the sight of a tiny islet in the midst of a black,
motionless, waveless tarn. The dimensions of the lagoon of darkness,
which is oval or pear-shaped, are eight degrees by five, so that it
occupies a space in the sky about one hundred and thirty times greater
than the area of the full moon. It attracts attention as soon as the
eye is directed toward the quarter where it exists, and by virtue of
the rarity of such phenomena it appears a far greater wonder than the
drifts of stars that are heaped around it. Now that observatories are
multiplying in the southern hemisphere, the great austral “Coal-sack”
will, no doubt, receive attention proportioned to its importance as one
of the most significant features of the sky. Already at the Sydney
Observatory photographs have shown that the southern portion of this
Dead Sea of Space is not quite “bottomless,” although its northern part
defies the longest sounding lines of the astronomer.

There is a similar, but less perfect, “coal-sack” in the northern
hemisphere, in the constellation of “The Swan,” which, strange to say,
also contains a well-marked figure of a cross outlined by stars. This
gap lies near the top of the cross-shaped figure. It is best seen by
averted vision, which brings out the contrast with the Milky Way, which
is quite brilliant around it. It does not, however, exercise the same
weird attraction upon the eye as the southern “Coal-sack,” for instead
of looking like an absolute void in the sky, it rather appears as if a
canopy of dark gauze had been drawn over the stars. We shall see the
possible significance of this appearance later.


[Illustration: The Milky Way. Region near M.S. Photographed by
Professor Barnard]


Just above the southern horizon of our northern middle latitudes, in
summer, where the Milky Way breaks up into vast sheets of nebulous
luminosity, lying over and between the constellations Scorpio and
Sagittarius, there is a remarkable assemblage of “coal-sacks,” though
none is of great size. One of them, near a conspicuous star-cluster in
Scorpio, M80, is interesting for having been the first of these strange
objects noted by Herschel. Probably it was its nearness to M80 which
suggested to his mind the apparent connection of such vacancies with
star-clusters which we have already mentioned.

But the most marvelous of the “coal-sacks” are those that have been
found by photography in Sagittarius. One of Barnard’s earliest and most
excellent photographs includes two of them, both in the star-cluster
M8. The larger, which is roughly rectangular in outline, contains one
little star, and its smaller neighbor is lune-shaped—surely a most
singular form for such an object. Both are associated with curious dark
lanes running through the clustered stars like trails in the woods.
Along the borders of these lanes the stars are ranked in parallel rows,
and what may be called the bottoms of the lanes are not entirely dark,
but pebbled with faint stellar points. One of them which skirts the two
dark gaps and traverses the cluster along its greatest diameter is
edged with lines of stars, recalling the alignment of the trees
bordering a French highway. This _road of stars_ cannot be less than
many billions of miles in length!

All about the cluster the bed of the Galaxy is strangely disturbed, and
in places nearly denuded, as if its contents had been raked away to
form the immense stack and the smaller accumulations of stars around
it. The well-known “Trifid Nebula” is also included in the field of the
photograph, which covers a truly marvelous region, so intricate in its
mingling of nebulæ, star-clusters, star-swarms, star-streams, and dark
vacancies that no description can do it justice. Yet, chaotic as it
appears, there is an unmistakable suggestion of unity about it,
impressing the beholder with the idea that all the different parts are
in some way connected, and have not been fortuitously thrown together.
Miss Agnes M. Clerke made the striking remark that the dusky lanes in
M8 are exemplified on the largest scale in the great rift dividing the
Milky Way, from Cygnus in the northern hemisphere all the way to the
“Cross” in the southern. Similar lanes are found in many other
clusters, and they are generally associated with flanking rows of
stars, resembling in their arrangement the thick-set houses and villas
along the roadways that traverse the approaches to a great city.

But to return to the black gaps. Are they really windows in the
star-walls of the universe? Some of them look rather as if they had
been made by a shell fired through a luminous target, allowing the eye
to range through the hole into the void space beyond. If science is
discretely silent about these things, what can the more venturesome and
less responsible imagination suggest? Would a huge “runaway sun,” like
Arcturus, for instance, make such an opening if it should pass like a
projectile through the Milky Way? It is at least a stimulating inquiry.
Being probably many thousands of times more massive than the galactic
stars, such a stellar missile would not be stopped by them, though its
direction of flight might be altered. It would drag the small stars
lying close to its course out of their spheres, but the ultimate
tendency of its attraction would be to sweep them round in its wake,
thus producing rather a star-swarm than a vacancy. Those that were very
close to it might be swept away in its rush and become its satellites,
careering away with it in its flight into outer space; but those that
were farther off, and they would, of course, greatly outnumber the
nearer ones, would tend inward from all sides toward the line of
flight, as dust and leaves collect behind a speeding motor (though the
forces operating would be different), and would fill up the hole, if
hole it were. A swarm thus collected should be rounded in outline and
bordered with a relatively barren ring from which the stars had been
“sucked” away. In a general sense the M8 cluster answers to this
description, but even if we undertook to account for its existence by a
supposition like the above, the black gaps would remain unexplained,
unless one could make a further draft on the imagination and suggest
that the stars had been thrown into a vast eddy, or system of eddies,
whose vortices appear as dark holes. Only a maelstrom-like motion could
keep such a funnel open, for without regard to the impulse derived from
the projectile, the proper motions of the stars themselves would tend
to fill it. Perhaps some other cause of the whirling motion may be
found. As we shall see when we come to the spiral nebulæ, gyratory
movements are exceedingly prevalent throughout the universe, and the
structure of the Milky Way is everywhere suggestive of them. But this
is hazardous sport even for the imagination—to play with _suns_ as if
they were but thistle-down in the wind or corks in a mill-race.

Another question arises: What is the thickness of the hedge of stars
through which the holes penetrate? Is the depth of the openings
proportionate to their width? In other words, is the Milky Way round in
section like a rope, or flat and thin like a ribbon? The answer is not
obvious, for we have little or no information concerning the relative
distances of the faint galactic stars. It would be easier, certainly,
to conceive of openings in a thin belt than in a massive ring, for in
the first case they would resemble mere rifts and breaks, while in the
second they would be like wells or bore-holes. Then, too, the fact that
the Milky Way is not a _continuous_ body but is made up of stars whose
actual distances apart is great, offers another quandary; persistent
and sharply bordered apertures in such an assemblage are _a priori_ as
improbable, if not impossible, as straight, narrow holes running
through a swarm of bees.

The difficulty of these questions indicates one of the reasons why it
has been suggested that the seeming gaps, or many of them, are not
openings at all, but opaque screens cutting off the light from stars
behind them. That this is quite possible in some cases is shown by
Barnard’s later photographs, particularly those of the singular region
around the star Rho Ophiuchi. Here are to be seen somber lanes and
patches, apparently forming a connected system which covers an immense
space, and which their discoverer thinks may constitute a “dark
nebula.” This seems at first a startling suggestion; but, after all,
why should their not be dark nebulæ as well as visible ones? In truth,
it has troubled some astronomers to explain the luminosity of the
bright nebulæ, since it is not to be supposed that matter in so diffuse
a state can be incandescent through heat, and phosphorescent light is
in itself a mystery. The supposition is also in accord with what we
know of the existence of dark solid bodies in space. Many bright stars
are accompanied by obscure companions, sometimes as massive as
themselves; the planets are non-luminous; the same is true of meteors
before they plunge into the atmosphere and become heated by friction;
and many plausible reasons have been found for believing that space
contains as many obscure as shining bodies of great size. It is not so
difficult, after all, then, to believe that there are immense
collections of shadowy gases and meteoric dust whose presence is only
manifested when they intercept the light coming from shining bodies
behind them.

This would account for the apparent extinguishment of light in open
space, which is indicated by the falling off in relative number of
telescopic stars below the tenth magnitude. Even as things are, the
amount of light coming to us from stars too faint to be seen with the
naked eye is so great that the statement of it generally surprises
persons who are unfamiliar with the inner facts of astronomy. It has
been calculated that on a clear night the total starlight from the
entire celestial sphere amounts to one-sixtieth of the light of the
full moon; but of this less than one-twenty-fifth is due to stars
separately distinguished by the eye. If there were no obscuring medium
in space, it is probable that the amount of starlight would be
noticeably and perhaps enormously increased.

But while it seems certain that some of the obscure spots in the Milky
Way are due to the presence of “dark nebulæ,” or concealing veils of
one kind or another, it is equally certain that there are many which
are true apertures, however they may have been formed, and by whatever
forces they may be maintained. These, then, are veritable windows of
the Galaxy, and when looking out of them one is face to face with the
great mystery of infinite space. _There_ the known universe visibly
ends, but manifestly space itself does not end there. It is not within
the power of thought to conceive an end to space, for the instant we
think of a terminal point or line the mind leaps forward to the
_beyond._ There must be space outside as well as inside. Eternity of
time and infinity of space are ideas that the intellect cannot fully
grasp, but neither can it grasp the idea of a limitation to either
space or time. The metaphysical conceptions of hypergeometry, or
fourth-dimensional space, do not aid us.

Having, then, discovered that the universe is a thing _contained_ in
something indefinitely greater than itself; having looked out of its
windows and found only the gloom of starless night outside—what
conclusions are we to draw concerning the beyond? It _seems_ as empty
as a vacuum, but is it really so? If it be, then our universe is a
single atom astray in the infinite; it is the only island in an ocean
without shores; it is the one oasis in an illimitable desert. Then the
Milky Way, with its wide-flung garland of stars, is afloat like a tiny
smoke-wreath amid a horror of immeasurable vacancy, or it is an
evanescent and solitary ring of sparkling froth cast up for a moment on
the viewless billows of immensity. From such conclusions the mind
instinctively shrinks. It prefers to think that there is _something_
beyond, though we cannot see it. Even the universe could not bear to be
alone—a Crusoe lost in the Cosmos! As the inhabitants of the most
elegant château, with its gardens, parks, and crowds of attendants,
would die of loneliness if they did not know that they have neighbors,
though not seen, and that a living world of indefinite extent surrounds
them, so we, when we perceive that the universe has limits, wish to
feel that it is not solitary; that beyond the hedges and the hills
there are other centers of life and activity. Could anything be more
terrible than the thought of an _isolated universe?_ The greater the
being, the greater the aversion to seclusion. Only the infinite
satisfies; in that alone the mind finds rest.

We are driven, then, to believe that the universal night which
envelopes us is not tenantless; that as we stare out of the star-framed
windows of the Galaxy and see nothing but uniform blackness, the fault
is with our eyes or is due to an obscuring medium. Since _our_ universe
is limited in extent, there must be _other_ universes beyond it on all
sides. Perhaps if we could carry our telescopes to the verge of the
great “Coal-sack” near the “Cross,” being then on the frontier of our
starry system, we could discern, sparkling afar off in the vast night,
some of the outer galaxies. They may be grander than ours, just as many
of the suns surrounding us are immensely greater than ours. If we could
take our stand somewhere in the midst of immensity and, with vision of
infinite reach, look about us, we should perhaps see a countless number
of stellar systems, amid which ours would be unnoticeable, like a
single star among the multitude glittering in the terrestial sky on a
clear night. Some might be in the form of a wreath, like our own; some
might be globular, like the great star-clusters in Hercules and
Centaurus; some might be glittering circles, or disks, or rings within
rings. If we could enter them we should probably find a vast variety of
composition, including elements unknown to terrestrial chemistry; for
while the _visible_ universe appears to contain few if any substances
not existing on the earth or in the sun, we have no warrant to assume
that others may not exist in infinite space.

And how as to gravitation? We do not _know_ that gravitation acts
beyond the visible universe, but it is reasonable to suppose that it
does. At any rate, if we let go _its_ sustaining hand we are lost, and
can only wander hopelessly in our speculations, like children astray.
If the empire of gravitation is infinite, then the various outer
systems must have _some,_ though measuring by our standards an
imperceptible, attractive influence upon each other, for gravitation
never lets go its hold, however great the space over which it is
required to act. Just as the stars about us are all in motion, so the
starry systems beyond our sight may be in motion, and our system as a
whole may be moving in concert with them. If this be so, then after
interminable ages the aspect of the entire system of systems must
change, its various members assuming new positions with respect to one
another. In the course of time we may even suppose that our universe
will approach relatively close to one of the others; and then, if men
are yet living on the earth, they may glimpse through the openings
which reveal nothing to us now, the lights of another nearing star
system, like the signals of a strange squadron, bringing them the
assurance (which can be but an inference at present) that the ocean of
space has other argosies venturing on its limitless expanse.

There remains the question of the luminiferous ether by whose agency
the waves of light are borne through space. The ether is as mysterious
as gravitation. With regard to ether we only infer its existence from
the effects which we ascribe to it. Evidently the ether must extend as
far as the most distant visible stars. But does it continue on
indefinitely in outer space? If it does, then the invisibility of the
other systems must be due to their distance diminishing the quantity of
light that comes from them below the limit of perceptibility, or to the
interposition of absorbing media; if it does not, then the reason why
we cannot see them is owing to the absence of a means of conveyance for
the light waves, as the lack of an interplanetary atmosphere prevents
us from hearing the thunder of sun-spots. (It is interesting to recall
that Mr Edison was once credited with the intention to construct a
gigantic microphone which should render the roar of sun-spots audible
by transforming the electric vibrations into sound-waves). On this
supposition each starry system would be enveloped in its own globule of
ether, and no light could cross from one to another. But the
probability is that both the ether and gravitation are ubiquitous, and
that all the stellar systems are immersed in the former like clouds of
phosphorescent organisms in the sea.

So astronomy carries the mind from height to greater height. Men were
long in accepting the proofs of the relative insignificance of the
earth; they were more quickly convinced of the comparative littleness
of the solar system; and now the evidence assails their reason that
what they had regarded as _the_ universe is only one mote gleaming in
the sunbeams of Infinity.




II
Star-Clouds, Star-Clusters, and Star-Streams


In the preceding chapter we have seen something of the strangely
complicated structure of the Galaxy, or Milky Way. We now proceed to
study more comprehensively that garlanded “Pathway of the Gods.”

Judged by the eye alone, the Milky Way is one of the most delicately
beautiful phenomena in the entire realm of nature—a shimmer of silvery
gauze stretched across the sky; but studied in the light of its
revelations, it is the most stupendous object presented to human ken.
Let us consider, first, its appearance to ordinary vision. Its apparent
position in the sky shifts according to the season. On a serene,
cloudless summer evening, in the absence of the moon, whose light
obscures it, one sees the Galaxy spanning the heavens from north to
southeast of the zenith like a phosphorescent arch. In early spring it
forms a similar but, upon the whole, less brilliant arch west of the
zenith. Between spring and summer it lies like a long, faint, twilight
band along the northern horizon. At the beginning of winter it again
forms an arch, this time spanning the sky from east to west, a little
north of the zenith. These are its positions as viewed from the mean
latitude of the United States. Even the beginner in star-gazing does
not have to watch it throughout the year in order to be convinced that
it is, in reality, a great circle, extending entirely around the
celestial sphere. We appear to be situated near its center, but its
periphery is evidently far away in the depths of space.

Although to the casual observer it seems but a delicate scarf of light,
brighter in some places than in others, but hazy and indefinite at the
best, such is not its appearance to those who study it with care. They
perceive that it is an organic whole, though marvelously complex in
detail. The telescope shows that it consists of stars too faint and
small through excess of distance to be separately visible. Of the
hundred million suns which some estimates have fixed as the probable
population of the starry universe, the vast majority (at least thirty
to one) are included in this strange belt of misty light. But they are
not uniformly distributed in it; on the contrary, they are arrayed in
clusters, knots, bunches, clouds, and streams. The appearance is
somewhat as if the Galaxy consisted of innumerable swarms of
silver-winged bees, more or less intermixed, some massed together, some
crossing the paths of others, but all governed by a single purpose
which leads them to encircle the region of space in which we are
situated.

From the beginning of the systematic study of the heavens, the fact has
been recognized that the form of the Milky Way denotes the scheme of
the sidereal system. At first it was thought that the shape of the
system was that of a vast round disk, flat like a cheese, and filled
with stars, our sun and his relatively few neighbors being placed near
the center. According to this view, the galactic belt was an effect of
perspective; for when looking in the direction of the plane of the
disk, the eye ranged through an immense extension of stars which
blended into a glimmering blur, surrounding us like a ring; while when
looking out from the sides of the disk we saw but few stars, and in
those directions the heavens appeared relatively blank. Finally it was
recognized that this theory did not correspond with the observed
appearances, and it became evident that the Milky Way was not a mere
effect of perspective, but an actual band of enormously distant stars,
forming a circle about the sphere, the central opening of the ring
(containing many scattered stars) being many times broader than the
width of the ring itself. Our sun is one of the scattered stars in the
central opening.

As already remarked, the ring of the Galaxy is very irregular, and in
places it is partly broken. With its sinuous outline, its pendant
sprays, its graceful and accordant curves, its bunching of masses, its
occasional interstices, and the manifest order of a general plan
governing the jumble of its details, it bears a remarkable resemblance
to a garland—a fact which appears the more wonderful when we recall its
composition. That an elm-tree should trace the lines of beauty with its
leafy and pendulous branches does not surprise us; but we can only gaze
with growing amazement when we behold _a hundred million suns imitating
the form of a chaplet!_ And then we have to remember that this form
furnishes the ground-plan of the universe.

As an indication of the extraordinary speculations to which the mystery
of the Milky Way has given rise, a theory recently (1909) proposed by
Prof. George C. Comstock may be mentioned. Starting with the data
(first) that the number of stars increases as the Milky Way is
approached, and reaches a maximum in its plane, while on the other hand
the number of nebulæ is greatest outside the Milky Way and increases
with distance from it, and (second) that the Milky Way, although a
complete ring, is broad and diffuse on one side through one-half its
course—that half alone containing nebulæ—and relatively narrow and well
defined on the opposite side, the author of this singular speculation
avers that these facts can best be explained by supposing that the
invisible universe consists of two interpenetrating parts, one of which
is a chaos of indefinite extent, strewn with stars and nebulous dust,
and the other a long, broad but comparatively thin cluster of stars,
including the sun as one of its central members. This flat star-cluster
is conceived to be moving edgewise through the chaos, and, according to
Professor Comstock, it acts after the manner of a snow-plough sweeping
away the cosmic dust and piling it on either hand above and below the
plane of the moving cluster. It thus forms a transparent rift, through
which we see farther and command a view of more stars than through the
intensified dust-clouds on either hand. This rift is the Milky Way. The
dust thrown aside toward the poles of the Milky Way is the substance of
the nebulæ which abound there. Ahead, where the front of the
star-plough is clearing the way, the chaos is nearer at hand, and
consequently there the rift subtends a broader angle, and is filled
with primordial dust, which, having been annexed by the vanguard of the
star-swarm, forms the nebulæ seen only in that part of the Milky Way.
But behind, the rift appears narrow because there we look farther away
between dust-clouds produced ages ago by the front of the plough, and
no scattered dust remains in that part of the rift.

In quoting an outline of this strikingly original theory the present
writer should not be understood as assenting to it. That it appears
bizarre is not, in itself, a reason for rejecting it, when we are
dealing with so problematical and enigmatical a subject as the Milky
Way; but the serious objection is that the theory does not sufficiently
accord with the observed phenomena. There is too much evidence that the
Milky Way is an organic system, however fantastic its form, to permit
the belief that it can only be a rift in chaotic clouds. As with every
organism, we find that its parts are more or less clearly repeated in
its ensemble. Among all the strange things that the Milky Way contains
there is nothing so extraordinary as itself. Every astronomer must many
times have found himself marveling at it in those comparatively rare
nights when it shows all its beauty and all its strangeness. In its
great broken rifts, divisions, and spirals are found the gigantic
prototypes of similar forms in its star-clouds and clusters. As we have
said, it determines the general shape of the whole sidereal system.
Some of the brightest stars in the sky appear to hang like jewels
suspended at the ends of tassels dropped from the Galaxy. Among these
pendants are the Pleiades and the Hyades. Orion, too, the “Mighty
Hunter,” is caught in “a loop of light” thrown out from it. The
majority of the great first-magnitude stars seem related to it, as if
they formed an inner ring inclined at an angle of some twenty degrees
to its plane. Many of the long curves that set off from it on both
sides are accompanied by corresponding curves of lucid stars. In a
word, it offers every appearance of structural connection with the
entire starry system. That the universe should have assumed the form of
a wreath is certainly a matter for astonishment; but it would have been
still more astonishing if it had been a cube, a rhomboid, or a
dodecahedron, for then we should have had to suppose that something
resembling the forces that shape crystals had acted upon the stars, and
the difficulty of explaining the universe by the laws of gravitation
would have been increased.

From the Milky Way as a whole we pass to the vast clouds, swarms, and
clusters of stars of which it is made up. It may be, as some
astronomers hold, that most of the galactic stars are much smaller than
the sun, so that their faintness is not due entirely to the effect of
distance. Still, their intrinsic brilliance attests their solar
character, and considering their remoteness, which has been estimated
at not less than ten thousand to twenty thousand light-years (a
light-year is equal to nearly six thousand thousand million miles)
their actual masses cannot be extremely small. The minutest of them are
entitled to be regarded as real suns, and they vary enormously in
magnitude. The effects of their attractions upon one another can only
be inferred from their clustering, because their relative movements are
not apparent on account of the brevity of the observations that we can
make. But imagine a being for whom a million years would be but as a
flitting moment; to him the Milky Way would appear in a state of
ceaseless agitation—swirling with “a fury of whirlpool motion.”

The cloud-like aspect of large parts of the Galaxy must always have
attracted attention, even from naked-eye observers, but the true
star-clouds were first satisfactorily represented in Barnard’s
photographs. The resemblance to actual clouds is often startling. Some
are close-packed and dense, like cumuli; some are wispy or mottled,
like cirri. The rifts and modulations, as well as the general outlines,
are the same as those of clouds of vapor or dust, and one notices also
the characteristic thinning out at the edges. But we must beware of
supposing that the component suns are thickly crowded as the particles
forming an ordinary cloud. They _look,_ indeed, as if they were matted
together, because of the irradiation of light, but in reality millions
and billions of miles separate each star from its neighbors.
Nevertheless they form real assemblages, whose members are far more
closely related to one another than is our sun to the stars around him,
and if we were in the Milky Way the aspect of the nocturnal sky would
be marvelously different from its present appearance.

Stellar clouds are characteristic of the Galaxy and are not found
beyond its borders, except in the “Magellanic Clouds” of the southern
hemisphere, which resemble detached portions of the Milky Way. These
singular objects form as striking a peculiarity of the austral heavens
as does the great “Coal-sack” described in Chapter 1. But it is their
isolation that makes them so remarkable, for their composition is
essentially galactic, and if they were included within its boundaries
they would not appear more wonderful than many other parts of the Milky
Way. Placed where they are, they look like masses fallen from the great
stellar arch. They are full of nebulæ and star-clusters, and show
striking evidences of spiral movement.

Star-swarms, which are also characteristic features of the Galaxy,
differ from star-clouds very much in the way that their name would
imply—_i.e.,_ their component stars are so arranged, even when they are
countless in number, that the idea of an exceedingly numerous
assemblage rather than that of a cloud is impressed on the observer’s
mind. In a star-swarm the separate members are distinguishable because
they are either larger or nearer than the stars composing a “cloud.” A
splendid example of a true star-swarm is furnished by Chi Persei, in
that part of the Milky Way which runs between the constellations
Perseus and Cassiopeia. This swarm is much coarser than many others,
and can be seen by the naked eye. In a small telescope it appears
double, as if the suns composing it had divided into two parties which
keep on their way side by side, with some commingling of their members
where the skirts of the two companies come in contact.

Smaller than either star-clouds or star-swarms, and differing from both
in their organization, are star-clusters. These, unlike the others, are
found outside as well as inside the Milky Way, although they are more
numerous inside its boundaries than elsewhere. The term star-cluster is
sometimes applied, though improperly, to assemblages which are rather
groups, such, for instance, as the Pleiades. In their most
characteristic aspect star-clusters are of a globular shape—globes of
suns! A famous example of a globular star-cluster, but one not included
in the Milky Way, is the “Great Cluster in Hercules.” This is barely
visible to the naked eye, but a small telescope shows its character,
and in a large one it presents a marvelous spectacle. Photographs of
such clusters are, perhaps, less effective than those of star-clouds,
because the central condensation of stars in them is so great that
their light becomes blended in an indistinguishable blur. The beautiful
effect of the incessant play of infinitesimal rays over the apparently
compact surface of the cluster, as if it were a globe of the finest
frosted silver shining in an electric beam, is also lost in a
photograph. Still, even to the eye looking directly at the cluster
through a powerful telescope, the central part of the wonderful
congregation seems almost a solid mass in which the stars are packed
like the ice crystals in a snowball.

The same question rises to the lips of every observer: How can they
possibly have been brought into such a situation? The marvel does not
grow less when we know that, instead of being closely compacted, the
stars of the cluster are probably separated by millions of miles; for
we know that their distances apart are slight as compared with their
remoteness from the Earth. Sir William Herschel estimated their number
to be about fourteen thousand, but in fact they are uncountable. If we
could view them from a point just within the edge of the assemblage,
they would offer the appearance of a hollow hemisphere emblazoned with
stars of astonishing brilliancy; the near-by ones unparalleled in
splendor by any celestial object known to us, while the more distant
ones would resemble ordinary stars. An inhabitant of the cluster would
not know, except by a process of ratiocination, that he was dwelling in
a globular assemblage of suns; only from a point far outside would
their spherical arrangement become evident to the eye. Imagine
fourteen-thousand fire-balloons with an approach to regularity in a
spherical space—say, ten miles in diameter; there would be an average
of less than thirty in every cubic mile, and it would be necessary to
go to a considerable distance in order to see them as a globular
aggregation; yet from a point sufficiently far away they would blend
into a glowing ball.


[Illustration: Star-cluster in Hercules
(Photographed with a two-foot reflector)]


Photographs show even better than the best telescopic views that the
great cluster is surrounded with a multitude of dispersed stars,
suggestively arrayed in more or less curving lines, which radiate from
the principle mass, with which their connection is manifest. These
stars, situated outside the central sphere, look somewhat like vagrant
bees buzzing round a dense swarm where the queen bee is sitting. Yet
while there is so much to suggest the operation of central forces,
bringing and keeping the members of the cluster together, the attentive
observer is also impressed with the idea that the whole wonderful
phenomenon may be _the result of explosion._ As soon as this thought
seizes the mind, confirmation of it seems to be found in the appearance
of the outlying stars, which could be as readily explained by the
supposition that they have been blown apart as that they have flocked
together toward a center. The probable fact that the stars constituting
the cluster are very much smaller than our sun might be regarded as
favoring the hypothesis of an explosion. Of their real size we know
nothing, but, on the basis of an uncertain estimate of their parallax,
it has been calculated that they may average forty-five thousand miles
in diameter—something more than half the diameter of the planet
Jupiter. Assuming the same mean density, fourteen thousand such stars
might have been formed by the explosion of a body about twice the size
of the sun. This recalls the theory of Olbers, which has never been
altogether abandoned or disproved, that the Asteroids were formed by
the explosion of a planet circulating between the orbits of Mars and
Jupiter. The Asteroids, whatever their manner of origin, form a ring
around the sun; but, of course, the explosion of a great independent
body, not originally revolving about a superior center of gravitational
force, would not result in the formation of a ring of small bodies, but
rather of a dispersed mass of them. But back of any speculation of this
kind lies the problem, at present insoluble: How could the explosion be
produced? (See the question of explosions in Chapters 6 and 14).

Then, on the other hand, we have the observation of Herschel, since
abundantly confirmed, that space is unusually vacant in the immediate
neighborhood of condensed star-clusters and nebulæ, which, as far as it
goes, might be taken as an indication that the assembled stars had been
drawn together by their mutual attractions, and that the tendency to
aggregation is still bringing new members toward the cluster. But in
that case there must have been an original condensation of stars at
that point in space. This could probably have been produced by the
coagulation of a great nebula into stellar nuclei, a process which
seems now to be taking place in the Orion Nebula.


[Illustration: Great southern star-cluster, Omega Centauri]


A yet more remarkable globular star-cluster exists in the southern
hemisphere, Omega Centauri. In this case the central condensation of
stars presents an almost uniform blaze of light. Like the Hercules
cluster, that in Centaurus is surrounded with stars scattered over a
broad field and showing an appearance of radial arrangement. In fact,
except for its greater richness, Omega Centauri is an exact duplicate
of its northern rival. Each appears to an imaginative spectator as a
veritable “city of suns.” Mathematics shrinks from the task of
disentangling the maze of motions in such an assemblage. It would seem
that the chance of collisions is not to be neglected, and this idea
finds a certain degree of confirmation in the appearance of “temporary
stars” which have more than once blazed out in, or close by, globular
star-clusters.

This leads up to the notable fact, first established by Professor
Bailey a few years ago, that such clusters are populous with variable
stars. Omega Centauri and the Hercules cluster are especially
remarkable in this respect. The variables found in them are all of
short period and the changes of light show a noteworthy tendency to
uniformity. The first thought is that these phenomena must be due to
collisions among the crowded stars, but, if so, the encounters cannot
be between the stars themselves, but probably between stars and meteor
swarms revolving around them. Such periodic collisions might go on for
ages without the meteors being exhausted by incorporation with the
stars. This explanation appears all the more probable because one would
naturally expect that flocks of meteors would abound in a close
aggregation of stars. It is also consistent with Perrine’s
discovery—that the globular star clusters are powdered with minute
stars strewn thickly among the brighter ones.

