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                                  THE
                              STORY OF THE
                              SOLAR SYSTEM


                                   BY
                      GEORGE F. CHAMBERS, F.R.A.S.
                 OF THE INNER TEMPLE, BARRISTER-AT-LAW
                    AUTHOR OF THE STORY OF THE STARS

[Illustration: ]

                    WITH TWENTY-EIGHT ILLUSTRATIONS

                                NEW YORK
                        McCLURE, PHILLIPS & CO.
                                 MCMIV


                            Copyright, 1895,
                      By D. APPLETON AND COMPANY.

[Illustration: Fig. 1.—The Planet Saturn.]




                                PREFACE.


Having in my “Story of the Stars” told of far distant suns, many of them
probably with planets revolving around them, I have in the present
volume, which is a companion to the former one, to treat of the Sun in
particular—our Sun as we may call him—and the body of attendants which
own his sway by revolving round him. The attendants are the planets,
commonly so called, together with a certain number of comets. I shall
deal with all these objects rather from a descriptive and practical than
from a speculative or essay point of view, and with special reference to
the convenience and opportunities of persons possessing, or having
access to, what may be called popular telescopes—telescopes say of from
two to four inches of aperture, and costing any sum between £10 and £50.
There is much pleasure and profit to be got out of telescopes of this
type, always presuming that they are used by persons possessed of
patience and perseverance. It is a very great mistake, though an
extremely common one, to suppose that unless a man can command a big
telescope he can do no useful work, and derive no pleasure from his
work. To all such croakers I always point as a moral the achievements of
Hermann Goldschmidt, who from an attic window at Fontenay-aux-Roses near
Paris, with a telescope of only 2½ inches aperture, discovered no fewer
than 14 minor planets.

As this volume is intended for general reading, rather than for
educational or technical purposes, I have kept statistical details and
numerical expressions within very narrow limits, mere figures being
always more or less unattractive.

John Richard Green, in the Preface to his book on _The Making of
England_, writes as follows:—“I may add, in explanation of the
reappearance of a few passages ... which my readers may have seen
before, that where I had little or nothing to add or to change, I have
preferred to insert a passage from previous work, with the requisite
connections and references, to the affectation of rewriting such a
passage for the mere sake of giving it an air of novelty.” I will
venture to adopt this thought as my own, and to apply it to the
repetition, here and there, of ideas and phrases which are already to be
found in my _Handbook of Astronomy_.

                                                                G. F. C.
  Northfield Grange,
    Eastbourne, 1895.




                               CONTENTS.


  CHAPTER                                                           PAGE
  I. Introductory Statement                                            7
  II. The Sun                                                         18
  III. Mercury                                                        57
  IV. Venus                                                           61
  V. The Earth                                                        69
  VI. The Moon                                                        89
  VII. Mars                                                          100
  VIII. The Minor Planets                                            110
  IX. Jupiter                                                        115
  X. Saturn                                                          122
  XI. Uranus                                                         138
  XII. Neptune                                                       143
  XIII. Comets                                                       150
  Appendix—Tables of the Solar System                                182
  General Index                                                      185




                         LIST OF ILLUSTRATIONS.


  FIGURE                                                            PAGE
  1. The Planet Saturn                                    _Frontispiece_
  2. Inclination of Planetary Orbits                                   9
  3. Comparative Sizes of Major Planets                               11
  4. Comparative Size of the Sun as seen from the Planets Named       17
  5. Ordinary Sun-spots, June 22, 1885                                22
  6. Change of Form in Sun-spots Owing to the Sun’s Rotation          29
  7. Sun-spots seen as a Notch                                        37
  8. The Sun Totally Eclipsed, July 18, 1860                          56
  9. Venus, Dec. 23, 1885                                             64
  10. Venus Near Conjunction as a Thin Crescent                       65
  11. Mare Crisium (Lick Observatory photographs)                     90
  12. Four Views of Mars (Barnard)                                   101
  13. Mars, Aug. 27, 1892 (Guyot)                                    107
  14. Jupiter, Nov. 27, 1857 (Dawes)                                 116
  15. Saturn, 1889                                                   123
  16. General View of the Phases of Saturn’s Rings                   126
  17. Phases of Saturn’s Rings at Specified Dates                    129
  18. Saturn with Titan and its Shadow                               137
  19. Telescopic Comet with a Nucleus                                154
  20. Comet seen in Daylight, Sept., 1882                            155
  21. Quenisset’s Comet, July 9, 1893                                156
  22. Holmes’s Comet, the Head on Nov. 9, 1892 (_Denning_)           159
  23. Holmes’s Comet, the Head on Nov. 16, 1892 (_Denning_)          159
  24. Comet III. of 1862, on Aug. 22, showing Jet of Luminous Matter
          (Challis)                                                  160
  25. Sawerthal’s Comet, June 4, 1888 (Charlois)                     165
  26. Biela’s Comet, 1846                                            169
  27. The Great Comet of 1811                                        177
  28. The Great Comet of 1882                                        179




                                  THE
                       STORY OF THE SOLAR SYSTEM.




                               CHAPTER I.
                        INTRODUCTORY STATEMENT.


By the term “Solar System” it is to be understood that an Astronomer,
speaking from the standpoint of an inhabitant of the Earth, wishes to
refer to that object, the Sun, which is to him the material and visible
centre of life and heat and control, and also to those bodies dependent
on the Sun which circulate round it at various distances, deriving their
light and heat from the Sun, and known as planets and comets. The
statement just made may be regarded as a general truth, but as the
strictest accuracy on scientific matters is of the utmost importance, a
trivial reservation must perhaps be put upon the foregoing broad
assertion. There is some reason for thinking that possibly one of the
planets (Jupiter) possesses a little inherent light of its own which is
not borrowed from the Sun; whilst of the comets it must certainly be
said that, as a rule, they shine with intrinsic, not borrowed light.
Respecting these reservations more hereafter.

The planets are divided into “primary” and “secondary.” By a “primary”
planet we mean one which directly circulates round the Sun; by a
“secondary” planet we mean one which in the first instance circulates
round a primary planet, and therefore only in a secondary sense
circulates round the Sun. The planets are also “major” or “minor”; this,
however, is only a distinction of size.

The secondary planets are usually termed “satellites,” or, very often,
in popular language, “moons,” because they own allegiance to their
respective primaries just as our Moon—_the_ Moon—does to the Earth. But
the use of the term “moon” is inconvenient, and it is better to stick to
“satellite.”

There is yet another method of classifying the planets which has its
advantages. They are sometimes divided into “inferior” and “superior.”
The “inferior” planets are those which travel round the Sun in orbits
which are inside the Earth’s orbit; the “superior” planets are those
whose orbits are outside the Earth.

The following is an enumeration of the major planets in the order of
their distances, reckoning from the Sun outwards:—

  1. Mercury.
  2. Venus.
  3. The Earth.
  4. Mars.
  5. Jupiter.
  6. Saturn.
  7. Uranus.
  8. Neptune.

All the above are major planets and also primary planets. In between
Nos. 4 and 5 circulate the “Minor” planets, an ever-increasing body, now
more than 400 in number, but all, except one or perhaps two, invisible
to the naked eye.

The “Inferior” planets it will be seen from the above table comprise
Mercury and Venus, whilst the “Superior” planets are Mars and all those
beyond.

Great differences exist in the inclinations of the orbits of the
different planets to the plane of the ecliptic, a fact which is better
shown by a diagram than by a table of mere figures. The orbit of Uranus
is indeed so much inclined that its motion is really _retrograde_
compared with the general run of the planets: and the same remark
applies, though much more forcibly, to the case of Neptune.

[Illustration: Fig. 2.—Inclination of Planetary Orbits.]

The actual movements of the planets round the Sun are extremely simple,
for they do nought else but go on, and on, and on, incessantly, always
in the same direction, and almost, though not quite, at a uniform pace,
though in orbits very variously inclined to the plane of the ecliptic.
But an element of extreme complication is introduced into their apparent
movements by reason of the fact that we are obliged to study the planets
from one of their own number, which is itself always in motion.

If the Earth itself were a fixture, the study of the movements of the
planets would be a comparatively easy matter, whilst to an observer on
the Sun it would be a supremely easy matter.

Greatly as the planets differ among themselves in their sizes, distances
from the Sun, and physical peculiarities, they have certain things in
common, and it will be well to make this matter clear before we go into
more recondite topics. For instance, not only do they move incessantly
round the Sun in the same direction at a nearly uniform pace, but the
planes of their orbits are very little inclined to the common plane of
reference, the ecliptic, or to one another.[1] The direction of motion
of the planets as viewed from the north side of the ecliptic is contrary
to the motion of the hands of a watch. Their orbits, unlike the orbits
of comets, are nearly circular, that is, they are only very slightly
oval. Agreeably to the principles of what is known as the Law of
Universal Gravitation, the speed with which they move in their orbits is
greatest in those parts which lie nearest the Sun, and least in those
parts which are most remote from the Sun; in other words, they move
quickest in Perihelion and slowest in Aphelion.

[Illustration: Fig. 3.—Comparative Sizes of the Major Planets.]

The physical peculiarities which the planets have in common include the
following points:—they are opaque bodies, and shine by reflecting light
which they receive from the Sun. Probably all of them are endued with an
axial rotation, hence their inhabitants, if there are any, have the
alternation of day and night, like the inhabitants of the Earth, but the
duration of their days, measured in absolute terrestrial hours, will in
most cases differ materially from the days and nights with which we are
familiar.

I stated on a previous page that, owing to the circumstances in which we
find ourselves on the Earth, the apparent and real movements of the
planets are widely different. It would be beyond the scope of this
little work to go into these differences in any considerable detail;
suffice it then to indicate only a few general points. In the first
place, an important distinction exists between the visible movements of
the inferior and superior planets. The inferior planets, Mercury and
Venus, lying as they do within the orbit of the Earth, are much
restricted in their movements, in the sky. We can never see them except
when they are more or less near to the rising (or risen) or setting (or
set) Sun. The extreme angular distance from the Sun in the sky to which
Mercury can attain is but 27°, and therefore we can never observe it
otherwise than in sunlight or twilight, for it never rises more than 1½
hours before sunrise nor sets later than 1½ hours after sunset. Of
course between these limits the planet is above the horizon all the time
that the Sun is above the horizon, but except in very large telescopes
is not usually to be detected during the day-time. These remarks
regarding Mercury apply likewise in principle to Venus; only the orbit
of Venus being larger than the orbit of Mercury, and Venus itself being
larger in size than Mercury, the application of these principles leads
to somewhat different results. The greatest possible distance of Venus
may be 47° instead of Mercury’s 27°. Venus is therefore somewhat more
emancipated from the effects of twilight. The body of Venus being also
very much larger and brighter than the body of Mercury, it may be more
often and more easily detected in broad daylight.

It follows from the foregoing statement that the inferior planets can
never be seen in those regions of the heavens which are, as it is
technically called, in “Opposition” to the Sun; that is, which are on
the meridian at midnight whilst the Sun is on the meridian in its midday
splendour to places on the opposite side of the Earth. On the other
hand, the two inferior planets on stated, though rare, occasions exhibit
to a terrestrial spectator certain phenomena of great interest and
importance in which no superior planet can ever take part. I am here
referring to the “Transits” of Mercury and Venus across the Sun. If
these planets and the Earth all revolved round the Sun exactly in the
plane of the ecliptic, transits of these planets would be perpetually
recurring after even intervals of only a few months; but the fact that
the orbit of Mercury is inclined 7°, and that of Venus about 3½, to the
ecliptic, involves such complications that transits of Mercury only
occur at unequal intervals of several years, whilst, in extreme cases,
more than a century may elapse between two successive transits of Venus.
For a transit of an inferior planet over the Sun to take place, the
Earth and the planet and the Sun must be exactly in the same straight
line, reckoned both vertically and horizontally. Twice in every
revolution round the Sun an inferior planet is vertically in the same
straight line with the Earth and the Sun; and it is said to be in
“inferior conjunction” when the planet comes between the Earth and the
Sun; and in “superior conjunction” when the planet is on the further
side of the Sun, the Sun intervening between the Earth and the planet.
But for all three to be horizontally in the same straight line is quite
another matter. It is the orbital inclinations of Mercury and Venus
which enable them, so to speak, to dodge an observer who is on the
lookout to see them pass exactly in front of the Sun, or to disappear
behind the Sun; and so it comes about that a favourable combination of
circumstances which is rare is needed before either of the aforesaid
planets can be seen as round black spots passing in front of the Sun. A
passage of either of these planets behind the Sun could never be seen by
human eye, because of the overpowering brilliancy of the Sun’s rays,
even though an Astronomer might know by his calculations the exact
moment that the planet was going to pass behind the Sun.

When an inferior planet attains its greatest angular distance from the
Sun, as we see it (which I have already stated to be about 27° in the
case of Mercury and 47° in the case of Venus), such planet is said to be
at its “greatest elongation,” “east” or “west,” as the case may be. At
eastern elongation or indeed whenever the planet is east of the Sun, it
is, to use a familiar phrase, an “evening star”; on the other hand, at
western elongation, or whenever it is on the western side of the Sun, it
is known as a “morning star.”

If the movements of an inferior planet are followed sufficiently long by
the aid of a star map, it will be seen that sometimes it appears to be
proceeding in a forward direction through the signs of the zodiac; then
for a while it will seem to stand still; then at another time it will
apparently go backwards, or possess a retrograde motion. All these
peculiarities have their originating cause in the motion of the Earth
itself, for the absolute movement of the planet never varies, being
always in the same direction, that is, forwards in the order of the
signs.

Turning now to the superior planets, we have to face an altogether
different succession of circumstances. A superior planet is not, as it
were, chained to the Sun so as to be unable to escape beyond the limits
of morning or evening twilight; it may have any angular distance from
the Sun up to 180°, reaching which point it approaches the Sun on the
opposite side, step by step, until it again comes into conjunction with
the Sun. As applied to a superior planet, the term “conjunction” means
the absolute moment when the Earth and the Sun and the planet are in the
same straight line, the Sun being in the middle. In such a case, to us
on the Earth the planet is lost in the Sun’s rays, whilst to a spectator
on the planet the Earth would appear similarly lost in the Sun’s rays,
as the Earth would be at that stage of her orbit which we, speaking of
_our_ inferior planets, call superior conjunction.

For a clear comprehension of all the various matters which we have just
been speaking of, a careful study of diagrams of a geometrical
character, or better still, of models, would be necessary.

Something must now be said about the phases of the planets. Mercury and
Venus, in regard to their orbital motions, stand very much on the same
footing with respect to the inhabitants of the Earth as the Moon does,
and accordingly both those planets in their periodical circuits round
the Sun exhibit the same succession of phases as the Moon does. In the
case, however, of the superior planets things are otherwise. Two only of
them, Mars and Jupiter, are sufficiently near the Earth to exhibit any
phase at all. When they are in quadrature (_i. e._, 90° from the Sun on
either side) there is a slight loss of light to be noticed along one
limb. In other words, the disc of each ceases for a short time, and to a
slight extent, to be truly circular; it becomes what is known as
“gibbous.” This occasional feature of Mars may be fairly conspicuous,
or, at least, noticeable; but in the case of Jupiter it will be less
obvious unless a telescope of some size is employed.

If the major planets are arbitrarily ranged in two groups, Mercury,
Venus, the Earth and Mars being taken as an interior group,
comparatively near the Sun; whilst Jupiter, Saturn, Uranus and Neptune
are regarded as an exterior group, being at a great distance from the
Sun, it will be found that some important physical differences exist
between the two groups.

[Illustration: Fig. 4.—Comparative size of the Sun as seen from the
Planets named.]

Of the interior planets, the Earth and Mars alone have satellites, and
between them make up a total of only three. The exterior planets, on the
other hand, all have satellites, the total number being certainly
seventeen, and possibly eighteen. In detail, Jupiter has four, Saturn
eight, Uranus four, and Neptune one, and perhaps two. These facts may be
regarded as an instance of the beneficence of the Creator of the
Universe if we consider that the satellites of these remoter planets are
so numerous, in order that by their numbers they may do something to
make up for the small amount of light which, owing to their distance
from the Sun, their primaries receive. Then again, the average density
of the first group of planets greatly exceeds the average density of the
second group in the approximate ratio of 5 to 1. Finally, there is
reason to believe that a marked difference exists in the axial rotations
of the planets forming the two groups. We do not know the precise
figures for all the exterior planets, but the knowledge which we do
possess seems to imply that the average length of the day in the case of
the interior planets is about twenty-four hours, but that in the case of
the exterior planets it is no more than about ten hours. These figures
can, however, only be presented as possibly true, because observations
on the rotation periods of Mercury and Venus on the one hand, and of
Uranus and Neptune on the other, are attended with so much difficulty
that the recorded results are of doubtful trustworthiness. It is,
however, reasonable to presume that the actual size of the respective
planets has more to do with the matter than their distances from the
Sun.

I think that the foregoing summary respecting the planets collectively
embraces as many points as are likely to be of interest to the
generality of readers; we will therefore pass on to consider somewhat in
detail the several constituent members of the solar system, beginning
with the Sun.




                              CHAPTER II.
                                THE SUN.


There was once a book published, the title of which was “The Sun, Ruler,
Fire, Light and Life of the Planetary System.” The title was by no means
a bad one, for without doubt the Sun may fairly be said to represent
practically all the ideas conveyed by the designations quoted.

There is certainly no one body in creation which is so emphatically
pre-eminent as the Sun. Whether or no there are stars which are
suns—centres of systems serving in their degree the purposes served by
our Sun, I need not now pause to enquire, though I think the idea is a
very probable one; but of those celestial objects with which our Earth
has a direct relationship, beyond doubt the Sun is unquestionably
entitled to the foremost place. It is, as it were, the pivot on which
the Earth and all the various bodies comprising the Solar System revolve
in their annual progress. It is our source of light and heat, and
therefore may be called the great agent by which an Almighty Providence
wills to sustain animal and vegetable life. The consideration of all the
complicated questions which arise out of these functions of the Sun
belongs to the domain of Physics rather than that of Astronomy; still
these matters are of such momentous interest that an allusion to them
must be made, for they ought not to be lost sight of by the student of
Astronomy. Half a century ago the actual state of our knowledge
respecting the Sun might without difficulty be brought within the
compass of a single chapter in any book on Astronomy, but so enormous
has been the development of knowledge respecting the Sun of late years,
that it is no longer a question of getting the materials properly into
one chapter, but it is a matter of a whole volume being devoted to the
Sun, or even, as in the case of Secchi, of two large octavo volumes of
500 pages each being required to cover the whole ground exhaustively.
The reader will therefore easily understand that in the space at my
disposal in this little work nothing but a passing glimpse can possibly
be obtained of this great subject. It is great not only in regard to the
vast array of purely astronomical facts which are at a writer’s command,
but also on account of the extensive ramifications which the subject has
into the domains of chemistry, photography, optics and cognate sciences.
I shall therefore endeavour to limit myself generally to what an amateur
can see for himself with a small telescope, and can readily understand,
rather than attempt to say a little something about everything, and fail
in the effort.

The mean distance of the Earth from the Sun may be taken to be about 93
millions of miles, and this distance is employed by Astronomers as the
unit by which most other long celestial distances are reckoned. The true
diameter of the Sun is about 866,000 miles. The surface area exceeds
that of the Earth 11,946 times, and the volume is 1,305,000 times
greater. The mass or weight of the Sun is 332,000 times that of the
Earth, or about 700 times that of all the planets put together. Bulk for
bulk the Sun is much lighter than the Earth: whilst a cubic foot of the
Earth on an average weighs rather more than 5 times as much as a cubic
foot of water, a cubic foot of Sun is only about 3½ times the weight of
the same bulk of water. This consideration of the comparative lightness
of the Sun (though in his day the Sun was thought to be lighter than it
is now supposed to be) led Sir J. Herschel to infer that an intense heat
prevails in its interior, independent it may be of its surface heat, so
to speak, of which alone we are directly cognizant by the evidence of
our senses.

The Sun is a sphere, and is surrounded by an extensive but attenuated
envelope, or rather series of envelopes, which taken together bear some
analogy to the atmosphere surrounding the Earth. These envelopes, which
we shall have to consider more in detail presently, throw out rays of
light and heat to the confines of the Solar System, though as to the
conditions and circumstances under which that light and heat are
generated we are entirely ignorant. Of the potency of the Sun’s rays we
can form but a feeble conception, for the amount received by the Earth
is, it has been calculated, but one 2300-millionth of the whole. Our
annual share would, it is supposed, be sufficient to melt a layer of ice
spread uniformly over the Earth to a depth of 100 feet, or to heat an
ocean of fresh water 60 feet deep from freezing point to boiling point.
The illuminating power of the Sun has to be expressed in language of
similar profundity. Thus it has been calculated to equal that which
would be afforded by 5563 wax candles concentrated at a distance of one
foot from the observer. Again, it has been concluded that no fewer than
half a million of full moons shining all at once would be required to
make up a mass of light equal to that of the Sun. I present all these
conclusions to the reader as they are furnished by various physicists
who have investigated such matters, but it is rather uncertain as to how
much reliance can safely be placed on such calculations in detail.

[Illustration: Fig. 5.—Ordinary Sun-spot, June 22, 1885.]

To an amateur possessed of a small telescope, the Sun offers (when the
weather is above the English average of recent years) a very great and
constant variety of matters for studious scrutiny in its so-called
“spots.” To the naked eye, or even on a hasty telescopic glance, the Sun
presents the appearance of a uniform disc of yellowish white colour,
though often a little attention will soon result in the discovery of a
few, or it may be many, little black, or blackish patches, scattered
here and there over the disc seemingly without order or method. We shall
presently find out, however, that this last-named suggestion is wholly
inaccurate. Though commonly called “spots,” these dark appearances are
not simple spots, as the word might imply, for around the rather black
patch which constitutes generally the main feature of the spot there is
almost invariably a fringe of paler tint; whilst within the confines of
the black patch which first catches the eye there is often a nucleus or
inner portion of far more intense depth of shade. The innermost and
darkest portion being termed the _nucleus_, the ordinary black portion
is known as _umbra_, whilst the encompassing fringe is the _penumbra_.
It is not always the case that each individual umbra has a penumbra all
to itself, for several spots are occasionally included in one common
penumbra. And it may further be remarked that cases of an umbra without
a penumbra and the contrary are on record, though these may be termed
exceptional, often having relation to material organic changes either
just commencing or just coming to a conclusion. A marked contrast
subsists in all cases between the luminosity of the penumbra and that of
the general surface of the Sun contiguous. Towards its exterior edge the
penumbra is usually darker than at its inner edge, where it comes in
contact with the umbra. The outline of the penumbra is usually very
irregular, but the umbra, especially in the larger spots, is often of
regular form (comparatively speaking of course) and the nucleus (or
nuclei) of the umbra still more noticeably partakes of a compactness of
outline.

Spots are for the most part confined to a zone extending 35° or so on
each side of the solar equator; and they are neither permanent in their
form nor stationary in their position. In their want of permanence, they
are subject, apparently, to no definite laws, for they frequently appear
and disappear with great suddenness.

Their motions are evidently of a two-fold nature; the Sun itself rotates
on its axis, and the spots collectively participate in this movement of
rotation; but over and above this it has been conclusively proved that
sometimes a spot has a proper motion of translation of its own
independently of the motion which it has in consequence of the Sun’s
axial rotation. Curiously enough, spots are very rare immediately under
the Sun’s equator. It is in the zone extending from 8° to 20° North or
South, as the case may be, that they are most abundant; or, to be more
precise still, their favourite latitude seems to be 17° or 18°. They are
often more numerous and of a greater general size in the northern
hemisphere, to which it may be added that the zone between 11° and 15°
North is particularly noted for large and enduring spots. A gregarious
tendency is often very obvious, and where the groups are very straggling
an imaginary line joining the extreme ends of the group will generally
be found more or less parallel to the solar equator; and not only so,
but extending a long way, or sometimes almost entirely, across the whole
of the visible disc. With respect to the foregoing matters Sir John
Herschel remarked:—“These circumstances ... point evidently to physical
peculiarities in certain parts of the Sun’s body more favourable than in
others to the production of the spots, on the one hand; and on the
other, to a general influence of its rotation on its axis as a
determining cause in their distribution and arrangement, and would
appear indicative of a system of movements in the fluids which
constitute its luminous surface; bearing no remote analogy to our
trade-winds—from whatever cause arising.” More often than not when a
main spot has a train of minor spots as followers that train will be
found extending eastwards from the east side of the spot, rather than in
any other direction.

Spots remain visible for very diverse lengths of time, from the extreme
of a few minutes up to a few months; but a few days up to, say, one
month, may, in a general way, be suggested as their ordinary limits of
endurance. As the Sun rotates on its axis in 25¼ days, and as the spots
may be said to be, practically speaking, fixed or nearly so with respect
to the Sun’s body, no spot can remain continuously visible for more than
about 12½ days, being half the duration of the Sun’s axial rotation.

With regard to their size, spots vary as much as they do in their
duration. The majority of them are telescopic, that is, are only visible
with the aid of a telescope; but instances are not uncommon of spots
sufficiently large to be visible to the naked eye. The ancients knew
nothing about the physical constitution of the Sun, and their few
allusions to the subject were mere guesses of the wildest character.
They were, however, able to notice now and then that when the Sun was
near the horizon certain black spots could sometimes be distinguished
with the naked eye, but they took these for planets in conjunction with
the Sun, or phenomena of unknown origin. Earliest in point of date of
those who have left on record accounts of naked eye sun-spots are
undoubtedly the Chinese. In a species of Cyclopædia ascribed to a
certain Ma-touan-lin (whose records of comets have been of the greatest
possible use to astronomers), we find an account of 45 sun-spots seen
during a period of 904 years, from 301 A. D. to 1205 A. D. In order to
convey an idea of the relative size of the spots, the observers compared
them to eggs, dates, plums, etc., as the case might be. The observations
often extended over several days; some indeed to as many as ten
consecutive days, and there seem no grounds for doubting the
authenticity of the observations thus handed down to us. A few stray
observations of sun-spots were recorded in Europe before the invention
of the telescope. Adelmus, a Benedictine monk, makes mention of a black
spot on the Sun on March 17, 807. It is also stated that such a spot was
seen by Averröes in 1161. Kepler himself seems to have unconsciously
once seen a spot on the Sun with the naked eye, though he supposed he
was looking at a transit of the planet Mercury. None of these early
observers have told us the way in which they made their observations,
but the smallest of boys who has any claim to scientific knowledge is
aware of the fact, that by the use of so simple an expedient as a piece
of glass blackened with smoke, spots which are of sufficient size can be
seen with the naked eye. Before telescopes came into use it was
customary to receive the solar rays in a dark chamber through a little
circular hole cut in a shutter. It was thus that J. Fabricius succeeded
in December 1610 in seeing a considerable spot and following its
movement sufficiently well to enable him to determine roughly the period
of the Sun’s rotation.

The spots may often be easily observed with telescopes of small
dimensions, taking care, however, to place in front of the eye-piece a
piece of strongly-coloured glass. For this purpose glasses of various
colours are used, but none so good as dark green or dark neutral tint.
It is not altogether easy to say positively how large a spot must be for
it to be visible with the naked eye, or an opera glass, but probably it
may be taken generally that no spot of lesser diameter than 1′ of arc
can be so seen. This measurement must be deemed to apply to that central
portion of a normal spot already mentioned as being what is called the
nucleus, because penumbræ may be more than 1′ in diameter without being
visible to the naked eye, for the reason that their shading is so much
less pronounced than the shading of umbræ. Very large and conspicuous
spots are comparatively rare, though during the years 1893 and 1894
there were an unusual number of such spots. It often happens that a
conspicuous group is the result of the merging or joining up of several
smaller groups. In such cases a group may extend over an area on the Sun
3′ or 4′ of arc in length by 2′ or 3′ in breadth. The largest spot on
record seems to have been one seen on September 30, 1858, the length of
which in one direction amounted to more than 140,000 miles.

The observation of spots on the Sun by projecting them on to a white
paper screen with the aid of a telescope is a method so convenient and
so exact as to deserve a detailed description, the more so as it is so
little used. Let there be made in the shutter of a darkened room a hole
so much larger than the diameter of the telescope to be used as will
allow a certain amount of play to the telescope tube, backwards and
forwards, up and down, and from right to left. Direct the telescope to
the Sun and draw out the eye-piece to such a distance from the
object-glass as that the image projected on a white screen held behind
may be sharply defined at its edges. If there are any spots on the Sun
at the time they will then be seen clearly exhibited on the screen. An
image obtained in this way is reversed as compared with the image seen
by looking at the Sun through a telescope directly. If therefore the
telescope is armed with the ordinary astronomical eye-piece, which
inverts, then the projection will be direct, that is to say, on the
screen the N. S. E. and W. points will correspond with the same
terrestrial points. Under such circumstances the spots will be seen to
enter the Sun’s disc on the E. side and to go off on the W. side. The
contrary condition of things would arise if a Galilean telescope or a
terrestrial telescope of any kind were made use of. These instruments
erect the image, and therefore will give by projection a reversed image,
in which we shall see the spots moving apparently in a direction
contrary to their true direction.

If the reader has grasped the broad general outlines now given
respecting the Sun and its spots he will perhaps be interested to learn
a few further details, but these must be presented in a somewhat
disjointed fashion, because the multitude of facts on record concerning
sun-spots are so great as to render a methodical treatment of them
extremely difficult within the limits here imposed on me. These matters
have been gone into in a very exhaustive way by Secchi in his great
treatise on the Sun, and in what follows I have made much use of his
observations.

Let us look a little further into the laws regulating the movement of
the spots. If it is not a question of seeing a spot spring into view,
but of watching one already in existence, we shall, in general, see such
a spot appear on the Eastern limb of the Sun just after having turned
the corner, so to speak. The spots traverse the Sun’s disc in lines
which are apparently oblique with reference to the diurnal movement and
the plane of the ecliptic, and after about 13 days they will disappear
at the Western limb if they have not done so before by reason of
physical changes in their condition. It is not uncommon for a spot after
remaining invisible for 13 days on the other side of the Sun, so to
speak, to reappear on the Eastern limb and make a second passage across
the Sun; sometimes a third, and indeed sometimes even a fourth, passage
may be observed, but more generally they change their form and vanish
altogether either before passing off the visible disc, or whilst they
are on the opposite side as viewed from the Earth.

[Illustration: Fig. 6.—Change of Form in Sun-spots owing to the Sun’s
rotation.]

When several spots appear simultaneously, they describe in the same
period of time similar paths which are sensibly parallel to one another
although they may be in very different latitudes. The conclusion from
this is inevitable, that spots are not bodies independent of the Sun, as
satellites would be, but that they are connected with the Sun’s surface,
and are affected by its movement of rotation. If we make every day for a
few days in succession a drawing of the Sun’s disc with any spots that
are visible duly marked thereon, we shall see that their apparent
progress is rapid near the centre of the Sun, but slow near either limb.
These differences, however, are apparent and not real, for their
movement appears to us to take place along a plane surface, whilst in
reality it takes place along a circle parallel to the solar equator. The
spots in approaching the Sun’s W. limb, if they happen to seem somewhat
circular in form when near the centre, first become oval, and then seem
to contract almost into mere lines. These changes are simple effects of
perspective, and are to be explained in the same manner as the apparent
decrease in the size of many of the spots is often explicable. But this
condition of things proves, however, that the spots belong to the actual
surface of the Sun, for, on a contrary supposition, we should have to
regard them as circular bodies greatly flattened like lozenges, and this
would be contrary to all we know of the forms affected by the heavenly
bodies. Of course besides the apparent changes of form just alluded to
as the effect of perspective, it is abundantly certain that solar spots
often undergo very real changes of form, not only from day to day, but
in the course of a few hours. Several spots will often become
amalgamated into one, and it was ephemeral changes of this character
which hindered generally the early observers from determining with
precision the duration of the Sun’s rotation.

The apparent movements of the spots vary also from month to month during
the year according to the season. In March their paths are very
elongated ellipses with the convexity towards the N., the longer axis of
the ellipse being almost parallel to the ecliptic. After that epoch the
curvature of the ellipse diminishes gradually, at the same time that the
major axis becomes inclined to the ecliptic, so that by June the
flattening of the ellipse has proceeded so far that the path has become
a straight line. Between June and September the elliptical form
reappears but in a reversed position; then, following these reversed
phases, the ellipticity decreases, and for the second time there is an
epoch of straight lines. This happens in December, but the straight
lines are inclined in a converse direction to that which was the case in
June. It must again be impressed on the reader that all these seemingly
different forms of path pursued by the spots are merely effects of
perspective, for in reality, the spots in crossing the Sun’s disc
describe lines which are virtually parallel to the solar equator. These
projections really depend of course on the position of the observer on
the Earth, and vary as his position varies during the Earth’s annual
circuit round the Sun. The number of the spots varies through wide
limits. Sometimes they are so numerous that a single observation will
enable us to recognise the position of the zones of maximum frequency.
Sometimes, on the other hand, they are so scarce, that many weeks may
pass away without hardly one being seen. A remarkable regularity is now
recognised in the succession of these periods of abundance and scarcity,
as we shall see later on.

It is both useful and interesting in studying the spots to record
methodically their number and their size, but it is not easy to teach
observers how to do this so systematically that observations by one
person can be brought into comparison with those of another. Photography
and hand-drawing on a screen alone furnish a trustworthy basis of
operations. Spots in general may naturally be classified into (1)
isolated spots or points, and (2) groups of spots; but often one
observer will describe as a small spot an object which another observer
would regard as a mere point; and one observer will record several
groups where another observer will see but one. A very few days’
experience with a telescope will bring home to the observer’s mind the
difficulty of dealing with the spots where it is a question of
systematic methodical observation of them.

Let us now take a brief survey of some of the theories which have been
put forth regarding the nature of the spots on the Sun. In the early
days of the telescope, that is to say, during the 17th century, two
general ideas were current. Some thought the spots to be shapeless
satellites revolving round the Sun; others that they were clouds, or
aggregations of smoke, floating about in a solar atmosphere. Scheiner,
the author of the first theory, abandoned it towards the close of his
life, having arrived at the conclusion that the spots were situated
below the general level of the Sun’s surface. Another idea, but of later
date, was that the Sun is a liquid and incandescent mass of matter, and
the spots immense fragments of _Scoriæ_, or clinkers, floating upon an
ocean of fire.