In speaking of Professor Comstock’s extraordinary theory of the Milky
Way, the fact was mentioned that, broadly speaking, the nebulæ are less
numerous in the galactic belt than in the comparatively open spaces on
either side of it, but that they are, nevertheless, abundant in the
broader half of the Milky Way which he designates as the front of the
gigantic “plough” supposed to be forcing its way through the enveloping
chaos. In and around the Sagittarius region the intermingling of nebulæ
and galactic star clouds and clusters is particularly remarkable. That
there is a causal connection no thoughtful person can doubt. We are
unable to get away from the evidence that a nebula is like a
seed-ground from which stars spring forth; or we may say that nebulæ
resemble clouds in whose bosom raindrops are forming. The wonderful
aspect of the admixtures of nebulæ and star-clusters in Sagittarius has
been described in Chapter 1. We now come to a still more extraordinary
phenomenon of this kind—the Pleiades nebulæ.


[Illustration: The Pleiades]


The group of the Pleiades, although lying outside the main course of
the Galaxy, is connected with it by a faint loop, and is the scene of
the most remarkable association of stars and nebulous matter known in
the visible universe. The naked eye is unaware of the existence of
nebulæ in the Pleiades, or, at the best, merely suspects that there is
something of the kind there; and even the most powerful telescopes are
far from revealing the full wonder of the spectacle; but in photographs
which have been exposed for many hours consecutively, in order to
accumulate the impression of the actinic rays, the revelation is
stunning. The principle stars are seen surrounded by, and, as it were,
_drowned in,_ dense nebulous clouds of an unparalleled kind. The forms
assumed by these clouds seem at first sight inexplicable. They look
like fleeces, or perhaps more like splashes and daubs of luminous paint
dashed carelessly from a brush. But closer inspection shows that they
are, to a large extent, _woven_ out of innumerable threads of filmy
texture, and there are many indications of spiral tendencies. Each of
the bright stars of the group—Alcyone, Merope, Maia, Electra, Taygeta,
Atlas—is the focus of a dense fog (totally invisible, remember, alike
to the naked eye and to the telescope), and these particular stars are
veiled from sight behind the strange mists. Running in all directions
across the relatively open spaces are nebulous wisps and streaks of the
most curious forms. On some of the nebular lines, which are either
straight throughout, or if they change direction do so at an angle,
little stars are strung like beads. In one case seven or eight stars
are thus aligned, and, as if to emphasize their dependence upon the
chain which connects them, when it makes a slight bend the file of
stars turns the same way. Many other star rows in the group suggest by
their arrangement that they, too, were once strung upon similar threads
which have now disappeared, leaving the stars spaced along their
ancient tracks. We seem forced to the conclusion that there was a time
when the Pleiades were embedded in a vast nebula resembling that of
Orion, and that the cloud has now become so rare by gradual
condensation into stars that the merest trace of it remains, and this
would probably have escaped detection but for the remarkable actinic
power of the radiant matter of which it consists. The richness of many
of these faint nebulous masses in ultra-violet radiations, which are
those that specifically affect the photographic plate, is the cause of
the marvelous revelatory power of celestial photography. So the
veritable unseen universe, as distinguished from the “unseen universe”
of metaphysical speculation, is shown to us.

A different kind of association between stars and nebulæ is shown in
some surprising photographic objects in the constellation Cygnus, where
long, wispy nebulæ, billions of miles in length, some of them looking
like tresses streaming in a breeze, lie amid fields of stars which seem
related to them. But the relation is of a most singular kind, for
notwithstanding the delicate structure of the long nebulæ they appear
to act as barriers, causing the stars to heap themselves on one side.
The stars are two, three, or four times as numerous on one side of the
nebulæ as on the other. These nebulæ, as far as appearance goes, might
be likened to rail fences, or thin hedges, against which the wind is
driving drifts of powdery snow, which, while scattered plentifully all
around, tends to bank itself on the leeward side of the obstruction.
The imagination is at a loss to account for these extraordinary
phenomena; yet there they are, faithfully giving us their images
whenever the photographic plate is exposed to their radiations.

Thus the more we see of the universe with improved methods of
observation, and the more we invent aids to human senses, each enabling
us to penetrate a little deeper into the unseen, the greater becomes
the mystery. The telescope carried us far, photography is carrying us
still farther; but what as yet unimagined instrument will take us to
the bottom, the top, and the end? And then, what hitherto untried power
of thought will enable us to comprehend the meaning of it all?




III
Stellar Migrations


To the untrained eye the stars and the planets are not distinguishable.
It is customary to call them all alike “stars.” But since the planets
more or less rapidly change their places in the sky, in consequence of
their revolution about the sun, while the stars proper seem to remain
always in the same relative positions, the latter are spoken of as
“fixed stars.” In the beginnings of astronomy it was not known that the
“fixed stars” had any motion independent of their apparent annual
revolution with the whole sky about the earth as a seeming center. Now,
however, we know that the term “fixed stars” is paradoxical, for there
is not a single really fixed object in the whole celestial sphere. The
apparent fixity in the positions of the stars is due to their immense
distance, combined with the shortness of the time during which we are
able to observe them. It is like viewing the plume of smoke issuing
from a steamer, hull down, at sea: if one does not continue to watch it
for a long time it appears to be motionless, although in reality it may
be traveling at great speed across the line of sight. Even the planets
seem fixed in position if one watches them for a single night only, and
the more distant ones do not sensibly change their places, except after
many nights of observation. Neptune, for instance, moves but little
more than two degrees in the course of an entire year, and in a month
its change of place is only about one-third of the diameter of the full
moon.

Yet, fixed as they seem, the stars are actually moving with a speed in
comparison with which, in some cases, the planets might almost be said
to stand fast in their tracks. Jupiter’s speed in his orbit is about
eight miles per second, Neptune’s is less than three and one-half
miles, and the earth’s is about eighteen and one-half miles; while
there are “fixed stars” which move two hundred or three hundred miles
per second. They do not all, however, move with so great a velocity,
for some appear to travel no faster than the planets. But in all cases,
notwithstanding their real speed, long-continued and exceedingly
careful observations are required to demonstrate that they are moving
at all. No more overwhelming impression of the frightful depths of
space in which the stars are buried can be obtained than by reflecting
upon the fact that a star whose actual motion across the line of sight
amounts to two hundred miles per second does not change its apparent
place in the sky, in the course of a thousand years, sufficiently to be
noticed by the casual observer of the heavens!

There is one vast difference between the motions of the stars and those
of the planets to which attention should be at once called: the
planets, being under the control of a central force emanating from
their immediate master, the sun, all move in the same direction and in
orbits concentric about the sun; the stars, on the other hand, move in
every conceivable direction and have no apparent center of motion, for
all efforts to discover such a center have failed. At one time, when
theology had finally to accept the facts of science, a grandiose
conception arose in some pious minds, according to which the Throne of
God was situated at the exact center of His Creation, and, seated
there, He watched the magnificent spectacle of the starry systems
obediently revolving around Him. Astronomical discoveries and
speculations seemed for a time to afford some warrant for this view,
which was, moreover, an acceptable substitute for the abandoned
geocentric theory in minds that could only conceive of God as a
superhuman artificer, constantly admiring his own work. No longer ago
than the middle of the nineteenth century a German astronomer, Maedler,
believed that he had actually found the location of the center about
which the stellar universe revolved. He placed it in the group of the
Pleiades, and upon his authority an extraordinary imaginative picture
was sometimes drawn of the star Alcyone, the brightest of the Pleiades,
as the very seat of the Almighty. This idea even seemed to gain a kind
of traditional support from the mystic significance, without known
historical origin, which has for many ages, and among widely separated
peoples, been attached to the remarkable group of which Alcyone is the
chief. But since Maedler’s time it has been demonstrated that the
Pleiades cannot be the center of revolution of the universe, and, as
already remarked, all attempts to find or fix such a center have proved
abortive. Yet so powerful was the hold that the theory took upon the
popular imagination, that even today astronomers are often asked if
Alcyone is not the probable site of “Jerusalem the Golden.”

If there were a discoverable center of predominant gravitative power,
to which the motions of all the stars could be referred, those motions
would appear less mysterious, and we should then be able to conclude
that the universe was, as a whole, a prototype of the subsidiary
systems of which it is composed. We should look simply to the law of
gravitation for an explanation, and, naturally, the center would be
placed within the opening enclosed by the Milky Way. If it were there
the Milky Way itself should exhibit signs of revolution about it, like
a wheel turning upon its hub. No theory of the star motions as a whole
could stand which failed to take account of the Milky Way as the basis
of all. But the very form of that divided wreath of stars forbids the
assumption of its revolution about a center. Even if it could be
conceived as a wheel having no material center it would not have the
form which it actually presents. As was shown in Chapter 2, there is
abundant evidence of motion in the Milky Way; but it is not motion of
the system as a whole, but motion affecting its separate parts. Instead
of all moving one way, the galactic stars, as far as their movements
can be inferred, are governed by local influences and conditions. They
appear to travel crosswise and in contrary directions, and perhaps they
eddy around foci where great numbers have assembled; but of a universal
revolution involving the entire mass we have no evidence.

Most of our knowledge of star motions, called “proper motions,” relates
to individual stars and to a few groups which happen to be so near that
the effects of their movements are measurable. In some cases the motion
is so rapid (not in appearance, but in reality) that the chief
difficulty is to imagine how it can have been imparted, and what will
eventually become of the “runaways.” Without a collision, or a series
of very close approaches to great gravitational centers, a star
traveling through space at the rate of two hundred or three hundred
miles per second could not be arrested or turned into an orbit which
would keep it forever flying within the limits of the visible universe.
A famous example of these speeding stars is “1830 Groombridge,” a star
of only the sixth magnitude, and consequently just visible to the naked
eye, whose motion across the line of sight is so rapid that it moves
upon the face of the sky a distance equal to the apparent diameter of
the moon every 280 years. The distance of this star is at least
200,000,000,000,000 miles, and may be two or three times greater, so
that its actual speed cannot be less than two hundred, and may be as
much as four hundred, miles per second. It could be turned into a new
course by a close approach to a great sun, but it could only be stopped
by collision, head-on, with a body of enormous mass. Barring such
accidents it must, as far as we can see, keep on until it has traversed
our stellar system, whence in may escape and pass out into space
beyond, to join, perhaps, one of those other universes of which we have
spoken. Arcturus, one of the greatest suns in the universe, is also a
runaway, whose speed of flight has been estimated all the way from
fifty to two hundred miles per second. Arcturus, we have every reason
to believe, possesses hundreds of times the mass of our sun—think,
then, of the prodigious momentum that its motion implies! Sirius moves
more moderately, its motion across the line of sight amounting to only
ten miles per second, but it is at the same time approaching the sun at
about the same speed, its actual velocity in space being the resultant
of the two displacements.

What has been said about the motion of Sirius brings us to another
aspect of this subject. The fact is, that in every case of stellar
motion the displacement that we observe represents only a part of the
actual movement of the star concerned. There are stars whose motion
carries them straight toward or straight away from the earth, and such
stars, of course, show no cross motion. But the vast majority are
traveling in paths inclined from a perpendicular to our line of sight.
Taken as a whole, the stars may be said to be flying about like the
molecules in a mass of gas. The discovery of the radial component in
the movements of the stars is due to the spectroscope. If a star is
approaching, its spectral lines are shifted toward the violet end of
the spectrum by an amount depending upon the velocity of approach; if
it is receding, the lines are correspondingly shifted toward the red
end. Spectroscopic observation, then, combined with micrometric
measurements of the cross motion, enables us to detect the real
movement of the star in space. Sometimes it happens that a star’s
radial movement is periodically reversed; first it approaches, and then
it recedes. This indicates that it is revolving around a near-by
companion, which is often invisible, and superposed upon this motion is
that of the two stars concerned, which together may be approaching or
receding or traveling across the line of sight. Thus the complications
involved in the stellar motions are often exceedingly great and
puzzling.

Yet another source of complication exists in the movement of our own
star, the sun. There is no more difficult problem in astronomy than
that of disentangling the effects of the solar motion from those of the
motions of the other stars. But the problem, difficult as it is, has
been solved, and upon its solution depends our knowledge of the speed
and direction of the movement of the solar system through space, for of
course the sun carries its planets with it. One element of the solution
is found in the fact that, as a result of perspective, the stars toward
which we are going appear to move apart toward all points of the
compass, while those behind appear to close up together. Then the
spectroscopic principle already mentioned is invoked for studying the
shift of the lines, which is toward the violet in the stars ahead of us
and toward the red in those that we are leaving behind. Of course the
effects of the independent motions of the stars must be carefully
excluded. The result of the studies devoted to this subject is to show
that we are traveling at a speed of twelve to fifteen miles per second
in a northerly direction, toward the border of the constellations
Hercules and Lyra. A curious fact is that the more recent estimates
show that the direction is not very much out of a straight line drawn
from the sun to the star Vega, one of the most magnificent suns in the
heavens. But it should not be inferred from this that Vega is drawing
us on; it is too distant for its gravitation to have such an effect.

Many unaccustomed thoughts are suggested by this mighty voyage of the
solar system. Whence have we come, and whither do we go? Every year of
our lives we advance at least 375,000,000 miles. Since the traditional
time of Adam the sun has led his planets through the wastes of space no
less than 225,000,000,000 miles, or more than 2400 times the distance
that separates him from the earth. Go back in imagination to the
geologic ages, and try to comprehend the distance over which the earth
has flown. Where was our little planet when it emerged out of the
clouds of chaos? Where was the sun when his “thunder march” began? What
strange constellations shone down upon our globe when its masters of
life were the monstrous beasts of the “Age of Reptiles”? A million
years is not much of a span of time in geologic reckoning, yet a
million years ago the earth was farther from its present place in space
than any of the stars with a measurable parallax are now. It was more
than seven times as far as Sirius, nearly fourteen times as far as
Alpha Centauri, three times as far as Vega, and twice as far as
Arcturus. But some geologists demand two hundred, three hundred, even
one thousand million years to enable them to account for the
evolutionary development of the earth and its inhabitants. In a
thousand million years the earth would have traveled farther than from
the remotest conceivable depths of the Milky Way!

Other curious reflections arise when we think of the form of the
earth’s track as it follows the lead of the sun, in a journey which has
neither known beginning nor conceivable end. There are probably many
minds which have found a kind of consolation in the thought that every
year the globe returns to the same place, on the same side of the sun.
This idea may have an occult connection with our traditional regard for
anniversaries. When that period of the year returns at which any great
event in our lives has occurred we have the feeling that the earth, in
its annual round, has, in a manner, brought us back to the scene of
that event. We think of the earth’s orbit as a well-worn path which we
traverse many times in the course of a lifetime. It seems familiar to
us, and we grow to have a sort of attachment to it. The sun we are
accustomed to regard as a fixed center in space, like the mill or pump
around which the harnessed patient mule makes his endless circuits. But
the real fact is that the earth never returns to the place in space
where it has once quitted. In consequence of the motion of the sun
carrying the earth and the other planets along, the track pursued by
our globe is a vast spiral in space continually developing and never
returning upon its course. It is probable that the tracks of the sun
and the others stars are also irregular, and possibly spiral, although,
as far as can be at present determined, they appear to be practically
straight. Every star, wherever it may be situated, is attracted by its
fellow-stars from many sides at once, and although the force is
minimized by distance, yet in the course of many ages its effects must
become manifest.

Looked at from another side, is there not something immensely
stimulating and pleasing to the imagination in the idea of so
stupendous a journey, which makes all of us the greatest of travelers?
In the course of a long life a man is transported through space thirty
thousand million miles; Halley’s Comet does not travel one-quarter as
far in making one of its immense circuits. And there are adventures on
this voyage of which we are just beginning to learn to take account.
Space is full of strange things, and the earth must encounter some of
them as it advances through the unknown. Many singular speculations
have been indulged in by astronomers concerning the possible effects
upon the earth of the varying state of the space that it traverses.
Even the alternation of hot and glacial periods has sometimes been
ascribed to this source. When tropical life flourished around the
poles, as the remains in the rocks assure us, the needed high
temperature may, it has been thought, have been derived from the
presence of the earth in a warm region of space. Then, too, there is a
certain interest for us in the thought of what our familiar planet has
passed through. We cannot but admire it for its long journeying as we
admire the traveler who comes to us from remote and unexplored lands,
or as we gaze with a glow of interest upon the first locomotive that
has crossed a continent, or a ship that has visited the Arctic or
Antarctic regions. If we may trust the indications of the present
course, the earth, piloted by the sun, has come from the Milky Way in
the far south and may eventually rejoin that mighty band of stars in
the far north.

While the stars in general appear to travel independently of one
another, except when they are combined in binary or trinary systems,
there are notable exceptions to this rule. In some quarters of the sky
we behold veritable migrations of entire groups of stars whose members
are too widely separated to show any indications of revolution about a
common center of gravity. This leads us back again to the wonderful
group of the Pleiades. All of the principle stars composing that group
are traveling in virtually parallel lines. Whatever force set them
going evidently acted upon all alike. This might be explained by the
assumption that when the original projective force acted upon them they
were more closely united than they are at present, and that in drifting
apart they have not lost the impulse of the primal motion. Or it may be
supposed that they are carried along by some current in space, although
it would be exceedingly difficult, in the present state of our
knowledge, to explain the nature of such a current. Yet the theory of a
current has been proposed. As to an attractive center around which they
might revolve, none has been found. Another instance of similar
“star-drift” is furnished by five of the seven stars constituting the
figure of the “Great Dipper.” In this case the stars concerned are
separated very widely, the two extreme ones by not less than fifteen
degrees, so that the idea of a common motion would never have been
suggested by their aspect in the sky; and the case becomes the more
remarkable from the fact that among and between them there are other
stars, some of the same magnitude, which do not share their motion, but
are traveling in other directions. Still other examples of the same
phenomenon are found in other parts of the sky. Of course, in the case
of compact star-clusters, it is assumed that all the members share a
like motion of translation through space, and the same is probably true
of dense star-swarms and star-clouds.

The whole question of star-drift has lately assumed a new phase, in
consequence of the investigations of Kapteyn, Dyson, and Eddington on
the “systematic motions of the stars.” This research will, it is hoped,
lead to an understanding of the general law governing the movements of
the whole body of stars constituting the visible universe. Taking about
eleven hundred stars whose proper motions have been ascertained with an
approach to certainty, and which are distributed in all parts of the
sky, it has been shown that there exists an apparent double drift, in
two independent streams, moving in different and nearly opposed
directions. The apex of the motion of what is called “Stream I” is
situated, according to Professor Kapteyn, in right ascension 85°,
declination south 11°, which places it just south of the constellation
Orion; while the apex of “Stream II” is in right ascension 260°,
declination south 48°, placing it in the constellation Ara, south of
Scorpio. The two apices differ very nearly 180° in right ascension and
about 120° in declination. The discovery of these vast star-streams, if
they really exist, is one of the most extraordinary in modern
astronomy. It offers the correlation of stellar movements needed as the
basis of a theory of those movements, but it seems far from revealing a
physical cause for them. As projected against the celestial sphere the
stars forming the two opposite streams appear intermingled, some
obeying one tendency and some the other. As Professor Dyson has said,
the hypothesis of this double movement is of a revolutionary character,
and calls for further investigation. Indeed, it seems at first glance
not less surprising than would be the observation that in a snow-storm
the flakes over our heads were divided into two parties and driving
across each other’s course in nearly opposite directions, as if urged
by interpenetrating winds.

But whatever explanation may eventually be found for the motions of the
stars, the knowledge of the existence of those motions must always
afford a new charm to the contemplative observer of the heavens, for
they impart a sense of life to the starry system that would otherwise
be lacking. A stagnant universe, with every star fixed immovably in its
place, would not content the imagination or satisfy our longing for
ceaseless activity. The majestic grandeur of the evolutions of the
celestial hosts, the inconceivable vastness of the fields of space in
which they are executed, the countless numbers, the immeasurable
distances, the involved convolutions, the flocking and the scattering,
the interpenetrating marches and countermarches, the strange community
of impulsion affecting stars that are wide apart in space and causing
them to traverse the general movement about them like aides and
despatch-bearers on a battle-field—all these arouse an intensity of
interest which is heightened by the mystery behind them.




IV
The Passing of the Constellations


From a historical and picturesque point of view, one of the most
striking results of the motions of the stars described in the last
chapter is their effect upon the forms of the constellations, which
have been watched and admired by mankind from a period so early that
the date of their invention is now unknown. The constellations are
formed by chance combinations of conspicuous stars, like figures in a
kaleidoscope, and if our lives were commensurate with the æons of
cosmic existence we should perceive that the kaleidoscope of the
heavens was ceaselessly turning and throwing the stars into new
symmetries. Even if the stars stood fast, the motion of the solar
system would gradually alter the configurations, as the elements of a
landscape dissolve and recombine in fresh groupings with the traveler’s
progress amid them. But with the stars themselves all in motion at
various speeds and in many directions, the changes occur more rapidly.
Of course, “rapid” is here understood in a relative sense; the wheel of
human history to an eye accustomed to the majestic progression of the
universe would appear to revolve with the velocity of a whirling
dynamo. Only the deliberation of geological movements can be contrasted
with the evolution and devolution of the constellations.

And yet this secular fluctuation of the constellation figures is not
without keen interest for the meditative observer. It is another
reminder of the swift mutability of terrestial affairs. To the passing
glance, which is all that we can bestow upon these figures, they appear
so immutable that they have been called into service to form the most
lasting records of ancient thought and imagination that we possess. In
the forms of the constellations, the most beautiful, and, in
imaginative quality, the finest, mythology that the world has ever
known has been perpetuated. Yet, in a broad sense, this scroll of human
thought imprinted on the heavens is as evanescent as the summer clouds.
Although more enduring than parchment, tombs, pyramids, and temples, it
is as far as they from truly eternizing the memory of what man has
fancied and done.

Before studying the effects that the motions of the stars have had and
will have upon the constellations, it is worth while to consider a
little further the importance of the stellar pictures as archives of
history. To emphasize the importance of these effects it is only
necessary to recall that the constellations register the oldest
traditions of our race. In the history of primeval religions they are
the most valuable of documents. Leaving out of account for the moment
the more familiar mythology of the Greeks, based on something older
yet, we may refer for illustration to that of the mysterious Maya race
of America. At Izamal, in Yucatan, says Mr Stansbury Hagar, is a group
of ruins perched, after the Mexican and Central-American plan, on the
summits of pyramidal mounds which mark the site of an ancient theogonic
center of the Mayas. Here the temples all evidently refer to a cult
based upon the constellations as symbols. The figures and the names, of
course, were not the same as those that we have derived from our Aryan
ancestors, but the star groups were the same or nearly so. For
instance, the loftiest of the temples at Izamal was connected with the
sign of the constellation known to us as Cancer, marking the place of
the sun at the summer solstice, at which period the sun was supposed to
descend at noon like a great bird of fire and consume the offerings
left upon the altar. Our Scorpio was known to the Mayas as a sign of
the “Death God.” Our Libra, the “Balance,” with which the idea of a
divine weighing out of justice has always been connected, seems to be
identical with the Mayan constellation Teoyaotlatohua, with which was
associated a temple where dwelt the priests whose special business it
was to administer justice and to foretell the future by means of
information obtained from the spirits of the dead. Orion, the “Hunter”
of our celestial mythology, was among the Mayas a “Warrior,” while
Sagittarius and others of our constellations were known to them (under
different names, of course), and all were endowed with a religious
symbolism. And the same star figures, having the same significance,
were familiar to the Peruvians, as shown by the temples at Cuzco. Thus
the imagination of ancient America sought in the constellations symbols
of the unchanging gods.

But, in fact, there is no nation and no people that has not recognized
the constellations, and at one period or another in its history
employed them in some symbolic or representative capacity. As handled
by the Greeks from prehistoric times, the constellation myths became
the very soul of poetry. The imagination of that wonderful race
idealized the principal star groups so effectively that the figures and
traditions thus attached to them have, for civilized mankind, displaced
all others, just as Greek art in its highest forms stands without
parallel and eclipses every rival. The Romans translated no heroes and
heroines of the mythical period of their history to the sky, and the
deified Cæsars never entered that lofty company, but the heavens are
filled with the early myths of the Greeks. Herakles nightly resumes his
mighty labors in the stars; Zeus, in the form of the white “Bull,”
Taurus, bears the fair Europa on his back through the celestial waves;
Andromeda stretches forth her shackled arms in the star-gemmed ether,
beseeching aid; and Perseus, in a blaze of diamond armor, revives his
heroic deeds amid sparkling clouds of stellar dust. There, too, sits
Queen Cassiopeia in her dazzling chair, while the Great King, Cepheus,
towers gigantic over the pole. Professor Young has significantly
remarked that a great number of the constellations are connected in
some way or other with the Argonautic Expedition—that strangely
fascinating legend of earliest Greek story which has never lost its
charm for mankind. In view of all this, we may well congratulate
ourselves that the constellations will outlast our time and the time of
countless generations to follow us; and yet they are very far from
being eternal. Let us now study some of the effects of the stellar
motions upon them.

We begin with the familiar figure of the “Great Dipper.” He who has not
drunk inspiration from its celestial bowl is not yet admitted to the
circle of Olympus. This figure is made up of seven conspicuous stars in
the constellation Ursa Major, the “Greater Bear.” The handle of the
“Dipper” corresponds to the tail of the imaginary “Bear,” and the bowl
lies upon his flank. In fact, the figure of a dipper is so evident and
that of a bear so unevident, that to most persons the “Great Dipper” is
the only part of the constellation that is recognizable. Of the seven
stars mentioned, six are of nearly equal brightness, ranking as of the
second magnitude, while the seventh is of only the third magnitude. The
difference is very striking, since every increase of one magnitude
involves an increase of two-and-a-half times in brightness. There
appears to be little doubt that the faint star, which is situated at
the junction of the bowl and the handle, is a variable of long period,
since three hundred years ago it was as bright as its companions. But
however that may be, its relative faintness at the present time
interferes but little with the perfection of the “Dipper’s” figure. In
order the more readily to understand the changes which are taking
place, it will be well to mention both the names and the Greek letters
which are attached to the seven stars. Beginning at the star in the
upper outer edge of the rim of the bowl and running in regular order
round the bottom and then out to the end of the handle, the names and
letters are as follows: Dubhe (α), Merak (β), Phaed (γ), Megrez (δ),
Alioth (ε), Mizar (ζ), and Benetnasch (η). Megrez is the faint star
already mentioned at the junction of the bowl and handle, and Mizar, in
the middle of the handle, has a close, naked-eye companion which is
named Alcor. The Arabs called this singular pair of stars “The Horse
and Rider.” Merak and Duhbe are called “The Pointers,” because an
imaginary line drawn northward through them indicates the Pole Star.


[Illustration: The “Great Dipper”]


Now it has been found that five of these stars—_viz.,_ Merak, Phaed,
Megrez, Alioth, and Mizar (with its comrade)—are moving with
practically the same speed in an easterly direction, while the other
two, Dubhe and Benetnasch, are simultaneously moving westward, the
motions of Benetnasch being apparently more rapid. The consequence of
these opposed motions is, of course, that the figure of the “Dipper”
cannot always have existed and will not continue to exist. In the
accompanying diagrams it has been thought interesting to show the
relative positions of these seven stars, as seen from the point which
the earth now occupies, both in the past and in the future. Arrows
attached to the stars in the figure representing the present appearance
of the “Dipper” indicate the directions of the motions and the
distances over which they will carry the stars in a period of about
five hundred centuries. The time, no doubt, seems long, but remember
the vast stretch of ages through which the earth has passed, and then
reflect that no reason is apparent why our globe should not continue to
be a scene of animation for ten thousand centuries yet to come. The
fact that the little star Alcor placed so close to Mizar should
accompany the latter in its flight is not surprising, but that two of
the principal stars of the group should be found moving in a direction
directly opposed to that pursued by the other five is surprising in the
highest degree; and it recalls the strange theory of a double drift
affecting all the stars, to which attention was called in the preceding
chapter. It would appear that Benetnasch and Dubhe belong to one
“current,” and Merak, Phaed, Megrez, Alioth, and Mizar to the other. As
far as is known, the motion of the seven stars are not shared by the
smaller stars scattered about them, but on the theory of currents there
should be such a community of motion, and further investigation may
reveal it.


[Illustration: Cassiopeia]


From the “Great Dipper” we turn to a constellation hardly less
conspicuous and situated at an equal distance from the pole on the
other side—Cassiopeia. This famous star-group commemorating the
romantic Queen of Ethiopia whose vain boasting of her beauty was
punished by the exposure of her daughter Andromeda to the “Sea
Monster,” is well-marked by five stars which form an irregular letter
“W” with its open side toward the pole. Three of these stars are
usually ranked as of the second magnitude, and two of the third; but to
ordinary observation they appear of nearly equal brightness, and
present a very striking picture. They mark out the chair and a part of
the figure of the beautiful queen. Beginning at the right-hand, or
western, end of the “W,” their Greek letter designations are: Beta (β),
Alpha (α), Gamma (γ), Delta (δ), and Epsilon (ε). Four of them, Beta,
Alpha, Delta, and Epsilon are traveling eastwardly at various speeds,
while the fifth, Gamma, moves in a westerly direction. The motion of
Beta is more rapid than that of any of the others. It should be said,
however, that no little uncertainty attaches to the estimates of the
rate of motion of stars which are not going very rapidly, and different
observers often vary considerably in their results.

In the beautiful “Northern Crown,” one of the most perfect and charming
of all the figures to be found in the stars, the alternate combining
and scattering effects of the stellar motions are shown by comparing
the appearance which the constellation must have had five hundred
centuries ago with that which it has at present and that which it will
have in the future. The seven principle stars of the asterism, forming
a surprisingly perfect coronet, have movements in three directions at
right angles to one another. That in these circumstances they should
ever have arrived at positions giving them so striking an appearance of
definite association is certainly surprising; from its aspect one would
have expected to find a community of movement governing the brilliants
of the “Crown,” but instead of that we find evidence that they will
inevitably drift apart and the beautiful figure will dissolve.