Somewhat more than a century after the spots had been generally studied
with the aid of a telescope a Scotchman named Wilson made a memorable
discovery. He showed by the clearest evidence that they are cavities,
and he propounded the first intelligible idea of the true physical
constitution of the Sun, when he compared to a strongly illuminated
cloud the luminous layer of solar material which we now term the
“photosphere.” On November 22, 1769, he observed on the Sun’s disc a
fine round spot encompassed by a penumbra, also circular, and concentric
with the nucleus. He watched that spot up to the time that it
disappeared, and he soon remarked that the penumbra ceased to be
symmetrical: the part turned towards the centre of the Sun became
smaller and smaller, and eventually disappeared altogether; whilst the
part on the opposite side preserved its fulness and dimensions almost
unchanged. Let us suppose we chanced to turn a telescope on to the Sun
on a given day, and were fortunate enough to discover a spot in the
centre of the disc, with a penumbra concentric with the nucleus. When
such a spot arrives about midway towards the limb, it will exhibit a
penumbra narrower on the left side than on the right; later on the
penumbra will disappear almost or quite completely on the left side:
then the nucleus itself will seem to be encroached upon. Finally, very
near the limb, there will remain only a slender thread of penumbra, and
the nucleus will have ceased to be directly visible. Such were the
phases of transformation observed by Wilson and often studied since.
Wilson suspected that he had come upon some great law that was ripe for
disclosure, and in order not to be misled he waited for the return of
the same spot, which indeed reappeared on the Sun’s W. limb after about
14 days. Then he found himself face to face with the same phases
reproduced, but in the reverse order: the penumbra contracted on one
side and full on the other, widening out on the contracted side as the
spot came up to the Sun’s centre. Henceforth doubt was no longer
possible; the spot had sensibly preserved the same shape during its
passage, and the alterations noticed were only apparent, and resulted
from an effect of perspective which was easy to be understood. The
different phases presented by such a spot as that just spoken of will be
so much the more sensible according as the depth of the cavity is
greater; but if the depth is inconsiderable the bottom of the cavity
will only disappear when a very oblique angle is attained, and this
cannot happen except when the spot is very near to the limb. By
observations carefully made under such circumstances it will be possible
to determine the depth of the cavity, and Wilson found that the depth of
a spot often amounted to about one-third of the Earth’s radius. Wilson’s
theory was not accepted without dispute; it was contested by several
astronomers, and in particular by Lalande. It was however taken up by
Sir W. Herschel, and as modified by him has met with general acceptance
down to the present time; though now and again challenged, perhaps most
recently and most vehemently by Howlett, a sun spot observer of great
experience. Wilson’s discovery was the point of departure for the grand
labours of Sir W. Herschel in the field of Solar Physics. Man of genius
that Herschel was, he was above all things an observer who took his own
line in what he did. He saw so many phenomena with the powerful
instruments constructed by himself, he described so minutely the marvels
which were revealed to him, that he left comparatively little for his
successors to do so far as regards mere telescopic observation.
Herschel’s main idea as to the Sun was based on Wilson’s discovery. He
remarked with reason, as that astronomer had done, that if the spots are
cavities the luminous matter could neither be properly called liquid nor
gaseous; for then it would precipitate itself with frightful rapidity to
fill up the void, and that would render it impossible that the spots
should endure as we often see they do during several revolutions of the
Sun. Moreover, the proper movements of the spots prove that the
photosphere is not solid. We can therefore only liken it to fogs or
clouds, and it must be suspended in an atmosphere similar to ours. Such
is, according to Herschel, the only hypothesis which can explain the
rapid changes which we witness. We shall see a little later on that
these phenomena do admit of another explanation.

In a second memoir Herschel followed up this inquiry with an acuteness
worthy of his genius. Unfortunately he allowed himself to be carried
away with the idea that the Sun was inhabited in order to sustain this
theory. He needed a solid kernel upon which his imaginary inhabitants
could dwell; and also a means whereby he could protect them from the
radiations of the photosphere. With this idea in view he conjectured the
existence above the Sun’s solid body of a layer of clouds always
contiguous to the photosphere which enveloped it, and which always being
rent when the photosphere was rent, thus enabled us to see the solid
body of the Sun lying behind. These notions can only be described as
very arbitrary, as unsupported by observation, and as involving
explanations quite out of harmony with the principles of modern physics.
However, the labours of Herschel resulted in so many positive
discoveries of visible facts, and in so many just conclusions, that they
contributed greatly to the growth of our present knowledge of the true
constitution of the Sun.

Since Wilson’s time, as Secchi pointedly remarks, astronomers generally
have verified his observations with good instruments, and by an
investigation of a great number of spots. De La Rue, discussing the Kew
observations, found that of 89 regular spots 72 gave results which
conformed to Wilson’s ideas, whilst the remaining 17 were opposed
thereto. There is nothing surprising in the existence of a contrarient
minority when we consider the great changes which in reality often occur
in the forms of the spots. De La Rue suggested a very simple expedient
for showing that the spots are cavities. Take two photographs of the Sun
made at an interval of one day: during that time every point on the
Sun’s surface will have been displaced, so far as the telescope is
concerned, by about 15°. Place these photographs in a stereoscope, and
we shall readily see the interior cavity, the edges of which will appear
raised above the photosphere. It is impossible therefore to entertain
the least doubt as to the truth of the theory that the spots are
excavations in the luminous stratum which envelopes the whole of the
solar globe.

If it be true that a spot is a cavity, it follows that when it reaches
the margin of the solar disc we ought to detect a hollow place; and this
will be so much the more easy to observe according as the cavity is
larger and deeper. As a matter of fact, numerous observations of this
sort have been recorded from the time of Cassini down to the present
time under the designation of “notches” on the Sun’s limb. On July 8,
1873, Secchi observed such a notch 8″, or 3600 miles deep.

Faye and some other astronomers are disposed to support a theory
according to which the spots are nothing else than aërial cyclones, but
this does not seem admissible. If the fundamental principle of a spot is
that it arises from a whirling movement, the rays (so to speak) which
compose the penumbræ must always be crooked, or the theory falls to the
ground. It is quite true that indications of cyclonic action do
sometimes appear, but they are at any rate very rare, for only a small
percentage exhibit in a distinct manner a spiral structure. Moreover,
when such a structure is seen it does not endure for the whole lifetime
of the spot but only for a day or two: the spot may last a long time
after it has lost its spiral features, if it ever had any. Sometimes
even the whirling movement, after having slackened, begins again, but in
the contrary direction. Under these circumstances, though this
occasional spiral structure is very curious and interesting, we are not
justified in taking it as the basis of a theory which has any
pretensions to explain the general nature of sun-spots.

[Illustration: Fig. 7.—Sun-spot seen as a Notch.]

When we examine the Sun with instruments of large aperture and high
magnifying power, we notice that its surface is far from being as smooth
and uniform as it appears in a small telescope. On the contrary, it
presents an irregular undulating appearance like a pond or other sheet
of water agitated by the wind. Careful scrutiny with a powerful
eye-piece reveals the fact that the Sun’s surface is marked by a
multitude of wrinkles and irregularities which it is well-nigh
impossible to describe in words. More or less everywhere there is a
general mottling visible; it is more distinct in some places than
others, and especially so towards the centre of the disc. This peculiar
appearance varies very much from time to time, and its distinctness
seems to depend a great deal on the state of the Earth’s atmosphere, for
it becomes invisible when the air is disturbed; but these variations
depend also on real variations of the photosphere—a fact which
observations made in very calm weather are thought clearly to indicate.

It is often said that the Sun exhibits a granulated structure. If we
wish to realise in the most precise manner what is meant by the word
“granulation” as applied to the structure of the Sun, we must abandon
the method of projection and examine the Sun directly with a powerful
eyepiece, taking advantage of a moment when the atmosphere is perfectly
calm, and before the eyepiece has had time to get hot. It may then be
seen that the Sun’s surface is covered with a multitude of little
grains, nearly all of about the same size, but of different shape,
though for the most part more or less oval. The small interstices which
separate these grains form a network which is dark without being
positively black. Secchi considered it difficult to name any known
object which exactly answers in appearance to this structure, but he
thought that we can find something resembling it in examining with a
microscope milk which has been a little dried up, and the globules of
which have lost their regular form. Exceptionally good atmospheric
conditions are under all circumstances indispensable for the study of
these details.

In point of fact, there is a mysterious uncertainty about the normal
condition of the Sun’s surface, in a visual sense, which a few years ago
engendered a very vehement controversy, and led to the use of such
expressions as “willow leaves,” “rice grains,” “sea beach,” and “straw
thatching,” to indicate what was seen. All these words are too precise
to be quite suitable to be taken literally, but perhaps, on the whole,
“rice grains” is not altogether a bad expression to recall what
certainly seems to be the granular surface of the Sun as we see it.

By making use of moderate magnifying powers, what we see will often
convey the impression of a multitude of white points on a black network.
This is very apparent during the first few moments that the telescope is
brought to bear on the Sun, but its clearness quickly passes away
because the eye gets fatigued, and the lenses becoming warm the air in
the telescope tube gets disturbed because also warmed. Sometimes the
appearance is a little different from that just described, and along
with the white and brilliant points little black holes are intermixed.
Oftentimes the grains appear as if suspended in a black network and
heaped together in knots more or less shaded and more or less broad.
Sometimes the grains exhibit a very elongated form, especially in the
neighbourhood of the spots. It is these elongated forms to which Nasmyth
applied the term “willow leaf,” whilst Huggins thought “rice grains” a
very suitable expression.

This granular or leaf-like structure—call it what we will—cannot be made
out except with considerable optical assistance, for the grains being
intrinsically very small, diffraction in enlarging them and causing them
to encroach on one another necessarily produces a general confusion of
image. The real dimensions of these grains cannot therefore readily be
determined by direct measurement, but by comparing them with the wires
used in micrometer eye-pieces it has been thought that their diameters
may usually be regarded as equal to ¼ or ⅓ of a second—say from 120 to
150 miles. The granules seem to be possessed of sensible movement, but
presumably it is not always or even generally a movement of translation
from place to place; only an undulatory movement like that of still
water when a stone is cast into it. Nevertheless, probably in certain
cases the granules actually are affected by a motion of translation, for
in the vicinity of spots they may sometimes be seen flowing over the
edges of the penumbræ. In order to explain the existence of the granules
the strangest theories have been broached. Sir William Herschel having
observed the granulations, applied to them the term “corrugations” or
“furrows”—words somewhat inexact, perhaps, but by which, as his
descriptions clearly show, he meant to designate the features which I am
now treating of. He even noticed the dark network which separates the
grains, and he applied to it the word “indentations.”

These granulations are without doubt prominences, probably of hydrogen
gas, which rise above the general surface, for this structure is much
more sharp and distinct at the centre of the sun’s disc than at the
limbs; that is to say, near the limbs of the Sun they partially overlap
one another, as indeed Herschel remarked. The idea of flames would
satisfy these appearances: and as the spectroscope suggests to us that
the Sun is habitually covered over with a multitude of little jets of
flame, the observations which have been made compel the opinion that the
grains are the summits of those prominences which exist all over the
Sun’s surface.

The surface is sometimes so thickly covered over with these
granulations—the network is so conspicuous—that we can readily imagine
that we see everywhere pores and the beginnings of spots, but this
aspect is not permanent, and seems to depend to some extent on
atmospheric causes combined also with actual changes in the Sun’s
surface itself. There seems however no doubt that the joints, so to
speak, of the dark network already referred to do sometimes burst
asunder and develope into spots.

The circumstances which accompany the formation of a spot cannot readily
be specified with certainty. It is impossible to say that there exists
any law as to this matter. Whilst some spots develope very slowly by the
expansion of certain pores, others spring into existence quite suddenly.
Yet it cannot be said that the formation of a spot is ever completely
instantaneous however rapid it may be. The phenomenon is often announced
some days in advance: we may perceive in the photosphere a great
agitation which often manifests itself by some very brilliant _faculæ_
(to be described presently) giving birth to one or more pores. Very
often we next notice some groups of little black spots, as if the
luminous stratum was becoming thinner in such a way as to disappear
little by little and leave a large black nucleus uncovered. At the
commencement of the business there is usually no clearly defined
penumbra. This developes itself gradually and acquires a regular
outline, just as the spot itself often takes a somewhat circular form.
This tranquil and peaceable formation of a spot only happens at a time
when calm seems to reign in the solar atmosphere: in general the
development is more tumultuous and the stages more complicated.

As a rule a spot passes through three stages of existence:—(1) the
Period of birth; (2) a Period of calm; and (3) the Period of
dissolution. When a spot is on the point of closing up, the flow of the
luminous matter which it, as it were, attracts, is not directed
uniformly towards the centre; it seems that the photospheric masses, no
longer meeting with resistance, are precipitated promiscuously anywhere
so as to fill up the hole. It is impossible to describe in detail the
phases which irregular spots go through, but two things may always be
remarked: that their structure is characterized by the existence of
luminous filaments, and that these filaments converge towards one or
several centres.

Secchi thus sums up certain conclusions which he arrived at relating to
spots generally:—(1) It is not on the surface of any solid body that the
solar spots are manifested; they are produced in a fluid mass, the
fluidity of which is represented by a gas, so that the constitution of
this medium may be likened to that of flames or clouds; (2) the known
details respecting the constitution of the penumbra and the phenomena
exhibited prove that the penumbra is not a mass of obscure matter which
floats across luminous matter, but that it is on the contrary a case of
luminous matter invading and floating about over darker materials and so
producing a half tint.

All the available evidence which we possess may be said to show that the
spots are not merely superficial appearances, but that they have their
origin deep in the interior of the Sun, and are produced by the
operation of causes still unknown to us which affect and disturb the
Sun’s mass to an extent which is sometimes very considerable. The spots
then are only the results of a great agitation in the materials of which
the Sun is composed, and this agitation extends far down below the
limits of the visible dark nucleus whatever that may consist of.

Besides the spots, streaks of light may frequently be remarked upon the
surface of the Sun towards the margin of the disc. These are termed
_faculæ_ (torches), and they are often found near the spots, or where
spots have previously existed or have afterwards appeared. When quite
near the Sun’s limb these faculæ are usually more or less parallel to
the limb. They are of irregular form and may be likened to certain kinds
of coral. They generally appear to be more luminous than the solar
surface immediately adjacent to them, but it is not improbable that this
is an optical illusion depending upon the fact that the edges of the Sun
always appear much more luminous than the centre. This last-named fact
may be readily recognised by the employment of a high magnifying power,
and moving the telescope rapidly from the limb to the centre of the
disc. If the Sun be projected on a screen, as already mentioned, this
degradation of the Sun’s light from centre to circumference becomes
particularly manifest.

After having studied the structure and the movement of the spots, one is
naturally led to ask if their apparitions at different periods are
subject to any general law. This question is one which has much engaged
the attention of modern astronomers. The older observers noticed that
the number of the spots visible differed in different years. There were
said to have been periods when months and even years passed away without
any spots being observed. Even allowing that this statement, so far as
“years” are concerned, might be exaggerated, and that the absence of
spots was due to the want of sufficient care in making the observations,
and especially to the want of efficient instruments, it is none the less
true that the number of the spots is extremely variable, and that there
have been epochs when they were very scarce.

Sir W. Herschel was the first who devoted himself to the question of
seeking to establish a relation between the variation of the spots and
terrestrial meteorology. For the want of any better object, he compared
the annual number of the spots with the price of wheat; but it is easy
to see that nothing could result from such a comparison. Without doubt
the meteorological phenomena of the globe must depend to some extent on
solar changes: but the term of comparison selected by Herschel had no
direct bearing on the state of the Sun.

In our time this question has been investigated to its very foundation
by Wolf, Director for many years at the Observatory of Zurich. It is to
his zeal that we owe a very interesting assemblage of old observations
which were buried in archives and chronicles. It was he who endeavoured
to reduce them into a systematic form, so as to supply as far as
possible the numerous gaps which exist in the different series.

The two most attentive observers at the period when the spots were
discovered were Marriott at Oxford and Scheiner at Ingoldstadt, but
Scheiner himself has informed us that he did not note down all the spots
which he saw; he only recorded those which were likely to assist him in
his special task of determining the period of the Sun’s rotation.
Several observers after him made isolated series of observations; but
some of these have been lost and the others show important gaps. J. G.
Staudacher, at Nuremburg, observed the Sun with great perseverance
during fifty years from 1749 to 1799. Before him the Cassinis, Maraldi,
and others were engaged in the same sort of work, but only in an
indirect way: that is to say, they contented themselves, whilst making
meridional observations of the Sun, with noting anything in the way of
spots which they deemed important. Zucconi and Flaugergues also left
behind them a good collection of observations which Wolf utilised,
rendering them comparable one with another by applying suitable
corrections. The great difficulty herein arises from the fact that the
observers were not provided with instruments of equal power; one man,
armed with a better telescope than his contemporaries, naturally
observed and recorded spots which would escape the others. The numbers
entered in their registers are therefore not comparable _inter se_. Wolf
endeavoured to replace these numbers by others which would represent the
spots which might have been seen if the observers had all employed
telescopes of a given kind and power. The result of his efforts in this
direction is an almost continuous series of Sun-spot records from an
epoch sufficiently remote, up to the time when this branch of science
was taken up with the vigour of modern scientific methods.

The observer who most assiduously devoted himself to this subject in
modern times was Schwabe of Dessau. From 1826 to 1868 he never failed to
make daily observations when the weather permitted him. His series of
records is specially valuable, for Carrington’s fits in with it, and
with that in turn Spörer’s is comparable, and the chain is complete by
the later photographic and other observations. All these Sun-spot
records, though differing in their details, may easily be used together
when it is a question of working out relative annual fluctuations.

At the present time there are many Astronomers who are engaged in
observing the spots with care; but just as formerly there are few who
possess sufficient perseverance. The photographic method is excellent,
but it takes much time and is costly. Some have decried, in a very
unreasonable manner, a drawing made by hand: such a drawing, of
sufficient size, and executed by projection by a skilful draughtsman
with a telescope driven by clockwork, may stand comparison with a
photograph, and this method has a better chance of being persevered in.
The Rev. F. Howlett’s name must be mentioned in this connection as a
draughtsman who has accomplished much by hand drawing. Though the once
famous Kew observations have been discontinued, they have been replaced
by a new series at Greenwich with similar appliances; whilst Janssen at
Meudon has also been carrying on for a number of years a splendid course
of photographic records.