A similar fate awaits such asterisms as the “Northern Cross” in Cygnus;
the “Crow” (Corvus), which stands on the back of the great “Sea
Serpent,” Hydra, and pecks at his scales; “Job’s Coffin” (Delphinus);
the “Great Square of Pegasus”; the “Twins” (Gemini); the beautiful
“Sickle” in Leo; and the exquisite group of the Hyades in Taurus. In
the case of the Hyades, two controlling movements are manifest: one,
affecting five of the stars which form the well-known figure of a
letter “V,” is directed northerly; the other, which controls the
direction of two stars, has an easterly trend. The chief star of the
group, Aldebaran, one of the finest of all stars both for its
brilliance and its color, is the most affected by the easterly motion.
In time it will drift entirely out of connection with its present
neighbors. Although the Hyades do not form so compact a group as the
Pleiades in the same constellation, yet their appearance of
relationship is sufficient to awaken a feeling of surprise over the
fact that, as with the stars of the “Dipper,” their association is only
temporary or apparent.


[Illustration: The “Northern Crown”]


The great figure of Orion appears to be more lasting, not because its
stars are physically connected, but because of their great distance,
which renders their movements too deliberate to be exactly ascertained.
Two of the greatest of its stars, Betelgeuse and Rigel, possess, as far
as has been ascertained, no perceptible motion across the line of
sight, but there is a little movement perceptible in the “Belt.” At the
present time this consists of an almost perfect straight line, a row of
second-magnitude stars about equally spaced and of the most striking
beauty. In the course of time, however, the two right-hand stars,
Mintaka and Alnilam (how fine are these Arabic star names!) will
approach each other and form a naked-eye double, but the third, Alnita,
will drift away eastward, so that the “Belt” will no longer exist.

For one more example, let us go to the southern hemisphere, whose most
celebrated constellation, the “Southern Cross,” has found a place in
all modern literatures, although it has no claim to consideration on
account of association with ancient legends. This most attractive
asterism, which has never ceased to fascinate the imagination of
Christendom since it was first devoutly described by the early
explorers of the South, is but a passing collocation of brilliant
stars. Yet even in its transfigurations it has been for hundreds of
centuries, and will continue to be for hundreds of centuries to come, a
most striking object in the sky. Our figures show its appearance in
three successive phases: first, as it was fifty thousand years ago
(viewed from the earth’s present location); second, as it is in our
day; and, third, as it will be an equal time in the future. The
nearness of these bright stars to one another—the length of the longer
beam of the “Cross” is only sixdegrees—makes this group very
noticeable, whatever the arrangement of its components may be. The
largest star, at the base of the “Cross,” is of the first magnitude,
two of the others are of the second magnitude, and the fourth is of the
third. Other stars, not represented in the figures, increase the effect
of a celestial blazonry, although they do not help the resemblance to a
cross.


[Illustration: The “Southern Cross”]


But since the motion of the solar system itself will, in the course of
so long a period as fifty thousand years, produce a great change in the
perspective of the heavens as seen from the earth, by carrying us
nearly nineteen trillion miles from our present place, why, it may be
asked, seek to represent future appearances of the constellations which
we could not hope to see, even if we could survive so long? The answer
is: Because these things aid the mind to form a picture of the effects
of the mobility of the starry universe. Only by showing the changes
from some definite point of view can we arrive at a due comprehension
of them. The constellations are more or less familiar to everybody, so
that impending changes of their forms must at once strike the eye and
the imagination, and make clearer the significance of the movements of
the stars. If the future history of mankind is to resemble its past and
if our race is destined to survive yet a million years, then our remote
descendents will see a “new heavens” if not a “new earth,” and will
have to invent novel constellations to perpetuate their legends and
mythologies.

If our knowledge of the relative distances of the stars were more
complete, it would be an interesting exercise in celestial geometry to
project the constellations probably visible to the inhabitants of
worlds revolving around some of the other suns of space. Our sun is too
insignificant for us to think that he can make a conspicuous appearance
among them, except, perhaps, in a few cases. As seen, for instance,
from the nearest known star, Alpha Centauri, the sun would appear of
the average first magnitude, and consequently from that standpoint he
might be the gem of some little constellation which had no Sirius, or
Arcturus, or Vega to eclipse him with its superior splendor. But from
the distance of the vast majority of the stars the sun would probably
be invisible to the naked eye, and as seen from nearer systems could
only rank as a fifth or sixth magnitude star, unnoticed and unknown
except by the star-charting astronomer.




V
Conflagrations in the Heavens


Suppose it were possible for the world to take fire and burn up—as some
pessimists think that it will do when the Divine wrath shall have
sufficiently accumulated against it—nobody out of our own little corner
of space would ever be aware of the catastrophe! With all their
telescopes, the astronomers living in the golden light of Arcturus or
the diamond blaze of Canopus would be unable to detect the least
glimmer of the conflagration that had destroyed the seat of Adam and
his descendents, just as now they are totally ignorant of its
existence.

But at least fifteen times in the course of recorded history men
looking out from the earth have beheld in the remote depths of space
great outbursts of fiery light, some of them more splendidly luminous
than anything else in the firmament except the sun! If _they_ were
conflagrations, how many million worlds like ours were required to feed
their blaze?

It is probable that “temporary” or “new” stars, as these wonderful
apparitions are called, really are conflagrations; not in the sense of
a bonfire or a burning house or city, but in that of a sudden eruption
of inconceivable heat and light, such as would result from the
stripping off the shell of an encrusted sun or the crashing together of
two mighty orbs flying through space with a hundred times the velocity
of the swiftest cannon-shot.

Temporary stars are the rarest and most erratic of astronomical
phenomena. The earliest records relating to them are not very clear,
and we cannot in every instance be certain that it was one of these
appearances that the ignorant and superstitious old chroniclers are
trying to describe. The first temporary star that we are absolutely
sure of appeared in 1572, and is known as “Tycho’s Star,” because the
celebrated Danish astronomer (whose remains, with his gold-and-silver
artificial nose—made necessary by a duel—still intact, were disinterred
and reburied in 1901) was the first to perceive it in the sky, and the
most assiduous and successful in his studies of it. As the first fully
accredited representative of its class, this new star made its entry
upon the scene with becoming _éclat._ It is characteristic of these
phenomena that they burst into view with amazing suddenness, and, of
course, entirely unexpectedly. Tycho’s star appeared in the
constellation Cassiopeia, near a now well-known and much-watched little
star named Kappa, on the evening of November 11, 1572. The story has
often been repeated, but it never loses interest, how Tycho, going home
that evening, saw people in the street pointing and staring at the sky
directly over their heads, and following the direction of their hands
and eyes he was astonished to see, near the zenith, an unknown star of
surpassing brilliance. It outshone the planet Jupiter, and was
therefore far brighter than the first magnitude. There was not another
star in the heavens that could be compared with it in splendor. Tycho
was not in all respects free from the superstitions of his time—and who
is?—but he had the true scientific instinct, and immediately he began
to study the stranger, and to record with the greatest care every
change in its aspect. First he determined as well as he could with the
imperfect instruments of his day, many of which he himself had
invented, the precise location of the phenomena in the sky. Then he
followed the changes that it underwent. At first it brightened until
its light equaled or exceeded that of the planet Venus at her
brightest, a statement which will be appreciated at its full value by
anyone who has ever watched Venus when she plays her dazzling rôle of
“Evening Star,” flaring like an arc light in the sunset sky. It even
became so brilliant as to be visible in full daylight, since, its
position being circumpolar, it never set in the latitude of Northern
Europe. Finally it began to fade, turning red as it did so, and in
March, 1574, it disappeared from Tycho’s searching gaze, and has never
been seen again from that day to this. None of the astronomers of the
time could make anything of it. They had not yet as many bases of
speculation as we possess today.


[Illustration: Chart showing location of Tycho’s star, 1572,  and Nova
Persei of 1901]


Tycho’s star has achieved a romantic reputation by being fancifully
identified with the “Star of Bethlehem,” said to have led the wondering
Magi from their eastern deserts to the cradle-manger of the Savior in
Palestine. Many attempts have been made to connect this traditional
“star” with some known phenomenon of the heavens, and none seems more
idle than this. Yet it persistently survives, and no astronomer is free
from eager questions about it addressed by people whose imagination has
been excited by the legend. It is only necessary to say that the
supposition of a connection between the phenomenon of the Magi and
Tycho’s star is without any scientific foundation. It was originally
based on an unwarranted assumption that the star of Tycho was a
variable of long period, appearing once every three hundred and fifteen
years, or thereabout. If that were true there would have been an
apparition somewhere near the traditional date of the birth of Christ,
a date which is itself uncertain. But even the data on which the
assumption was based are inconsistent with the theory. Certain monkish
records speak of something wonderful appearing in the sky in the years
1264 and 945, and these were taken to have been outbursts of Tycho’s
star. Investigation shows that the records more probably refer to
comets, but even if the objects seen were temporary stars, their dates
do not suit the hypothesis; from 945 to 1264 there is a gap of 319
years, and from 1264 to 1572 one of only 308 years; moreover 337 years
have now (1909) elapsed since Tycho saw the last glimmer of his star.
Upon a variability so irregular and uncertain as that, even if we felt
sure that it existed, no conclusion could be found concerning an
apparition occurring 2000 years ago.

In the year 1600 (the year in which Giordano Bruno was burned at the
stake for teaching that there is more than one physical world), a
temporary star of the third magnitude broke out in the constellation
Cygnus, and curiously enough, considering the rarity of such phenomena,
only four years later another surprisingly brilliant one appeared in
the constellation Ophiuchus. This is often called “Kepler’s star,”
because the great German astronomer devoted to it the same attention
that Tycho had given to the earlier phenomenon. It, too, like Tycho’s,
was at first the brightest object in the stellar heavens, although it
seems never to have quite equaled its famous predecessor in splendor.
It disappeared after a year, also turning of a red color as it became
more faint. We shall see the significance of this as we go on. Some of
Kepler’s contemporaries suggested that the outburst of this star was
due to a meeting of atoms in space, and idea bearing a striking
resemblance to the modern theory of “astronomical collisions.”

In 1670, 1848, and 1860 temporary stars made their appearance, but none
of them was of great brilliance. In 1866 one of the second magnitude
broke forth in the “Northern Crown” and awoke much interest, because by
that time the spectroscope had begun to be employed in studying the
composition of the stars, and Huggins demonstrated that the new star
consisted largely of incandescent hydrogen. But this star, apparently
unlike the others mentioned, was not absolutely new. Before its
outburst it had shown as a star of the ninth magnitude (entirely
invisible, of course, to the naked eye), and after about six weeks it
faded to its original condition in which it has ever since remained. In
1876 a temporary star appeared in the constellation Cygnus, and
attained at one time the brightness of the second magnitude. Its
spectrum and its behavior resembled those of its immediate predecessor.
In 1885, astronomers were surprised to see a sixth-magnitude star
glimmering in the midst of the hazy cloud of the great Andromeda
Nebula. It soon absolutely disappeared. Its spectrum was remarkable for
being “continuous,” like that of the nebula itself. A continuous
spectrum is supposed to represent a body, or a mass, which is either
solid or liquid, or composed of gas under great pressure. In January,
1892, a new star was suddenly seen in the constellation Auriga. It
never rose much above the fourth magnitude, but it showed a peculiar
spectrum containing both bright and dark lines of hydrogen.

But a bewildering surprise was now in store; the world was to behold at
the opening of the twentieth century such a celestial spectacle as had
not been on view since the times of Tycho and Kepler. Before daylight
on the morning of February 22, 1901, the Rev. Doctor Anderson, of
Edinburgh, an amateur astronomer, who had also been the first to see
the new star in Auriga, beheld a strange object in the constellation
Perseus not far from the celebrated variable star Algol. He recognized
its character at once, and immediately telegraphed the news, which
awoke the startled attention of astronomers all over the world. When
first seen the new star was no brighter than Algol (less than the
second magnitude), but within twenty-four hours it was ablaze,
outshining even the brilliant Capella, and far surpassing the first
magnitude. At the spot in the sky where it appeared nothing whatever
was visible on the night before its coming. This is known with
certainty because a photograph had been made of that very region on
February 21, and this photograph showed everything down to the twelfth
magnitude, but not a trace of the stranger which burst into view
between the 21st and the 22nd like the explosion of a rocket.

Upon one who knew the stars the apparition of this intruder in a
well-known constellation had the effect of a sudden invasion. The new
star was not far west of the zenith in the early evening, and in that
position showed to the best advantage. To see Capella, the hitherto
unchallenged ruler of that quarter of the sky, abased by comparison
with this stranger of alien aspect, for there was always an unfamiliar
look about the “nova,” was decidedly disconcerting. It seemed to
portend the beginning of a revolution in the heavens. One could
understand what the effect of such an apparition must have been in the
superstitious times of Tycho. The star of Tycho had burst forth on the
northern border of the Milky Way; this one was on its southern border,
some forty-five degrees farther east.

Astronomers were well-prepared this time for the scientific study of
the new star, both astronomical photography and spectroscopy having
been perfected, and the results of their investigations were calculated
to increase the wonder with which the phenomenon was regarded. The star
remained at its brightest only a few days; then, like a veritable
conflagration, it began to languish; and, like the reflection of a
dying fire, as it sank it began to glow with the red color of embers.
But its changes were spasmodic; once about every three days it flared
up only to die away again. During these fluctuations its light varied
alternately in the ratio of one to six. Finally it took a permanent
downward course, and after a few months the naked eye could no longer
perceive it; but it remained visible with telescopes, gradually fading
until it had sunk to the ninth magnitude. Then another astonishing
change happened: in August photographs taken at the Yerkes Observatory
and at Heidelberg showed that the “nova” was _surrounded by a spiral
nebula!_ The nebula had not been there before, and no one could doubt
that it represented a phase of the same catastrophe that had produced
the outburst of the new star. At one time the star seemed virtually to
have disappeared, as if all its substance had been expanded into the
nebulous cloud, but always there remained a stellar nucleus about which
the misty spiral spread wider and ever wider, like a wave expanding
around a center of disturbance. The nebula too showed a variability of
brightness, and four condensations which formed in it seemed to have a
motion of revolution about the star. As time went on the nebula
continued to expand at a rate which was computed to be not less than
twenty thousand miles per second! And now the star itself, showing
indications of having turned into a nebula, behaved in a most erratic
manner, giving rise to the suspicion that it was about to burst out
again. But this did not occur, and at length it sunk into a state of
lethargy from which it has to the present time not recovered. But the
nebulous spiral has disappeared, and the entire phenomena as it now
(1909) exists consists of a faint nebulous star of less than the ninth
magnitude.

The wonderful transformations just described had been forecast in
advance of the discovery of the nebulous spiral encircling the star by
the spectroscopic study of the latter. At first there was no suggestion
of a nebular constitution, but within a month or two characteristic
nebular lines began to appear, and in less than six months the whole
spectrum had been transformed to the nebular type. In the mean time the
shifting of the spectral lines indicated a complication of rapid
motions in several directions simultaneously. These motions were
estimated to amount to from one hundred to five hundred miles per
second.

The human mind is so constituted that it feels forced to seek an
explanation of so marvelous a phenomenon as this, even in the absence
of the data needed for a sound conclusion. The most natural hypothesis,
perhaps, is that of a collision. Such a catastrophe could certainly
happen. It has been shown, for instance, that in infinity of time the
earth is sure to be hit by a comet; in the same way it may be asserted
that, if no time limit is fixed, the sun is certain to run against some
obstacle in space, either another star, or a dense meteor swarm, or one
of the dark bodies which there is every reason to believe abound around
us. The consequences of such a collision are easy to foretell, provided
that we know the masses and the velocities of the colliding bodies. In
a preceding chapter we have discussed the motions of the sun and stars,
and have seen that they are so swift that an encounter between any two
of them could not but be disastrous. But this is not all; for as soon
as two stars approached within a few million miles their speed would be
enormously increased by their reciprocal attractions and, if their
motion was directed radially with respect to their centers, they would
come together with a crash that would reduce them both to nebulous
clouds. It is true that the chances of such a “head-on” collision are
relatively very small; two stars approaching each other would most
probably fall into closed orbits around their common center of gravity.
If there were a collision it would most likely be a grazing one instead
of a direct front-to-front encounter. But even a close approach,
without any actual collision, would probably prove disastrous, owing to
the tidal influence of each of the bodies on the other. Suns, in
consequence of their enormous masses and dimensions and the
peculiarities of their constitution, are exceedingly dangerous to one
another at close quarters. Propinquity awakes in them a mutually
destructive tendency. Consisting of matter in the gaseous, or perhaps,
in some cases, liquid, state, their tidal pull upon each other if
brought close together might burst them asunder, and the photospheric
envelope being destroyed the internal incandescent mass would gush out,
bringing fiery death to any planets that were revolving near. Without
regard to the resulting disturbance of the earth’s orbit, the close
approach of a great star to the sun would be in the highest degree
perilous to us. But this is a danger which may properly be regarded as
indefinitely remote, since, at our present location in space, we are
certainly far from every star except the sun, and we may feel confident
that no great invisible body is near, for if there were one we should
be aware of its presence from the effects of its attraction. As to dark
nebulæ which may possibly lie in the track that the solar system is
pursuing at the rate of 375,000,000 miles per year, that is another
question—and they, too, could be dangerous!


[Illustration: Nova Persei, with its nebular rings]


This brings us directly back to “Nova Persei,” for among the many
suggestions offered to explain its outburst, as well as those of other
temporary stars, one of the most fruitful is that of a collision
between a star and a vast invisible nebula. Professor Seeliger, of
Munich, first proposed this theory, but it afterward underwent some
modifications from others. Stated in a general form, the idea is that a
huge dark body, perhaps an extinguished sun, encountered in its
progress through space a widespread flock of small meteors forming a
dark nebula. As it plunged into the swarm the friction of the
innumerable collisions with the meteors heated its surface to
incandescence, and being of vast size it then became visible to us as a
new star. Meanwhile the motion of the body through the nebula, and its
rotation upon itself, set up a gyration in the blazing atmosphere
formed around it by the vaporized meteors; and as this atmosphere
spread wider, under the laws of gyratory motion a rotation in the
opposite direction began in the inflamed meteoric cloud outside the
central part of the vortex. Thus the spectral lines were caused to show
motion in opposite directions, a part of the incandescent mass
approaching the earth simultaneously with the retreat of another part.
So the curious spectroscopic observations before mentioned were
explained. This theory might also account for the appearance of the
nebulous spiral first seen some six months after the original outburst.
The sequent changes in the spectrum of the “nova” are accounted for by
this theory on the assumption, reasonable enough in itself, that at
first the invading body would be enveloped in a vaporized atmosphere of
relatively slight depth, producing by its absorption the fine dark
lines first observed; but that as time went on and the incessant
collisions continued, the blazing atmosphere would become very deep and
extensive, whereupon the appearance of the spectral lines would change,
and bright lines due to the light of the incandescent meteors
surrounding the nucleus at a great distance would take the place of the
original dark ones. The vortex of meteors once formed would protect the
flying body within from further immediate collisions, the latter now
occurring mainly among the meteors themselves, and then the central
blaze would die down, and the original splendor of the phenomenon would
fade.

But the theories about Nova Persei have been almost as numerous as the
astronomers who have speculated about it. One of the most startling of
them assumed that the outburst was caused by the running amuck of a
dark star which had encountered another star surrounded with planets,
the renewed outbreaks of light after the principal one had faded being
due to the successive running down of the unfortunate planets! Yet
another hypothesis is based on what we have already said of the tidal
influence that two close approaching suns would have upon each other.
Supposing two such bodies which had become encrusted, but remained
incandescent and fluid within, to approach within almost striking
distance; they would whirl each other about their common center of
gravity, and at the same time their shells would burst under the tidal
strain, and their glowing nuclei being disclosed would produce a great
outburst of light. Applying this theory to a “nova,” like that of 1866
in the “Northern Crown,” which had been visible as a small star before
the outbreak, and which afterward resumed its former aspect, we should
have to assume that a yet shining sun had been approached by a dark
body whose attraction temporarily burst open its photosphere. It might
be supposed that in this case the dark body was too far advanced in
cooling to suffer the same fate from the tidal pull of its victim. But
a close approach of that kind would be expected to result in the
formation of a binary system, with orbits of great eccentricity,
perhaps, and after the lapse of a certain time the outburst should be
renewed by another approximation of the two bodies. A temporary star of
that kind would rather be ranked as a variable.

The celebrated French astronomer, Janssen, had a different theory of
Nova Persei, and of temporary stars in general. According to his idea,
such phenomena might be the result of chemical changes taking place in
a sun without interference by, or collision with, another body. Janssen
was engaged for many years in trying to discover evidence of the
existence of oxygen in the sun, and he constructed his observatory on
the summit of Mount Blanc specially to pursue that research. He
believed that oxygen must surely exist in the sun since we find so many
other familiar elements included in the constitution of the solar
globe, and as he was unable to discover satisfactory evidence of its
presence he assumed that it existed in a form unknown on the earth. If
it were normally in the sun’s chromosphere, or coronal atmosphere, he
said, it would combine with the hydrogen which we know is there and
form an obscuring envelope of water vapor. It exists, then, in a
special state, uncombined with hydrogen; but let the temperature of the
sun sink to a critical point and the oxygen will assume its normal
properties and combine with the hydrogen, producing a mighty outburst
of light and heat. This, Janssen thought, might explain the phenomena
of the temporary stars. It would also, he suggested, account for their
brief career, because the combination of the elements would be quickly
accomplished, and then the resulting water vapor would form an
atmosphere cutting off the radiation from the star within.

This theory may be said to have a livelier human interest than some of
the others, since, according to it, the sun may carry in its very
constitution a menace to mankind; one does not like to think of it
being suddenly transformed into a gigantic laboratory for the explosive
combination of oxygen and hydrogen! But while Janssen’s theory might do
for some temporary stars, it is inadequate to explain all the phenomena
of Nova Persei, and particularly the appearance of the great spiral
nebula that seemed to exhale from the heart of the star. Upon the
whole, the theory of an encounter between a star and a dark nebula
seems best to fit the observations. By that hypothesis the expanding
billow of light surrounding the core of the conflagration is very well
accounted for, and the spectroscopic peculiarities are also explained.

Dr Gustov Le Bon offers a yet more alarming theory, suggesting that
temporary stars are the result of _atomic explosion;_ but we shall
touch upon this more fully in Chapter 14.

Twice in the course of this discussion we have called attention to the
change of color invariably undergone by temporary stars in the later
stages of their career. This was conspicuous with Nova Persei which
glowed more and more redly as it faded, until the nebulous light began
to overpower that of the stellar nucleus. Nothing could be more
suggestive of the dying out of a great fire. Moreover, change of color
from white to red is characteristic of all variable stars of long
period, such as “Mira” in Cetus. It is also characteristic of stars
believed to be in the later stages of evolution, and consequently
approaching extinction, like Antares and Betelgeuse, and still more
notably certain small stars which “gleam like rubies in the field of
the telescope.” These last appear to be suns in the closing period of
existence as self-luminous bodies. Between the white stars, such as
Sirius and Rigel, and the red stars, such as Aldebaran and Alpha
Herculis, there is a progressive series of colors from golden yellow
through orange to deep red. The change is believed to be due to the
increase of absorbing vapors in the stellar atmosphere as the body
cools down. In the case of ordinary stars these changes no doubt occupy
many millions of years, which represent the average duration of solar
life; but the temporary stars run through similar changes in a few
months: they resemble ephemeral insects—born in the morning and doomed
to perish with the going down of the sun.




VI
Explosive and Whirling Nebulæ


One of the most surprising triumphs of celestial photography was
Professor Keeler’s discovery, in 1899, that the great majority of the
nebulæ have a distinctly spiral form. This form, previously known in
Lord Rosse’s great “Whirlpool Nebula,” had been supposed to be
exceptional; now the photographs, far excelling telescopic views in the
revelation of nebular forms, showed the spiral to be the typical shape.
Indeed, it is a question whether all nebulæ are not to some extent
spiral. The extreme importance of this discovery is shown in the effect
that it has had upon hitherto prevailing views of solar and planetary
evolution. For more than three-quarters of a century Laplace’s
celebrated hypothesis of the manner of origin of the solar system from
a rotating and contracting nebula surrounding the sun had guided
speculation on that subject, and had been tentatively extended to cover
the evolution of systems in general. The apparent forms of some of the
nebulæ which the telescope had revealed were regarded, and by some are
still regarded, as giving visual evidence in favor of this theory.
There is a “ring nebula” in Lyra with a central star, and a “planetary
nebula” in Gemini bearing no little resemblance to the planet Saturn
with its rings, both of which appear to be practical realizations of
Laplace’s idea, and the elliptical rings surrounding the central
condensation of the Andromeda Nebula may be cited for the same kind of
proof.


[Illustration: Lord Rosse’s nebula]


But since Keeler’s discovery there has been a decided turning away of
speculation another way. The form of the spiral nebulæ seems to be
entirely inconsistent with the theory of an originally globular or
disk-shaped nebula condensing around a sun and throwing or leaving off
rings, to be subsequently shaped into planets. Some astronomers,
indeed, now reject Laplace’s hypothesis _in toto,_ preferring to think
that even our solar system originated from a spiral nebula. Since the
spiral type prevails among the existing nebulæ, we must make any
mechanical theory of the development of stars and planetary systems
from them accord with the requirements which that form imposes. A
glance at the extraordinary variations upon the spiral which Professor
Keeler’s photographs reveal is sufficient to convince one of the
difficulty of the task of basing a general theory upon them. In truth,
it is much easier to criticize Laplace’s hypothesis than to invent a
satisfactory substitute for it. If the spiral nebulæ seem to oppose it
there are other nebulæ which appear to support it, and it may be that
no one fixed theory can account for all the forms of stellar evolution
in the universe. Our particular planetary system may have originated
very much as the great French mathematician supposed, while others have
undergone, or are now undergoing, a different process of development.
There is always a too strong tendency to regard an important new
discovery and the theories and speculations based upon it as
revolutionizing knowledge, and displacing or overthrowing everything
that went before. Upon the plea that “Laplace only made a guess” more
recent guesses have been driven to extremes and treated by injudicious
exponents as “the solid facts at last.”


[Illustration: Wonderful spiral in triangulum]


Before considering more recent theories than Laplace’s, let us see what
the nature of the photographic revelations is. The vast celestial
maelstrom discovered by Lord Rosse in the “Hunting Dogs” may be taken
as the leading type of the spiral nebulæ, although there are less
conspicuous objects of the kind which, perhaps, better illustrate some
of their peculiarities. Lord Rosse’s nebula appears far more wonderful
in the photographs than in his drawings made with the aid of his giant
reflecting telescope at Parsonstown, for the photographic plate records
details that no telescope is capable of showing. Suppose we look at the
photograph of this object as any person of common sense would look at
any great and strange natural phenomenon. What is the first thing that
strikes the mind? It is certainly the appearance of violent whirling
motion. One would say that the whole glowing mass had been spun about
with tremendous velocity, or that it had been set rotating so rapidly
that it had become the victim of “centrifugal force,” one huge fragment
having broken loose and started to gyrate off into space. Closer
inspection shows that in addition to the principal focus there are
various smaller condensations scattered through the mass. These are
conspicuous in the spirals. Some of them are stellar points, and but
for the significance of their location we might suppose them to be
stars which happen to lie in a line between us and the nebula. But when
we observe how many of them follow most faithfully the curves of the
spirals we cannot but conclude that they form an essential part of the
phenomenon; it is not possible to believe that their presence in such
situations is merely fortuitous. One of the outer spirals has at least
a dozen of these star-like points strung upon it; some of them sharp,
small, and distinct, others more blurred and nebulous, suggesting
different stages of condensation. Even the part which seems to have
been flung loose from the main mass has, in addition to its central
condensation, at least one stellar point gleaming in the half-vanished
spire attached to it. Some of the more distant stars scattered around
the “whirlpool” look as if they too had been shot out of the mighty
vortex, afterward condensing into unmistakable solar bodies. There are
at least two curved rows of minute stars a little beyond the periphery
of the luminous whirl which clearly follow lines concentric with those
of the nebulous spirals. Such facts are simply dumbfounding for anyone
who will bestow sufficient thought upon them, for these are _suns,_
though they may be small ones; and what a birth is that for a sun!

Look now again at the glowing spirals. We observe that hardly have they
left the central mass before they begin to coagulate. In some places
they have a “ropy” aspect; or they are like peascods filled with
growing seeds, which eventually will become stars. The great focus
itself shows a similar tendency, especially around its circumference.
The sense that it imparts of a tremendous shattering force at work is
overwhelming. There is probably more matter in that whirling and
bursting nebula than would suffice to make a hundred solar systems! It
must be confessed at once that there is no confirmation of the
Laplacean hypothesis here; but what hypothesis will fit the facts?
There is one which it has been claimed does so, but we shall come to
that later. In the meanwhile, as a preparation, fix in the memory the
appearance of that second spiral mass spinning beside its master which
seems to have spurned it away.


[Illustration: Spiral in Ursa Major]


For a second example of the spiral nebulæ look at the one in the
constellation Triangulum. _God, how hath the imagination of puny man
failed to comprehend Thee!_ Here is creation through destruction with a
vengeance! The spiral form of the nebula is unmistakable, but it is
half obliterated amid the turmoil of flying masses hurled away on all
sides with tornadic fury. The focus itself is splitting asunder under
the intolerable strain, and in a little while, as time is reckoned in
the Cosmos, it will be gyrating into stars. And then look at the
cyclonic rain of already finished stars whirling round the outskirts of
the storm. Observe how scores of them are yet involved in the fading
streams of the nebulous spirals; see how they have been thrown into
vast loops and curves, of a beauty that half redeems the terror of the
spectacle enclosed within their lines—like iridescent cirri hovering
about the edges of a hurricane. And so again are suns born!