Schwabe, when he had collected a considerable number of observations,
recognised clear indications of periodicity. Very definite epochs of
maxima and minima succeeded one another at intervals of 10 or 11 years.
It is true that in following out such a study the observations are
certain to be in a sense a little defective. At first it was not
possible to observe the Sun every day, and the gaps which resulted from
bad weather necessarily added to the number of days which had to be set
down as being without spots. Moreover, every method of numbering the
spots must be a little arbitrary: there are often groups which, in
consequence of their sub-divisions, may be counted in different ways:
but in a mass of observations so considerable as those of Schwabe’s,
such uncertainties will compensate for one another and will disappear in
the final result. In fact the law is so striking that it suffices to
cast one’s eye over his table[2] to see that.

That table is both interesting and instructive at the same time. The
numbers exhibited in it speak for themselves, and it is sufficient to
examine them with even a small amount of attention to realise the
certainty of the conclusions which have been drawn.

It is therefore now to be deemed an ascertained fact that there are
periodical maxima and minima in the display of spots, and that the
extent of the period is between 10 and 12 years. In order to determine
this value with the utmost exactness, some astronomers have had recourse
to early observations. Wolf of Zurich made this the subject of some very
interesting inquiries. He was able to establish the chronology of the
phases which the Sun has passed through from the time of the first
discovery of the spots to the present day—more than 2½ centuries. His
calculations led him to a period of 11-1/9 years. Lamont fixed upon
10.43 years, but this number does not represent the more recent
observations with sufficient precision.

In order to exhibit this law in the plainest possible manner the dates
of maxima and minima should be laid down on ruled paper in proper
mathematical form, the _abscissæ_ of the curve representing the years,
and the _ordinates_ the number of spots observed.

An examination of a curve thus plotted shows two things:—(1) That the
period is clearly an eleven-year one, as has been already stated; (2)
that it is not however quite as simple in its form as it was at first
thought to be; for in reality there are two periods superposed, the one
rather more than half a century long, and the other extending over the
11 years already spoken of. We do not possess early observations
sufficiently numerous and sufficiently good to enable us to draw any
unimpeachable conclusions as to the nature of the long period; we can
only be certain that it exists. The later labours of Wolf, however,
fixed that period at 55½ years. It is a result of this that, according
to Loomis, a period of comparative calm on the Sun existed between 1810
and 1825.

Each maximum lies nearer to the minimum which precedes it than to the
minimum which follows it, for the spots increase during 3.7 years, and
then diminish during 7.4 years. According to De La Rue the increase
occupies 3.52 years, and diminution 7.55 years. This concurrence between
De La Rue and Wolf is surprising considering the diversity of the
methods which led to results almost identical, the one set being based
on the number of the spots, and the other on the superficial extent of
the spots. The different periods in succession are not absolutely
identical: but it has been remarked that if during any one period the
decrease is retarded or accelerated, then the increase next following
will be lengthened or contracted to a corresponding extent. In
consequence of this we are sometimes able to predict with fair accuracy
when the next ensuing maximum or minimum will take place.

The most striking feature of such a curve as that just alluded to is the
very sensible secondary augmentation which happens very soon after the
principal maximum.

A very curious circumstance has come to light in connection with the
epochs of maxima and minima. In arranging the spots according to their
latitude and longitude on a diagram sufficiently contracted, Carrington
found that their latitude decreases gradually as the period of minimum
draws near; then when their number begins to increase they begin to
appear again at a higher latitude. This seems to be a definite law. At
any rate Carrington’s conclusion has been found to hold good by the
observations of Spörer and Secchi.

The variations of the spots which we now recognise naturally recall
those obscurations of the Sun which are recorded in history; but it is
necessary to accept many of these with caution. A great number of these
phenomena which attracted the attention of people in early times are
only eclipses badly observed and still more badly described. In other
instances the obscuration has been produced by very protracted dry fogs.
It is probably to this last-named cause that we must ascribe the
obscuration which, according to Kepler and Gemma Frisius, took place in
1547.

It was in some such way as this that, according to Virgil (Georg. i,
630), who has echoed a tradition which he found in history, the Sun was
obscured at the death of Cæsar:—

  Ille etiam extincto miseratus Cæsare Romam
  Quum caput obscura nitidum ferrugine texit,
  Impiaque æternam timuerunt sæcula noctem.

In the year 553 A.D., and again in the year 626 A.D. the Sun remained
obscured for several months; but these facts (if facts they are) besides
being ill-observed, and clothed, no doubt, in extremely exaggerated
language, are brought to our notice as having occurred at epochs which
are quite independent of one another, whilst the variations in the
markings on the Sun, which we have just been talking about, present an
almost mathematical regularity of sequence.

We must now institute some inquiries as to the causes of the periodicity
of the spots. A periodicity so well established would naturally invite
astronomers to seek the causes which produced it. The presence of spots
only in the Zodiacal regions led Galileo to suspect the existence of
some relation between the spots and the position of the planets; but
there is in this a mere surmise, which, when it was made, had nothing to
justify it, and it is still impossible for us to say anything for
certain on the point. The determining cause of the periodicity may exist
in the interior of the Sun, and may depend on circumstances which will
for ever remain unknown to us. Or it may be something external: it may
be due after all to the influence of the planets. It remains for us,
therefore, to search and see if any such influence can be traced.

According to Wolf, the attraction of the planets, or of some of them, is
the real cause of the periodicity which we are dealing with; that
attraction producing on the surface of the solar globe true tides, which
give birth to the spots, these tides themselves experiencing periodic
variations owing to the periodic changes of position of the celestial
bodies which cause them. It has even been thought safe to assert that
the fact of the principal period coinciding with the revolution of
Jupiter is of momentous significance; but this coincidence seems purely
accidental, and no certain conclusion can be drawn as to this matter.
The influence of Mercury and Venus would perhaps be much more potent,
for their distance from the Sun is not very great, and this should
render their influence more sensible. On the other hand, their masses
appear to be too small to be capable of producing any sufficient effect.

De La Rue, Balfour Stewart, and Löwy most perseveringly studied this
point of solar physics. They seem to have arrived at the conclusion that
the conjunctions of Venus and Jupiter do exercise a certain amount of
influence on the number of the spots and on their latitude; and that
this influence is less considerable when Venus is situated in the plane
of the solar equator. At any rate it is a fact, that a great number of
the visible inequalities in a duly plotted curve of the spots do really
correspond to special positions of these two planets.

In order to determine with more precision these coincidences and the
importance which attaches to them, De La Rue extended his inquiries. He
separately analysed many different groups of spots, selecting for his
purpose more particularly those of which the observations happened to
have been specially continuous and complete, giving a preference
moreover to those which had been observed in the central portions of the
Sun’s disc. From an investigation of 794 groups De La Rue arrived at the
following conclusions:—(1) If we take a meridian passing through the
middle of the disc and represented by a diameter perpendicular to the
equator, we find that the mean size of the spots is not the same with
regard to that meridian. It appears certain that the correction required
for perspective does not suffice to explain this difference; and that
another element must be introduced in order to secure that the apparent
dimensions of the spots may be the same on both sides. We do not yet
possess a very clear explanation of this fact; but the most probable is
this:—the spots are surrounded by a projecting bank, which seems to
disappear in part during their transit across the Sun. This bank is more
elevated on the preceding than on the following side; accordingly, the
spots ought to seem smaller when they are in the eastern half of the
disc; larger when they are in the western half; for in the first
position the observer’s eye meets an elevated obstacle, which hides a
portion of the spot itself. (2) De La Rue specially studied the spots
observed at the times when the planets Venus and Mars were at a
heliocentric distance from the Earth equal to 0, 90, 180, and 270
degrees, and arrived at this result; the spots are larger in the part of
the Sun which is away from Venus and Mars, and they are smaller on the
side on which these planets happen to be. The same result was obtained,
whether Carrington’s figures or the Kew photographs were employed. (3)
Meanwhile it does not appear that Jupiter emits any similar influence.
This influence should be easily perceived, for if we calculate the
action of the planets in the way that we calculate the tides, treating
it as directly proportional to the masses and inversely proportional to
the cubes of the distances, the influence of Jupiter should greatly
outweigh that of Venus.

Wolf thought that he had noticed traces of some influence being exerted
by Saturn; but this remains altogether without confirmation.

De La Rue noticed that large spots are generally situated at extremities
of the same diameter. This law also often applies to the development of
large prominences. The coincidence agrees well with the theory that
there exists on the Sun some action resembling that of our tides.

Whatever may be the amount of probability which attaches to these
explanations we ought not to forget that we are still far off from
possessing the power of giving a vigorous demonstration of them. If we
consider with attention the periodical variations of the spots we shall
not be long in coming to the conclusion that it is impossible to connect
them directly with any one astronomical function in particular, for the
spots appear in a sudden and irregular manner which contrasts in a
striking degree with the continuous and progressive action of the
ordinary perturbations which we meet with in the study of Celestial
Mechanics. There is but one reply possible to this objection. The spots
and their changes must be visible manifestations of the periodical
activity of the Sun—an activity which itself depends (as assumed) on the
action of the planets and on their relative positions. The cause, thus
defined, of the Sun’s activity may be very regular; the activity itself
may vary in a continuous manner without the resulting phenomena
possessing the same continuity and the same regularity. We see this in
the periodical succession of the Seasons on the Earth. The position of
the Sun, and consequently its manner of acting upon our globe, varies
with a remarkable uniformity, but nevertheless the meteorological
phenomena which result are irregular and capricious. Thus it comes about
that physicists are more and more inclined to believe that the spots are
only secondary effects produced by causes more important and more
fundamental.

Whatever may be our ignorance as to the causes which produce variations
in the Sun’s activity we may at least draw one conclusion from the
preceding remarks: it is, that the Sun is a very long way from having
arrived at a state of tranquillity and freedom from internal commotion.
On the contrary, it is the seat of great movements. Its activity is
subject to numberless periodical changes which ought in their turn to
influence the intensity of the heat and light given out by the Sun; and
so re-act on the planets which receive their heat, light, and life from
the Sun.

No account of the periodicity of the spots on the Sun can be deemed
complete which does not include information respecting certain other
periodical phenomena which have been found to exhibit features of
alternation closely resembling in their sequence and character the
periodical changes which take place in regard to the spots on the Sun.
There is evidently a deep mystery lying hid under the curious fact
(which is clearly established) that the 11-year period of the spots
coincides in a manner as unexpected as it is certain with the period of
the variation of terrestrial magnetism. The magnetic needle is subject
to a diurnal variation which reaches its extreme amount every 11 years,
and not only so, but the epoch of maximum variation corresponds with the
epoch of the maximum prevalence of Sun spots. And similarly years in
which the needle is least disturbed are also years in which the Sun
spots are fewest. Two other very curious discoveries have also been made
which are in evident close connection with the foregoing. The
manifestation of the Aurora Borealis and of those strange currents of
electricity known as magnetic earth currents (which travel below the
Earth’s surface and frequently interfere with telegraphic operations),
likewise exhibit periodical changes which take 11 years to go through
all their stages. This fact alone would be sufficiently curious, but
when we come to find that the curve which exhibits the changes these two
manifestations of force go through, also shows that their maxima and
minima are contemporaneous with the maxima and minima of the Sun spots
and magnetic needle variations, we cannot doubt that (to use Balfour
Stewart’s words) “a bond of union exists between these four phenomena.
The question next arises, what is the nature of this bond? Now, with
respect to that which connects Sun spots with magnetic disturbances we
can as yet form no conjecture.” To cut a long story short, it may be
said generally that whilst without doubt electricity is the common basis
of the three last-named of the four phenomena just mentioned, it seems
scarcely too great a stretch of the imagination to go one step further
and suggest that electricity has in some or other occult manner
something to do with all these things and therefore with the spots on
the Sun.

[Illustration: Fig. 8.—The Sun totally eclipsed, July 18, 1860
(Feilitzsch).]

The reader who has followed me thus far will by this time be in a
position to appreciate a remark made in an earlier part of this chapter,
that the multitude of facts known to us in connection with the Sun and
its spots is so great, as to render it impossible to exhibit in a single
chapter anything more than the barest outline of them. The numerous
observations of recent eclipses of the Sun, especially since that of
1860, and the extensive application of the spectroscope to the Sun both
in connection with these eclipses, and generally, may be said to have
completely revolutionised our knowledge of solar phenomena during the
present generation; or perhaps it might be more correct to say have
enormously increased our knowledge of the facts of the case and have
revolutionised in no small degree the conclusions deduced from the
facts.




                              CHAPTER III.
                                MERCURY.


So far as we know at present, Mercury is the nearest planet to the Sun.
The circumstances under which it presents itself to us and a brief
general account of its movements have already been stated. In the
present chapter, therefore (and this remark applies in substance to each
of the succeeding chapters appropriated to particular planets), I shall
limit myself to such topics as seem to be of interest to an observer
armed with a telescope. Mercury, as already mentioned, exhibits from
time to time phases which may be said to be the same as those of the
moon; but as the only chance of seeing it is when it is at its greatest
distance east or west of the Sun, practically it can only be studied
when in, or rather near to, what may be called the half-moon phase; and
even then observations on its physical appearance can only be obtained
with difficulty. Perhaps its most definite feature is its colour. This,
undoubtedly, is more or less pink. Strange to say, in spite of the
multiplication of telescopes and observers, comparatively little
attention has been paid to this planet, and we really know very little
more about it than Schröter told us nearly a hundred years ago. He
obtained what he conceived to be satisfactory evidence of the existence
of at any rate one mountain, having a height of about 11 English miles—a
height which it will be noted, far exceeds, not only relatively but
absolutely, any mountain on the earth. What Schröter based this
conclusion upon was the fact that when the planet was near inferior
conjunction, the southern horn presented a truncated appearance, which
might be the result of a lofty projection arresting the Sun’s light.
Schröter also announced that Mercury rotated on its axis in 24 hours 5
minutes. Sir W. Herschel failed to satisfy himself that Schröter’s
conclusions were well-founded, but it must certainly be admitted that
some support for them is furnished by certain observations made within
the last few years. It is matter for regret, however, that most of these
were made with instruments of sizes which, for the most part, cannot be
said to have been equal to the task to which they were applied. The
truncature of the southern horn first spoken of by Schröter, was thought
by Denning, in 1882, to be obvious; and in the same year, by watching
the displacement of certain bright and dusky spaces on the disc, the
same observer concluded that a rotation period of about 25 hours was
indicated.

In 1882 Schiaparelli at Milan commenced a prolonged study of Mercury.
Believing that it was essential to observe through a good condition of
atmosphere, and that this was impossible if the planet were only looked
at in twilight, when it was necessarily at a low altitude, Schiaparelli
made all his observations with the Sun and planet high up in the
heavens. He considered, in effect, that the blaze of the Sun’s light was
a lesser evil than the tremors inseparable from observations of the
planet, clear it might be in some degree of inconvenient Sun-light, but
viewed through the vapours and atmospheric disturbances, which always
spoil all observations near the horizon. Schiaparelli’s observations
yielded various results, most of them novel, and one of them very
startling. He considers Mercury to be a much spotted globe and to be
enveloped in a tolerably dense atmosphere. He thought he noticed
brownish stripes and streaks (which might be regarded as permanent
markings), more clearly visible on some occasions than on others; and
that these systematically disappeared near the limb, owing to the
increased depth there of the atmosphere through which they had to be
looked at.

The foregoing observations may be regarded as not unreasonable; they may
even be accepted without further question. But what are we to say to
Schiaparelli’s conclusions that these markings are so nearly permanent,
taking one day with another, that Mercury’s rotation cannot be measured
in hours at all, but is a matter of days,—in point of fact, of 88 days;
and that in reality Mercury occupies in its rotation on its axis the
whole of the 88 days which constitute its sidereal year, or period of
revolution round the Sun. The counterpart of this for us would be that,
instead of the inhabitants of the earth having a day of 24 hours, they
would have only one day and night every 365 days. Astronomers are not at
present satisfied to accept this conclusion in regard to Mercury.

Some observers have thought that Mercury is more easy to observe than
Venus, and that, speaking generally, its surface, if we could only get
to see it constantly under favourable circumstances, might be considered
to resemble in most respects that of Mars. Mercury revolves round the
Sun at a mean distance of 36 millions of miles. Owing, however, to the
fact that the eccentricity of its orbit (or its departure from the
circular form) is greater than that of any of the other major planets,
it may approach to within 28½ millions of miles or recede to more than
43 millions of miles. Its apparent diameter varies between 4½″ in
superior conjunction to 13″ in inferior conjunction. The real diameter
may be taken at about 3000 miles.




                              CHAPTER IV.
                                 VENUS.


The planet Venus has two things in common with Mercury. One is, that
being an inferior planet, that is to say, a planet revolving round the
Sun in an orbit within that of the Earth, it is never very far distant
from the Sun, and therefore can never be seen on a distinctly dark sky.
The second point alluded to arises out of the first; Venus exhibits from
time to time a series of phases which are identical in character with
those of Mercury, and therefore with those of the Moon. Venus differs,
however, from Mercury in the very important point of size. Inasmuch as
its diameter is considerably more than double the diameter of Mercury it
has a surface more than six times as great, and therefore exhibits a far
larger area of illumination than Mercury does. The result of this
(coupled with another fact which will be stated presently) is that the
planet may often be easily seen in broad daylight, and sometimes casts a
sensible shadow at night. Under special circumstances, which recur every
8 years, this planet shines with very peculiar brilliancy. True, that
only about ¼th of the whole disc is then illuminated, but that fraction
transmits to us more light than phases of greater extent do, because
these latter coincide with epochs when the planet is more remote from
the Earth.

Spots and shadings have on various occasions been noticed on Venus, and
though it is not easy to harmonise the various accounts, there seems no
doubt of the reality of the facts, or that they must be ascribed to the
existence of mountains. Schröter found very much the same state of
things to exist on Venus that he found on Mercury, and putting together
what he saw he arrived at the conclusion that Venus possesses mountains
of considerable height, and that his observations must be taken to imply
that the planet revolved on its axis in rather more than 23 hours. This
conclusion as regards the planet’s axial rotation was not first arrived
at by Schröter, for the two Cassinis, one about 1666, and the other
about 1740, both ascribed to Venus a rotation period of about 23 hours,
an evaluation which was fully confirmed by Di Vico at Rome between 1839
and 1841, and by Flammarion in 1894.

What has been already said with respect to Mercury is true also of
Venus, namely that it has been much neglected by modern observers; and
accordingly an announcement made by Schiaparelli in 1890, that the
rotation period of Venus is to be measured not by hours but by months,
came upon the astronomical world as a startling revelation; but it is a
revelation which has been keenly contested, and certainly awaits legal
proof. Schiaparelli has not ventured to assert as he has done in the
case of Mercury, that Venus’s rotation period is identical with the
period of 7½ months in which it revolves round the Sun; he only claims
this as a strong probability arising out of what he says he is certain
of, namely that its period of rotation cannot be less than six months
and may be as much as nine months. His assumption is that previous
observers in endeavouring to ascertain Venus’s rotation period have used
and relied upon evanescent shadings which probably were of atmospheric
origin and scarcely recognisable from day to day, whereas he fixed his
attention upon round defined white spots, which, whatever their origin,
are so far permanent that their existence has been spoken of for two
centuries. Miss Clarke thus puts the matter:—“His steady watch over them
showed the invariability of their position with regard to the
terminator; and this is as much as to say that the regions of day and
night do not shift on the surface of the planet. In other words she
keeps the same face always turned towards the Sun.”

Various recent observations, some of them made with the express object
of throwing light upon Schiaparelli’s conclusions, are strangely
contradictory. Perrotin at Nice in 1890 thought his observations
confirmed Schiaparelli’s; on the other hand Niesten at Brussels
considered that numerous drawings of Venus made by himself and Stuyvaert
between 1881 and 1890 harmonised well with Di Vico’s rotation period of
23h. 21m. 22s.; which Trouvelot in 1892 only wished to increase to about
24 hours.

There is a general consensus of opinion that great irregularities exist
on the surface of Venus. These are made specially manifest to us in
connection with the terminator or visible edge of the planet seen as an
illuminated crescent. If the planet had a smooth surface this line would
at all times be a perfect and continuous curve, instead of which it is
frequently to be noticed as a jagged or broken line. Observations to
this effect go back as far as 1643, when Fontana at Naples observed this
to be the condition of the terminator. La Hire, Schröter, Mädler, Di
Vico and many others down to the present epoch have noted the same
thing. The fact that the southern horn of Venus is constantly to be seen
blunted is so well established as to admit of no doubt, and this
blunting is commonly ascribed to the existence of a lofty mountain, to
which Schröter ascribed a height of 27 miles. Whatever we may think as
to the precise accuracy of this figure, it seems impossible to doubt the
main fact on which it depends; whilst a Belgian observer, Van Ertborn,
in 1876 repeatedly saw a point of light in this locality which he
regarded as due to Sun-light impinging on a detached peak, adjacent
valleys remaining in shadow. This effect is common enough in the case of
the Moon, and is familiar to all who are in the habit of studying the
Moon.

[Illustration: Fig. 9.—Venus, Dec. 23, 1885.]

The existence on Venus of an atmosphere of considerable density and
extent is well established. Proof of this is to be found in the marked
diminution of the planet’s brilliancy towards the terminator; and in the
faint curved line of light which occasionally may be seen when the
planet is near inferior conjunction. When so situated, so much of the
planet itself as can be seen illuminated shows as a narrow radiant
crescent of light, ending off in two points called indifferently cusps
or horns. It sometimes happens, however, that from the point of each
cusp there runs round to the other cusp a faint continuation of the
crescent, resulting in the general appearance of the planet being that
of a nearly uniform ring of light. There is no known way in which the
Sun can illuminate so much more than the half of Venus so as to permit
of a perfect circle being visible except by supposing that an atmosphere
exists on the planet and refracts (or transmits by bending, as it were,
round the corner) a sufficient amount of Sun-light to give rise to the
appearance in question. Further proof of the existence of an atmosphere
on Venus is obtainable on those very rare occasions when the planet is
seen passing across the disc of the Sun—a phenomenon known as a “Transit
of Venus.” It then nearly always happens that a hazy nebulous ring of
feeble light may be detected encompassing the planet’s disc indicative
of course of the fact that the Sun’s rays are there slightly obstructed
in reaching the eye of an observer on the Earth. Some observers
scrutinising Venus when in transit have thought that they were able to
obtain, by means of the spectroscope, traces of aqueous vapour on the
planet, but the evidence of this does not appear to be altogether clear
or conclusive.

[Illustration: Fig. 10.—Venus near conjunction as a thin crescent, Sept.
21, 1887 (Flammarion).]

Everybody may be presumed to be acquainted with the spectacle popularly
known as “The Old Moon in the New Moon’s Arms” whereby when the Moon is
only about two or three days old and exhibits but a narrow crescent of
bright light, yet the whole outline of the disc is traceable on the sky.
A phenomenon analogous to this may often be seen in the case of Venus
when near its inferior conjunction. With the Moon the cause is due to
the reflection of Earth-light (so to speak) to the Moon, but that
explanation seems inadequate in respect of Venus, because it is
conceived that the amount of Earth-light available is altogether
insufficient for the purpose. Many other explanations have been put
forward including phosphorescence on the surface of Venus, electrical
displays in the nature of terrestrial auroræ, and what not, but it must
be frankly confessed that astronomers are all at sea on the subject.

The existence of snow at the poles of Venus has been suspected by
observers of tried skill and experience such as Phillips and Webb,
though the idea was first broached by Gruithuisen in 1813. Flammarion’s
observations during 1892 and the two following years are distinctly
confirmatory of this idea. He adds that as both polar caps are visible
at the same time the planet’s axis cannot be much inclined to the plane
of its orbit.

Compared with all the other planets the absolute brightness of Venus
stands very high. Of course it must be understood that by this phrase
“absolute brightness” no more is meant than its reflective power. Venus
is what it is by virtue of its power of reflecting Sun-light; presumably
it has no inherent brightness of its own. What its reflective power is
was probably never more effectively brought under the notice of a human
eye than on September 26, 1878, when Nasmyth enjoyed an opportunity of
seeing Venus and Mercury side by side for several hours in the same
field of view. He speaks of Venus as resembling clean silver and Mercury
as nothing better than lead or zinc. Seeing that owing to its greater
proximity to the Sun the light incident on Mercury must be some 3½ times
as strong as the light incident on Venus, it follows that the reflective
power of Venus must be very great. As a matter of fact it has been
calculated to be nearly equal to newly fallen snow; in other words to
reflect fully 70 per cent. of the light which impinges on it.

Venus has no satellite; this fact seems certain. Yet half a dozen or
more observers between 1645 and 1768 discovered such a satellite;
observed it; followed it! This startling mystery, as it really was,
attracted some years ago the attention of a very careful Belgian
observer, Stroobant, who examined in a most painstaking manner all the
recorded observations. His conclusions were that in almost all cases
particular stars (which he identified) were mistaken for a satellite.
Where the object seen was not capable of identification, possibly it was
a minor planet; whilst in one instance it was probable that it was
Uranus which had been seen and regarded as a satellite of Venus.

Venus is perhaps the planet which has most impressed the popular mind.
For the earliest illustration of this statement we must go as far back
as Homer who makes two references to it in the _Iliad_. These, in Pope’s
version, run as follows:—

  “As radiant Hesper shines with keener light,
  Far beaming o’er the silver host of night.”
                                                       —xxii. 399 [318].

  “The morning planet told th’ approach of light;
  And fast behind, Aurora’s warmer ray
  O’er the broad ocean pour’d the golden day.”
                                                      —xxiii. 281 [226].

The phases of Venus were first discovered by Galileo and were made known
to the world, or rather to Kepler, in a mystic sentence which has often
been quoted:—

  “_Hæc immatura, a me jam frustra leguntur—oy._”

  “These things not ripe; at present [read] in vain [by others] are read
              by me.”

The former sentence transposed becomes—

  _Cynthiæ figuras æmulatur mater amorum._

  The mother of loves [Venus] imitates the phases of Cynthia [the Moon].

Venus revolves round the Sun in 224½ days at a mean distance of about 67
millions of miles. Its apparent diameter varies between 9½″ in superior
conjunction, and 62″ in inferior conjunction. The real diameter is about
7500 miles; in other words Venus is nearly as large as the Earth.




                               CHAPTER V.
                               THE EARTH.


To us, as its inhabitants, the Earth appeals in two characters, and in
writing a book on astronomy it is necessary, yet difficult, to keep
these two characters separate. The Earth is an ordinary planet member of
the solar system, amenable to the same laws, impelled by the same
forces, and going through the same movements as the other members of the
Sun’s _entourage_. Yet, by reason of the fact that we are ourselves on
the Earth and are not spectators of it looking at it from at a distance,
there are many phenomena coming under our notice which require special
treatment, and it is often very difficult to say where the province of
the astronomer ends and that of the geographer begins. This volume being
specially designed to deal with astronomical matters, I shall pass over
many subjects which may be said to be on the border line, and which some
of my readers may therefore be disappointed not to find discussed.
Besides the geographer, the geologist and his scientific brother the
mineralogist are concerned with the Earth regarded as a planet moving
through space as the other planets do. The geologist studies the actual
structure of the Earth, its circumstances and history so far as they
have been revealed to us, whilst the mineralogist investigates and names
the materials of which it is composed, and classifies such materials
with the assistance of the geologist on the one hand and of the chemist
on the other. All these subordinate sciences—subordinate I mean from an
astronomer’s point of view—open up very varied, instructive, and
interesting fields of study, but they are of course foreign to the
purpose of the present volume.

Though the Earth is commonly regarded as a sphere it is not that in
reality, because it is not of identical dimensions from east to west and
from north to south. It is somewhat flattened at the poles; its polar
diameter is less than its equatorial diameter, in the ratio of about 298
to 299, or, expressed in miles, its polar diameter is about 26 miles
less than its equatorial diameter. If a globe 3 feet in diameter be
taken to represent the Earth, then the polar diameter will, on this
scale, be ⅛ inch too long. This flattening of the poles of the Earth
finds its counterpart, so far as we know, in most, and probably in all
of the planets. It is most considerable and therefore most conspicuous
in the case of Jupiter. It ought here to be added that a suspicion
exists that the equatorial section of the Earth is not a perfect circle,
but that the diameter of the Earth, taken through the points on the
equator marked by the meridians 13° 58′ and 193° 58′ east of Greenwich,
is one mile longer than the diameter at right angles to these two
points.

The science which inquires into matters of this kind, including besides
the figure of the Earth, the length of the degree at different
latitudes, and the distances of places from one another, alike in
angular measure and in time, is called Geodesy; it is, in point of fact,
land-surveying on a very large scale, in which instruments and processes
of astronomical origin are brought into operation, and in which
astronomers are more or less required to take the lead.

Although we all of us now perfectly understand that the Earth is a
planet moving round the Sun as a centre, it is, comparatively speaking,
but recently that this fact has become generally recognised and
understood. It is true that we can discover here and there in ancient
writings some trace of the idea, yet it is doubtful whether 2000 years
ago more than a few “advanced” thinkers thoroughly and clearly accepted
it as a distinct truth. It was much more in consonance with popular
thought and the actual appearance of things that the Earth should be the
centre round which the Sun revolved and on which the planets depended;
and accordingly, sometimes in one shape and sometimes in another, the
notion of the Earth being the centre of the universe was generally
accepted. The contrary opinion had, however, a few sympathisers. For
instance, Aristarchus of Samos, who lived in the third century before
the Christian era, supposed, if we may trust the testimony of Archimedes
and Plutarch, that the Earth revolved round the Sun; this, however, was
regarded as a “heresy,” in respect of which he was accused of “impiety.”
Some few years elapsed and a certain Cleanthes of Assos is said by
Plutarch to have suggested that the great phenomena of the universe
might be explained by assuming that the Earth was endued with a motion
of translation round the Sun together with one of rotation on its own
axis. The historian states that this idea was so contrary to the
received opinions that it was proposed to put Cleanthes on his trial for
impiety.

In former times the philosophers who studied the solar system ranged
themselves in several “schools of thought,” to use a modern hackneyed
phrase. Some upheld the Ptolemaic system, which took its name from a
great Egyptian astronomer, Claudius Ptolemy, though it does not appear
that he was actually the first to suggest it. The Ptolemaic system
regarded the Earth as the centre, with the following bodies, all called
planets, revolving round it in the order stated:—the Moon, Mercury,
Venus, the Sun, Mars, Jupiter, and Saturn. It will be observed that
there are seven bodies here named, and as seven was regarded as the
“number of perfection,” it was in later times considered that only these
seven bodies (neither more nor less) could really be the Earth’s
celestial attendants. Though Ptolemy was in one sense an Egyptian, there
yet prevailed amongst the Egyptians at large another theory slightly
different from Ptolemy’s. According to the “Egyptian theory,” Mercury
and Venus were regarded as satellites of the Sun, and not as primary
planets appurtenant to the Earth.

After Ptolemy’s era many centuries elapsed, during which the whole
subject of the solar system lay practically dormant, and it continued so
until the revival of learning brought new theorists upon the scene. The
most important of these was Copernicus, who, in the sixteenth century,
propounded a theory which eventually superseded all others, and, with
slight modifications, is the one now accepted. Copernicus placed the Sun
in the centre of the system, and treated it as the point around which
all the primary planets revolved. So far, so good; but Copernicus went
astray on the question of the orbits of the planets. He failed to
realise the true character of the curves which they follow and treated
these curves as “epicycles,” which word may be described as representing
a complicated combination of little circles which taken together form a
big one. It was left to Kepler and Newton to settle all such details on
a true and firm basis. But before this stage was reached a man of the
highest astronomical attainments and practical experience, Tycho Brahe,
made shipwreck of his reputation as an astronomer by solemnly reviving
the idea of the Earth being the immovable centre of everything. He
treated the Moon as revolving round the Earth at no great distance and
the Sun as doing the same thing a little farther off; the five planets
revolving round the sun as solar satellites. The “Tychonic system,” as
it is called, has something in common with the Ptolemaic system without
being by any means as logical as the latter. That such far-fetched ideas
as Tycho’s should have been palmed off on the world of science so
recently as 300 years ago is passing strange; but the explanation
appears to be that his action arose out of a misconception of certain
passages of Holy Scripture, which seemed irreconcilable with the
Copernican theory. It must not be forgotten that Copernicus’s famous
book, published in 1543, in which he had announced his views, had been
condemned by the Papal “Congregation of the Index;” and therefore Tycho
might have had as a further motive a desire to curry favour with the
authorities of the Church of Rome, and to gratify his own vanity at the
same time.

With these explanations it will no longer be misleading if, for
convenience sake, I speak of a certain great circle of the heavens as
apparently traversed by the Sun every year, owing to the revolution of
our Earth round that body. This circle is called the “Ecliptic,” and its
plane is usually employed by astronomers as a fixed plane of reference.
It must be distinguished from that other great circle called the
“celestial equator,” which is the plane of the Earth’s equator extended
towards the stars. The plane of the equator is inclined to the ecliptic
at an angle of about 23½°, which angle is known as the “obliquity of the
ecliptic.” It is this inclination which gives rise to the seasons which
follow one another in succession during our annual journey round the
Sun. The two points where the celestial equator and the ecliptic
intersect are called the “equinoxes,” of spring or autumn as the case
may be; the points midway between these being the “solstices,” of summer
or winter as the case may be. These words need but little explanation,
at any rate, as regards those persons who are able to trace the Latin
origin of the words. “Equinox” is simply the place occupied by the Sun
twice every year (namely about March 20 and September 22), when day and
night are theoretically equal throughout the world, when also the sun
rises exactly in the east and sets exactly in the west. The “solstices”
represent the standing still of the sun at the given times and places,
and are the neutral points where the Sun attains its greatest northern
or southern declination. This usually occurs about June 21 and December
21. It must not be forgotten by the way, that the above application of
the words “summer” and “winter” to the solstices is only correct so far
as concerns places in northern terrestrial latitudes—Europe and the
United States, for instance. In southern terrestrial latitudes—for
instance, when speaking of what happens at the Cape of Good Hope and in
Australia—the words must be reversed.

We have seen in a previous chapter that whilst the orbits of the planets
are nearly true circles, none of them are quite such: and the departure
from the truly circular form results in some important consequences.
Whilst some of these are too technical to be explained in detail here,
one at least must be referred to because of what it involves. Not only
is the Earth’s orbit eccentric in form, but its eccentricity varies
within narrow limits; and besides this the orbit itself, as a whole, is
subject to a periodical shift of place, from the joint effect of all
which changes it comes about that our seasons are now of unequal length,
the spring and summer quarters of the year unitedly extending to 186
days, whilst the autumn and winter quarters comprise only 178 days. The
sun therefore has the chance of shining for a longer absolute period of
time over the northern hemisphere than over the southern hemisphere;
hence the northern is the warmer of the two hemispheres, because it has
a better, because a longer, chance of storing up an accumulation of
solar radiant heat. Probably it is one result of this that the north
polar regions of the Earth are easier of access than the south polar
regions. In the northern hemisphere navigators have reached to 81° of
latitude, whereas 71° is the highest limit yet attained in the southern
hemisphere. Readers who have studied the history of explorations in the
Arctic regions will not need to be reminded of the controversy which has
so often arisen respecting the existence or nonexistence of an “Open
Polar Sea.”