Let us turn to the exquisite spiral in Ursa Major; how different its
aspect from that of the other! One would say that if the terrific coil
in Triangulum has all but destroyed itself in its fury, this one on the
contrary has just begun its self-demolition. As one gazes one seems to
see in it the smooth, swift, accelerating motion that precedes
catastrophe. The central part is still intact, dense, and uniform in
texture. How graceful are the spirals that smoothly rise from its oval
rim and, gemmed with little stars, wind off into the darkness until
they have become as delicate as threads of gossamer! But at bottom the
story told here is the same—creation by gyration!

Compare with the above the curious mass in Cetus. Here the plane of the
whirling nebula nearly coincides with our line of sight and we see the
object at a low angle. It is far advanced and torn to shreds, and if we
could look at it perpendicularly to its plane it is evident that it
would closely resemble the spectacle in Triangulum.

Then take the famous Andromeda Nebula (see Frontispiece), which is so
vast that notwithstanding its immense distance even the naked eye
perceives it as an enigmatical wisp in the sky. Its image on the
sensitive plate is the masterpiece of astronomical photography; for
wild, incomprehensible beauty there is nothing that can be compared
with it. Here, if anywhere, we look upon the spectacle of creation in
one of its earliest stages. The Andromeda Nebula is apparently less
advanced toward transformation into stellar bodies than is that in
Triangulum. The immense crowd of stars sprinkled over it and its
neighborhood seem in the main to lie this side of the nebula, and
consequently to have no connection with it. But incipient stars (in
some places clusters of them) are seen in the nebulous rings, while one
or two huge masses seem to give promise of transformation into stellar
bodies of unusual magnitude. I say “rings” because although the loops
encompassing the Andromeda Nebula have been called spirals by those who
wish utterly to demolish Laplace’s hypothesis, yet they are not
manifestly such, as can be seen on comparing them with the undoubted
spirals of the Lord Rosse Nebula. They look quite as much like circles
or ellipses seen at an angle of, say, fifteen or twenty degrees to
their plane. If they are truly elliptical they accord fairly well with
Laplace’s idea, except that the scale of magnitude is stupendous, and
if the Andromeda Nebula is to become a solar system it will surpass
ours in grandeur beyond all possibility of comparison.


[Illustration: Nebula in Cetus]


There is one circumstance connected with the spiral nebulæ, and
conspicuous in the Andromeda Nebula on account of its brightness, which
makes the question of their origin still more puzzling; they all show
continuous spectra, which, as we have before remarked, indicate that
the mass from which the light comes is either solid or liquid, or a gas
under heavy pressure. Thus nebulæ fall into two classes: the “white”
nebulæ, giving a continuous spectrum; and the “green” nebulæ whose
spectra are distinctly gaseous. The Andromeda Nebula is the great
representative of the former class and the Orion Nebula of the latter.
The spectrum of the Andromeda Nebula has been interpreted to mean that
it consists not of luminous gas, but of a flock of stars so distant
that they are separately indistinguishable even with powerful
telescopes, just as the component stars of the Milky Way are
indistinguishable with the naked eye; and upon this has been based the
suggestion that what we see in Andromeda is an outer universe whose
stars form a series of elliptical garlands surrounding a central mass
of amazing richness. But this idea is unacceptable if for no other
reason than that, as just said, all the spiral nebulæ possess the same
kind of spectrum, and probably no one would be disposed to regard them
all as outer universes. As we shall see later, the peculiarity of the
spectra of the spiral nebulæ is appealed to in support of a modern
substitute for Laplace’s hypothesis.

Finally, without having by any means exhausted the variety exhibited by
the spiral nebulæ, let us turn to the great representative of the other
species, the Orion Nebula. In some ways this is even more marvelous
than the others. The early drawings with the telescope failed to convey
an adequate conception either of its sublimity or of its complication
of structure. It exists in a nebulous region of space, since
photographs show that nearly the whole constellation is interwoven with
faintly luminous coils. To behold the entry of the great nebula into
the field even of a small telescope is a startling experience which
never loses its novelty. As shown by the photographs, it is an
inscrutable chaos of perfectly amazing extent, where spiral bands,
radiating streaks, dense masses, and dark yawning gaps are strangely
intermingled without apparent order. In one place four conspicuous
little stars, better seen in a telescope than in the photograph on
account of the blurring produced by over-exposure, are suggestively
situated in the midst of a dark opening, and no observer has ever felt
any doubt that these stars have been formed from the substance of the
surrounding nebula. There are many other stars scattered over its
expanse which manifestly owe their origin to the same source. But
compare the general appearance of this nebula with the others that we
have studied, and remark the difference. If the unmistakably spiral
nebulæ resemble bursting fly-wheels or grindstones from whose
perimeters torrents of sparks are flying, the Orion Nebula rather
recalls the aspect of a cloud of smoke and fragments produced by the
explosion of a shell. This idea is enforced by the look of the outer
portion farthest from the bright half of the nebula, where sharply
edged clouds with dark spaces behind seem to be billowing away as if
driven by a wind blowing from the center.


[Illustration: The Orion nebula]


Next let us consider what scientific speculation has done in the effort
to explain these mysteries. Laplace’s hypothesis can certainly find no
standing ground either in the Orion Nebula or in those of a spiral
configuration, whatever may be its situation with respect to the grand
Nebula of Andromeda, or the “ring” and “planetary” nebulæ. Some other
hypothesis more consonant with the appearances must be found. Among the
many that have been proposed the most elaborate is the “Planetesimal
Hypothesis” of Professors Chamberlin and Moulton. It is to be remarked
that it applies to the spiral nebulæ distinctively, and not to an
apparently chaotic mass of gas like the vast luminous cloud in Orion.
The gist of the theory is that these curious objects are probably the
result of close approaches to each other of two independent suns,
reminding us of what was said on this subject when we were dealing with
temporary stars. Of the previous history of these appulsing suns the
theory gives us no account; they are simply supposed to arrive within
what may be called an effective tide-producing distance, and then the
drama begins. Some of the probable consequences of such an approach
have been noticed in Chapter 5; let us now consider them a little more
in detail.

Tides always go in couples; if there is a tide on one side of a globe
there will be a corresponding tide on the other side. The cause is to
be found in the law that the force of gravitation varies inversely as
the square of the distance; the attraction on the nearest surface of
the body exercised by another body is greater than on its center, and
greater yet than on its opposite surface. If two great globes attract
each other, each tends to draw the other out into an ellipsoidal
figure; they must be more rigid than steel to resist this—and even then
they cannot altogether resist. If they are liquid or gaseous they will
yield readily to the force of distortion, the amount of which will
depend upon their distance apart, for the nearer they are the greater
becomes the tidal strain. If they are encrusted without and liquid or
gaseous in the interior, the internal mass will strive to assume the
figure demanded by the tidal force, and will, if it can, burst the
restraining envelope. Now this is virtually the predicament of the body
we call a sun when in the immediate presence of another body of
similarly great mass. Such a body is presumably gaseous throughout, the
component gases being held in a state of rigidity by the compression
produced by the tremendous gravitational force of their own aggregate
mass. At the surface such a body is enveloped in a shell of relatively
cool matter. Now suppose a great attracting body, such as another sun,
to approach near enough for the difference in its attraction on the two
opposite sides of the body and on its center to become very great; the
consequence will be a tidal deformation of the whole body, and it will
lengthen out along the line of the gravitational pull and draw in at
the sides, and if its shell offers considerable resistance, but not
enough to exercise a complete restraint, it will be violently burst
apart, or blown to atoms, and the internal mass will leap out on the
two opposite sides in great fiery spouts. In the case of a sun further
advanced in cooling than ours the interior might be composed of molten
matter while the exterior crust had become rigid like the shell of an
egg; then the force of the “tidal explosion” produced by the appulse of
another sun would be more violent in consequence of the greater
resistance overcome. Such, then, is the mechanism of the first phase in
the history of a spiral nebula according to the Planetesimal
Hypothesis. Two suns, perhaps extinguished ones, have drawn near
together, and an explosive outburst has occured in one or both. The
second phase calls for a more agile exercise of the imagination.

To simplify the case, let us suppose that only one of the tugging suns
is seriously affected by the strain. Its vast wings produced by the
outburst are twisted into spirals by their rotation and the contending
attractions exercised upon them, as the two suns, like battleships in
desperate conflict, curve round each other, concentrating their
destructive energies. Then immense quantities of débris are scattered
about in which eddies are created, and finally, as the sun that caused
the damage goes on its way, leaving its victim to repair its injuries
as it may, the dispersed matter cools, condenses, and turns into
streams of solid particles circling in elliptical paths about their
parent sun. These particles, or fragments, are the “planetesimals” of
the theory. In consequence of the inevitable intersection of the orbits
of the planetesimals, nodes are formed where the flying particles meet,
and at these nodes large masses are gradually accumulated. The larger
the mass the greater its attraction, and at last the nodal points
become the nuclei of great aggregations from which planets are shaped.

This, in very brief form, is the Planetesimal Hypothesis which we are
asked to substitute for that based on Laplace’s suggestion as an
explanation of the mode of origin of the solar system; and the
phenomena of the spiral nebulæ are appealed to as offering evident
support to the new hypothesis. We are reminded that they are elliptical
in outline, which accords with the hypothesis; that their spectra are
not gaseous, which shows that they may be composed of solid particles
like the planetesimals; and that their central masses present an oval
form, which is what would result from the tidal effects, as just
described. We also remember that some of them, like the Lord Rosse and
the Andromeda nebulæ, are visually double, and in these cases we might
suppose that the two masses represent the tide-burst suns that ventured
into too close proximity. It may be added that the authors of the
theory do not insist upon the appulse of two suns as the _only_ way in
which the planetesimals may have originated, but it is the only
supposition that has been worked out.

But serious questions remain. It needs, for instance, but a glance at
the Triangulum monster to convince the observer that it cannot be a
solar system which is being evolved there, but rather a swarm of stars.
Many of the detached masses are too vast to admit of the supposition
that they are to be transformed into planets, in our sense of planets,
and the distances of the stars which appear to have been originally
ejected from the focal masses are too great to allow us to liken the
assemblage that they form to a solar system. Then, too, no nodes such
as the hypothesis calls for are visible. Moreover, in most of the
spiral nebulæ the appearances favor the view that the supposititious
encountering suns have not separated and gone each rejoicing on its
way, after having inflicted the maximum possible damage on its
opponent, but that, on the contrary, they remain in close association
like two wrestlers who cannot escape from each other’s grasp. And this
is exactly what the law of gravitation demands; stars cannot approach
one another with impunity, with regard either to their physical make-up
or their future independence of movement. The theory undertakes to
avoid this difficulty by assuming that in the case of our system the
approach of the foreign body to the sun was not a close one—just close
enough to produce the tidal extrusion of the relatively insignificant
quantity of matter needed to form the planets. But even then the effect
of the appulse would be to change the direction of flight, both of the
sun and of its visitor, and there is no known star in the sky which can
be selected as the sun’s probable partner in their ancient _pas deux._
That there are unconquered difficulties in Laplace’s hypothesis no one
would deny, but in simplicity of conception it is incomparably more
satisfactory, and with proper modifications could probably be made more
consonant with existing facts in our solar system than that which is
offered to replace it. Even as an explanation of the spiral nebulæ, not
as solar systems in process of formation, but as the birthplaces of
stellar clusters, the Planetesimal Hypothesis would be open to many
objections. Granting its assumptions, it has undoubtedly a strong
mathematical framework, but the trouble is not with the mathematics but
with the assumptions. Laplace was one of the ablest mathematicians that
ever lived, but he had never seen a spiral nebula; if he had, he might
have invented a hypothesis to suit its phenomena. His actual hypothesis
was intended only for our solar system, and he left it in the form of a
“note” for the consideration of his successors, with the hope that they
might be able to discover the full truth, which he confessed was hidden
from him. It cannot be said that that truth has yet been found, and
when it is found the chances are that intuition and not logic will have
led to it.

The spiral nebulæ, then, remain among the greatest riddles of the
universe, while the gaseous nebulæ, like that of Orion, are no less
mysterious, although it seems impossible to doubt that both forms give
birth to stars. It is but natural to look to them for light on the
question of the origin of our planetary system; but we should not
forget that the scale of the phenomena in the two cases is vastly
different, and the forces in operation may be equally different. A hill
may have been built up by a glacier, while a mountain may be the
product of volcanic forces or of the upheaval of the strata of the
planet.




VII
The Banners of the Sun


As all the world knows, the sun, a blinding globe pouring forth an
inconceivable quantity of light and heat, whose daily passage through
the sky is caused by the earth’s rotation on its axis, constitutes the
most important phenomenon of terrestial existence. Viewed with a dark
glass to take off the glare, or with a telescope, its rim is seen to be
a sharp and smooth circle, and nothing but dark sky is visible around
it. Except for the interference of the moon, we should probably never
have known that there is any more of the sun than our eyes ordinarily
see.

But when an eclipse of the sun occurs, caused by the interposition of
the opaque globe of the moon, we see its immediate surroundings, which
in some respects are more wonderful than the glowing central orb. These
surroundings, although not in the sense in which we apply the term to
the gaseous envelope of the earth, may be called the sun’s atmosphere.
They consist of two very different parts—first, the red “prominences,”
which resemble tongues of flame ascending thousands of miles above the
sun’s surface; and, second, the “corona,” which extends to distances of
millions of miles from the sun, and shines with a soft, glowing light.
The two combined, when well seen, make a spectacle without parallel
among the marvels of the sky. Although many attempts have been made to
render the corona visible when there is no eclipse, all have failed,
and it is to the moon alone that we owe its revelation. To cover the
sun’s disk with a circular screen will not answer the purpose because
of the illumination of the air all about the observer. When the moon
hides the sun, on the other hand, the sunlight is withdrawn from a
great cylinder of air extending to the top of the atmosphere and
spreading many miles around the observer. There is then no glare to
interfere with the spectacle, and the corona appears in all its
surprising beauty. The prominences, however, although they were
discovered during an eclipse, can now, with the aid of the
spectroscope, be seen at any time. But the prominences are rarely large
enough to be noticed by the naked eye, while the streamers of the
corona, stretching far away in space, like ghostly banners blown out
from the black circle of the obscuring moon, attract every eye, and to
this weird apparition much of the fear inspired by eclipses has been
due. But if the corona has been a cause of terror in the past it has
become a source of growing knowledge in our time.


[Illustration: The Corona]


The story of the first scientific observation of the corona and the
prominences is thrillingly interesting, and in fact dramatic. The
observation was made during the eclipse of 1842, which fortunately was
visible all over Central and Southern Europe so that scores of
astronomers saw it. The interest centers in what happened at Pavia in
Northern Italy, where the English astronomer Francis Baily had set up
his telescope. The eclipse had begun and Bailey was busy at his
telescope when, to quote his own words in the account which he wrote
for the _Memoirs of the Royal Astronomical Society:_


I was astounded by a tremendous burst of applause from the streets
below, and at the same moment was electrified by the sight of one of
the most brilliant and splendid phenomena that can well be imagined;
for at that instant the dark body of the moon was suddenly surrounded
with a corona, or kind of bright glory, similar in shape and magnitude
to that which painters draw round the heads of saints...

Pavia contains many thousand inhabitants, the major part of whom were
at this early hour walking about the streets and squares or looking out
of windows in order to witness this long-talked-of phenomenon; and when
the total obscuration took place, which was _instantaneous,_ there was
a universal shout from every observer which “made the welkin ring,” and
for the moment withdrew my attention from the object with which I was
immediately occupied. I had, indeed, expected the appearance of a
luminous circle round the moon during the time of total obscurity; but
I did not expect, from any of the accounts of preceding eclipses that I
had read, to witness so magnificent an exhibition as that which took
place...

Splendid and astonishing, however, as this remarkable phenomenon really
was, and although it could not fail to call forth the admiration and
applause of every beholder, yet I must confess that there was at the
same time something in its singular and wonderful appearance that was
appalling...

But the most remarkable circumstance attending the phenomenon was the
appearance of _three large protuberances_ apparently emanating from the
circumference of the moon, but evidently forming a portion of the
corona. They had the appearance of mountains of a prodigious elevation;
their color was red tinged with lilac or purple; perhaps the color of
the peach-blossom would more nearly represent it. They somewhat
resembled the tops of the snowy Alpine mountains when colored by the
rising or the setting sun. They resembled the Alpine mountains in
another respect, inasmuch as their light was perfectly steady, and had
none of that flickering or sparkling motion so visible in other parts
of the corona...

The whole of these protuberances were visible even to the last moment
of total obscuration, and when the first ray of light was admitted from
the sun they vanished, with the corona, altogether, and daylight was
instantly restored.


I have quoted nearly all of this remarkable description not alone for
its intrinsic interest, but because it is the best depiction that can
be found of the general phenomena of a total solar eclipse. Still, not
every such eclipse offers an equally magnificent spectacle. The
eclipses of 1900 and 1905, for instance, which were seen by the writer,
the first in South Carolina and the second in Spain, fell far short of
that described by Bailey in splendor and impressiveness. Of course,
something must be allowed for the effect of surprise; Bailey had not
expected to see what was so suddenly disclosed to him. But both in 1900
and 1905 the amount of scattered light in the sky was sufficient in
itself to make the corona appear faint, and there were no very
conspicuous prominences visible. Yet on both occasions there was
manifest among the spectators that mingling of admiration and awe of
which Bailey speaks. The South Carolinians gave a cheer and the ladies
waved their handkerchiefs when the corona, ineffably delicate of form
and texture, _melted_ into sight and then in two minutes melted away
again. The Spaniards, crowded on the citadel hill of Burgos, with their
king and his royal retinue in their midst, broke out with a great
clapping of hands as the awaited spectacle unfolded itself in the sky;
and on both occasions, before the applause began, after an awed silence
a low murmur ran through the crowds. At Burgos it is said many made the
sign of the cross.

It was not long before Bailey’s idea that the prominences were a part
of the corona was abandoned, and it was perceived that the two
phenomena were to a great extent independent. At the eclipse of 1868,
which the astronomers, aroused by the wonderful scene of 1842, and
eager to test the powers of the newly invented spectroscope, flocked to
India to witness, Janssen conceived the idea of employing the
spectroscope to render the prominences visible when there was no
eclipse. He succeeded the very next day, and these phenomena have been
studied in that way ever since.

There are recognized two kinds of prominences—the “erruptive” and the
“quiescent.” The latter, which are cloud-like in form, may be seen
almost anywhere along the edge of the sun; but the former, which often
shoot up as if hurled from mighty volcanoes, appear to be associated
with sun-spots, and appear only above the zones where spots abound.
Either of them, when seen in projection against the brilliant solar
disk, appears white, not red, as against a background of sky. The
quiescent prominences, whose elevation is often from forty thousand to
sixty thousand miles, consist, as the spectroscope shows, mainly of
hydrogen and helium. The latter, it will be remembered, is an element
which was known to be in the sun many years before the discovery that
it also exists in small quantities on the earth. A fact which may have
a significance which we cannot at present see is that the emanation
from radium gradually and spontaneously changes into helium, an
alchemistical feat of nature that has opened many curious vistas to
speculative thinkers. The eruptive prominences, which do not spread
horizontally like the others, but ascend with marvelous velocity to
elevations of half a million miles or more, are apparently composed
largely of metallic vapors—_i.e._ metals which are usually solid on the
earth, but which at solar temperatures are kept in a volatilized state.
The velocity of their ascent occasionally amounts to three hundred or
four hundred miles per second. It is known from mathematical
considerations that the gravitation of the sun would not be able to
bring back any body that started from its surface with a velocity
exceeding three hundred and eighty-three miles per second; so it is
evident that some of the matter hurled forth in eruptive prominences
may escape from solar control and go speeding out into space, cooling
and condensing into solid masses. There seems to be no reason why some
of the projectiles from the sun might not reach the planets. Here,
then, we have on a relatively small scale, _explosions_ recalling those
which it has been imagined may be the originating cause of some of the
sudden phenomena of the stellar heavens.


[Illustration: A solar “prominence.” Photographed May 21, 1907]


Of the sun-spots it is not our intention here specifically to speak,
but they evidently have an intimate connection with eruptive
prominences, as well as some relation, not yet fully understood, with
the corona. Of the real cause of sun-spots we know virtually nothing,
but recent studies by Professor Hale and others have revealed a strange
state of things in the clouds of metallic vapors floating above them
and their surroundings. Evidences of a cyclonic tendency have been
found, and Professor Hale has proved that sun-spots are strong magnetic
fields, and consist of columns of ionized vapors rotating in opposite
directions in the two hemispheres. A fact which may have the greatest
significance is that titanium and vanadium have been found both in
sun-spots and in the remarkable variable Mira Ceti, a star which every
eleven months, or thereabout, flames up with great brilliancy and then
sinks back to invisibility with the naked eye. It has been suggested
that sun-spots are indications of the beginning of a process in the sun
which will be intensified until it falls into the state of such a star
as Mira. Stars very far advanced in evolution, without showing
variability, also exhibit similar spectra; so that there is much reason
for regarding sunspots as emblems of advancing age.

The association of the corona with sun-spots is less evident than that
of the eruptive prominences; still such an association exists, for the
form and extent of the corona vary with the sun-spot period of which we
shall presently speak. The constitution of the corona remains to be
discovered. It is evidently in part gaseous, but it also probably
contains matter in the form of dust and small meteors. It includes one
substance altogether mysterious—“coronium.” There are reasons for
thinking that this may be the lightest of all the elements, and
Professor Young, its discoverer, said that it was “absolutely unique in
nature; utterly distinct from any other known form of matter,
terrestial, solar, or cosmical.” The enormous extent of the corona is
one of its riddles. Since the development of the curious subject of the
“pressure of light” it has been proposed to account for the
sustentation of the corona by supposing that it is borne upon the
billows of light continually poured out from the sun. Experiment has
proved, what mathematical considerations had previously pointed out as
probable, that the waves of light exert a pressure or driving force,
which becomes evident in its effects if the body acted upon is
sufficiently small. In that case the light pressure will prevail over
the attraction of gravitation, and propel the attenuated matter away
from the sun in the teeth of its attraction. The earth itself would be
driven away if, instead of consisting of a solid globe of immense
aggregate mass, it were a cloud of microscopic particles. The reason is
that the pressure varies in proportion to the _surface_ of the body
acted upon, while the gravitational attraction is proportional to the
_volume,_ or the total amount of matter in the body. But the surface of
any body depends upon the _square_ of its diameter, while the volume
depends upon the _cube_ of the diameter. If, for instance, the diameter
is represented by 4, the surface will be proportional to 4 × 4, or 16,
and the volume to 4 × 4 × 4, or 64; but if the diameter is taken as 2,
the surface will be 2 × 2, or 4, and the volume 2 × 2 × 2, or 8. Now,
the ratio of 4 to 8 is twice as great as that of 16 to 64. If the
diameter is still further decreased, the ratio of the surface to the
volume will proportionally grow larger; in other words, the pressure
will gain upon the attraction, and whatever their original ratio may
have been, a time will come, if the diminution of size continues, when
the pressure will become more effective than the attraction, and the
body will be driven away. Supposing the particles of the corona to be
below the critical size for the attraction of a mass like that of the
sun to control them, they would be driven off into the surrounding
space and appear around the sun like the clouds of dust around a mill.
We shall return to this subject in connection with the Zodiacal Light,
the Aurora, and Comets.

On the other hand, there are parts of the corona which suggest by their
forms the play of electric or magnetic forces. This is beautifully
shown in some of the photographs that have been made of the corona
during recent eclipses. Take, for instance, that of the eclipse of
1900. The sheaves of light emanating from the poles look precisely like
the “lines of force” surrounding the poles of a magnet. It will be
noticed in this photograph that the corona appears to consist of two
portions: one comprising the polar rays just spoken of, and the other
consisting of the broader, longer, and less-defined masses of light
extending out from the equatorial and middle-latitude zones. Yet even
in this more diffuse part of the phenomenon one can detect the presence
of submerged curves bearing more or less resemblance to those about the
poles. Just what part electricity or electro-magnetism plays in the
mechanism of the solar radiation it is impossible to say, but on the
assumption that it is a very important part is based the hypothesis
that there exists a direct solar influence not only upon the magnetism,
but upon the weather of the earth. This hypothesis has been under
discussion for half a century, and still we do not know just how much
truth it represents. It is certain that the outbreak of great
disturbances on the sun, accompanied by the formation of sun-spots and
the upshooting of eruptive prominences (phenomena which we should
naturally expect to be attended by action), have been instantly
followed by corresponding “magnetic storms” on the earth and brilliant
displays of the auroral lights. There have been occasions when the
influence has manifested itself in the most startling ways, a great
solar outburst being followed by a mysterious gripping of the cable and
telegraph systems of the world, as if an invisible and irresistible
hand had seized them. Messages are abruptly cut off, sparks leap from
the telegraph instruments, and the entire earth seems to have been
thrown into a magnetic flurry. These occurrences affect the mind with a
deep impression of the dependence of our planet on the sun, such as we
do not derive from the more familiar action of the sunlight on the
growth of plants and other phenomena of life depending on solar
influences.

Perhaps the theory of solar magnetic influence upon the weather is best
known in connection with the “sun-spot cycle.” This, at any rate, is,
as already remarked, closely associated with the corona. Its existence
was discovered in 1843 by the German astronomer Schwabe. It is a period
of variable length, averaging about eleven years, during which the
number of spots visible on the sun first increases to a maximum, then
diminishes to a minimum, and finally increases again to a maximum. For
unknown reasons the period is sometimes two or three years longer than
the average and sometimes as much shorter. Nevertheless, the phenomena
always recur in the same order. Starting, for instance, with a time
when the observer can find few or no spots, they gradually increase in
number and size until a maximum, in both senses, is reached, during
which the spots are often of enormous size and exceedingly active.
After two or three years they begin to diminish in number, magnitude,
and activity until they almost or quite disappear. A strange fact is
that when a new period opens, the spots appear first in high northern
and southern latitudes, far from the solar equator, and as the period
advances they not only increase in number and size, but break out
nearer and nearer to the equator, the last spots of a vanishing period
sometimes lingering in the equatorial region after the advance-guard of
its successor has made its appearance in the high latitudes. Spots are
never seen on the equator nor near the poles. It was not very long
after the discovery of the sun-spot cycle that the curious observation
was made that a striking coincidence existed between the period of the
sun-spots and another period affecting the general magnetic condition
of the earth. When a curved line representing the varying number of
sun-spots was compared with another curve showing the variations in the
magnetic state of the earth the two were seen to be in almost exact
accord, a rise in one curve corresponding to a rise in the other, and a
fall to a fall. Continued observation has proved that this is a real
coincidence and not an accidental one, so that the connection, although
as yet unexplained, is accepted as established. But does the influence
extend further, and directly affect the weather and the seasons as well
as the magnetic elements of the earth? A final answer to this question
cannot yet be given, for the evidence is contradictory, and the
interpretations put upon it depend largely on the predilections of the
judges.

But, in a broad sense, the sun-spots and the phenomena connected with
them _must_ have a relation to terrestial meteorology, for they prove
the sun to be a variable star. Reference was made, a few lines above,
to the resemblance of the spectra of sun-spots to those of certain
stars which seem to be failing through age. This in itself is extremely
suggestive; but if this resemblance had never been discovered, we
should have been justified in regarding the sun as variable in its
output of energy; and not only variable, but probably increasingly so.
The very inequalities in the sun-spot cycle are suspicious. When the
sun is most spotted its total light may be reduced by one-thousandth
part, although it is by no means certain that its outgiving of thermal
radiations is then reduced. A loss of one-thousandth of its luminosity
would correspond to a decrease of .0025 of a stellar magnitude,
considering the sun as a star viewed from distant space. So slight a
change would not be perceptible; but it is not alone sun-spots which
obscure the solar surface, its entire globe is enveloped with an
obscuring veil. When studied with a powerful telescope the sun’s
surface is seen to be thickly mottled with relatively obscure specks,
so numerous that it has been estimated that they cut off from one-tenth
to one-twentieth of the light that we should receive from it if the
whole surface were as brilliant as its brightest parts. The condition
of other stars warrants the conclusion that this obscuring envelope is
the product of a process of refrigeration which will gradually make the
sun more and more variable until its history ends in extinction.
Looking backward, we see a time when the sun must have been more
brilliant than it is now. At that time it probably shone with the
blinding white splendor of such stars as Sirius, Spica, and Vega; now
it resembles the relatively dull Procyon; in time it will turn ruddy
and fall into the closing cycle represented by Antares. Considering
that once it must have been more radiantly powerful than at present,
one is tempted to wonder if that could have been the time when tropical
life flourished within the earth’s polar circles, sustained by a
vivific energy in the sun which it has now lost.

The corona, as we have said, varies with the sun-spot cycle. When the
spots are abundant and active the corona rises strong above the spotted
zones, forming immense beams or streamers, which on one occasion, at
least, had an observed length of _ten million miles._ At the time of a
spot minimum the corona is less brilliant and has a different outline.
It is then that the curved polar rays are most conspicuous. Thus the
vast banners of the sun, shaken out in the eclipse, are signals to tell
of its varying state, but it will probably be long before we can read
correctly their messages.




VIII
The Zodiacal Light Mystery


There is a singular phenomenon in the sky—one of the most puzzling of
all—which has long arrested the attention of astronomers, defying their
efforts at explanation, but which probably not one in a hundred, and
possibly not one in a thousand, of the readers of this book has ever
seen. Yet its name is often spoken, and it is a conspicuous object if
one knows when and where to look for it, and when well seen it exhibits
a mystical beauty which at the same time charms and awes the beholder.
It is called “The Zodiacal Light,” because it lies within the broad
circle of the Zodiac, marking the sun’s apparent annual path through
the stars. What it is nobody has yet been able to find out with
certainty, and books on astronomy usually speak of it with singular
reserve. But it has given rise to many remarkable theories, and a true
explanation of it would probably throw light on a great many other
celestial mysteries. The Milky Way is a more wonderful object to look
upon, but its nature can be comprehended, while there is a sort of
uncanniness about the Zodiacal Light which immediately impresses one
upon seeing it, for its part in the great scheme of extra-terrestrial
affairs is not evident.