It has already been hinted that it is not an easy matter to determine,
when dealing with the Earth, where astronomy and its allied sciences,
geography, geodesy and geology respectively, begin and end. But as
certain topics connected with these sciences, such as the rotundity of
the Earth and its rotation on its axis, will come more conveniently
under consideration in other volumes of this series, I shall pass them
over and only treat of a few things which more directly concern the
student of nature observing either with or without the assistance of a
telescope.

The fact that the Earth is surrounded by a considerable atmosphere
largely composed of aqueous vapour has a material bearing on the success
or failure of observations made on the Earth of bodies situated at a
distance. It may be taken as a general rule that the nearer an observer
is to the surface of the sea, or otherwise to the surface of the land at
the sea-level, the greater will be the difficulty which will confront
him in carrying on astronomical observations. Hence such observations
are generally made with unsatisfactory results on the sea coast or on
the banks of rivers. An interesting but rather ancient illustration of
this last-named fact is to be found in the circumstance that Copernicus,
who died at the age of 70, complained in his last moments that much as
he had tried he had never succeeded in detecting the planet Mercury, a
failure due, as Gassendi supposed, to the vapours prevailing near the
horizon at the town of Thorn on the banks of the Vistula where the
illustrious philosopher lived.

The phenomena depending on the presence of aqueous vapour in the
atmosphere which especially under the notice of the astronomer are
Refraction, Twilight, and the Twinkling of the Stars.

Refraction is what it professes to be, a bending, and what is bent is
the ray of light coming from a celestial object to a terrestrial
station. Olmsted has put the matter in this way:—“We must consider that
any such object always appears in the direction in which the _last_ ray
of light comes to the eye. If the light which comes from a star were
bent into fifty directions before it reached the eye, the star would
nevertheless appear in a line described by the ray nearest the eye. The
operation of this principle is seen when an oar, or any stick, is thrust
into the water. As the rays of light by which the oar is seen have their
direction changed as they pass out of water into air, the apparent
direction in which the body is seen is changed in the same degree,
giving it a bent appearance—the part below the water having apparently a
different direction from the part above.” The direction of this
refraction is determined by the general law of optics that when a ray of
light passes out of a rarer into a denser medium (for instance out of
air into water, or out of space into the Earth’s atmosphere) it is bent
_towards_ a perpendicular to the surface of the medium; but when it
passes out of a denser into a rarer medium it is bent _from_ the
perpendicular. The effect of refraction is to make a heavenly body
appear to have an apparent altitude greater than its true altitude, so
that, for example, an object situated actually _in_ the horizon will
appear _above_ it. Indeed it sometimes happens that objects which are
actually below the horizon and which otherwise would be invisible were
it not for refraction are thus brought into sight. It was in consequence
of this that on April 20, 1837, the Moon rose eclipsed before the Sun
had set.

Sir Henry Holland thus alludes to the phenomenon:—“I am tempted to
notice a spectacle, having a certain association with this science,
which I do not remember to have seen recorded either in prose or poetry,
though well meriting description in either way. This spectacle requires,
however, a combination of circumstances rarely occurring—a perfectly
clear Eastern and Western horizon, and an entirely level intervening
surface, such as that of the sea or the African desert—the former
rendering the illusion, if such it may be called, most complete to the
eye. The view I seek to describe embraces the orb of the setting Sun,
and that of the full Moon rising in the East—_both above the horizon at
the same time_. The spectator on the sea between, if he can discard from
mental vision the vessel on which he stands, and regard only these two
great globes of Heaven and the sea-horizon circling unbroken around him,
gains a conception through this spectacle clearer than any other
conjunction can give, of those wonderful relations which it is the
triumph of astronomy to disclose. All objects are excluded save the Sun,
the Moon, and our own Globe between, but these objects are such in
themselves that their very simplicity and paucity of number enhances the
sense of the sublime. Only twice or thrice, however, have I witnessed
the sight in its completeness—once on a Mediterranean voyage between
Minorca and Sardinia—once in crossing the desert from Suez to Cairo,
when the same full Moon showed me, a few hours later, the very different
but picturesque sight of one of the annual caravans of Mecca pilgrims,
with a long train of camels making their night march towards the Red
Sea.”[3]

It is due to the same cause that the Sun and the Moon when very near the
horizon may often be noticed to exhibit a distorted oval outline. The
fact simply is, that the upper and the lower limbs undergo a different
degree of refraction. The lower limb being nearer the horizon is more
affected and is consequently raised to a greater extent than the upper
limb, the resulting effect being that the two limbs are seemingly
squeezed closer together by the difference of the two refractions. The
vertical diameter is compressed and the circular outline becomes thereby
an oval outline with the lesser axis vertical and the greater axis
horizontal.

Though the foregoing information merely embraces a few general
principles and facts, the reader will have no difficulty in
understanding that refraction exercises a very inconvenient disturbing
influence on observations which relate to the exact places of celestial
objects. No such observations are available for mutual comparison,
however great the skill of the observer, or the perfection of his
instrument, unless, and until certain corrections are applied to the
observed positions in order to neutralise the disturbing effects of
refraction. In practice this is usually done by means of tables of
corrections, those in most general use being Bessel’s. Inasmuch as
refraction depends upon the aqueous vapour in the atmosphere, its amount
at any given moment is affected by the height of the barometer and the
temperature of the air. Accordingly when, for any purpose, the utmost
precision is required, it is necessary to take into account the height
of the barometer and the position of the mercury in the thermometer at
the moment in question. At the zenith there is no refraction whatever,
objects appearing projected on the background of the sky exactly in the
position they would occupy were the earth altogether destitute of an
atmosphere at all. The amount of the refraction increases gradually, but
in accordance with a very complex law, from the zenith to the horizon.
Thus the displacement due to refraction which at the zenith is nothing
and at an altitude of 45° is only 57″ becomes at the horizon more than
½°. One very curious consequence is involved in the fact that the
displacement due to refraction is at the horizon what it is; the
diameter both of the Sun and Moon may be said to be ½°, more or less, so
that when we see the lower edge of either of these luminaries just
_touching_ the horizon _in reality_ the whole disc is completely _below_
it, and would be altogether hidden by the convexity of the earth were it
not for the existence of the earth’s atmosphere and the consequent
refraction of the rays of light passing through it from the Sun (or
Moon) to the observer.

Twilight is another phenomenon associated with astronomical principles
and effects which depends in some degree on the Earth’s atmosphere and
on the laws which regulate the reflection and refraction of light. After
the Sun has set it continues to illuminate the clouds and upper strata
of the air just as it may often be seen shining on the tops of hills
long after it has disappeared from the view of the inhabitants of the
plains below, and indeed may illuminate the chimneys of a house when it
is no longer visible to a person standing in the garden below. The air
and clouds thus illuminated reflect some of the Sun’s light to the
surface of the earth lying immediately underneath, and thus produce
after sun-set and before sun-rise, in a degree more or less considerable
according as the Sun is only a little or is much depressed below the
horizon, that luminous glow which we call “twilight.” This word is of
Saxon origin and implies the presence of a twin, or double, light. As
soon as the Sun has disappeared below the horizon all the clouds
overhead continue for a few minutes so highly illuminated as to reflect
scarcely less light than the direct light of the Sun. As, however, the
Sun gradually sinks lower and lower, less and less of the visible
atmosphere receives any portion of its light, and consequently less and
less is reflected minute by minute to the Earth at the observer’s
station until at length the time comes when there is no sunlight to be
reflected—and it is night. The converse of all this happens before and
up to sun-rise; night ceases, twilight ensues, gradually becoming more
definite; the dawn appears, and finally the full Sun bursts forth. It
may here be stated as a note by the way that the circumstances under
which the Sun first shows itself after it has risen above the horizon
has some bearing on the probable character of the weather which is at
hand. When the first indications of day-light are seen above a bank of
clouds it is thought to be a sign of wind; but if the first streaks of
light are discovered low down, that is in, or very near the horizon,
fair weather may be expected.

Twilight is usually reckoned to last until the Sun has sunk 18° below
the horizon, but the question of its duration depends on where the
observer is stationed, on the season of the year, and (in a slight
degree) on the condition of the atmosphere. The general rule is that the
twilight is least in the tropics and increases as the observer moves
away from the equator towards either pole. Whilst in the tropics a
depression of 16° or 17° is sufficient to put an end to the phenomenon,
in the latitude of England a depression of from 17° to 20° is required.
As implied above, it varies with the latitude; and as regards the
different seasons of the year, it is least on March 1 and October 12,
being three weeks before the vernal equinox and three weeks after the
autumnal equinox. The duration at the equator may be about 1 hour 12
minutes; it amounts to nearly 2 hours at the latitude of Greenwich, and
so on towards the pole. At each pole in turn the Sun is below the
horizon for 6 months, but as it is less than 18° below the horizon for
about 3½ of those 6 months it may be said that there is a continual
twilight for those 3½ months. Something of the same sort of thing as
this occurs in the latitude of Greenwich, for there is no true night at
Greenwich from May 22 to July 21, but constant twilight from sunset to
sunrise, or 2 months of twilight in all. Though twilight at the equator
is commonly set down as lasting about an hour, this period is there, as
elsewhere, affected by the elevation of the observer above the
sea-level. Where the air is very rarified, as at places situated as
Quito and Lima are, the twilight is said to last no more than 20
minutes, and this would accord with the theory that where there is no
air at all (_e.g._, on the Moon) there is no twilight at all. The
greater purity and clearness of mountain air, rarified as it is, is
another cause which contributes to vary by reducing the duration of
twilight.

It is sometimes stated that a secondary twilight may be noticed, and Sir
John Herschel has spoken of it as “consequent on a re-reflection of the
rays dispersed through the atmosphere in the primary one. The phenomenon
seen in the clear atmosphere of the Nubian Desert, described by
travellers under the name of the ‘afterglow,’ would seem to arise from
this cause.” I am not acquainted with any records which throw light on
these remarks of Sir John Herschel.

The phenomenon of twinkling is a subject which has been much neglected,
possibly on account of its apparent, but only apparent, simplicity. The
familiar verse of our days of childhood—

  “Twinkle, twinkle little star,
  How I wonder what you are,
  Up above the earth so high,
  Like a diamond in the sky,”

contains even in this simple form a good deal of food for reflection;
whilst the new version—

  “Twinkle, twinkle little star,
  Now we’ve found out what you are,
  When unto the midnight sky
  We the spectroscope apply,”

does so yet more.

As an optical phenomenon the twinkling, or to use the more scientific
phrase, the scintillation, of the stars is a matter which has been
strangely ignored by physicists. Indeed, the only investigators who seem
to have dealt with it in any sort of detail are two Italians, Secchi and
Respighi, Dufour, a Frenchman, Montigny, a Belgian, and the Rev. E.
Ledger, an Englishman. Secchi has truly remarked that the twinkling of
the stars is one of the most beautiful of the minor phenomena of the
heavens. Light, sometimes bright, sometimes feeble, sometimes white,
sometimes red, darts about in intermittent gleams, like the sparkling
flashes of a well-cut diamond, and works upon the feelings of even the
most stolid spectator. The theory of twinkling is still surrounded by
many difficulties. One thing, however, is certain—it has nothing to do
with recurrent changes in the intrinsic light or physical condition of
the star itself, but arises during the passage of its rays through our
atmosphere; it depends, therefore, in some way or other on the varying
conditions of the atmosphere. On the summit of high mountains, according
to the observations of all careful observers (notably Tacchini, who
studied the subject on Mount Etna), the light of the stars is steady,
like that of the planets; and it is so likewise during the hours of calm
which often precede terrestrial storms. The vibrations are usually more
frequent near the horizon, and diminish with the elevation of the star
above the horizon; in other words, with the lessening of the thickness
of the atmospheric strata which the rays of light have to traverse.
Nevertheless, during windy weather, and specially with northerly wind,
it may be noticed that the stars twinkle high up above the horizon, and
even as far as the zenith. From these and other similar considerations
we are justified in drawing the conclusion that twinkling largely
depends on the condition and movements of the atmosphere.

Secchi further points out that it is impossible to study carefully with
the naked eye all the features of twinkling, and that telescopic
assistance is imperatively necessary. When, with the aid of a telescope,
we scrutinise a star during a disturbed evening marked by much twinkling
we see an image diffused and undefined and surrounded by rays, as if
several images were superposed, and were jumping about rapidly. On such
occasions we do not see that little defined disc surrounded by
motionless diffraction rings, ordinarily indicative of a tranquil
atmosphere. With a telescope armed with a medium power, the field of
view of which is more extensive than that of a high power, we find that
if a light tap is given to the telescope, the ordinary simple image is
changed into a luminous curve, the perimeter of which is formed entirely
of a succession of arcs exhibiting the colours of the rainbow. This
coloured curve does not, in principle, differ from what one sees on
swinging round and round in the air such a thing as a stick, the end of
which is alight, having been freshly taken from a fire. The glowing tip
produces in appearance a continuous arc, the result of the persistence
of the image of the tip on the retina. In such a case the colour is
constant, because the illumination resulting from the blazing wood does
not vary; but in the case of a star the arcs are differently coloured
during the very brief space of time in which the vibrating telescope
transports the image from one side to another of the visible field. This
experiment is from its nature very crude, but the idea was improved upon
and reduced to a systematic shape by Montigny, who introduced into his
telescope, at a certain distance from the eyepiece, a concave lens
eccentrically placed with respect to the axis of the instrument, and
endued with a rapid movement of rotation imparted by suitable mechanism.
He thus obtained images which revolved with regularity, and so was able
to submit certain features of the phenomenon to a definite system of
measurement. To cut a long story short, Montigny started with the
assumption (made good by the sequel) that possibly stars were affected
in their twinkling by intrinsic constitutional differences; and that
possibly Secchi’s classification of stars into four types (a
classification which depends on the spectra which they yield) might put
him on the track of some intelligible conclusions with respect to the
theory of twinkling.[4]

The results he ultimately arrived at were, that the yellow and red stars
of the IInd and IIIrd types twinkle less rapidly than the white stars of
the Ist type. Whilst the average number of scintillations per second of
the stars of type III. were 56, those of type II. were 69, and those of
type I. 86. These differences may be confidently said to depend upon too
many observations of too many different stars to be fortuitous. Montigny
also arrived at a number of incidental conclusions of considerable
interest. The one main thread running through them, is that there is a
connection between the twinkling of a star and its spectrum, which had
never before been thought of. We are justified, indeed, in going so far
as to say, that Montigny’s observations point distinctly to a law on
this subject, the law being that the more the spectrum of a star is
interrupted by dark lines, the less frequent are its scintillations. The
individual character of the light, therefore, emitted by any given star
appears to affect its twinkling, both as regards the frequency thereof
and the colours displayed.

Montigny collected some other interesting facts with reference to
twinkling, which may here be stated in a concise form. There is a
greater display of twinkling in showery weather, than when the
atmosphere is in a normal condition; and in winter than in summer,
whatever may be the weather. In dry weather in Spring and Autumn the
twinkling is about the same, but wet has more effect in Autumn than in
Spring in developing the phenomenon. Variations in the barometric
pressure and in the humidity of the air also affect the amount of
twinkling; there is more before a rainy period, likely to last 2 or 3
days, than before a single, or, so to speak, casual rainy day. Twinkling
also varies with the aggregate total rain-fall of any group of days,
being more pronounced as the rain-fall is greater, but decreasing
suddenly and considerably as soon as the rainy condition of the
atmosphere has passed away. The number of scintillations found to be
observable with the aid of Montigny’s instrument (which he called a
“_scintillomètre_”), varied from a minimum of 50 during June and July,
to 97 in January, and 101 in February, increasing and decreasing in
regular sequence from month to month. When an Aurora Borealis is
visible, there is a marked increase in the amount of twinkling. It would
be interesting to follow up this last named discovery by an endeavour to
ascertain whether the fluctuations which are coincident in point of time
with an Auroral display depend upon optical considerations connected
with the Aurora, or on physical considerations having any relation to
the increased development of terrestrial magnetism.

I have been thus particular in unfolding somewhat fully the present
state of our knowledge concerning the twinkling of the stars, because it
is evident that there are many interesting points connected with it,
which may be studied by any patient and attentive star-gazer, and which
do not need the instrumental appliances and technical refinements which
are only to be found in fully-equipped public and private observatories.

It should be mentioned in conclusion that the planets twinkle very
little, or, more often, not at all. This is mainly due to the fact that
they exhibit discs of sensible diameter and therefore that there is, as
Young puts it, “a general unchanging average of brightness for the sum
total of all the luminous points of which the disc is composed. When,
for instance, point A of the disc becomes dark for a moment, point B,
very near to it, is just as likely to become bright; the interference
conditions being different for the 2 points. The different points of the
disc _do not keep step_, so to speak, in their twinkling.” The
non-twinkling of planets because they possess sensible discs is often
available as a means for determining when a planet is looked for, which,
of several objects looked at, is the planet wanted and which are merely
stars.




                              CHAPTER VI.
                               THE MOON.


The Moon being merely the satellite of a planet, to wit, the Earth, it
should, according to the plan of this book, be included in the chapter
which deals with its primary; but for us inhabitants of the Earth the
Moon has so many special features of interest that it will be better to
give it a special chapter to itself.