If you are out-of-doors soon after sunset—say, on an evening late in
the month of February—you may perceive, just after the angry flush of
the dying winter’s day has faded from the sky, a pale ghostly presence
rising above the place where the sun went down. The writer remembers
from boyhood the first time it was pointed out to him and the unearthly
impression that it made, so that he afterward avoided being out alone
at night, fearful of seeing the spectral thing again. The phenomenon
brightens slowly with the fading of the twilight, and soon distinctly
assumes the shape of an elongated pyramid of pearly light, leaning
toward the south if the place of observation is in the northern
hemisphere. It does not impress the observer at all in the same manner
as the Milky Way; that looks far off and is clearly among the stars,
but the Zodiacal Light seems closer at hand, as if it were something
more intimately concerning the earth. To all it immediately suggests a
connection, also, with the sunken sun. If the night is clear and the
moon absent (and if you are in the country, for city lights ruin the
spectacles of the sky), you will be able to watch the apparition for a
long time. You will observe that the light is brightest near the
horizon, gradually fading as the pyramidal beam mounts higher, but in
favorable circumstances it may be traced nearly to the meridian south
of the zenith, where its apex at last vanishes in the starlight. It
continues visible during the evenings of March and part of April, after
which, ordinarily, it is seen no more, or if seen is relatively faint
and unimpressive. But when autumn comes it appears again, this time not
like a wraith hovering above the westward tomb of the day-god, but
rather like a spirit of the morning announcing his reincarnation in the
east.

The reason why the Zodiacal Light is best seen in our latitudes at the
periods just mentioned is because at those times the Zodiac is more
nearly perpendicular to the horizon, first in the west and then in the
east; and, since the phenomenon is confined within the borders of the
Zodiac, it cannot be favorably placed for observation when the zodiacal
plane is but slightly inclined to the horizon. Its faint light requires
the contrast of a background of dark sky in order to be readily
perceptible. But within the tropics, where the Zodiac is always at a
favorable angle, the mysterious light is more constantly visible.
Nearly all observant travelers in the equatorial regions have taken
particular note of this phenomenon, for being so much more conspicuous
there than in the temperate zones it at once catches the eye and holds
the attention as a novelty. Humboldt mentions it many times in his
works, for his genius was always attracted by things out of the
ordinary and difficult of explanation, and he made many careful
observations on its shape, its brilliancy, and its variations; for
there can be no doubt that it does vary, and sometimes to an
astonishing degree. It is said that it once remained practically
invisible in Europe for several years in succession. During a trip to
South Africa in 1909 an English astronomer, Mr E. W. Maunder, found a
remarkable difference between the appearance of the Zodiacal Light on
his going and coming voyages. In fact, when crossing the equator going
south he did not see it at all; but on returning he had, on March 6th,
when one degree south of the equator, a memorable view of it.

It was a bright, clear night, and the Zodiacal Light was
extraordinarily brilliant—brighter than he had ever seen it before. The
Milky Way was not to be compared with it. The brightest part extended
75° from the sun. There was a faint and much narrower extension which
they could just make out beyond the Pleiades along the ecliptic, but
the greater part of the Zodiacal Light showed as a broad truncated
column, and it did not appear nearly as conical as he had before seen
it.


When out of the brief twilight of intertropical lands, where the sun
drops vertically to the horizon and night rushes on like a wave of
darkness, the Zodiacal Light shoots to the very zenith, its color is
described as a golden tint, entirely different from the silvery sheen
of the Milky Way. If I may venture again to refer to personal
experiences and impressions, I will recall a view of the Zodiacal Light
from the summit of the cone of Mt Etna in the autumn of the year 1896
(more briefly described in _Astronomy with the Naked Eye_). There are
few lofty mountains so favorably placed as Etna for observations of
this kind. It was once resorted to by Prof. George E. Hale, in an
attempt to see the solar corona without an eclipse. Rising directly
from sea-level to an elevation of nearly eleven thousand feet, the
observer on its summit at night finds himself, as it were, lost in the
midst of the sky. But for the black flanks of the great cone on which
he stands he might fancy himself to be in a balloon. On the occasion to
which I refer the world beneath was virtually invisible in the moonless
night. The blaze of the constellations overhead was astonishingly
brilliant, yet amid all their magnificence my attention was immediately
drawn to a great tapering light that sprang from the place on the
horizon where the sun would rise later, and that seemed to be blown out
over the stars like a long, luminous veil. It was the finest view of
the Zodiacal light that I had ever enjoyed—thrilling in its
strangeness—but I was almost disheartened by the indifference of my
guide, to whom it was only a light and nothing more. If he had no
science, he had less poetry—rather a remarkable thing, I thought, for a
child of his clime. The Light appeared to me to be distinctly brighter
than the visible part of the Milky Way which included the brilliant
stretches in Auriga and Perseus, and its color, if one may speak of
color in connection with such an object, seemed richer than that of the
galactic band; but I did not think of it as yellow, although Humboldt
has described it as resembling a golden curtain drawn over the stars,
and Du Chaillu in Equatorial Africa found it of a bright yellow color.
It may vary in color as in conspicuousness. The fascination of that
extraordinary sight has never faded from my memory. I turned to regard
it again and again, although I had never seen the stellar heavens so
brilliant, and it was one of the last things I looked for when the
morning glow began softly to mount in the east, and Sicily and the
Mediterranean slowly emerged from the profound shadow beneath us.

The Zodiacal Light seems never to have attracted from astronomers in
general the amount of careful attention that it deserves; perhaps
because so little can really be made of it as far as explanation is
concerned. I have referred to the restraint that scientific writers
apparently feel in speaking of it. The grounds for speculation that it
affords may be too scanty to lead to long discussions, yet it piques
curiosity, and as we shall see in a moment has finally led to a most
interesting theory. Once it was the subject of an elaborate series of
studies which carried the observer all round the world. That was in
1845—46, during the United States Exploring Expedition that visited the
then little known Japan. The chaplain of the fleet, the Rev. Mr Jones,
went out prepared to study the mysterious light in all its phases. He
saw it from many latitudes on both sides of the equator, and the
imagination cannot but follow him with keen interest in his
world-circling tour, keeping his eyes every night fixed upon the
phantasm overhead, whose position shifted with that of the hidden sun.
He demonstrated that the flow extends at times completely across the
celestial dome, although it is relatively faint directly behind the
earth. On his return the government published a large volume of his
observations, in which he undertook to show that the phenomenon was due
to the reflection of sunlight from a ring of meteoric bodies encircling
the earth. But, after all, this elaborate investigation settled
nothing.

Prof. E. E. Barnard has more recently devoted much attention to the
Zodiacal Light, as well as to a strange attendant phenomenon called the
“Gegenschein,” or Counterglow, because it always appears at that point
in the sky which is exactly opposite the sun. The Gegenschein is an
extremely elusive phenomenon, suitable only for eyes that have been
specially trained to see it. Professor Newcomb has cautiously remarked
that

it is said that in that point of the heavens directly opposite the sun
there is an elliptical patch of light... This phenomenon is so
difficult to account for that its existence is sometimes doubted; yet
the testimony in its favor is difficult to set aside.


It certainly cannot be set aside at all since the observations of
Barnard. I recall an attempt to see it under his guidance during a
visit to Mount Hamilton, when he was occupied there with the Lick
telescope. Of course, both the Gegenschein and the Zodiacal Light are
too diffuse to be studied with telescopes, which, so to speak, magnify
them out of existence. They can only be successfully studied with the
naked eye, since every faintest glimmer that they afford must be
utilized. This is especially true of the Gegenschein. At Mount
Hamilton, Mr Barnard pointed out to me its location with reference to
certain stars, but with all my gazing I could not be sure that I saw
it. To him, on the contrary, it was obvious; he had studied it for
months, and was able to indicate its shape, its boundaries, its
diameter, and the declination of its center with regard to the
ecliptic. There is not, of course, the shadow of a doubt of the
existence of the Gegenschein, and yet I question if one person in a
million has ever seen or ever will see it. The Zodiacal Light, on the
other hand, is plain enough, provided that the time and the
circumstances of the observation are properly chosen.

In the attempts to explain the Zodiacal Light, the favorite hypothesis
has been that it is an appendage of the sun—perhaps simply an extension
of the corona in the plane of the ecliptic, which is not very far from
coinciding with that of the sun’s equator. This idea is quite a natural
one, because of the evident relation of the light to the position of
the sun. The vast extension of the equatorial wings of the corona in
1878 gave apparent support to this hypothesis; if the substance of the
corona could extend ten million miles from the sun, why might it not
extend even one hundred million, gradually fading out beyond the orbit
of the earth? A variation of this hypothesis assumes that the
reflection is due to swarms of meteors circling about the sun, in the
plane of its equator, all the way from its immediate neighborhood to a
distance exceeding that of the earth. But in neither form is the
hypothesis satisfactory; there is nothing in the appearance of the
corona to indicate that it extends even as far as the planet Mercury,
while as to meteors, the orbits of the known swarms do not accord with
the hypothesis, and we have no reason to believe that clouds of others
exist traveling in the part of space where they would have to be in
order to answer the requirements of the theory. The extension of the
corona in 1878 did not resemble in its texture the Zodiacal Light.

Now, it has so often happened in the history of science that an
important discovery in one branch has thrown unexpected but most
welcome light upon some pending problem in some other branch, that a
strong argument might be based upon that fact alone against the too
exclusive devotion of many investigators to the narrow lines of their
own particular specialty; and the Zodiacal Light affords a case in
point, when it is considered in connection with recent discoveries in
chemistry and physics. From the fact that atoms are compound bodies
made up of corpuscles at least a thousand times smaller than the
smallest known atom—a fact which astounded most men of science when it
was announced a few years ago—a new hypothesis has been developed
concerning the nature of the Zodiacal Light (as well as other
astronomical riddles), and this hypothesis comes not from an
astronomer, but from a chemist and physicist, the Swede, Svante
Arrhenius. In considering an outline of this new hypothesis we need
neither accept nor reject it; it is a case rather for suspension of
judgment.

To begin with, it carries us back to the “pressure of light” mentioned
in the preceding chapter. The manner in which this pressure is believed
generally to act was there sufficiently explained, and it only remains
to see how it is theoretically extended to the particles of matter
supposed to constitute the Zodiacal Light. We know that corpuscles, or
“fragments of atoms” negatively electrified, are discharged from hot
bodies. Streams of these “ions” pour from many flames and from molten
metals; and the impact of the cathode and ultra-violet rays causes them
to gush even from cold bodies. In the vast laboratory of the sun it is
but reasonable to suppose that similar processes are taking place. “As
a very hot metal emits these corpuscles,” says Prof. J. J. Thomson, “it
does not seem an improbable hypothesis that they are emitted by that
very hot body, the sun.” Let it be assumed, then, that the sun does
emit them; what happens next? Negatively charged corpuscles, it is
known, serve as nuclei to which particles of matter in the ordinary
state are attracted, and it is probable that those emitted from the sun
immediately pick up loads in this manner and so grow in bulk. If they
grow large enough the gravitation of the sun draws them back, and they
produce a negative charge in the solar atmosphere. But it is probable
that many of the particles do not attain the critical size which,
according to the principles before explained, would enable the
gravitation of the sun to retain them in opposition to the pressure of
the waves of light, and with these particles the light pressure is
dominant. Clouds of them may be supposed to be continually swept away
from the sun into surrounding space, moving mostly in or near the plane
of the solar equator, where the greatest activity, as indicated by
sunspots and related phenomena, is taking place. As they pass outward
into space many of them encounter the earth. If the earth, like the
moon, had no atmosphere the particles would impinge directly on its
surface, giving it a negative electric charge. But the presence of the
atmosphere changes all that, for the first of the flying particles that
encounter it impart to it their negative electricity, and then, since
like electric charges repel like, the storm of particles following will
be sheered off from the earth, and will stream around it in a maze of
hyperbolic paths. Those that continue on into space beyond the earth
may be expected to continue picking up wandering particles of matter
until their bulk has become so great that the solar attraction prevails
again over the light pressure acting upon them, and they turn again
sunward. Passing the earth on their return they will increase the
amount of dust-clouds careering round it; and these will be further
increased by the action of the ultra-violet rays of the sunlight
causing particles to shoot radially away from the earth when the
negative charge of the upper atmosphere has reached a certain amount,
which particles, although starting sunward, will be swept back to the
earth with the oncoming streams. As the final result of all this
accumulation of flying and gyrating particles in the earth’s
neighborhood, we are told that the latter must be transformed into the
semblance of a gigantic solid-headed comet provided with streaming
tails, the longest of them stretching away from the direction of the
sun, while another shorter one extends toward the sun. This shorter
tail is due to the particles that we have just spoken of as being
driven sunward from the earth by the action of ultra-violet light. No
doubt this whole subject is too technical for popular statement; but at
any rate the general reader can understand the picturesque side of the
theory, for its advocates assure us that if we were on the moon we
would doubtless be able to see the comet-like tails of the earth, and
then we could appreciate the part that they play in producing the
phenomenon of the Zodiacal Light.

That the Light as we see it could be produced by the reflection of
sunlight from swarms of particles careering round the earth in the
manner supposed by Arrhenius’ hypothesis is evident enough; and it will
be observed that the new theory, after all, is only another variant of
the older one which attributes the Zodiacal Light to an extension of
the solar corona. But it differs from the older theory in offering an
explanation of the manner in which the extension is effected, and it
differentiates between the corona proper and the streams of negative
particles shot away from the sun. In its details the hypothesis of
Arrhenius also affords an explanation of many peculiarities of the
Zodiacal Light, such as that it is confined to the neighborhood of the
ecliptic, and that it is stronger on the side of the earth which is
just turning away from a position under the sun than on the other side;
but it would carry us beyond our limits to go into these particulars.
The Gegenschein, according to this theory, is a part of the same
phenomenon as the Zodiacal Light, for by the laws of perspective it is
evident that the reflection from the streams of particles situated at a
point directly opposite to the sun would be at a maximum, and this is
the place which the Gegenschein occupies. Apart from its geometrical
relations to the position of the sun, the variability of the Zodiacal
Light appears to affirm its solar dependence, and this too would be
accounted for by Arrhenius’ hypothesis better than by the old theory of
coronal extension. The amount of corpuscular discharge from the sun
must naturally be governed by the state of relative activity or
inactivity of the latter, and this could not but be reflected in the
varying splendor of the Zodiacal Light. But much more extended study
than has yet been given to the subject will be required before we can
feel that we know with reasonable certainty what this mysterious
phenomenon really is. By the hypothesis of Arrhenius every planet that
has an atmosphere must have a Zodiacal Light attending it, but the
phenomenon is too faint for us to be able to see it in the case, for
instance, of Venus, whose atmosphere is very abundant. The moon has no
corresponding “comet’s tail” because, as already explained, of the lack
of a lunar atmosphere to repel the streams by becoming itself
electrified; but if there were a lunar Zodiacal Light, no doubt we
could see it because of the relative nearness of our satellite.




IX
Marvels of the Aurora


One of the most vivid recollections of my early boyhood is that of
seeing my father return hastily into the house one evening and call out
to the family: “Come outside and look at the sky!” Ours was a country
house situated on a commanding site, and as we all emerged from the
doorway we were dumbfounded to see the heavens filled with pale flames
which ran licking and quivering over the stars. Instantly there sprang
into my terrified mind the recollection of an awful description of “the
Day of Judgment” (the _Dies Iræ_), which I had heard with much
perturbation of spirit in the Dutch Reformed church from the lips of a
tall, dark-browed, dreadfully-in-earnest preacher of the old-fashioned
type. My heart literally sank at sight of the spectacle, for it
recalled the preacher’s very words; it was just as he had said it would
be, and it needed the assured bearing of my elders finally to convince
me that

That Day of Wrath, O dreadful day,
When Heaven and Earth shall pass away,
As David and the Sibyl say


had not actually come upon us. And even the older members of the
household were not untouched with misgivings when menacing spots of
crimson appeared, breaking out now here, now there, in the shuddering
sky. Toward the north the spectacle was appalling. A huge arch spanned
an unnaturally dark segment resting on the horizon, and above this arch
sprang up beams and streamers in a state of incessant agitation,
sometimes shooting up to the zenith with a velocity that took one’s
breath, and sometimes suddenly falling into long ranks, and _marching,
marching, marching,_ like an endless phalanx of fiery specters, and
moving, as I remember, always from east to west. The absolute silence
with which these mysterious evolutions were performed and the quavering
reflections which were thrown upon the ground increased the awfulness
of the exhibition. Occasionally enormous curtains of lambent flame
rolled and unrolled with a majestic motion, or were shaken to and fro
as if by a mighty, noiseless wind. At times, too, a sudden billowing
rush would be made toward the zenith, and for a minute the sky overhead
would glow so brightly that the stars seemed to have been consumed. The
spectacle continued with varying intensity for hours.


[Illustration: Auroral beams seen in England]


This exhibition occurred in Central New York, a latitude in which the
Aurora Borealis is seldom seen with so much splendor. I remember
another similar one seen from the city of New York in November, 1882.
On this last occasion some observers saw a great upright beam of light
which majestically moved across the heavens, stalking like an
apparition in the midst of the auroral pageant, of whose general
movements it seemed to be independent, maintaining always its upright
posture, and following a magnetic parallel from east to west. This
mysterious beam was seen by no less than twenty-six observers in
different parts of the country, and a comparison of their observations
led to a curious calculation indicating that the apparition was about
_one hundred and thirty-three miles tall_ and moved at the speed of ten
miles per second!


[Illustration: Auroral arches of an eliptic form seen in the Arctic
regions]


But, as everybody knows, it is in the Arctic regions that the Aurora,
or the “Northern Lights,” can best be seen. There, in the long polar
night, when for months together the sun does not rise, the strange
coruscations in the sky often afford a kind of spectral daylight in
unison with the weird scenery of the world of ice. The pages in the
narratives of Arctic exploration that are devoted to descriptions of
the wonderful effects of the Northern Lights are second to none that
man has ever penned in their fascination. The lights, as I have already
intimated, display astonishing colors, particularly shades of red and
green, as they flit from place to place in the sky. The discovery that
the magnetic needle is affected by the Aurora, quivering and darting
about in a state of extraordinary excitement when the lights are
playing in the sky, only added to the mystery of the phenomenon until
its electro-magnetic nature had been established. This became evident
as soon as it was known that the focus of the displays was the magnetic
pole; and when the far South was visited the Aurora Australis was
found, having its center at the South Magnetic Pole. Then, if not
before, it was clear that the earth was a great globular magnet, having
its poles of opposite magnetism, and that the auroral lights, whatever
their precise cause might be, were manifestations of the magnetic
activity of our planet. After the invention of magnetic telegraphy it
was found that whenever a great Aurora occurred the telegraph lines
were interrupted in their operation, and the ocean cables ceased to
work. Such a phenomenon is called a “magnetic storm.”

The interest excited by the Aurora in scientific circles was greatly
stimulated when, in the last half of the nineteenth century, it was
discovered that it is a phenomenon intimately associated with
disturbances on the sun. The ancient “Zurich Chronicles,” extending
from the year 1000 to the year 1800, in which both sun-spots visible to
the naked eye and great displays of the auroral lights were recorded,
first set Rudolf Wolf on the track of this discovery. The first notable
proof of the suspected connection was furnished with dramatic emphasis
by an occurrence which happened on September 1, 1859. Near noon on that
day two intensely brilliant points suddenly broke out in a group of
sun-spots which were under observation by Mr R. C. Carrington at his
observatory at Redhill, England. The points remained visible for not
more than five minutes, during which interval they moved _thirty-five
thousand miles_ across the solar disk. Mr R. Hodgson happened to see
the same phenomenon at his observatory at Highgate, and thus all
possibility of deception was removed. But neither of the startled
observers could have anticipated what was to follow, and, indeed, it
was an occurrence which has never been precisely duplicated. I quote
the eloquent account given by Miss Clerke in her _History of Astronomy
During the Nineteenth Century._

This unique phenomenon seemed as if specially designed to accentuate
the inference of a sympathetic relation between the earth and the sun.
From August 28 to September 4, 1859, a magnetic storm of unparalleled
intensity, extent, and duration was in progress over the entire globe.
Telegraphic communication was everywhere interrupted—except, indeed,
that it was in some cases found practicable to work the lines _without
batteries_ by the agency of the earth-currents alone; sparks issued
from the wires; gorgeous auroras draped the skies in solemn crimson
over both hemispheres, and even in the tropics; the magnetic needle
lost all trace of continuity in its movements and darted to and fro as
if stricken with inexplicable panic. The coincidence was even closer.
_At the very instant_ of the solar outburst witnessed by Carrington and
Hodgson the photographic apparatus at Kew registered a marked
disturbance of all the three magnetic elements; while shortly after the
ensuing midnight the electric agitation culminated, thrilling the whole
earth with subtle vibrations, and lighting up the atmosphere from pole
to pole with coruscating splendors which perhaps dimly recall the times
when our ancient planet itself shone as a star.


If this amazing occurrence stood alone, and as I have already said it
has never been exactly duplicated, doubt might be felt concerning some
of the inferences drawn from it; but in varying forms it has been
repeated many times, so that now hardly anyone questions the reality of
the assumed connection between solar outbursts and magnetic storms
accompanied by auroral displays on the earth. It is true that the late
Lord Kelvin raised difficulties in the way of the hypothesis of a
direct magnetic action of the sun upon the earth, because it seemed to
him that an inadmissible quantity of energy was demanded to account for
such action. But no calculation like that which he made is final, since
all calculations depend upon the validity of the data; and no authority
is unshakable in science, because no man can possess omniscience. It
was Lord Kelvin who, but a few years before the thing was actually
accomplished, declared that aerial navigation was an impracticable
dream, and demonstrated its impracticability by calculation. However
the connection may be brought about, it is as certain as evidence can
make it that solar outbursts are coincident with terrestial magnetic
disturbances, and coincident in such a way as to make the inference of
a causal connection irresistible. The sun is only a little more than a
hundred times its own diameter away from the earth. Why, then, with the
subtle connection between them afforded by the ether which conveys to
us the blinding solar light and the life-sustaining solar heat, should
it be so difficult to believe that the sun’s enormous electric energies
find a way to us also? No doubt the impulse coming from the sun acts
upon the earth after the manner of a touch upon a trigger, releasing
energies which are already stored up in our planet.

But besides the evidence afforded by such occurrences as have been
related of an intimate connection between solar outbreaks and
terrestial magnetic flurries, attended by magnificent auroral displays,
there is another line of proof pointing in the same direction. Thus, it
is known that the sun-spot period, as remarked in a preceding chapter,
coincides in a most remarkable manner with the periodic fluctuations in
the magnetic state of the earth. This coincidence runs into the most
astonishing details. For instance, when the sun-spot period shortens,
the auroral period shortens to precisely the same extent; as the short
sun-spot periods usually bring the most intense outbreaks of solar
activity, so the corresponding short auroral periods are attended by
the most violent magnetic storms; a secular period of about two hundred
and twenty-two years affecting sun-spots is said to have its auroral
duplicate; a shorter period of fifty-five and a half years, which some
observers believe that they have discovered appears also to be common
to the two phenomena; and yet another “superposed” period of about
thirty-five years, which some investigators aver exists, affects
sun-spots and aurora alike. In short, the coincidences are so numerous
and significant that one would have to throw the doctrine of
probability to the winds in order to be able to reject the conclusion
to which they so plainly lead.


[Illustration: Auroral curtain seen in Scandinavia]


But still the question recurs: How is the influence transmitted? Here
Arrhenius comes once more with his hypothesis of negative corpuscles,
or ions, driven away from the sun by light-pressure—a hypothesis which
seems to explain so many things—and offers it also as an explanation of
the way in which the sun creates the Aurora. He would give the Aurora
the same lineage with the Zodiacal Light. To understand the application
of this theory we must first recall the fact that the earth is a great
magnet having its two opposite poles of magnetism, one near the Arctic
and the other near the Antarctic Circle. Like all magnets, the earth is
surrounded with “lines of force,” which, after the manner of the curved
rays we saw in the photograph of a solar eclipse, start from a pole,
rising at first nearly vertically, then bend gradually over, passing
high above the equator, and finally descending in converging sheaves to
the opposite pole. Now the axis of the earth is so placed in space that
it lies at nearly a right angle to the direction of the sun, and as the
streams of negatively charged particles come pouring on from the sun
(see the last preceding chapter), they arrive in the greatest numbers
over the earth’s equatorial regions. There they encounter the lines of
magnetic force at the place where the latter have their greatest
elevation above the earth, and where their direction is horizontal to
the earth’s surface. Obeying a law which has been demonstrated in the
laboratory, the particles then follow the lines of force toward the
poles. While they are above the equatorial regions they do not become
luminescent, because at the great elevation that they there occupy
there is virtually no atmosphere; but as they pass on toward the north
and the south they begin to descend with the lines of force, curving
down to meet at the poles; and, encountering a part of the atmosphere
comparable in density with what remains in an exhausted Crookes tube,
they produce a glow of cathode rays. This glow is conceived to
represent the Aurora, which may consequently be likened to a gigantic
exhibition of vacuum-tube lights. Anybody who recalls his student days
in the college laboratory and who has witnessed a display of Northern
Lights will at once recognize the resemblance between them in colors,
forms, and behavior. This resemblance had often been noted before
Arrhenius elaborated his hypothesis.

Without intending to treat his interesting theory as more than a
possibly correct explanation of the phenomena of the Aurora, we may
call attention to some apparently confirmatory facts. One of the most
striking of these relates to a seasonal variation in the average number
of auroræ. It has been observed that there are more in March and
September than at any other time of the year, and fewer in June and
December; moreover (and this is a delicate test as applied to the
theory), they are slightly rarer in June than in December. Now all
these facts seem to find a ready explanation in the hypothesis of
Arrhenius, thus: (1) The particles issuing from the sun are supposed to
come principally from the regions whose excitement is indicated by the
presence of sun-spots (which accords with Hale’s observation that
sun-spots are columns of ionized vapors), and these regions have a
definite location on either side of the solar equator, seldom
approaching it nearer than within 5° or 10° north or south, and never
extending much beyond 35° toward either pole; (2) The equator of the
sun is inclined about 7° to the plane of the earth’s orbit, from which
it results that twice in a year—_viz.,_ in June and December—the earth
is directly over the solar equator, and twice a year—_viz.,_ in March
and September—when it is farthest north or south of the solar equator,
it is over the inner edge of the sun-spot belts. Since the corpuscles
must be supposed to be propelled radially from the sun, few will reach
the earth when the latter is over the solar equator in June and
December, but when it is over, or nearly over, the spot belts, in March
and September, it will be in the line of fire of the more active parts
of the solar surface, and relatively rich streams of particles will
reach it. This, as will be seen from what has been said above, is in
strict accord with the observed variations in the frequency of auroræ.
Even the fact that somewhat fewer auroræ are seen in June than in
December also finds its explanation in the known fact that the earth is
about three million miles nearer the sun in the winter than in the
summer, and the number of particles reaching it will vary, like the
intensity of light, inversely as the square of the distance. These
coincidences are certainly very striking, and they have a cumulative
force. If we accept the theory, it would appear that we ought to
congratulate ourselves that the inclination of the sun’s equator is so
slight, for as things stand the earth is never directly over the most
active regions of the sun-spots, and consequently never suffers from
the maximum bombardment of charged particles of which the sun is
capable. Incessant auroral displays, with their undulating draperies,
flitting colors, and marching columns might not be objectionable from
the point of view of picturesqueness, but one magnetic storm of extreme
intensity following closely upon the heels of another, for months on
end, crazing the magnetic needle and continually putting the telegraph
and cable lines out of commission, to say nothing of their effect upon
“wireless telegraphy”, would hardly add to the charms of terrestrial
existence.


[Illustration: Auroral arches seen in Scandinavia]


One or two other curious points in connection with Arrhenius’
hypothesis may be mentioned. First, the number of auroræ, according to
his explanation, ought to be greatest in the daytime, when the face of
the earth on the sunward side is directly exposed to the atomic
bombardment. Of course visual observation can give us no information
about this, since the light of the Aurora is never sufficiently intense
to be visible in the presence of daylight, but the records of the
magnetic observatories can be, and have been, appealed to for
information, and they indicate that the facts actually accord with the
theory. Behind the veil of sunlight in the middle of the afternoon,
there is good reason to believe, auroral exhibitions often take place
which would eclipse in magnificence those seen at night if we could
behold them. Observation shows, too, that auroræ are more frequent
before than after midnight, which is just what we should expect if they
originate in the way that Arrhenius supposes. Second, the theory offers
an explanation of the alleged fact that the formation of clouds in the
upper air is more frequent in years when auroræ are most abundant,
because clouds are the result of the condensation of moisture upon
floating particles in the atmosphere (in an absolutely dustless
atmosphere there would be no clouds), and it has been proved that
negative ions like those supposed to come from the sun play a master
part in the phenomena of cloud formation.