We may regard the Moon in a twofold aspect, and consider what it is as a
mere object to look at, and what it does for us; probably my present
readers will prefer that most prominence shall be given to the former
aspect. The Moon as seen with the naked eye exhibits a silvery mass of
light, which at the epoch of what is called “full Moon” has a seemingly
even circular outline. Full or not full, its surface appears to be
irregularly shaded or mottled. The immediate cause of this shading is
the fact that the surface of the Moon, not being really smooth, reflects
irregularly the Sun’s light which falls upon it. The _causa causans_ of
this is the existence of numerous mountains and valleys on its surface,
and which were first discovered to be such by Galileo. That there are
mountains is proved by the shadows cast by their peaks on the
surrounding plains, when the Sun illuminates the Moon obliquely—that is,
when the Moon is shining either as a crescent or gibbous. Such shadows,
however, disappear at the phase of “Full-Moon,” because the Sun’s rays
then fall perpendicularly on the Moon’s surface. When the Moon presents
either a crescent or a gibbous form (in point of fact when it presents
any form except that of “Full-Moon”), the boundary line which separates
the illuminated from the unilluminated portion (and which boundary line
is generally spoken of as the “terminator”) has a rough, jagged
appearance; this is due to the fact that the Sun’s light falls first on
the summits of the peaks, and that the adjacent valleys and declivities
are in shade. These remain so till by reason of the Moon’s progress in
its orbit a sufficient time has elapsed for the Sun to penetrate to the
bottom of the valleys. With this explanation the reader will have no
difficulty in realising why the terminator always exhibits an irregular
or jagged edge.

[Illustration: Fig. 11.—Mare Crisium. (Lick Observatory photographs.)]

Various mountains on the Moon to the number of more than a thousand have
been mapped, and their elevations calculated. Of these fully half have
received names, being those of men of various dates and nationalities,
who have figured conspicuously in the annals of science, including some,
however, who have not done so. Whilst many of these mountains are
isolated elevations, not a few form definite chains of mountains, and to
certain of these chains definite names, borrowed from the Earth, have
been given. Thus we find on maps of the Moon the “Apennines,” the
“Alps,” the “Altai Mountains,” the “Dörfel Mountains,” the “Caucasus
Mountains,” and so on.

Besides the mountains there exist on the Moon a number of plains
analogous in some sense to the “steppes” of Asia and the “prairies” of
North America. These were termed “seas” in the early days of the
telescope, because it was assumed that as they were so large and so
smooth they were vast tracts of water. This supposition has long ago
been overthrown, but the names have been retained as a matter of
convenience. Hence it comes about that in descriptions of the Moon one
meets with such names as _Mare Imbrium_, the “Sea of Showers”; _Mare
Serenitatis_, the “Sea of Serenity”; _Mare Tranquillitatis_, the “Sea of
Tranquillity”; and so on. It seems probable that the so-called seas
represent in nearly its original form what was once the original surface
of the Moon before the mountains were formed. A confirmation of this
idea is to be found in the fact that though these plains are fairly
level surfaces compared with the masses of mountains which hedge them in
on all sides, yet the plains themselves are dotted over with
inequalities (small elevations and pits), which seem to suggest that
some of them might eventually have developed into mountains if the
further formation of mountains had not been arrested by the fiat of the
Creator.

Though hitherto we have been speaking of the mountains of the Moon under
that generic title, it is necessary for the reader to understand that
the Moon’s surface exhibits everywhere remarkable illustrations of those
geological processes which we on the earth associate with the word
“volcano.” There cannot be the least doubt that the existing surface of
the Moon, as we see it, owes all its striking features to volcanic
action, differing little from the volcanic action to which we are
accustomed on the earth. That this theory is well founded may be very
easily inferred by comparing the structural details of certain
terrestrial volcanoes and their surroundings with a typical lunar
mountain, or indeed, I might say, with any lunar mountain. This point
was very well worked out some 40 years ago by Professor Piazzi Smyth,
who placed on pictorial record his results of an examination and survey
of the Peak of Teneriffe. Any person seeing side by side one of Smyth’s
pictures of Teneriffe and a picture of any average lunar crater would
find great difficulty if the pictures were not labelled in determining
which was which.

The one special feature of the Moon, which never fails to attract the
attention of everybody who looks at our satellite for the first time
through a telescope, are the crater mountains, which indeed constitute
an immense majority of all the lunar mountains. Their outline almost
always conforms, more or less, to that of the circle, but when seen near
either limb of the Moon they often appear considerably oval simply
because they are then seen considerably foreshortened. In their normal
form they exhibit a basin bounded by a ridge, with a conical elevation
in the centre of the basin, the basin and the cone together being
evidently the result of an uprush of gases breaking through the outer
crust of the Moon and carrying with them masses of molten lava. This
lava, with perhaps the materials in fragments, projected in the first
instance up into the air, fell back on to the Moon forming first of all
the outer edge of the basin, and subsequently, as the eruptive force
became weakened, the small central accumulation, which took, as it
naturally would do, a conical shape. An experimental imitation of the
process thus inferred was carried out some years ago by a French
physicist, Bergeron, who acted upon a very fusible mixture of metals
known as Wood’s alloy by forcing through it a current of hot air. The
success of this experiment was complete, and Bergeron considered that
his experiments, taken as a whole, were calculated to throw much light
on the past history of the Moon.

Several observers at various times have fancied they have seen signs
that the lunar mountain Aristarchus was an active volcano even up to the
present century; but it admits of no doubt that this idea is altogether
a misconception, and that what they saw as a faint illumination of the
summit of Aristarchus was no more than an effect of earth-shine. On the
general question of volcanic action on the Moon, Sir John Herschel
summed up as follows:—“Decisive marks of volcanic stratification arising
from successive deposits of ejected matter, and evident indications of
lava currents, streaming outwards in all directions, may be clearly
traced with powerful telescopes. In Lord Rosse’s magnificent Reflector
the flat bottom of the crater called Albategnius is seen to be strewed
with blocks not visible in inferior telescopes, while the exterior ridge
of another (Aristillus) is all hatched over with deep gulleys radiating
towards its centre.”

The valleys and clefts or rills visible on the Moon’s surface constitute
another remarkable feature in the topography of our satellite. The
valleys, properly so-called, require no particular comment, because they
are just what their name implies—hollows often many miles long and
several miles wide. The clefts or rills, however, are more mysterious,
by reason of their great length and remarkable narrowness. One is almost
led to infer that they are naught else but cracks in the lunar crust,
the result of sudden cooling, how caused is of course not known.

There is another lunar feature to be mentioned somewhat akin to the
foregoing in appearance but apparently, however, owing its origin to a
different cause. I refer to the systems of bright streaks which,
especially at or near the time of full Moon, are seen to radiate from
several of the largest craters, and in particular from Tycho,
Copernicus, Kepler and Aristarchus. These bright streaks extend in many
cases far beyond what may fairly be considered as the neighbourhood of
the craters from which they start, traversing distant mountains, valleys
and other craters in a way which renders it very difficult to assign an
explanation of their origin.

There are 13 areas on the Moon, which used to be regarded as “seas,” one
of them, however, bearing the name of “_Oceanus Procellarum_,” the
“Ocean of Storms”; but besides these there are several bays, termed in
Latin _Sinus_, of which the most important is the _Sinus Iridum_ or the
“Bay of Rainbows,” a beautiful spot on the northern border of the _Mare
Imbrium_, and best seen when the Moon is between 9 and 10 days old. The
summits of the semi-circular range of rocks which enclose the bay are
then strongly illuminated and a greenish shadow marks the valley at its
base. By the way, it is worth mentioning that not a few of the lunar
seas, so-called, seem to be pervaded by a greenish hue, though no
particular explanation of this fact is forthcoming.

Much controversy has ranged round the question whether or not the Moon
has an atmosphere. Without doubt the preponderance of opinion is on the
negative side, though it must be admitted that some observers of
eminence have suggested that there are indeed traces of an atmosphere to
be had, but that it is extremely attenuated and of no great extent,
otherwise it must render its presence discoverable by optical phenomena
which it is certain cannot be detected.

A brief reference may here be made to a curious phenomenon sometimes
seen in connection with occultations of stars by the Moon. Premising
that an “occultation” is the disappearance of a star behind the solid
body of the Moon by reason of the forward movement of the Moon in her
orbit, it must be stated that though generally the Moon extinguishes the
star’s light instantaneously, yet this does not invariably happen, for
sometimes the star seems to hang upon the Moon’s limb as if reluctant to
disappear. No very clear or satisfactory explanation of this phenomenon
has yet been given; the existence of a lunar atmosphere would be an
explanation, and accordingly this anomalous appearance, seen on
occasions, has been advanced in support of the theory that a lunar
atmosphere does exist; but, nevertheless, astronomers do not accept that
idea.

Any one desirous of carrying out a careful study of the Moon’s surface
must be provided with a good map, and for general purposes none is so
convenient or accessible as Webb’s, reduced from Beer and Mädler’s
_Mappa Selenographica_ published in 1837, of which another reproduction
is given in Lardner’s _Astronomy_. Those, however, who would desire to
study the Moon with the utmost attention to detail must provide
themselves with Schmidt’s map published in 1878 at the expense of the
German Government. When it is stated that this map represents the Moon
on a circle 7½ feet in diameter, the size and amount of detail in it
will be readily understood. Special books on the Moon furnishing
numerous engravings and detailed descriptions have been written by
Carpenter and Nasmyth (jointly) and by Neison.

Various attempts have been made to determine the amount of light
reflected by the Moon, and also the question whether it yields any
measurable amount of heat. As regards the light of the full Moon
compared with that of the Sun, the estimates range from 1/300000 to
1/800000, a discrepancy not perhaps greater than might be expected under
the circumstances of the case.

With respect to the heat possessed by, or radiated from the Moon’s
surface, the conclusions of those who have attempted to deal with the
matter are less consistent. As regards the surface of the Moon itself
Sir John Herschel was of opinion that it is heated at least to the
temperature of boiling water, but that owing to the radiant heat having
to pass through our atmosphere, which acts as an obstacle, it is no
wonder that it should be difficult for us to become conscious of its
existence. In 1846 Melloni, by concentrating the rays of the Moon with a
lens 3 feet in diameter, thought he detected a sensible elevation of
temperature; and in 1856 C. P. Smyth at Teneriffe, but with inferior
instrumental appliances, arrived at the same conclusion. Though
Professor Tyndall in 1861 obtained a contrary result, yet the most
recent experiments by the younger Earl of Rosse, Professor Langley, and
others, all tend to show that the Moon does really radiate a certain
infinitesimally small amount of heat. Perhaps, however, it will be best
to give Langley’s ideas as to this in his own words:—“While we have
found abundant evidence of heat from the Moon, every method we have
tried, or that has been tried by others, for determining the character
of this heat appears to us inconclusive; and without questioning that
the Moon radiates heat earthward from its soil, we have not yet found
any experimental means of discriminating with such certainty between
this and reflected heat that it is not open to misinterpretation.” It is
obvious from the foregoing that we on the Earth need not concern
ourselves very much about lunar heat; and I will only add that F. W.
Very, by an ingenious endeavour to localise the Moon’s radiant heat, has
been able, he thinks, to establish the fact that on the part of the Moon
to which the Sun is setting, what he calls the heat-gradient (using a
phrase suggested by terrestrial meteorology) appears to be steeper than
on that part to which the Sun is rising. Generally, Very’s observations
accord fairly with Lord Rosse’s.

The Moon revolves round the Earth in 27 d. 7 h. 43 m. 11 s. at a mean
distance of 237,300 miles, in an orbit which is somewhat, but not very,
eccentric. Its angular diameter at mean distance is 31′ 5″, or, say,
just over ½°. The real diameter may be called 2160 miles.

A few words will probably be expected by the reader on the subject of
lunar influences on the weather, and generally; this being a matter
highly attractive to the popular mind. The truth appears to lie, as
usual, between two extremes of thought. The Moon, of course, is the main
cause of the tides of the Ocean, and it is not entirely inconceivable
that tidal changes imparted to vast masses of water may be either
synchronous with, or may in some way engender, analogous movements in
the Earth’s atmosphere; though no distinct proofs of this, as a
determinate fact, can be brought forward.

There is no doubt whatever that at or near the time of full Moon,
evening clouds tend to disperse as the Moon comes up to the meridian,
and that by the time the Moon has reached the meridian a sky previously
overcast will have become almost or quite clear. Sir John Herschel has
alluded to this by speaking of a “tendency to disappearance of clouds
under a full Moon”; and he considers this “fully entitled to rank as a
meteorological fact.” He goes on, not unnaturally, to suggest the
obvious thought that such dissipation of terrestrial clouds is due to
the circumstance that, assuming heat really comes by radiation from the
Moon (and we have seen on a previous page the probability of this) such
radiant heat will be more potential if it falls on the Earth
perpendicularly, as from a Meridian Moon, than if it comes to us at any
one locality from a Moon low down in the observer’s horizon, and
therefore has to pass through the denser strata of the Earth’s
atmosphere and suffer material enfeeblement accordingly. I am aware that
Mr. Ellis, late of the Royal Observatory, Greenwich, has sought to show
by a seemingly powerful array of statistics that the idea now under
consideration is unfounded, but I consider that we have here only one
more illustration of the familiar statement that you can prove anything
you like by statistics. I am firmly convinced, as the result of more
than 30 years’ observation, that terrestrial clouds do disperse under
the circumstances stated. Sir J. Herschel added that his statement
proceeded from his own observation “made quite independently of any
knowledge of such a tendency having been observed by others. Humboldt,
however, in his _Personal Narrative_, speaks of it as well known to the
pilots and seamen of Spanish America.” Sir John Herschel further
remarked:—“Arago has shown from a comparison of rain, registered as
having fallen during a long period, that a slight preponderance in
respect of quantity falls near the ‘new’ Moon over that which falls near
the ‘full.’ This would be a natural and necessary consequence of a
preponderance of a cloudless sky about the ‘full,’ and forms, therefore,
part and parcel of the same meteorological fact.”

Bernadin has asserted it to be a fact that many thunderstorms occur
about the period of “new” or “full” Moon. But what I want most to warn
the reader against is that popular idea (wonderfully wide-spread it must
be admitted) that at the epochs of what are called, most illogically,
the Moon’s “changes,” changes of weather may certainly be expected.
There is absolutely no foundation whatever for this, and still more void
of authority (if such a phrase is admissible) is a table of imaginary
weather to be expected at changes of the Moon, often met with in books
published half a century ago, and still occasionally reprinted in
third-rate almanacs, and designated “Dr. Herschel’s Weather Table.” This
precious production is not only devoid of authenticity as regards its
name, but may easily be seen to be fraudulent in its reputed facts any
month in the year.

It would be beyond both my present available space and the legitimate
objects of this work to attempt even an outline of the influences over
things terrestrial ascribed to, or associated, rightly or wrongly, with
the Moon, and of which the word “lunatic” perhaps affords the most
familiar exponent.




                              CHAPTER VII.
                                 MARS.


Mars, though considerably smaller than the Earth, is commonly regarded
as the planet which, taken all in all, bears most resemblance to the
Earth, though only one-fourth its size. Under circumstances which have
already been briefly alluded to in Chapter I., Mars exhibits from time
to time a slight phase, but nothing approaching in amount the phases
presented by the two inferior planets, Mercury and Venus. When in
opposition to the Sun, that is to say when on the meridian at midnight,
it has a truly circular disc; but between opposition and its two
positions of quadrature it is gibbous. At the minimum phase, which is at
each quadrature, E. or W. as the case may be, the planet resembles the
Moon 3 days from its “full.” These phases are an indication that Mars
shines by the reflected light of the Sun. It is a remarkable tribute to
Galileo’s powers of observation that with his trumpery telescope, only a
few inches long, he should have been able to suspect the existence of a
Martial phase. Writing to a friend in 1610 he says:—“I dare not affirm
that I can observe the phases of Mars; however, if I mistake not, I
think I already perceive that he is not perfectly round.”

[Illustration: Fig. 12.—Four views of Mars differing 90° in longitude
(Barnard).]

The period in which Mars performs its journey round the Sun (called the
sidereal period) is about 687 days; but owing to the Earth’s motion we
are more concerned with what is called the planet’s synodical period of
780 days than with its sidereal period of 687 days. The synodical period
is the interval between two successive conjunctions or oppositions of
the planet as regards the Earth, and 780 days being twice 365 and 50
days over, it follows that we have an opportunity of seeing the planet
at its best about every 2 years; and this is one of the reasons why Mars
has been so much and so thoroughly studied as regards its physical
appearance. Of course Mars is not equally well seen every 2 years,
because it may so happen at a given opposition that it may be at its
nearest to the Sun (perihelion), and the Earth at its farthest from the
Sun (aphelion), in which case the actual distance between the two bodies
will be the greatest possible. What is therefore wanted is for the
planet to be nearest to the Sun and nearest to the Earth at the same
time, under which circumstances it shines with a brilliancy rivalling
Jupiter. This favourable combination occurs once in 7 synodical
revolutions, or about every 15 years. The most favourable oppositions
occur at the end of August, and the least favourable at the end of
February. The next very favourable opposition will not occur until 1909.
Mars may approach to within about 35 millions of miles from the Earth at
a favourable opposition, whilst under extreme circumstances the other
way it may be no nearer than 61 millions of miles at opposition.

Mars in opposition is a very conspicuous object in the Heavens, shining
with a fiery red light which has always been regarded as a peculiar
attribute of the planet, so much so that its name, or epithet, in many
languages conveys the idea of “fiery” or “blazing.” It is recorded that
in August 1719 its brilliancy was such as to cause a panic amongst the
public.