Yet another singular fact, almost mystical in its suggestions, may be
mentioned. It seems that the dance of the auroral lights occurs most
frequently during the absence of the moon from the hemisphere in which
they appear, and that they flee, in greater part, to the opposite
hemisphere when the moon’s revolution in an orbit considerably inclined
to the earth’s equator brings her into that where they have been
performing. Arrhenius himself discovered this curious relation of
auroral frequency to the position of the moon north or south of the
equator, and he explains it in this way. The moon, like the earth, is
exposed to the influx of the ions from the sun; but having no
atmosphere, or almost none, to interfere with them, they descend
directly upon her surface and charge her with an electric negative
potential to a very high degree. In consequence of this she affects the
electric state of the upper parts of the earth’s atmosphere where they
lie most directly beneath her, and thus prevents, to a large extent,
the negative discharges to which the appearance of the Aurora is due.
And so “the extravagant and erring spirit” of the Aurora avoids the
moon as Hamlet’s ghost fled at the voice of the cock announcing the
awakening of the god of day.

There are even other apparent confirmations of the hypothesis, but we
need not go into them. We shall, however, find one more application of
it in the next chapter, for it appears to be a kind of cure-all for
astronomical troubles; at any rate it offers a conceivable solution of
the question, How does the sun manage to transmit its electric
influence to the earth? And this solution is so grandiose in
conception, and so novel in the mental pictures that it offers, that
its acceptance would not in the least detract from the impression that
the Aurora makes upon the imagination.




X
Strange Adventures of Comets


The fears and legends of ancient times before Science was born, and the
superstitions of the Dark Ages, sedulously cultivated for theological
purposes by monks and priests, have so colored our ideas of the
influence that comets have had upon the human mind that many readers
may be surprised to learn that it was the apparition of a wonderful
comet, that of 1843, which led to the foundation of our greatest
astronomical institution, the Harvard College Observatory. No doubt the
comet superstition existed half a century ago, as, indeed, it exists
yet today, but in this case the marvelous spectacle in the sky proved
less effective in inspiring terror than in awakening a desire for
knowledge. Even in the sixteenth century the views that enlightened
minds took of comets tended powerfully to inspire popular confidence in
science, and Halley’s prediction, after seeing and studying the motion
of the comet which appeared in 1682, that it would prove to be a
regular member of the sun’s family and would be seen returning after a
period of about seventy-six years, together with the fulfillment of
that prediction, produced a revulsion from the superstitious notions
which had so long prevailed.


[Illustration: Swift’s comet. Taken at Arequipa, March 30 1892]


Then the facts were made plain that comets are subject to the law of
gravitation equally with the planets; that there are many which
regularly return to the neighborhood of the sun (perihelion); and that
these travel in orbits differing from those of the planets only in
their greater eccentricity, although they have the peculiarity that
they do not, like the planets, all go round the sun in the same
direction, and do not keep within the general plane of the planetary
system, but traverse it sometimes from above and sometimes from below.
Other comets, including most of the “great” ones, appear to travel in
parabolic or, in a few cases, hyperbolic orbits, which, not being
closed curves, never bring them back again. But it is not certain that
these orbits may not be extremely eccentric ellipses, and that after
the lapse of hundreds, or thousands, of years the comets that follow
them may not reappear. The question is an interesting one, because if
all orbits are really ellipses, then all comets must be permanent
members of the solar system, while in the contrary case many of them
are simply visitors, seen once and never to be seen again. The
hypothesis that comets are originally interlopers might seem to derive
some support from the fact that the certainly periodic ones are
associated, in groups, with the great outer planets, whose attraction
appears to have served as a trap for them by turning them into
elliptical orbits and thus making them prisoners in the solar system.
Jupiter, owing to his great mass and his commanding situation in the
system, is the chief “comet-catcher;” but he catches them not for
himself, but for the sun. Yet if comets do come originally from without
the borders of the planetary system, it does not, by any means, follow
that they were wanderers at large in space before they yielded to the
overmastering attraction of the sun. Investigation of the known
cometary orbits, combined with theoretical considerations, has led some
astronomers to the conclusion that as the sun travels onward through
space he “picks up _en route_” cometary masses which, without belonging
strictly to his empire, are borne along in the same vast “cosmical
current” that carries the solar system.

But while no intelligent person any longer thinks that the appearance
of a great comet is a token from the heavenly powers of the approaching
death of a mighty ruler, or the outbreak of a devastating war, or the
infliction of a terrible plague upon wicked mankind, science itself has
discovered mysteries about comets which are not less fascinating
because they are more intellectual than the irrational fancies that
they have displaced. To bring the subject properly before the mind, let
us see what the principal phenomena connected with a comet are.

At the present day comets are ordinarily “picked up” with the telescope
or the photographic plate before any one except their discoverer is
aware of their existence, and usually they remain so insignificant in
appearance that only astronomers ever see them. Yet so great is the
prestige of the word “comet” that the discovery of one of these
inconspicuous wanderers, and its subsequent movements, become items of
the day’s news which everybody reads with the feeling, perhaps, that at
least he knows what is going on in the universe even if he doesn’t
understand it. But a truly great comet presents quite a different
proposition. It, too, is apt to be detected coming out of the depths of
space before the world at large can get a glimpse of it, but as it
approaches the sun its aspect undergoes a marvelous change. Agitated
apparently by solar influence, it throws out a long streaming tail of
nebulous light, directed away from the sun and looking as if blown out
like a pennon by a powerful wind. Whatever may be the position of the
comet with regard to the sun, as it circles round him it continually
keeps its tail on the off side. This, as we shall soon see, is a fact
of capital importance in relation to the probable nature of comets’
tails. Almost at the same time that the formation of the tail is
observed a remarkable change takes place in the comet’s head, which, by
the way, is invariably and not merely occasionally its most important
part. On approaching the sun the head usually contracts. Coincidently
with this contraction a nucleus generally makes its appearance. This is
a bright, star-like point in the head, and it probably represents the
totality of solid matter that the comet possesses. But it is regarded
as extremely unlikely that even the nucleus consists of a uniformly
solid mass. If it were such, comets would be far more formidable
visitors when they pass near the planets than they have been found to
be. The diameter of the nucleus may vary from a few hundred up to
several thousand miles; the heads, on the average, are from twenty-five
thousand to one hundred thousand miles in diameter, although a few have
greatly exceeded these dimensions; that of the comet of 1811, one of
the most stupendous ever seen, was a million and a quarter miles in
diameter! As to the tails, not withstanding their enormous length—some
have been more than a hundred million miles long—there is reason to
believe that they are of extreme tenuity, “as rare as vacuum.” The
smallest stars have been seen shining through their most brilliant
portions with undiminished luster.

After the nucleus has been formed it begins to throw out bright jets
directed toward the sun. A stream, and sometimes several streams, of
light also project sunward from the nucleus, occasionally appearing
like a stunted tail directed oppositely to the real tail. Symmetrical
envelopes which, seen in section, appear as half circles or parabolas,
rise sunward from the nucleus, forming a concentric series. The ends of
these stream backward into the tail, to which they seem to supply
material. Ordinarily the formation of these ejections and envelopes is
attended by intense agitation of the nucleus, which twists and turns,
swinging and gyrating with an appearance of the greatest violence.
Sometimes the nucleus is seen to break up into several parts. The
entire heads of some comets have been split asunder in passing close
around the sun; The comet of 1882 retreated into space after its
perihelion passage with _five heads_ instead of the one that it had
originally, and each of these heads had its own tail!

The possession of the spectroscope has enabled astronomers during later
years to study the chemical composition of comets by analyzing their
light. At first the only substances thus discovered in them were
hydro-carbon compounds, due evidently to the gaseous envelopes in which
some combination of hydrogen with carbon existed. Behind this gaseous
spectrum was found a faint continuous spectrum ascribed to the nucleus,
which apparently both reflects the sunlight and gives forth the light
of a glowing solid or liquid. Subsequently sodium and iron lines were
found in cometary spectra. The presence of iron would seem to indicate
that some of these bodies may be much more massive than observations on
their attractive effects have indicated. In some recent comets, such as
Morehouse’s, in 1908, several lines have been found, the origin of
which is unknown.

Without going back of the nineteenth century we may find records of
some of the most extraordinary comets that man has ever looked upon. In
1811, still spoken of as “the year of the comet,” because of the
wonderful vintage ascribed to the skyey visitor, a comet shaped like a
gigantic sword amazed the whole world, and, as it remained visible for
seventeen months, was regarded by superstitious persons as a symbol of
the fearful happenings of Napoleon’s Russian campaign. This comet, the
extraordinary size of whose head, greatly exceeding that of the sun
itself, has already been mentioned, was also remarkable for exhibiting
so great a brilliancy without approaching even to the earth’s distance
from the sun. But there was once a comet (and only once—in the year
1729) which never got nearer to the sun than four times the distance of
the earth and yet appeared as a formidable object in the sky. As
Professor Young has remarked, “it must have been an enormous comet to
be visible from such a distance.” And we are to remember that there
were no great telescopes in the year 1729. That comet affects the
imagination like a phantom of space peering into the solar system,
displaying its enormous train afar off (which, if it had approached as
near as other comets, would probably have become _the_ celestial wonder
of all human memory), and then turning away and vanishing in the depths
of immensity.

In 1843 a comet appeared which was so brilliant that it could be seen
in broad day close beside the sun! This was the first authenticated
instance of that kind, but the occurrence was to be repeated, as we
shall see in a moment, less than forty years later.

The splendid comet of 1858, usually called Donati’s, is remembered by
many persons yet living. It was, perhaps, both as seen by the naked eye
and with the telescope, the most beautiful comet of which we have any
record. It too marked a rich vintage year, still remembered in the
vineyards of France, where there is a popular belief that a great comet
ripens the grape and imparts to the wine a flavor not attainable by the
mere skill of the cultivator. There are “comet wines,” carefully
treasured in certain cellars, and brought forth only when their owner
wishes to treat his guests to a sip from paradise.

The year 1861 saw another very remarkable comet, of an aspect strangely
vast and diffuse, which is believed to have swept the earth with its
immense tail when it passed between us and the sun on the night of June
30th, an event which produced no other known effect than the appearance
of an unwonted amount of scattered light in the sky.

The next very notable comet was the “Great Southern Comet” of 1880,
which was not seen from the northern hemisphere. It mimicked the aspect
of the famous comet of 1843, and to the great surprise of astronomers
appeared to be traveling in the same path. This proved to be the rising
of the curtain for an astronomical sensation unparalleled in its kind;
for two years later another brilliant comet appeared, first in the
southern hemisphere, _and it too followed the same track._ The
startling suggestion was now made that this comet was identical with
those of 1843 and 1880, its return having been hastened by the
resistance experienced in passing twice through the coronal envelope,
and there were some who thought that it would now swing swiftly round
and then plunge straight into the sun, with consequences that might be
disastrous to us on account of the “flash of heat” that would be
produced by the impact. Nervous people were frightened, but observation
soon proved that the danger was imaginary, for although the comet
almost grazed the sun, and must have rushed through two or three
million miles of the coronal region, no retardation of its immense
velocity was perceptible, and it finally passed away in a damaged
condition, as before remarked, and has never since appeared.

Then the probable truth was perceived—_viz.,_ that the three comets
(1843, 1880, and 1882) were not one identical body, but three separate
ones all traveling in the same orbit. It was found, too, that a comet
seen in 1668 bore similar insignia of relationship. The natural
inference was that these four bodies had once formed a single mass
which had been split apart by the disruptive action of the sun.
Strength was lent to this hypothesis by the fact that the comet of 1882
was apparently torn asunder during its perihelion passage, retreating
into space in a dissevered state. But Prof. George Forbes has a theory
that the splitting of the original cometary mass was effected by an
unknown planet, probably greater than Jupiter, situated at a hundred
times the earth’s distance from the sun, and revolving in a period of a
thousand years. He supposes that the original comet was not that of
1668, but one seen in 1556, which has since been “missing,” and that
its disruption occurred from an encounter with the supposititious
planet about the year 1700. Truly from every point of view comets are
the most extraordinary of adventurers!

The comet of 1882 was likewise remarkable for being visible, like its
predecessor of 1843, in full daylight in close proximity to the sun.
The story of its detection when almost in contact with the solar disk
is dramatic. It had been discovered in the southern hemisphere only a
couple of weeks before its perihelion, which occurred on September
17th, and on the forenoon of that day it was seen by Doctor Common in
England, and by Doctor Elkin and Mr Finlay at the Cape of Good Hope,
almost touching the sun. It looked like a dazzling white bird with
outspread wings. The southern observers watched it go _right into the
sun,_ when it instantly disappeared. What had happened was that the
comet in passing its perihelion point had swung exactly between the
earth and the sun. On the following morning it was seen from all parts
of the world close by the sun on the opposite side, and it remained
thus visible for three days, gradually receding from the solar disk. It
then became visible for northern observers in the morning sky before
sunrise, brandishing a portentous sword-shaped tail which, if it had
been in the evening sky, would have excited the wonder of hundreds of
millions, but situated where it was, comparatively few ever saw it.


[Illustration: Daniels’ comet. August 11, 1907]


The application of photography to the study of comets has revealed many
curious details which might otherwise have escaped detection, or at
best have remained subject to doubt. It has in particular shown not
only the precise form of the tails, but the remarkable vicissitudes
that they undergo. Professor Barnard’s photographs of Brooks’ comet in
1893 suggested, by the extraordinary changes in the form of the tail
which they revealed, that the comet was encountering a series of
obstructions in space which bent and twisted its tail into fantastic
shapes. The reader will observe the strange form into which the tail
was thrown on the night of October 21st. A cloud of meteors through
which the comet was passing might have produced such deformations of
its tail. In the photograph of Daniels’ comet of 1907, a curious
striping of the tail will be noticed. The short bright streaks seen in
the photograph, it may be explained, are the images of stars which are
drawn out into lines in consequence of the fact that the photographic
telescope was adjusted to follow the motion of the comet while the
stars remained at rest.

But the adventures of comets are not confined to possible encounters
with unknown obstacles. We have referred to the fact that the great
planets, and especially Jupiter, frequently interfere with the motions
of comets. This interference is not limited to the original alteration
of their orbits from possible parabolas to ellipses, but is sometimes
exercised again and again, turning the bewildered comets into
elliptical paths of all degrees of eccentricity. A famous example of
this kind of planetary horse-play is furnished by the story of Lexell’s
missing comet. This comet was first seen in 1770. Investigation showed
that it was moving in an orbit which should bring it back to perihelion
every five and a half years; yet it had never been seen before and,
although often searched for, has never been seen since. Laplace and
Leverrier proved mathematically that in 1767 it had approached so close
to Jupiter as to be involved among the orbits of his satellites. What
its track had been before is not known, but on that occasion the giant
planet seized the interloper, threw it into a short elliptic orbit and
sent it, like an arrested vagrant, to receive sentence at the bar of
the sun. On this journey it passed within less than 1,500,000 miles of
the earth. The form of orbit which Jupiter had impressed required, as
we have said, its return in about five and a half years; but soon after
1770 it had the misfortune a second time to encounter Jupiter at close
range, and he, as if dissatisfied with the leniency of the sun, or
indignant at the stranger’s familiarity, seized the comet and hurled it
out of the system, or at any rate so far away that it has never since
been able to rejoin the family circle that basks in the immediate rays
of the solar hearth. Nor is this the only instance in which Jupiter has
dealt summarily with small comets that have approached him with too
little deference.


[Illustration: Brooks’ comet. Photographed by Barnard, October 21,
1893]


The function which Jupiter so conspicuously fulfills as master of the
hounds to the sun is worth considering a little more in detail. To
change the figure, imagine the sun in its voyage through space to be
like a majestic battleship surrounded by its scouts. Small vessels (the
comets, as they are overhauled by the squadron, are taken in charge by
the scouts, with Jupiter for their chief, and are forced to accompany
the fleet, but not all are impressed. If a strange comet undertakes to
run across Jupiter’s bows the latter brings it to, and makes prize of
it by throwing it into a relatively small ellipse with the sun for its
focus. Thenceforth, unless, as happened to the unhappy comet of Lexell,
it encounters Jupiter again in such a way as to be diverted by him into
a more distant orbit, it can never get away. About thirty comets are
now known to have thus been captured by the great planet, and they are
called “Jupiter’s Comet Family.” But, on the other hand, if a wandering
comet crosses the wake of the chief planetary scout the latter simply
drives it away by accelerating its motion and compels it to steer off
into open space. The transformation of comets into meteors will be
considered in the next chapter, but here, in passing, mention may be
made of the strange fate of one member of Jupiter’s family, Biela’s
comet, which, having become over bold in its advances to its captor,
was, after a few revolutions in is impressed orbit, torn to pieces and
turned into a flock of meteors.

And now let us return to the mystery of comets’ tails. That we are
fully justified in speaking of the tails of comets as mysterious is
proved by the declaration of Sir John Herschel, who averred, in so many
words, that “there is some profound secret and mystery of nature
concerned in this phenomenon,” and this profound secret and mystery has
not yet been altogether cleared up. Nevertheless, the all-explaining
hypothesis of Arrhenius offers us once more a certain amount of aid.
Comets’ tails, Arrhenius assures us, are but another result of the
pressure of light. The reader will recall the applications of this
theory to the Zodiacal Light and the Aurora. In the form in which we
now have to deal with it, the supposition is made that as a comet
approaches the sun eruptions of vapor, due to the solar heat, occur in
its nucleus. These are naturally most active on the side which is
directly exposed to the sun, whence the appearance of the immense
glowing envelopes that surround the nucleus on the sunward side. Among
the particles of hydro-carbon, and perhaps solid carbon in the state of
fine dust, which are thus set free there will be many whose size is
within the critical limit which enables the light-waves from the sun to
drive them away. Clouds of such particles, then, will stream off behind
the advancing comet, producing the appearance of a tail. This accounts
for the fact that the tails of comets are always directed away from the
sun, and it also explains the varying forms of the tails and the
extraordinary changes that they undergo. The speed of the particles
driven before the light-waves must depend upon their size and weight,
the lightest of a given size traveling the most swiftly. By accretion
certain particles might grow, thus losing velocity and producing the
appearance of bunches in the tail, such as have been observed. The
hypothesis also falls in with the researches of Bredichin, who has
divided the tails of comets into three principal classes—_viz.:_ (1)
Those which appear as long, straight rays; (2) Those which have the
form of curved plumes or scimitars; (3) Those which are short, brushy,
and curved sharply backward along the comet’s path. In the first type
he calculates the repulsive force at from twelve to fifteen times the
force of gravity; in the second at from two to four times; and in the
third at about one and a half times. The straight tails he ascribes to
hydrogen because the hydrogen atom is the lightest known; the
sword-shaped tails to hydro-carbons; and the stumpy tails to vaporized
iron. It will be seen that, if the force driving off the tails is that
which Arrhenius assumes it to be, the forms of those appendages would
accord with those that Bredichin’s theory calls for. At the same time
we have an explanation of the multiple tails with which some comets
have adorned themselves. The comet of 1744, for instance, had at one
time no less than seven tails spread in a wide curved brush behind it.
Donati’s comet of 1858 also had at least two tails, the principal one
sword-shaped and the other long, narrow, and as straight as a rule.
According to Bredichin, the straight tail must have been composed of
hydrogen, and the other of some form of hydro-carbon whose atoms are
heavier than those of hydrogen, and, consequently, when swept away by
the storm of light-waves, followed a curvature depending upon the
resultant of the forces operating upon them. The seven tails of the
comet of 1744 presented a kind of diagram graphically exhibiting its
complex composition, and, if we knew a little more about the
constituents of a comet, we might be able to say from the amount of
curvature of the different tails just what were the seven substances of
which that comet consisted.

If these theories seem to the reader fantastic, at any rate they are no
more fantastic than the phenomena that they seek to explain.




XI
Meteors, Fire-Balls, and Meteorites


One of the most terrorizing spectacles with which the heavens have ever
caused the hearts of men to quake occurred on the night of November 13,
1833. On that night North America, which faced the storm, was under a
continual rain of fire from about ten o’clock in the evening until
daybreak.

_The fragments of a comet had struck the earth._

But the meaning of what had happened was not discovered until long
afterward. To the astronomers who, with astonishment not less than that
of other people, watched the wonderful scene, it was an unparalleled
“shower of meteors.” They did not then suspect that those meteors had
once formed the head of a comet. Light dawned when, a year later, Prof.
Denison Olmsted, of Yale College, demonstrated that the meteors had all
moved in parallel orbits around the sun, and that these orbits
intersected that of the earth at the point where our planet happened to
be on the memorable night of November 13th. Professor Olmsted even went
so far as to suggest that the cloud of meteors that had encountered the
earth might form a diffuse comet; but full recognition of the fact that
they were cometary débris came later, as the result of further
investigation. The key to the secret was plainly displayed in the
spectacle itself, and was noticed without being understood by thousands
of the terror-stricken beholders. It was _an umbrella of fire_ that had
opened overhead and covered the heavens; in other words, the meteors
all radiated from a particular point in the constellation Leo, and,
being countless as the snowflakes in a winter tempest, they ribbed the
sky with fiery streaks. Professor Olmsted showed that the radiation of
the meteors from a fixed point was an effect of perspective, and in
itself a proof that they were moving in parallel paths when they
encountered the earth. The fact was noted that there had been a
similar, but incomparably less brilliant, display of meteors on the
same day of November, 1832, and it was rightly concluded that these had
belonged to the same stream, although the true relationship of the
phenomena was not immediately apprehended. Olmsted ascribed to the
meteors a revolution about the sun once in every six months, bringing
them to the intersection of their orbit with that of the earth every
November 13th; but later investigators found that the real period was
about thirty-three and one-quarter years, so that the great displays
were due three times in a century, and their return was confidently
predicted for the year 1866. The appearance of the meteors in 1832, a
year before the great display, was ascribed to the great length of the
stream which they formed in space—so great that they required more than
two years to cross the earth’s orbit. In 1832 the earth had encountered
a relatively rare part of the stream, but in 1833, on returning to the
crossing-place, it found there the richest part of the stream pouring
across its orbit. This explanation also proved to be correct, and the
predicted return in 1866 was duly witnessed, although the display was
much less brilliant than in 1833. It was followed by another in 1867.


[Illustration: Curious forms of meteorite trains
Nos. 1 to 6 show the changes undergone by a train left by a meteorite
which passed near the “Great Dipper”; 7 shows the changes and drift of
a train seen in the constellation Virgo; 8 is the singular train of the
meteorite of February 22, 1909, near the Pole Star. (From _La
Nature_.)]


In the mean time Olmsted’s idea of a cometary relationship of the
meteors was demonstrated to be correct by the researches of
Schiaparelli and others, who showed that not only the November meteors,
but those of August, which are seen more or less abundantly every year,
traveled in the tracks of well-known comets, and had undoubtedly an
identical origin with those comets. In other words the comets and the
meteor-swarms were both remnants of original masses which had probably
been split up by the action of the sun, or of some planet to which they
had made close approaches. The annual periodicity of the August meteors
was ascribed to the fact that the separation had taken place so long
ago that the meteors had become distributed all around the orbit, in
consequence of which the earth encountered some of them every year when
it arrived at the crossing-point. Then Leverrier showed that the
original comet associated with the November meteors was probably
brought into the system by the influence of the planet Uranus in the
year 126 of the Christian era. Afterward Alexander Herschel identified
the tracks of no less than seventy-six meteor-swarms (most of them
inconspicuous) with those of comets. The still more recent researches
of Mr W. F. Denning make it probable that there are no meteors which do
not belong to a flock or system probably formed by the disintegration
of a cometary mass; even the apparently sporadic ones which shoot
across the sky, “lost souls in the night,” being members of flocks
which have become so widely scattered that the earth sometimes takes
weeks to pass through the region of space where their paths lie.

The November meteors should have exhibited another pair of spectacles
in 1899 and 1900, and their failure to do so caused at first much
disappointment, until it was made plain that a good reason existed for
their absence. It was found that after their last appearance, in 1867,
they had been disturbed in their movements by the planets Jupiter and
Saturn, whose attractions had so shifted the position of their orbit
that it no longer intersected that of the earth, as it did before.
Whether another planetary interference will sometime bring the
principal mass of the November meteors back to the former point of
intersection with the earth’s orbit is a question for the future to
decide. It would seem that there may be several parallel streams of the
November meteors, and that some of them, like those of August, are
distributed entirely around the orbit, so that every mid-November we
see a few of them.

We come now to a very remarkable example of the disintegration of a
comet and the formation of a meteor-stream. In 1826 Biela, of
Josephstadt, Austria, discovered a comet to which his name was given.
Calculation showed that it had an orbital period of about six and a
half years, belonging to Jupiter’s “family.” On one of its returns, in
1846, it astonished its watchers by suddenly splitting in two. The two
comets thus formed out of one separated to a distance of about one
hundred and sixty thousand miles, and then raced side by side,
sometimes with a curious ligature connecting them, like Siamese twins,
until they disappeared together in interplanetary space. In 1852 they
came back, still nearly side by side, but now the distance between them
had increased to a million and a quarter of miles. After that, at every
recurrence of their period, astronomers looked for them in vain, until
1872, when an amazing thing happened. On the night of November 28th,
when the earth was crossing the plane of the orbit of the missing
comet, a brilliant shower of meteors burst from the northern sky,
traveling nearly in the track which the comet should have pursued. The
astronomers were electrified. Klinkerfues, of Göttingen, telegraphed to
Pogson, of Madras: _“Biela touched earth; search near Theta Centauri.”_
Pogson searched in the place indicated and saw a cometary mass
retreating into the southern heavens, where it was soon swallowed from
sight!


[Illustration: Section of the atmosphere up to 100 kilometers.
Showing the mean elevation at which meteorites and meteors make their
appearance. Below are shown the elevation of Mount Everest, the highest
manned balloon ascent by M. Berson; the height of cirrus clouds; the
highest free balloon ascent; and the elevation attained by the clouds
of fire-dust ejected by the Krakatoa eruption in 1883. (From _La
Nature_)]


Since then the Biela meteors have been among the recognized periodic
spectacles of the sky, and few if any doubt that they represent a
portion of the missing comet whose disintegration began with the
separation into two parts in 1846. The comet itself has never since
been seen. The first display of these meteors, sometimes called the
“Andromedes,” because they radiate from the constellation Andromeda,
was remarkable for the great brilliancy of many of the fire-balls that
shot among the shower of smaller sparks, some of which were described
as equaling the full moon in size. None of them is known to have
reached the earth, but during the display of the same meteors in 1885 a
meteoric mass fell at Mazapil in Northern Mexico (it is now in the
Museum at Vienna), which many have thought may actually be a piece of
the original comet of Biela. This brings us to the second branch of our
subject.

More rare than meteors or falling stars, and more startling, except
that they never appear in showers, are the huge balls of fire which
occasionally dart through the sky, lighting up the landscapes beneath
with their glare, leaving trains of sparks behind them, often producing
peals of thunder when they explode, and in many cases falling upon the
earth and burying themselves from a few inches to several feet in the
soil, from which, more than once, they have been picked up while yet
hot and fuming. These balls are sometimes called bolides. They are not
really round in shape, although they often look so while traversing the
sky, but their forms are fragmentary, and occasionally fantastic. It
has been supposed that their origin is different from that of the true
meteors; it has even been conjectured that they may have originated
from the giant volcanoes of the moon or have been shot out from the sun
during some of the tremendous explosions that accompany the formation
of eruptive prominences. By the same reasoning some of them might be
supposed to have come from some distant star. Others have conjectured
that they are wanderers in space, of unknown origin, which the earth
encounters as it journeys on, and Lord Kelvin made a suggestion which
has become classic because of its imaginative reach—_viz.,_ that the
first germs of life may have been brought to the earth by one of these
bodies, “a fragment of an exploded world.”

It is a singular fact that astronomers and scientific men in general
were among the last to admit the possibility of solid masses falling
from the sky. The people had believed in the reality of such phenomena
from the earliest times, but the savants shook their heads and talked
of superstition. This was the less surprising because no scientifically
authenticated instance of such an occurrence was known, and the stones
popularly believed to have fallen from the sky had become the objects
of worship or superstitious reverence, a fact not calculated to
recommend them to scientific credence. The celebrated “black stone”
suspended in the Kaaba at Mecca is one of these reputed gifts from
heaven; the “Palladium” of ancient Troy was another; and a stone which
fell near Ensisheim, in Germany, was placed in a church as an object to
be religiously venerated. Many legends of falling stones existed in
antiquity, some of them curiously transfigured by the imagination, like
the “Lion of the Peloponnesus,” which was said to have sprung down from
the sky upon the Isthmus of Corinth. But near the beginning of the
nineteenth century, in 1803, a veritable shower of falling stones
occurred at L’Aigle, in Northern France, and this time astronomers took
note of the phenomenon and scientifically investigated it. Thousands of
the strange projectiles came from the sky on this occasion, and were
scattered over a wide area of country, and some buildings were hit.
Four years later another shower of stones occurred at Weston, Conn.,
numbering thousands of individuals. The local alarm created in both
cases was great, as well it might be, for what could be more
intimidating than to find the blue vault of heaven suddenly hurling
solid missiles at the homes of men? After these occurrences it was
impossible for the most skeptical to doubt any longer, and the regular
study of “aerolites,” or “meteorites,” began.

One of the first things recognized was the fact that fire-balls are
solid meteorites in flight, and not gaseous exhalations in the air, as
some had assumed. They burn in the air during their flight, and
sometimes, perhaps, are entirely consumed before reaching the ground.
Their velocity before entering the earth’s atmosphere is equal to that
of the planets in their orbits—_viz.,_ from twenty to thirty miles per
second—a fact which proves that the sun is the seat of the central
force governing them. Their burning in the air is not difficult to
explain; it is the heat of friction which so quickly brings them to
incandescence. Calculation shows that a body moving through the air at
a velocity of about a mile per second will be brought, superficially,
to the temperature of “red heat” by friction with the atmosphere. If
its velocity is twenty miles per second the temperature will become
thousands of degrees. This is the state of affairs with a meteorite
rushing into the earth’s atmosphere; its surface is liquefied within a
few seconds after the friction begins to act, and the melted and
vaporized portion of its mass is swept backward, forming the train of
sparks that follows every great fire-ball. However, there is one
phenomenon connected with the trains of meteorites which has never been
satisfactorily explained: they often persist for long periods of time,
drifting and turning with the wind, but not ceasing to glow with a
phosphorescent luminosity. The question is, Whence comes this light? It
must be light without heat, since the fine dust or vapor of which the
train can only consist would not retain sufficient heat to render it
luminous for so long a time. An extremely remarkable incident of this
kind occurred on February 22, 1909, when an immense fire-ball that
passed over southern England left a train that remained visible during
two hours, assuming many curious shapes as it was drifted about by
currents in the air.


[Illustration: A meteor photographed in flight]


But notwithstanding the enormous velocity with which meteorites enter
the air they are soon slowed down to comparatively moderate speed, so
that when they disappear they are usually traveling not faster than a
mile a second. The courses of many have been traced by observers
situated along their track at various points, and thus a knowledge has
been obtained of their height above the ground during their flight and
of the length of their visible courses. They generally appear at an
elevation of eighty or a hundred miles, and are seldom visible after
having descended to within five miles of the ground, unless the
observer happens to be near the striking-point, when he may actually
witness the fall. Frequently they burst while high in the air and their
fragments are scattered like shrapnel over the surface of the ground,
sometimes covering an area of several square miles, but of course not
thickly; different fragments of the same meteorite may reach the ground
at points several miles apart. The observed length of their courses in
the atmosphere varies from fifty to five hundred miles. If they
continued a long time in flight after entering the air, even the
largest of them would probably be consumed to the last scrap, but their
fiery career is so short on account of their great speed that the heat
does not have time to penetrate very deeply, and some that have been
picked up immediately after their fall have been found cold as ice
within. Their size after reaching the ground is variable within wide
limits; some are known which weigh several tons, but the great majority
weigh only a few pounds and many only a few ounces.

Meteorites are of two kinds: _stony_ meteorites and _iron_ meteorites.
The former outnumber the latter twenty to one; but many stone
meteorites contain grains of iron. Nickel is commonly found in iron
meteorites, so that it might be said that that redoubtable alloy
nickel-steel is of cosmical invention. Some twenty-five chemical
elements have been found in meteorites, including carbon and the
“sun-metal,” helium. The presence of the latter is certainly highly
suggestive in connection with the question of the origin of meteorites.
The iron meteorites, besides metallic iron and nickel, of which they
are almost entirely composed, contain hydrogen, helium, and carbonic
oxide, and about the only imaginable way in which these gases could
have become absorbed in the iron would be through the immersion of the
latter while in a molten or vaporized state in a hot and dense
atmosphere composed of them, a condition which we know to exist only in
the envelopes of the sun and the stars.

The existence of carbon in the Canyon Diablo iron meteorites is
attended by a circumstance of the most singular character—a very “fairy
tale of science.” In some cases _the carbon has become diamond!_ These
meteoric diamonds are very small; nevertheless, they are true diamonds,
resembling in many ways the little black gems produced by Moissan’s
method with the aid of the electric furnace. The fact that they are
found embedded in these iron meteorites is another argument in favor of
the hypothesis of the solar or stellar origin of the latter. To
appreciate this it is necessary to recall the way in which Moissan made
his diamonds. It was by a combination of the effects of great heat,
great pressure, and sudden or rapid superficial cooling on a mass of
iron containing carbon. When he finally broke open his iron he found it
a pudding stuffed with miniature black diamonds. When a fragment of the
Canyon Diablo meteoric iron was polished in Philadelphia over fifteen
years ago it cut the emery-wheel to pieces, and examination showed that
the damage had been effected by microscopic diamonds peppered through
the mass. How were those diamonds formed? If the sun or Sirius was the
laboratory that prepared them, we can get a glimpse at the process of
their formation. There is plenty of heat, plenty of pressure, and an
abundance of vaporized iron in the sun and the stars. When a great
solar eruption takes place, masses of iron which have absorbed carbon
may be shot out with a velocity which forbids their return. Plunged
into the frightful cold of space, their surfaces are quickly cooled, as
Moissan cooled his prepared iron by throwing it into water, and thus
the requisite stress is set up within, and, as the iron solidifies, the
included carbon crystallizes into diamonds. Whether this explanation
has a germ of truth in it or not, at any rate it is evident that iron
meteorites were not created in the form in which they come to us; they
must once have been parts of immeasurably more massive bodies than
themselves.

The fall of meteorites offers an appreciable, though numerically
insignificant, peril to the inhabitants of the earth. Historical
records show perhaps three or four instances of people being killed by
these bodies. But for the protection afforded by the atmosphere, which
acts as a very effective shield, the danger would doubtless be very
much greater. In the absence of an atmosphere not only would more
meteorites reach the ground, but their striking force would be
incomparably greater, since, as we have seen, the larger part of their
original velocity is destroyed by the resistance of the air. A
meteorite weighing many tons and striking the earth with a velocity of
twenty or thirty miles per second, would probably cause frightful
havoc.


[Illustration: Looking across Coon Butte crater from northern rim]


It is a singular fact that recent investigations seem to have proved
that an event of this kind actually happened in North America—perhaps
not longer than a thousand or two thousand years ago. The scene of the
supposed catastrophe is in northern central Arizona, at Coon Butte,
where there is a nearly circular crater in the middle of a circular
elevation or small mountain. The crater is somewhat over four thousand
feet in diameter, and the surrounding rim, formed of upturned strata
and ejected rock fragments, rises at its highest point one hundred and
sixty feet above the plain. The crater is about six hundred feet in
depth—that is, from the rim to the visible floor or bottom of the
crater. There is no evidence that volcanic action has ever taken place
in the immediate neighborhood of Coon Butte. The rock in which the
crater has been made is composed of horizontal sandstone and limestone
strata. Between three hundred and four hundred million tons of rock
fragments have been detached, and a large portion hurled by some cause
out of the crater. These fragments lie concentrically distributed
around the crater, and in large measure form the elevation known as
Coon Butte. The region has been famous for nearly twenty years on
account of the masses of meteoric iron found scattered about and known
as the “Canyon Diablo” meteorites. It was one of these masses, which
consist of nickel-iron containing a small quantity of platinum, and of
which in all some ten tons have been recovered for sale to the various
collectors throughout the world, that as before mentioned destroyed the
grinding-tool at Philadelphia through the cutting power of its embedded
diamonds. These meteoric irons are scattered about the crater-hill, in
concentric distribution, to a maximum distance of about five miles.
When the suggestion was first made in 1896 that a monster meteorite
might have created by its fall this singular lone crater _in stratified
rocks,_ it was greeted with incredulous smiles; but since then the
matter has assumed a different aspect. The Standard Iron Company,
formed by Messrs. D. M. Barringer, B. C. Tilghman, E. J. Bennitt, and
S. J. Holsinger, having become, in 1903, the owner of this freak of
nature, sunk shafts and bored holes to a great depth in the interior of
the crater, and also trenched the slopes of the mountain, and the
result of their investigations has proved that the meteoric hypothesis
of origin is correct. (See the papers published in the _Proceedings of
the Academy of Natural Sciences of Philadelphia,_ December, 1905,
wherein it is proved that the United States Geological Survey was wrong
in believing this crater to have been due to a steam explosion. Since
that date there has been discovered a great amount of additional
confirmatory proof). Material of unmistakably meteoric origin was found
by means of the drills, mixed with crushed rock, to a depth of six
hundred to seven hundred feet below the floor of the crater, and a
great deal of it has been found admixed with the ejected rock fragments
on the outer slopes of the mountain, absolutely proving synchronism
between the two events, the formation of this great crater and the
falling of the meteoric iron out of the sky. The drill located in the
bottom of the crater was sent, in a number of cases, much deeper (over
one thousand feet) into unaltered horizontal red sandstone strata, but
no meteoric material was found below this depth (seven hundred feet, or
between eleven and twelve hundred feet below the level of the
surrounding plain), which has been assumed as being about the limit of
penetration. It is not possible to sink a shaft at present, owing to
the water which has drained into the crater, and which forms, with the
finely pulverized sandstone, a very troublesome quicksand encountered
at about two hundred feet below the visible floor of the crater. As
soon as this water is removed by pumping it will be easy to explore the
depths of the crater by means of shafts and drifts. The rock strata
(sandstone and limestone) of which the walls consist present every
appearance of having been violently upturned by a huge body penetrating
the earth like a cannon-ball. The general aspect of the crater
strikingly resembles the impression made by a steel projectile shot
into an armor-plate. Mr Tilghman has estimated that a meteorite about
five hundred feet in diameter and moving with a velocity of about five
miles per second would have made just such a perforation upon striking
rocks of the character of those found at this place. There was some
fusion of the colliding masses, and the heat produced some steam from
the small amount of water in the rocks. As a result there has been
found at depth a considerable amount of fused quartz (original
sandstone), and with it innumerable particles or sparks of fused
nickel-iron (original meteorite). A projectile of that size penetrating
eleven to twelve hundred feet into the rocky shell of the globe must
have produced a shock which was perceptible several hundred miles away.


[Illustration: Trail on south side, Coon Butte crater]


The great velocity ascribed to the supposed meteorite at the moment of
striking could be accounted for by the fact that it probably plunged
nearly vertically downward, for it formed a circular crater in the
rocky crust of the earth. In that case it would have been less retarded
by the resistance of the atmosphere than are meteorites which enter the
air at a lower angle and shoot ahead hundreds of miles until friction
has nearly destroyed their original motion when they drop upon the
earth. Some meteoric masses of great size, such as Peary’s iron
meteorite found at Cape York, Greenland, and the almost equally large
mass discovered at Bacubirito, Mexico, appear to have penetrated but
slightly on striking the earth. This may be explained by supposing that
they pursued a long, horizontal course through the air before falling.
The result would be that, their original velocity having been
practically destroyed, they would drop to the ground with a velocity
nearly corresponding to that which gravity would impart within the
perpendicular distance of their final fall. A
six-hundred-and-sixty-pound meteorite, which fell at Knyahinya,
Hungary, striking at an angle of 27° from the vertical, penetrated the
ground to a depth of eleven feet.

It has been remarked that the Coon Butte meteorite may have fallen not
longer ago than a few thousand years. This is based upon the fact that
the geological indications favor the supposition that the event did not
occur more than five thousand years ago, while on the other hand the
rings of growth in the cedar-trees growing on the slopes of the crater
show that they have existed there about seven hundred years. Prof.
William H. Pickering has recently correlated this with an ancient
chronicle which states that at Cairo, Egypt, in the year 1029, “many
stars passed with a great noise.” He remarks that Cairo is about 100°,
by great circle, from Coon Butte, so that if the meteorite that made
the crater was a member of a flock of similar bodies which encountered
the earth moving in parallel lines, some of them might have traversed
the sky tangent to the earth’s surface at Cairo. That the spectacle
spoken of in the chronicle was caused by meteorites he deems
exceedingly probable because of what is said about “a great noise;”
meteorites are the only celestial phenomena attended with perceptible
sounds. Professor Pickering conjectures that this supposed flock of
great meteorites may have formed the nucleus of a comet which struck
the earth, and he finds confirmation of the idea in the fact that out
of the ten largest meteorites known, no less than seven were found
within nine hundred miles of Coon Butte. It would be interesting if we
could trace back the history of that comet, and find out what malicious
planet caught it up in its innocent wanderings and hurled it with so
true an aim at the earth! This remarkable crater is one of the most
interesting places in the world, for there is absolutely no record of
such a mass, possibly an iron-headed comet, from outer space having
come into collision with our earth. The results of the future
exploration of the depths of the crater will be awaited with much
interest.




XII
The Wrecking of the Moon


There are sympathetic moods under whose influence one gazes with a
certain poignant tenderness at the worn face of the moon; that little
“fossil world” (the child of our mother earth, too) bears such terrible
scars of its brief convulsive life that a sense of pity is awakened by
the sight. The moon is the wonder-land of the telescope. Those towering
mountains, whose “proud aspiring peaks” cast silhouettes of shadow that
seem drawn with india-ink; those vast plains, enchained with gentle
winding hills and bordered with giant ranges; those oval “oceans,”
where one looks expectant for the flash of wind-whipped waves; those
enchanting “bays” and recesses at the seaward feet of the Alps; those
broad straits passing between guardian heights incomparably mightier
than Gibraltar; those locket-like valleys as secluded among their
mountains as the Vale of Cashmere; those colossal craters that make us
smile at the pretensions of Vesuvius, Etna, and Cotopaxi; those strange
white ways which pass with the unconcern of Roman roads across
mountain, gorge, and valley—all these give the beholder an irresistible
impression that it is truly a world into which he is looking, a world
akin to ours, and yet no more like our world than Pompeii is like
Naples. Its air, its waters, its clouds, its life are gone, and only a
skeleton remains—a mute but eloquent witness to a cosmical tragedy
without parallel in the range of human knowledge.

One cannot but regret that the moon, if it ever was the seat of
intelligent life, has not remained so until our time. Think what the
consequences would have been if this other world at our very door had
been found to be both habitable and inhabited! We talk rather airily of
communicating with Mars by signals; but Mars never approaches nearer
than 35,000,000 miles, while the moon when nearest is only a little
more than 220,000 miles away. Given an effective magnifying power of
five thousand diameters, which will perhaps be possible at the mountain
observatories as telescopes improve, and we should be able to bring the
moon within an apparent distance of about forty miles, while the
corresponding distance for Mars would be more than seven thousand
miles. But even with existing telescopic powers we can see details on
the moon no larger than some artificial constructions on the earth. St
Peter’s at Rome, with the Vatican palace and the great piazza, if
existing on the moon, would unquestionably be recognizable as something
else than a freak of nature. Large cities, with their radiating lines
of communication, would at once betray their real character. Cultivated
tracts, and the changes produced by the interference of intelligent
beings, would be clearly recognizable. The electric illumination of a
large town at night would probably be markedly visible. Gleams of
reflected sunlight would come to us from the surfaces of the lakes and
oceans, and a huge “liner” traversing a lunar sea could probably be
followed by its trail of smoke. As to communications by “wireless”
signals, which certain enthusiasts have thought of in connection with
Mars, in the case of the moon they should be a relatively simple
matter, and the feat might actually be accomplished. Think what a
literature would grow up about the moon if it were a living world! Its
very differences from the earth would only accentuate its interest for
us. Night and day on the moon are each two weeks in length; how
interesting it would be to watch the manner in which the lunarians
dealt with such a situation as that. Lunar and terrestrial history
would keep step with each other, and we should record them both. Truly
one might well wish to have a neighbor world to study; one would feel
so much the less alone in space.

It is not impossible that the moon did at one time have inhabitants of
some kind. But, if so, they vanished with the disappearance of its
atmosphere and seas, or with the advent of its cataclysmic age. At the
best, its career as a living world must have been brief. If the water
and air were gradually absorbed, as some have conjectured, by its
cooling interior rocks, its surface might, nevertheless, have retained
them for long ages; but if, as others think, their disappearance was
due to the escape of their gaseous molecules in consequence of the
inability of the relatively small lunar gravitation to retain them,
then the final catastrophe must have been as swift as it was
inevitable. Accepting Darwin’s hypothesis, that the moon was separated
from the earth by tidal action while both were yet plastic or nebulous,
we may reasonably conclude that it began its career with a good supply
of both water and air, but did not possess sufficient mass to hold them
permanently. Yet it may have retained them long enough for life to
develop in many forms upon its surface; in fact, there are so many
indications that air and water have not always been lacking to the
lunar world that we are driven to invent theories to explain both their
former presence and their present absence.


[Illustration: The craters Clavius, Longomontanus, Tycho, etc.]


But whatever the former condition of the moon may have been, its
existing appearance gives it a resistless fascination, and it bears so
clearly the story of a vast catastrophe sculptured on its rocky face
that the thoughtful observer cannot look upon it without a feeling of
awe. The gigantic character of the lunar features impresses the
beholder not less than the universality of the play of destructive
forces which they attest. Let us make a few comparisons. Take the lunar
crater called “Tycho”, which is a typical example of its kind. In the
telescope Tycho appears as a perfect ring surrounding a circular
depression, in the center of which rises a group of mountains. Its
superficial resemblance to some terrestrial volcanic craters is very
striking. Vesuvius, seen from a point vertically above, would no doubt
look something like that (the resemblance would have been greater when
the Monte del Cavallo formed a more complete circuit about the crater
cone). But compare the dimensions. The remains of the outer crater ring
of Vesuvius are perhaps half a mile in diameter, while the active
crater itself is only two or three hundred feet across at the most;
Tycho has a diameter of fifty-four miles! The group of relatively
insignificant peaks in the center of the crater floor of Tycho is far
more massive than the entire mountain that we call Vesuvius. The
largest known volcanic crater on the earth, Aso San, in Japan, has a
diameter of seven miles; it would take _sixty_ craters like Aso San to
equal Tycho in area! And Tycho, though one of the most perfect, is by
no means the largest crater on the moon. Another, called “Theophilus,”
has a diameter of sixty-four miles, and is eighteen thousand feet deep.
There are hundreds from ten to forty miles in diameter, and thousands
from one to ten miles. They are so numerous in many places that they
break into one another, like the cells of a crushed honeycomb.

The lunar craters differ from those of the earth more fundamentally
than in the matter of mere size; _they are not situated on the tops of
mountains._ If they were, and if all the proportions were the same, a
crater like Tycho might crown a conical peak fifty or one hundred miles
high! Instead of being cavities in the summits of mountains, the lunar
craters are rather gigantic sink-holes whose bottoms in many cases lie
two or three miles below the general surface of the lunar world. Around
their rims the rocks are piled up to a height of from a few hundred to
two or three thousand feet, with a comparatively gentle inclination,
but on the inner side they fall away in gigantic broken precipices
which make the dizzy cliffs of the Matterhorn seem but “lover’s leaps.”
Down they drop, ridge below ridge, crag under crag, tottering wall
beneath wall, until, in a crater named “Newton,” near the south lunar
pole, they attain a depth where the rays of the sun never reach.
Nothing more frightful than the spectacle which many of these terrible
chasms present can be pictured by the imagination. As the lazy lunar
day slowly advances, the sunshine, unmitigated by clouds or atmospheric
veil of any kind, creeps across their rims and begins to descend the
opposite walls. Presently it strikes the ragged crest of a ridge which
had lain hidden in such darkness as we never know on the earth, and
runs along it like a line of kindling fire. Rocky pinnacles and needles
shoot up into the sunlight out of the black depths. Down sinks the line
of light, mile after mile, and continually new precipices and cliffs
are brought into view, until at last the vast floor is attained and
begins to be illuminated. In the meanwhile the sun’s rays, darting
across the gulf, have touched the summits of the central peaks, twenty
or thirty miles from the crater’s inmost edge, and they immediately
kindle and blaze like huge stars amid the darkness. So profound are
some of these awful craters that days pass before the sun has risen
high enough above them to chase the last shadows from their depths.

Although several long ranges of mountains resembling those of the earth
exist on the moon, the great majority of its elevations assume the
crateriform aspect. Sometimes, instead of a crater, we find an immense
mountain ring whose form and aspect hardly suggest volcanic action. But
everywhere the true craters are in evidence, even on the sea-beds,
although they attain their greatest number and size on those parts of
the moon—covering sixty per cent of its visible surface—which are
distinctly mountainous in character and which constitute its most
brilliant portions. Broadly speaking, the southwestern half of the moon
is the most mountainous and broken, and the northeastern half the least
so. Right down through the center, from pole to pole, runs a wonderful
line of craters and crateriform valleys of a magnitude stupendous even
for the moon. Another similar line follows the western edge. Three or
four “seas” are thrust between these mountainous belts. By the effects
of “libration” parts of the opposite hemisphere of the moon which is
turned away from the earth are from time to time brought into view, and
their aspect indicates that that hemisphere resembles in its surface
features the one which faces the earth. There are many things about the
craters which seem to give some warrant for the hypothesis which has
been particularly urged by Mr G. K. Gilbert, that they were formed by
the impact of meteors; but there are also many things which militate
against that idea, and, upon the whole, the volcanic theory of their
origin is to be preferred.

The enormous size of the lunar volcanoes is not so difficult to account
for when we remember how slight is the force of lunar gravity as
compared with that of the earth. With equal size and density, bodies on
the moon weigh only one-sixth as much as on the earth. Impelled by the
same force, a projectile that would go ten miles on the earth would go
sixty miles on the moon. A lunar giant thirty-five feet tall would
weigh no more than an ordinary son of Adam weighs on his greater
planet. To shoot a body from the earth so that it would not drop back
again, we should have to start it with a velocity of seven miles per
second; a mile and a half per second would serve on the moon. It is by
no means difficult to believe, then, that a lunar volcano might form a
crater ring eight or ten times broader than the greatest to be found on
the earth, especially when we reflect that in addition to the
relatively slight force of gravity, the materials of the lunar crust
are probably lighter than those of our terrestrial rocks.


[Illustration: Western part of the Mare Serenitatis]


For similar reasons it seems not impossible that the theory mentioned
in a former chapter—that some of the meteorites that have fallen upon
the earth originated from the lunar volcanoes—is well founded. This
would apply especially to the stony meteorites, for it is hardly to be
supposed that the moon, at least in its superficial parts, contains
much iron. It is surely a scene most strange that is thus presented to
the mind’s eye—that little attendant of the earth’s (the moon has only
one-fiftieth of the volume, and only one-eightieth of the mass of the
earth) firing great stones back at its parent planet! And what can have
been the cause of this furious outbreak of volcanic forces on the moon?
Evidently it was but a passing stage in its history; it had enjoyed
more quiet times before. As it cooled down from the plastic state in
which it parted from the earth, it became incrusted after the normal
manner of a planet, and then oceans were formed, its atmosphere being
sufficiently dense to prevent the water from evaporating and the
would-be oceans from disappearing continually in mist. This, if any,
must have been the period of life in the lunar world. As we look upon
the vestiges of that ancient world buried in the wreck that now covers
so much of its surface, it is difficult to restrain the imagination
from picturing the scenes which were once presented there; and, in such
a case, should the imagination be fettered? We give it free rein in
terrestrial life, and it rewards us with some of our greatest
intellectual pleasures. The wonderful landscapes of the moon offer it
an ideal field with just enough half-hidden suggestions of facts to
stimulate its powers.

The great plains of the _Mare Imbrium_ and the _Mare Serenitatis_ (the
“Sea of Showers” and the “Sea of Serenity”), bordered in part by lofty
mountain ranges precisely like terrestrial mountains, scalloped along
their shores with beautiful bays curving back into the adjoining
highlands, and united by a great strait passing between the nearly
abutting ends of the “Lunar Apennines” and the “Lunar Caucasus,” offer
the elements of a scene of world beauty such as it would be difficult
to match upon our planet. Look at the finely modulated bottom of the
ancient sea in Mr Ritchey’s exquisite photograph of the western part of
the _Mare Serenitatis,_ where one seems to see the play of the watery
currents heaping the ocean sands in waving lines, making shallows,
bars, and deeps for the mariner to avoid or seek, and affording a
playground for the creatures of the main. What geologist would not wish
to try his hammer on those rocks with their stony pages of fossilized
history? There is in us an instinct which forbids us to think that
there was never any life there. If we could visit the moon, there is
not among us a person so prosaic and unimaginative that he would not,
the very first thing, begin to search for traces of its inhabitants. We
would look for them in the deposits on the sea bottoms; we would
examine the shores wherever the configuration seemed favorable for
harbors and the sites of maritime cities—forgetting that it may be a
little ridiculous to ascribe to the ancient lunarians the same ideas
that have governed the development of our race; we would search through
the valleys and along the seeming courses of vanished streams; we would
explore the mountains, not the terrible craters, but the pinnacled
chains that recall our own Alps and Rockies; seeking everywhere some
vestige of the transforming presence of intelligent life. Perhaps we
should find such traces, and perhaps, with all our searching, we should
find nothing to suggest that life had ever existed amid that universal
ruin.


[Illustration: Mare Tranquilitatis and surroundings]


Look again at the border of the “Sea of Serenity”—what a name for such
a scene!—and observe how it has been rent with almost inconceivable
violence, the wall of the colossal crater Posidonius dropping
vertically upon the ancient shore and obliterating it, while its giant
neighbor, Le Monnier, opens a yawning mouth as if to swallow the sea
itself. A scene like this makes one question whether, after all, those
may not be right who have imagined that the so-called sea bottoms are
really vast plains of frozen lava which gushed up in floods so
extensive that even the mighty volcanoes were half drowned in the fiery
sea. This suggestion becomes even stronger when we turn to another of
the photographs of Mr Ritchey’s wonderful series, showing a part of the
_Mare Tranquilitatis_ (“Sea of Tranquility”!). Notice how near the
center of the picture the outline of a huge ring with radiating ridges
shows through the sea bottom; a fossil volcano submerged in a petrified
ocean! This is by no means the only instance in which a buried world
shows itself under the great lunar plains. Yet, as the newer craters in
the sea itself prove, the volcanic activity survived this other
catastrophe, or broke out again subsequently, bringing more ruin to
pile upon ruin.

Yet notwithstanding the evidence which we have just been considering in
support of the hypothesis that the “seas” are lava floods, Messrs.
Loewy and Puiseux, the selenographers of the Paris Observatory, are
convinced that these great plains bear characteristic marks of the
former presence of immense bodies of water. In that case we should be
forced to conclude that the later oceans of the moon lay upon vast
sheets of solidified lava; and thus the catastrophe of the lunar world
assumes a double aspect, the earliest oceans being swallowed up in
molten floods issuing from the interior, while the lands were reduced
to chaos by a universal eruption of tremendous volcanoes; and then a
period of comparative quiet followed, during which new seas were
formed, and new life perhaps began to flourish in the lunar world, only
to end in another cataclysm, which finally put a term to the existence
of the moon as a life-supporting world.

Suppose we examine two more of Mr Ritchey’s illuminating photographs,
and, first, the one showing the crater Theophilus and its surroundings.
We have spoken of Theophilus before, citing the facts that it is
sixty-four miles in diameter and eighteen thousand feet deep. It will
be noticed that it has two brother giants—Cyrillus the nearer, and
Catharina the more distant; but Theophilus is plainly the youngest of
the trio. Centuries, and perhaps thousands of years, must have elapsed
between the periods of their upheaval, for the two older craters are
partly filled with débris, while it is manifest at a glance that when
the south eastern wall of Theophilus was formed, it broke away and
destroyed a part of the more ancient ring of Cyrillus. There is no more
tremendous scene on the moon than this; viewed with a powerful
telescope, it is absolutely appalling.


[Illustration: Lunar craters Theophilus and surrounding region]


The next photograph shows, if possible, a still wilder region. It is
the part of the moon lying between Tycho and the south pole. Tycho is
seen in the lower left-hand part of the picture. To the right, at the
edge of the illuminated portion of the moon, are the crater-rings,
Longomontanus and Wilhelm I, the former being the larger. Between them
are to be seen the ruins of two or three more ancient craters which,
together with portions of the walls of Wilhelm I and Longomontanus,
have been honeycombed with smaller craters. The vast crateriform
depression above the center of the picture is Clavius, an unrivaled
wonder of lunar scenery, a hundred and forty-two miles in its greatest
length, while its whole immense floor has sunk two miles below the
general surface of the moon outside the ring. The monstrous
shadow-filled cavity above Clavius toward the right is Blancanus, whose
aspect here gives a good idea of the appearance of these chasms when
only their rims are in the sunlight. But observe the indescribable
savagery of the entire scene. It looks as though the spirit of
destruction had gone mad in this spot. The mighty craters have broken
forth one after another, each rending its predecessor; and when their
work was finished, a minor but yet tremendous outbreak occurred, and
the face of the moon was gored and punctured with thousands of smaller
craters. These relatively small craters (small, however, only in a
lunar sense, for many of them would appear gigantic on the earth)
recall once more the theory of meteoric impact. It does not seem
impossible that some of them may have been formed by such an agency.

One would not wish for our planet such a fate as that which has
overtaken the moon, but we cannot be absolutely sure that something of
the kind may not be in store for it. We really know nothing of the
ultimate causes of volcanic activity, and some have suggested that the
internal energies of the earth may be accumulating instead of dying
out, and may never yet have exhibited their utmost destructive power.
Perhaps the best assurance that we can find that the earth will escape
the catastrophe that has overtaken its satellite is to be found in the
relatively great force of its gravitation. The moon has been the victim
of its weakness; given equal forces, and the earth would be the better
able to withstand them. It is significant, in connection with these
considerations, that the little planet Mercury, which seems also to
have parted with its air and water, shows to the telescope some
indications that it is pitted with craters resembling those that have
torn to pieces the face of the moon.

Upon the whole, after studying the dreadful lunar landscapes, one
cannot feel a very enthusiastic sympathy with those who are seeking
indications of the continued existence of some kind of life on the
moon; such a world is better without inhabitants. It has met its fate;
let it go! Fortunately, it is not so near that it cannot hide its scars
and appear beautiful—except when curiosity impels us to look with the
penetrating eyes of the astronomer.


[Illustration: Mare Crisium]




XIII
The Great Mars Problem


Let any thoughtful person who is acquainted with the general facts of
astronomy look up at the heavens some night when they appear in their
greatest splendor, and ask himself what is the strongest impression
that they make upon his mind. He may not find it easy to frame an
answer, but when he has succeeded it will probably be to the effect
that the stars give him an impression of the universality of
intelligence; they make him feel, as the sun and the moon cannot do,
that his world is not alone; that all this was not made simply to form
a gorgeous canopy over the tents of men. If he is of a devout turn of
mind, he thinks, as he gazes into those fathomless deeps and among
those bewildering hosts, of the infinite multitude of created beings
that the Almighty has taken under his care. The narrow ideas of the old
geocentric theology, which made the earth God’s especial footstool, and
man his only rational creature, fall away from him like a veil that had
obscured his vision; they are impossible in the presence of what he
sees above. Thus the natural tendency, in the light of modern progress,
is to regard the universe as everywhere filled with life.

But science, which is responsible for this broadening of men’s thoughts
concerning the universality of life, itself proceeds to set limits. Of
spiritual existences it pretends to know nothing, but as to physical
beings, it declares that it can only entertain the supposition of their
existence where it finds evidence of an environment suited to their
needs, and such environment may not everywhere exist. Science, though
repelled by the antiquated theological conception of the supreme
isolation of man among created beings, regards with complacency the
probability that there are regions in the universe where no organic
life exists, stars which shine upon no inhabited worlds, and planets
which nourish no animate creatures. The astronomical view of the
universe is that it consists of matter in every stage of evolution:
some nebulous and chaotic; some just condensing into stars (suns) of
every magnitude and order; some shaped into finished solar bodies
surrounded by dependent planets; some forming stars that perhaps have
no planets, and will have none; some constituting suns that are already
aging, and will soon lose their radiant energy and disappear; and some
aggregated into masses that long ago became inert, cold, and rayless,
and that can only be revivified by means about which we can form
conjectures, but of which we actually know nothing.

As with the stars, so with the planets, which are the satellites of
stars. All investigations unite to tell us that the planets are not all
in the same state of development. As some are large and some small, so
some are, in an evolutionary sense, young, and some old. As they depend
upon the suns around which they revolve for their light, heat, and
other forms of radiant energy, so their condition varies with their
distance from those suns. Many may never arrive at a state suitable for
the maintenance of life upon their surfaces; some which are not at
present in such a state may attain it later; and the forms of life
themselves may vary with the peculiar environment that different
planets afford. Thus we see that we are not scientifically justified in
affirming that life is ubiquitous, although we are thus justified in
saying that it must be, in a general sense, universal. We might liken
the universe to a garden known to contain every variety of plant. If on
entering it we see no flowers, we examine the species before us and
find that they are not of those which bloom at this particular season,
or perhaps they are such as never bear flowers. Yet we feel no doubt
that we shall find flowers somewhere in the garden, because there _are_
species which bloom at this season, and the garden contains _all_
varieties.

While it is tacitly assumed that there are planets revolving around
other stars than the sun, it would be impossible for us to see them
with any telescope yet invented, and no instrument now in the
possession of astronomers could assure us of their existence; so the
only planetary system of which we have visual knowledge is our own.
Excluding the asteroids, which could not from any point of view be
considered as habitable, we have in the solar system eight planets of
various sizes and situated at various distances from the sun. Of these
eight we know that one, the earth, is inhabited. The question, then,
arises: Are there any of the others which are inhabited or habitable?
Since it is our intention to discuss the habitability of only one of
the seven to which the question applies, the rest may be dismissed in a
few words. The smallest of them, and the nearest to the sun, is
Mercury, which is regarded as uninhabitable because it has no
perceptible supply of water and air, and because, owing to the
extraordinary eccentricity of its orbit, it is subjected to excessive
and very rapid alterations in the amount of solar heat and light poured
upon its surface, such alterations being inconsistent with the
supposition that it can support living beings. Even its average
temperature is more than six and a half times that prevailing on the
earth! Another circumstance which militates against its habitability is
that, according to the results of the best telescopic studies, it
always keeps the same face toward the sun, so that one half of the
planet is perpetually exposed to the fierce solar rays, and the other
half faces the unmitigated cold of open space. Venus, the next in
distance from the sun, is almost the exact twin of the earth in size,
and many arguments may be urged in favor of its habitability, although
it is suspected of possessing the same peculiarity as Mercury, in
always keeping the same side sunward. Unfortunately its atmosphere
appears to be so dense that no permanent markings on its surface are
certainly visible, and the question of its actual condition must, for
the present, be left in abeyance. Mars, the first planet more distant
from the sun than the earth, is the special subject of this chapter,
and will be described and discussed a few lines further on. Jupiter,
Saturn, Uranus, and Neptune, the four giant planets, all more distant
than Mars, and each more distant than the other in the order named, are
all regarded as uninhabitable because none of them appears to possess
any degree of solidity. They may have solid or liquid nuclei, but
exteriorly they seem to be mere balls of cloud. Of course, one can
imagine what he pleases about the existence of creatures suited to the
physical constitution of such planets as these, but they must be
excluded from the category of habitable worlds in the ordinary sense of
the term. We go back, then, to Mars.

It will be best to begin with a description of the planet. Mars is 4230
miles in diameter; its surface is not much more than one-quarter as
extensive as that of the earth (.285). Its mean distance from the sun
is 141,500,000 miles, 48,500,000 miles greater than that of the earth.
Since radiant energy varies inversely as the square of distance, Mars
receives less than half as much solar light and heat as the earth gets.
Mars’ year (period of revolution round the sun) is 687 days. Its mean
density is 71 per cent of the earth’s, and the force of gravity on its
surface is 38 per cent of that on the surface of the earth; _i.e.,_ a
body weighing one hundred pounds on the earth would, if transported to
Mars, weigh but thirty-eight pounds. The inclination of its equator to
the plane of its orbit differs very little from that of the earth’s
equator, and its axial rotation occupies 24 hours 37 minutes. so that
the length of day and night, and the extent of the seasonal changes on
Mars, are almost precisely the same as on the earth. But owing to the
greater length of its year, the seasons of Mars, while occurring in the
same order, are almost twice as long as ours. The surface of the planet
is manifestly solid, like that of our globe, and the telescope reveals
many permanent markings on it, recalling the appearance of a globe on
which geographical features have been represented in reddish and dusky
tints. Around the poles are plainly to be seen rounded white areas,
which vary in extent with the Martian seasons, nearly vanishing in
summer and extending widely in winter. The most recent spectroscopic
determinations indicate that Mars has an atmosphere perhaps as dense as
that to be found on our loftiest mountain peaks, and there is a
perceptible amount of watery vapor in this atmosphere. The surface of
the planet appears to be remarkably level, and it has no mountain
ranges. No evidences of volcanic action have been discovered on Mars.
The dusky and reddish areas were regarded by the early observers as
respectively seas and lands, but at present it is not believed that
there are any bodies of water on the planet. There has never been much
doubt expressed that the white areas about the poles represent snow.

It will be seen from this brief description that many remarkable
resemblances exist between Mars and the earth, and there is nothing
wonderful in the fact that the question of the habitability of the
former has become one of extreme and wide-spread interest, giving rise
to the most diverse views, to many extraordinary speculations, and
sometimes to regrettably heated controversy. The first champion of the
habitability of Mars was Sir William Herschel, although even before his
time the idea had been suggested. He was convinced by the revelations
of his telescopes, continually increasing in power, that Mars was more
like the earth than any other planet. He could not resist the testimony
of the polar snows, whose suggestive conduct was in such striking
accord with what occurs upon the earth. Gradually, as telescopes
improved and observers increased in number, the principal features of
the planet were disclosed and charted, and “areography,” as the
geography of Mars was called, took its place among the recognized
branches of astronomical study. But it was not before 1877 that a
fundamentally new discovery in areography gave a truly sensational turn
to speculation about life on “the red planet.” In that year Mars made
one of its nearest approaches to the earth, and was so situated in its
orbit that it could be observed to great advantage from the northern
hemisphere of the earth. The celebrated Italian astronomer,
Schiaparelli, took advantage of this opportunity to make a
trigonometrical survey of the surface of Mars—as coolly and confidently
as if he were not taking his sights across a thirty-five-million-mile
gulf of empty space—and in the course of this survey he was astonished
to perceive that the reddish areas, then called continents, were
crossed in many directions by narrow, dusky lines, to which he gave the
suggestive name of “canals.” Thus a kind of firebrand was cast into the
field of astronomical speculation, which has ever since produced
disputes that have sometimes approached the violence of political
faction. At first the accuracy of Schiaparelli’s observations was
contested; it required a powerful telescope, and the most excellent
“seeing,” to render the enigmatical lines visible at all, and many
searchers were unable to detect them. But Schiaparelli continued his
studies in the serene sky of Italy, and produced charts of the
gridironed face of Mars containing so much astonishing detail that one
had either to reject them _in toto_ or to confess that Schiaparelli was
right. As subsequent favorable oppositions of Mars occurred, other
observers began to see the “canals” and to confirm the substantial
accuracy of the Italian astronomer’s work, and finally few were found
who would venture to affirm that the “canals” did not exist, whatever
their meaning might be.


[Illustration: Schiaparelli’s chart of Mars, showing the so-called
system of canals]


When Schiaparelli began his observations it was generally believed, as
we have said, that the dusky areas on Mars were seas, and since
Schiaparelli thought that the “canals” invariably began and ended at
the shores of the “seas,” the appropriateness of the title given to the
lines seemed apparent. Their artificial character was immediately
assumed by many, because they were too straight and too suggestively
geometrical in their arrangement to permit the conclusion that they
were natural watercourses. A most surprising circumstance noted by
Schiaparelli was that the “canals” made their appearance _after_ the
melting of the polar snow in the corresponding hemisphere had begun,
and that they grew darker, longer, and more numerous in proportion as
the polar liquidation proceeded; another very puzzling observation was
that many of them became double as the season advanced; close beside an
already existing “canal,” and in perfect parallelism with it, another
would gradually make its appearance. That these phenomena actually
existed and were not illusions was proved by later observations, and
today they are seen whenever Mars is favorably situated for
observation.

In the closing decade of the nineteenth century, Mr Percival Lowell
took up the work where Schiaparelli had virtually dropped it, and soon
added a great number of “canals” to those previously known, so that in
his charts the surface of the wonderful little planet appears covered
as with a spider’s web, the dusky lines criss-crossing in every
direction, with conspicuous knots wherever a number of them come
together. Mr Lowell has demonstrated that the areas originally called
seas, and thus named on the earlier charts, are not bodies of water,
whatever else they may be. He has also found that the mysterious lines
do not, as Schiaparelli supposed, begin and end at the edges of the
dusky regions, but often continue on across them, reaching in some
cases far up into the polar regions. But Schiaparelli was right in his
observation that the appearance of the “canals” is synchronous with the
gradual disappearance of the polar snows, and this fact has become the
basis of the most extraordinary theory that the subject of life in
other worlds has ever given birth to.

Now, the effect of such discoveries, as we have related, depends upon
the type of mind to whose attention they are called. Many are content
to accept them as strange and inexplicable at present, and to wait for
further light upon them; others insist upon an immediate inquiry
concerning their probable nature and meaning. Such an inquiry can only
be based upon inference proceeding from analogy. Mars, say Mr Lowell
and those who are of his opinion, is manifestly a solidly incrusted
planet like the earth; it has an atmosphere, though one of great
rarity; it has water vapor, as the snows in themselves prove; it has
the alternation of day and night, and a succession of seasons closely
resembling those of the earth; its surface is suggestively divided into
regions of contrasting colors and appearance, and upon that surface we
see an immense number of lines geometrically arranged, with a system of
symmetrical intersections where the lines expand into circular and oval
areas—and all connected with the annual melting of the polar snows in a
way which irresistibly suggests the interference of intelligence
directed to a definite end. Why, with so many concurrent circumstances
to support the hypothesis, should we not regard Mars as an inhabited
globe?

But the differences between Mars and the earth are in many ways as
striking as their resemblances. Mars is relatively small; it gets less
than half as much light and heat as we receive; its atmosphere is so
rare that it would be distressing to us, even if we could survive in it
at all; it has no lakes, rivers, or seas; its surface is an endless
prairie. and its “canals” are phenomena utterly unlike anything on the
earth. Yet it is precisely upon these divergences between the earth and
Mars, this repudiation of terrestrial standards, that the theory of
“life on Mars,” for which Mr Lowell is mainly responsible, is based.
Because Mars is smaller than the earth, we are told it must necessarily
be more advanced in planetary evolution, the underlying cause of which
is the gradual cooling and contraction of the planet’s mass. Mars has
parted with its internal heat more rapidly than the earth; consequently
its waters and its atmosphere have been mostly withdrawn by chemical
combinations, but enough of both yet remain to render life still
possible on its surface. As the globe of Mars is evolutionally older
than that of the earth, so its forms of organic life may be
proportionally further advanced, and its inhabitants may have attained
a degree of cultivated intelligence much superior to what at present
exists upon the earth. Understanding the nature and the causes of the
desiccation of their planet, and possessing engineering science and
capabilities far in advance of ours, they may be conceived to have
grappled with the stupendous problem of keeping their world in a
habitable condition as long as possible. Supposing them to have become
accustomed to live in their rarefied atmosphere (a thing not
inconceivable, since men can live for a time at least in air hardly
less rare), the most pressing problem for them is that of a
water-supply, without which plant life cannot exist, while animal life
in turn depends for its existence upon vegetation. The only direction
in which they can seek water is that of the polar regions, where it is
alternately condensed into snow and released in the liquid form by the
effect of the seasonal changes. It is, then, to the annual melting of
the polar snow-fields that the Martian engineers are supposed to have
recourse in supplying the needs of their planet, and thus providing the
means of prolonging their own existence. It is imagined that they have
for this purpose constructed a stupendous system of irrigation
extending over the temperate and equatorial regions of the planet. The
“canals” represent the lines of irrigation, but the narrow streaks that
we see are not the canals themselves, but the irrigated bands covered
by them. Their dark hue, and their gradual appearance after the polar
melting has begun, are due to the growth of vegetation stimulated by
the water. The rounded areas visible where several “canals” meet and
cross are called by Mr Lowell “oases.” These are supposed to be the
principal centers of population and industry. It must be confessed that
some of them, with their complicated systems of radiating lines, appear
to answer very well to such a theory. No attempt to explain them by
analogy with natural phenomena on the earth has proved successful.

But a great difficulty yet remains: How to explain the seemingly
miraculous powers of the supposed engineers? Here recourse is had once
more to the relative smallness of the planet. We have remarked that the
force of gravity on Mars is only thirty-eight per cent of that on the
earth. A steam-shovel driven by a certain horse-power would be nearly
three times as effective there as here. A man of our stature on Mars
would find his effective strength increased in the same proportion. But
just because of the slight force of gravity there, a Martian might
attain to the traditional stature of Goliath without finding his own
weight an encumbrance to his activity, while at the same time his huge
muscles would come into unimpeded play, enabling him single-handed to
perform labors that would be impossible to a whole gang of terrestrial
workmen. The effective powers of huge machines would be increased in
the same way; and to all this must be added the fact that the mean
density of the materials of which Mars is composed is much less than
that of the constituents of the earth. Combining all these
considerations, it becomes much less difficult to conceive that public
works might be successfully undertaken on Mars which would be
hopelessly beyond the limits of human accomplishment.

Certain other difficulties have also to be met; as, for instance, the
relative coldness of the climate of Mars. At its distance it gets
considerably less than half as much light and heat as we receive. In
addition to this, the rarity of its atmosphere would naturally be
expected to decrease the effective temperature at the planet’s surface,
since an atmosphere acts somewhat like the glass cover of a hot-house
in retaining the solar heat which has penetrated it. It has been
calculated that, unless there are mitigating circumstances of which we
know nothing, the average temperature at the surface of Mars must be
far below the freezing-point of water. To this it is replied that the
possible mitigating circumstances spoken of evidently exist in fact,
because we can _see_ that the watery vapor condenses into snow around
the poles in winter, but melts again when summer comes. The mitigating
agent may be supposed to exist in the atmosphere where the presence of
certain gases would completely alter the temperature gradients.

It might also be objected that it is inconceivable that the Martian
engineers, however great may be their physical powers, and however
gigantic the mechanical energies under their control, could force water
in large quantities from the poles to the equator. This is an
achievement that measures up to the cosmical standard. It is admitted
by the champions of the theory that the difficulty is a formidable one;
but they call attention to the singular fact that on Mars there can be
found no chains of mountains, and it is even doubtful if ranges of
hills exist there. The entire surface of the planet appears to be
almost “as smooth as a billiard ball,” and even the broad regions which
were once supposed to be seas apparently lie at practically the same
level as the other parts, since the “canals” in many cases run
uninterruptedly across them. Lowell’s idea is that these sombre areas
may be expanses of vegetation covering ground of a more or less marshy
character, for while the largest of them appear to be permanent, there
are some which vary coincidently with the variations of the canals.

As to the kind of machinery employed to force the water from the poles,
it has been conjectured that it may have taken the form of a gigantic
system of pumps and conduits; and since the Martians are assumed to be
so far in advance of us in their mastery of scientific principles, the
hypothesis will at least not be harmed by supposing that they have
learned to harness forces of nature whose very existence in a
manageable form is yet unrecognized on the earth. If we wish to let the
imagination loose, we may conjecture that they have conquered the
secret of those intra-atomic forces whose resistless energy is
beginning to become evident to us, but the possibility of whose
utilization remains a dream, the fulfillment of which nobody dares to
predict.

Such, in very brief form, is the celebrated theory of Mars as an
inhabited world. It certainly captivates the imagination, and if we
believe it to represent the facts, we cannot but watch with the deepest
sympathy this gallant struggle of an intellectual race to preserve its
planet from the effects of advancing age and death. We may, indeed,
wonder whether our own humanity, confronted by such a calamity, could
be counted on to meet the emergency with equal stoutness of heart and
inexhaustibleness of resource. Up to the present time we certainly have
shown no capacity to confront Nature toe to toe, and to seize her by
the shoulders and turn her round when she refuses to go our way. If we
could get into wireless telephonic communication with the Martians we
might learn from their own lips the secret of their more than “Roman
recovery.”




XIV
The Riddle of the Asteroids


Between the orbits of Mars and Jupiter revolves the most remarkable
system of little bodies with which we are acquainted—the Asteroids, or
Minor Planets. Some six hundred are now known, and they may actually
number thousands. They form virtually a ring about the sun. The most
striking general fact about them is that they occupy the place in the
sky which should be occupied, according to Bode’s Law, by a single
large planet. This fact, as we shall see, has led to the invention of
one of the most extraordinary theories in astronomy—_viz.,_ that of the
explosion of a world!

Bode’s Law, so-called, is only an empiric formula, but until the
discovery of Neptune it accorded so well with the distances of the
planets that astronomers were disposed to look upon it as really
representing some underlying principle of planetary distribution. They
were puzzled by the absence of a planet in the space between Mars and
Jupiter, where the “law” demanded that there should be one, and an
association of astronomers was formed to search for it. There was a
decided sensation when, in 1801, Piazzi, of Palermo, announced that he
had found a little planet which apparently occupied the place in the
system which belonged to the missing body. He named it Ceres, and it
was the first of the Asteroids. The next year Olbers, of Bremen, while
looking for Ceres with his telescope, stumbled upon another small
planet which he named Pallas. Immediately he was inspired with the idea
that these two planets were fragments of a larger one which had
formerly occupied the vacant place in the planetary ranks, and he
predicted that others would be found by searching in the neighborhood
of the intersection of the orbits of the two already discovered. This
bold prediction was brilliantly fulfilled by the finding of two
more—Juno in 1804, and Vesta in 1807. Olbers would seem to have been
led to the invention of his hypothesis of a planetary explosion by the
faith which astronomers at that time had in Bode’s Law. They appear to
have thought that several planets revolving in the gap where the “law”
called for but one could only be accounted for upon the theory that the
original _one_ had been broken up to form the several. Gravitation
demanded that the remnants of a planet blown to pieces, no matter how
their orbits might otherwise differ, should all return at stated
periods to the point where the explosion had occurred; hence Olbers’
prediction that any asteroids that might subsequently be discovered
would be found to have a common point of orbital intersection. And
curiously enough all of the first asteroids found practically answered
to this requirement. Olbers’ theory seemed to be established.

After the first four, no more asteroids were found until 1845, when one
was discovered; then, in 1847, three more were added to the list; and
after that searchers began to pick them up with such rapidity that by
the close of the century hundreds were known, and it had become almost
impossible to keep track of them. The first four are by far the largest
members of the group, but their actual sizes remained unknown until
less than twenty years ago. It was long supposed that Vesta was the
largest, because it shines more brightly than any of the others; but
finally, in 1895, Barnard, with the Lick telescope, definitely measured
their diameters, and proved to everybody’s surprise that Ceres is
really the chief, and Vesta only the third in rank. His measures are as
follows: Ceres, 477 miles; Pallas, 304 miles; Vesta, 239 miles; and
Juno, 120 miles. They differ greatly in the reflective power of their
surfaces, a fact of much significance in connection with the question
of their origin. Vesta is, surface for surface, rather more than three
times as brilliant as Ceres, whence the original mistake about its
magnitude.

Nowadays new asteroids are found frequently by photography, but
physically they are most insignificant bodies, their average diameter
probably not exceeding twenty miles, and some are believed not to
exceed ten. On a planet only ten miles in diameter, assuming the same
mean density as the earth’s, which is undoubtedly too much, the force
of gravity would be so slight that an average man would not weigh more
than three ounces, and could jump off into space whenever he liked.

Although the asteroids all revolve around the sun in the same direction
as that pursued by the major planets, their orbits are inclined at a
great variety of angles to the general plane of the planetary system,
and some of them are very eccentric—almost as much so as the orbits of
many of the periodic comets. It has even been conjectured that the two
tiny moons of Mars and the four smaller satellites of Jupiter may be
asteroids gone astray and captured by those planets. Two of the
asteroids are exceedingly remarkable for the shapes and positions of
their orbits; these are Eros, discovered in 1898, and T. G., 1906,
found eight years later. The latter has a mean distance from the sun
slightly greater than that of Jupiter, while the mean distance of Eros
is less than that of Mars. The orbit of Eros is so eccentric that at
times it approaches within 15,000,000 miles of the earth, nearer than
any other regular member of the solar system except the moon, thus
affording an unrivaled means of measuring the solar parallax. But for
our present purpose the chief interest of Eros lies in its
extraordinary changes of light.

These changes, although irregular, have been observed and photographed
many times, and there seems to be no doubt of their reality. Their
significance consists in their possible connection with the form of the
little planet, whose diameter is generally estimated at not more than
twenty miles. Von Oppolzer found, in 1901, that Eros lost three-fourths
of its brilliancy once in every two hours and thirty-eight minutes.
Other observers have found slightly different periods of variability,
but none as long as three hours. The most interesting interpretation
that has been offered of this phenomenon is that it is due to a great
irregularity of figure, recalling at once Olbers’ hypothesis. According
to some, Eros may be double, the two bodies composing it revolving
around each other at very close quarters; but a more striking, and it
may be said probable, suggestion is that Eros has a form not unlike
that of a dumb-bell, or hour-glass, turning rapidly end over end so
that the area of illuminated surface presented to our eyes continually
changes, reaching at certain times a minimum when the amount of light
that it reflects toward the earth is reduced to a quarter of its
maximum value. Various other bizarre shapes have been ascribed to Eros,
such, for instance, as that of a flat stone revolving about one of its
longer axes, so that sometimes we see its face and sometimes its edge.

All of these explanations proceed upon the assumption that Eros cannot
have a simple globular figure like that of a typical planet, a figure
which is prescribed by the law of gravitation, but that its shape is
what may be called accidental; in a word, it is a _fragment,_ for it
seems impossible to believe that a body formed in interplanetary space,
either through nebular condensation or through the aggregation of
particles drawn together by their mutual attractions, should not be
practically spherical in shape. Nor is Eros the only asteroid that
gives evidence by variations of brilliancy that there is something
abnormal in its constitution; several others present the same
phenomenon in varying degrees. Even Vesta was regarded by Olbers as
sufficiently variable in its light to warrant the conclusion that it
was an angular mass instead of a globe. Some of the smaller ones show
very notable variations, and all in short periods, of three or four
hours, suggesting that in turning about one of their axes they present
a surface of variable extent toward the sun and the earth.

The theory which some have preferred—that the variability of light is
due to the differences of reflective power on different parts of the
surface—would, if accepted, be hardly less suggestive of the origin of
these little bodies by the breaking up of a larger one, because the
most natural explanation of such differences would seem to be that they
arose from variations in the roughness or smoothness of the reflecting
surface, which would be characteristic of fragmentary bodies. In the
case of a large planet alternating expanses of land and water, or of
vegetation and desert, would produce a notable variation in the amount
of reflection, but on bodies of the size of the asteroids neither water
nor vegetation could exist, and an atmosphere would be equally
impossible.

One of the strongest objections to Olbers’ hypothesis is that only a
few of the first asteroids discovered travel in orbits which measurably
satisfy the requirement that they should all intersect at the point
where the explosion occurred. To this it was at first replied that the
perturbations of the asteroidal orbits, by the attractions of the major
planets, would soon displace them in such a manner that they would
cease to intersect. One of the first investigations undertaken by the
late Prof. Simon Newcomb was directed to the solution of this question,
and he arrived at the conclusion that the planetary perturbations could
not explain the actual situation of the asteroidal orbits. But
afterward it was pointed out that the difficulty could be avoided by
supposing that not one but a series of explosions had produced the
asteroids as they now are. After the primary disruption the fragments
themselves, according to this suggestion, may have exploded, and then
the resulting orbits would be as “tangled” as the heart could wish.
This has so far rehabilitated the explosion theory that it has never
been entirely abandoned, and the evidence which we have just cited of
the probably abnormal shapes of Eros and other asteroids has lately
given it renewed life. It is a subject that needs a thorough
rediscussion.

We must not fail to mention, however, that there is a rival hypothesis
which commends itself to many astronomers—_viz.,_ that the asteroids
were formed out of a relatively scant ring of matter, situated between
Mars and Jupiter and resembling in composition the immensely more
massive rings from which, according to Laplace’s hypothesis, the
planets were born. It is held by the supporters of this theory that the
attraction of the giant Jupiter was sufficient to prevent the small,
nebulous ring that gave birth to the asteroids from condensing like the
others into a single planet.

But if we accept the explosion theory, with its corollary that minor
explosions followed the principal one, we have still an unanswered
question before us: What caused the explosions? The idea of _a world
blowing up_ is too Titanic to be shocking; it rather amuses the
imagination than seriously impresses it; in a word, it seems
essentially chimerical. We can by no appeal to experience form a mental
picture of such an occurrence. Even the moon did not blow up when it
was wrecked by volcanoes. The explosive nebulæ and new stars are far
away in space, and suggest no connection with such a catastrophe as the
bursting of a planet into hundreds of pieces. We cannot conceive of a
great globe thousands of miles in diameter resembling a pellet of
gunpowder only awaiting the touch of a match to cause its sudden
disruption. Somehow the thought of human agency obtrudes itself in
connection with the word “explosion,” and we smile at the idea that
giant powder or nitro-glycerine could blow up a planet. Yet it would
only need _enough_ of them to do it.

After all, we may deceive ourselves in thinking, as we are apt to do,
that explosive energies lock themselves up only in small masses of
matter. There are many causes producing explosions in nature, every
volcanic eruption manifests the activity of some of them. Think of the
giant power of confined steam; if enough steam could be suddenly
generated in the center of the earth by a downpour of all the waters of
the oceans, what might not the consequences be for our globe? In a
smaller globe, and it has never been estimated that the original
asteroid was even as large as the moon, such a catastrophe would,
perhaps, be more easily conceivable; but since we are compelled in this
case to assume that there was a series of successive explosions, steam
would hardly answer the purpose; it would be more reasonable to suppose
that the cause of the explosion was some kind of chemical reaction, or
something affecting the atoms composing the exploding body. Here Dr
Gustav Le Bon comes to our aid with a most startling suggestion, based
on his theory of the dissipation of intra-atomic energy. It will be
best to quote him at some length from his book on _The Evolution of
Forces._

“It does not seem at first sight,” says Doctor Le Bon,


very comprehensible that worlds which appear more and more stable as
they cool could become so unstable as to afterward dissociate entirely.
To explain this phenomenon, we will inquire whether astronomical
observations do not allow us to witness this dissociation.

We know that the stability of a body in motion, such as a top or a
bicycle, ceases to be possible when its velocity of rotation descends
below a certain limit. Once this limit is reached it loses its
stability and falls to the ground. Prof. J. J. Thomson even interprets
radio-activity in this manner, and points out that when the speed of
the elements composing the atoms descends below a certain limit they
become unstable and tend to lose their equilibria. There would result
from this a commencement of dissociation, with diminution of their
potential energy and a corresponding increase of their kinetic energy
sufficient to launch into space the products of intra-atomic
disintegration.

It must not be forgotten that the atom being an enormous reservoir of
energy is by this very fact comparable with explosive bodies. These
last remain inert so long as their internal equilibria are undisturbed.
So soon as some cause or other modifies these, they explode and smash
everything around them after being themselves broken to pieces.

Atoms, therefore, which grow old in consequence of the diminution of a
part of their intra-atomic energy gradually lose their stability. A
moment, then, arrives when this stability is so weak that the matter
disappears by a sort of explosion more or less rapid. The bodies of the
radium group offer an image of this phenomenon—a rather faint image,
however, because the atoms of this body have only reached a period of
instability when the dissociation is rather slow. It probably precedes
another and more rapid period of dissociation capable of producing
their final explosion. Bodies such as radium, thorium, etc., represent,
no doubt, a state of old age at which all bodies must some day arrive,
and which they already begin to manifest in our universe, since all
matter is slightly radio-active. It would suffice for the dissociation
to be fairly general and fairly rapid for an explosion to occur in a
world where it was manifested.

These theoretical considerations find a solid support in the sudden
appearances and disappearances of stars. The explosions of a world
which produce them reveal to us, perhaps, how the universes perish when
they become old.

As astronomical observations show the relative frequency of these rapid
destructions, we may ask ourselves whether the end of a universe by a
sudden explosion after a long period of old age does not represent its
most general ending.


Here, perhaps, it will be well to stop, since, entrancing as the
subject may be, we know very little about it, and Doctor Le Bon’s
theory affords a limitless field for the reader’s imagination.