Telescopically examined, Mars is always found to exhibit patches of
shade of various sizes and shapes, and, on the whole, fairly permanent
from year to year. During the last few years in particular these
markings have been subjected to very careful scrutiny and measurement at
the hands of numerous observers of skill and experience, and armed in
many cases with very powerful telescopes. The conjoint effect of the
observations obtained has been largely to augment our knowledge of the
planet’s geography, or (to use the proper term) “areography.” Before
describing the minutest details recorded and pencilled by the best
observers, it will be best to speak of the leading general features
which are within the grasp of comparatively small telescopes—say,
refractors of 6 inches and reflectors of 12 inches in aperture. The
first thing which presents itself as very obvious on the disc of Mars,
is the fact that certain portions are ruddy, whilst others are greenish
in hue. It is generally assumed that the red areas represent land and
the green areas water. On this subject Sir John Herschel’s remarks,
penned about half a century ago, may be said still to stand good. He
ascribes the ruddy colour to “an ochrey tinge in the general soil, like
what the red sandstone districts on the Earth may possibly offer to the
inhabitants of Mars, only more decided.” The propriety of this thought
will be best appreciated by a reader who has travelled through parts of
North Gloucestershire, and seen a succession of ploughed fields in that
locality. The deep red colour of the soil is in many places very
conspicuous. It has often been remarked that the redness of Mars is much
more noticeable with the naked eye than with a telescope; and Arago
carried this idea one step further in suggesting that the higher the
optical power the less the colour. This, however, might naturally be
expected.

The most prominent surface marking on Mars is that known as the “Kaiser
Sea,” sometimes called the “V-mark” from its resemblance to that letter,
though a leg of mutton would be quite as good a simile. East of the
Kaiser Sea and a little north of the planet’s equator is a well-defined
dark streak known as “Herschel II. Strait”; whilst on the west side is
another shaded area which has been called “Flammarion Sea.” These three
features are so very conspicuous, that, provided the hemisphere in which
they are situated is fairly in front of the observer, his telescope, if
it will show anything on Mars, will show these. The white patches seen
on certain occasions _at_ Mars’s N. pole and _close to_ its S. pole form
another special feature of interest connected with this planet. It
admits of no doubt whatever that these are immense masses of snow and
ice which undergo at stated intervals changes analogous to the changes
which we know happen in the great fields of ice situated in the regions
of the Earth surrounding the Earth’s two poles. Not only do these white
patches look like snow, but if attention is paid to the changes they
undergo and the epochs at which the changes take place there will be
found abundant confirmation of this theory, for these patches decrease
in size when brought under the Sun’s influence on the approach of summer
and increase again in size when the summer is over and winter draws
near. In the second half of 1892 the Southern Pole was in full view, and
during especially July and August the diminution of the snow area from
week to week was very evident. Schiaparelli, who observed it with great
attention during that season, noted at the commencement of the season
that the snow reached at the first as far as latitude 70° and formed a
polar cap some 1200 miles in diameter. Its subsequent decrease, however,
was so marked that two or three months later the diameter of the snow
patch had dwindled to no more than 180 miles, and became indeed still
smaller at a later period. The summer solstice on Mars occurred on
October 13, 1892, which was therefore the epoch of midsummer for Mars’s
southern hemisphere. Whilst these changes were taking place in the
southern hemisphere, no doubt changes of the reverse character were
going on in the northern hemisphere, but they were not visible from the
Earth because the North Pole was situated in that hemisphere of Mars
which was turned away from the Earth. In previous years, however, the
North Pole being turned towards the Earth its snow was also seen to
undergo the same sort of change; in other words, was seen to melt. This
happened, and was seen in 1882, 1884, and 1886. These observations of
the alternate increase and decrease of the polar snow on Mars may be
viewed with telescopes of moderate power, but of course it is more
interesting and profitable to watch them with a large telescope. The
fact (for it is an undoubted fact) that the north polar snow is
concentric with the planet’s axis whilst the southern polar patch is
eccentric to the extent of about 180 miles from the southern pole is one
which has not yet received a satisfactory explanation. If both patches
were eccentric so as to be exactly opposite to one another an
explanation would be much more easy for we might say that the poles of
rotation lay in one direction and the poles of cold in another.

I have spoken on a previous page of three specially conspicuous shadings
of Mars, and other similar shadings to the number perhaps of a couple of
dozen were generally recognised by astronomers (having been mapped and
named) down to about the year 1877. In that year the astronomical world
was startled by the announcement that Schiaparelli of Milan, an able and
competent observer, had discovered that those shaded areas which all
previous astronomers had regarded as continents or vast tracts of land,
were in reality islands, that is to say, so far, that the continents in
question were cut up by innumerable channels intersecting one another at
various angles. When this discovery was announced, and older
observations and drawings came to be examined, it was found, or at
any-rate thought, that these so-called canals might be traced in
drawings of earlier dates by Dawes, Secchi, and Holden. So much for
1877. In December, 1881, the planet was again in opposition, but farther
off in distance, and therefore smaller in size than in 1877. It was,
however, higher up in the Heavens as seen at Milan and the weather
appears to have been more favourable. In these altered circumstances
Schiaparelli again saw his canals, but this time they were in at least
as many as twenty instances seen in duplicate; that is to say, a twin
canal was seen to run parallel to the original one at a distance of from
200 to 400 miles, as the case might be. The existence of not only single
canals but of twin canals seems an established fact, for Schiaparelli’s
drawings and descriptions have been confirmed by competent testimony;
but explanation is nowhere; especially in view of Schiaparelli’s own
idea that the duplication of his canals is perhaps not a permanent
feature but a periodical phenomenon depending on, or connected in some
way with, Mars’s seasons.

[Illustration: Fig. 13.—Mars, August 27, 1892 (Guiot).]

Several points stand out clearly established by the observations of Mars
during the opposition of 1894, especially the correctness of
Schiaparelli’s discoveries and maps. Most of the canals originally seen
by him were again seen, and thus their existence was confirmed, whilst
new ones were also noticed. Many of these canals were double. The great
extent of the S. Polar cap and its rapid disappearance as Mars’s summer
approached was also a special feature of the observations of 1894. It
dwindled until it became almost invisible, or at best showed as a tiny
speck. It is thought by some observers that as the Polar cap melts, the
water collects round the Pole, and thence flows over the planet’s
surface, giving rise to the phenomenon of canals, and that this is the
way the planet’s surface is irrigated. It may here be remarked that the
word “canal,” which has been given to these dark streaks crossing and
cutting up the large areas of land in Mars, is an unfortunate one,
suggesting as it does artificial agency. But these Martial canals are
probably, especially the largest, a great many miles in width and
hundreds of miles in length, though some are smaller; and they are
probably nature’s method of distributing over the continents and lands
of Mars the water which collects round the Pole during the rapid melting
of the Polar snows.

The idea of the presence of cloud or mist on Mars also received strong
confirmation in 1894. Large portions of the planet’s disc were found to
be hidden from view. “Herschel I. Continent” and the “Maraldi Sea” (both
well-known markings on Mars, readily visible with small telescopes) were
at times quite obscured by cloud. Indeed, the Maraldi Sea was
occasionally quite blotted out: other well-known markings were also
either blotted out or only faintly seen. These facts seem almost to
prove conclusively the existence of cloud and vapour in Mars, especially
as some of these markings subsequently again assumed their ordinary form
and colour. Bright projections too were seen at times on the terminator
of Mars, giving rise to the belief that there are high mountains on the
planet, though some observers regarded these projections as high clouds
powerfully reflecting the Sun’s light.

Mars rotates on its axis in 24h. 37m. 22s., a period so nearly
coincident with the period of the Earth’s rotation as greatly to
facilitate the mapping of Mars’s features by work continued from day to
day by observers who have the necessary instrumental means and artistic
skill in handling the pencil.

Mars has an atmosphere which may be said to be no more than moderately
dense; that is to say much less dense than the Earth’s atmosphere. Of
course the existence of snow, which has been taken for granted on a
previous page, carries with it the existence of water and aqueous
vapour—a fact capable of independent spectroscopic proof.

The inclination of Mars’s axis to the ecliptic has not been ascertained
with all desirable certainty, but if Sir W. Herschel’s estimate that the
obliquity on Mars is 28¾° (the Earth’s obliquity being 23½°) is correct,
it is evident that there must be a very close similarity between the
seasons of the Earth and the seasons of Mars, thereby furnishing another
link of proof to support the statement made at the commencement of this
chapter that, taken all in all, Mars is the planet which bears most
resemblance to the Earth.

The apparent absence of satellites in the case of Mars was long a matter
of regret to astronomers; they seemed to think that such a planet ought
to have at least one companion. At last, in 1887, two were found by Hall
at Washington, U. S., using a very fine refractor of 26 inches aperture.
These satellites, which have been named Phobos and Deimos, are, however,
very small, for Phobos at its best only resembles a star of mag. 11½,
whilst Deimos is no brighter than a star of mag. 13½; from this it will
be understood that only very large telescopes will show either of them.
Phobos revolves round Mars in 7½ hours at a distance of about 6000
miles, whilst Deimos revolves in 30 hours at a distance of about 15,000
miles. It has been thought that neither of them can be more than about 6
or 7 miles in diameter, and therefore that they can not afford much
light to their primary.

Mars revolves round the Sun in 686d. 23h. 30m., at a mean distance of
141 million of miles, which the eccentricity of its orbit may increase
to 154 millions or diminish to 128 millions. The planet’s apparent
diameter varies between 4″ in conjunction and 30″ in opposition. Owing
to the great eccentricity of the orbit the planet’s apparent diameter as
seen from the Earth varies very much at different oppositions. The real
diameter is rather more than 4000 miles.




                             CHAPTER VIII.
                           THE MINOR PLANETS.


In 1772 a German astronomer named Bode, of Berlin, drew attention to
certain curious numerical relations subsisting between the distances of
the various planets. This “law,” as it has been sometimes called,
usually bears Bode’s name, though it was not he but J. D. Titius of
Wittemberg who really first discovered it.

Take the numbers—

  0, 3, 6, 12, 24, 48, 96, 192, 384;

each of which (the second excepted) is double the preceding; adding to
each of these numbers 4 we obtain—

  4, 7, 10, 16, 28, 52, 100, 196, 388;

which numbers approximately represent the distances of the planets from
the sun expressed in radii of the Earth’s orbit. A little table will
make the matter more clear.

  Planets.     Distance: Bode’s Law.     True distance from Sun.
  Mercury                4                         3.9
  Venus                  7                         7.2
  Earth                  10                        10.0
  Mars                   16                        15.2
  [Ceres]               [28]                      [27.7]
  Jupiter                52                        52.0
  Saturn                100                        95.4
  [Uranus]             [196]                     [191.8]
  [Neptune]            [388]                     [300.0]

Bode having examined these relations and noticing the void between 16
and 52 (Ceres and the other minor planets, and Uranus and Neptune also,
being then unknown) ventured to predict the discovery of new planets,
and this idea stimulated him to organise a little company of astronomers
to hunt for new planets. Before, however, this scheme was got into
working order, Piazzi, director of the Observatory at Palermo, on
January 1, 1801, noted an 8th magnitude star in Taurus, which on the
next and succeeding nights he saw again, and found had moved. He
observed the strange object for 6 weeks, when illness interrupted him.
However he wrote letters announcing what he had seen, one of them to
Bode himself; but this letter, though dated Jan. 24, did not reach Bode
at Berlin, till March 20—a striking illustration of the state of the
Postal service on the Continent less than 100 years ago. The new body,
at first assumed to be a tailless comet, was eventually recognised to be
a new planet; and the name of Ceres, the tutelary goddess of Sicily, was
at Piazzi’s instance bestowed upon it.

Looking for Ceres in March, 1802, Olbers at Bremen, came upon another
new planet, which was afterwards named Pallas. At first he thought he
had got hold of a new variable star, but two hours sufficed to show that
the object under notice was in motion. The two new bodies were found to
be so much alike in size and appearance, and in their orbits, that
Olbers suggested both were but fragments of some larger body which had
been shattered by some great convulsion of nature. The idea was a daring
one, and it was an attractive one, though now regarded as untenable.
However it served the purpose of stimulating research, and the discovery
of Pallas was followed by that of Juno, by Harding, at Lilienthal 1804;
and of Vesta, by Olbers, at Bremen in 1807.

The organised search for minor planets was relinquished in 1816,
presumably because no more planets seemed to be forthcoming, and it does
not appear that any further attempts were made by anybody till about
1830, when a Prussian amateur, named Hencke of Driessen, profiting by
the publication of some new star maps put forth by the Berlin Academy,
commenced a methodical search for small planets. These Berlin maps, one
for each hour of R. A., were only completed in 1859, and, therefore,
Hencke had only a small number of them at his command during the early
years of his labours. Still it is strange that 15 years elapsed before
his zeal and perseverance were rewarded, his first discovery, the planet
Astræa, not taking place till December 1845. Once however the ice was
broken new planets followed with considerable rapidity, and beginning
with 1847, no year has elapsed without several or many having been
found. During the last decade the number detected annually has been very
great—sometimes as many as 20 in a year, but this has been the result of
photography being brought to bear on the work. It is obvious that if a
photograph of a given field taken on any one day is compared with a
photograph taken a few days earlier or later, and any of the objects
photographed have moved, their change of place will soon be noticed and
will be a distinct proof of their planetary nature.

It seems quite certain that all the larger of these planets have now
been found, for the average brilliancy (and this no doubt means the
average size) of those recently discovered has been steadily diminishing
year by year, and it looks as if the limit of visibility will soon be
reached, if it has not been reached already.

The three largest of these bodies, in order of size, have generally been
thought to be Vesta, Ceres, and Pallas; but Barnard, from observations
made in 1894, concluded that Ceres is 520 miles in diameter; Pallas, 304
miles; and Vesta, 241 miles. As to all the rest of the minor planets,
excepting Juno, Hornstein is of opinion that those having a greater
diameter than 25 geographical miles are few in number, and that the
majority of them are no larger than from 5 to 15 miles in diameter.

From what has gone before the reader will readily infer that these minor
planets are of no sort of interest to the casual amateur who dabbles in
Astronomy; and indeed that they are of very little interest to anybody.
With a few general statistics, therefore, this chapter may be concluded.
The total number of minor planets now known nearly reaches 500, and
every year increases the list; but not, however, at as rapid a rate as
was once the case, because the German mathematicians, who alone latterly
have been willing to trouble themselves with the computation of the
orbits, are understood to have announced that they are no longer able to
keep pace with the discoveries made. Those who care to investigate in
detail the circumstances of these planets will find great extremes in
the nature of the orbits. Whilst the planet nearest to the Sun has a
period of only 3 years, the most distant occupies nearly 9 years in
performing its journey round the Sun. So, also, there are great
differences in the eccentricities of the orbits and in their
inclinations to the ecliptic. Whilst one planet revolves almost in the
plane of the ecliptic, another (Pallas) has an orbit which is inclined
no less than 34° to the ecliptic. One word, in conclusion, as to the
names applied to these bodies. At the outset the names given were,
without exception, chosen from the mythologies of ancient Greece and
Rome, but, latterly, the most fantastic and ridiculous names have in
many cases been selected, names which in too many instances have served
no other purpose than that of displaying the national or personal vanity
of the astronomers who applied them to the several planets. The French
are great offenders in this matter.




                              CHAPTER IX.
                                JUPITER.


The planet Jupiter occupies, in one sense, the first position in the
planetary world, it being the largest of all the planets. Moreover, with
the exception of Venus, it is the brightest of the planets. As with
Mars, and for the like reason, Jupiter, when in the positions known as
the Quadratures (or near thereto), exhibits a slight phase, but owing to
the far greater distance from the Sun of Jupiter, compared with Mars,
the deviation of the illuminated surface from that of a complete circle
is very small; it is, however, perceptible at or near the time of
quadrature, a slight shading off of the limb farthest from the Sun being
traceable.

Jupiter is noteworthy on account of two features, both of them more or
less familiar, at least by name, to most people—its belts and its
satellites,—both of which will be described in due course.

The belts are dusky streaks, which vary from time to time both in
breadth and number: most commonly two broad belts will be seen with two
or three narrower ones on either side; but sometimes all are rather
narrow, and their narrowness is made up for by an increase in their
number.

[Illustration: Fig. 14.—Jupiter, November 27, 1857 (Dawes).]

Under all circumstances they lie practically parallel, or nearly so, to
the planet’s equator. It is generally thought that the planet, whatever
may be its actual structure or constitution, is surrounded by a dense
cloudy envelope, and that the shaded streaks which we call belts are
rifts in this atmosphere, which expose to view the solid body of the
planet underneath. Whether, however, the term “solid body” is an
accurate one to be used in this connection is thought by some to be open
to doubt. The laws which regulate the existence of these belts are quite
unknown; indeed it seems doubtful whether any laws exist at all, for the
belts at one time appear to undergo constant change, whilst at another
time they remain almost unchanged for several months. It has been
suggested that when the changes are rapid it must be presumed that great
atmospheric storms are to be considered as in progress, and possibly
this may be the true explanation. Belts are commonly non-existent
immediately under the equator; whilst north and south of this void space
it most usually happens that there is one broad belt and several
narrower ones in each hemisphere. At each pole the planet’s brightness
is less than the average brightness, but it cannot exactly be said that
this is due to the existence there of belts properly so called.

It was formerly considered that no tinges of colour could be traced on
Jupiter except a silvery gray of different degrees of intensity; but
during the last thirty years there can be no doubt that shades of brown,
red, and orange, of no great depth, but yet quite definite have been
traceable. Many observers concur in this opinion. Whether this detection
of colour is due to an absolute development of colour during the period
in question; or whether its detection is merely the result of more
careful scrutiny with better instruments is a matter as to which the
evidence is not clear. Though the general position of the belts is such
that they are parallel to the planet’s equator, yet there are sometimes
exceptions to this rule, for in a few very rare instances a streak in
the nature of a narrow belt has been seen, inclined to the equator at a
decided angle, perhaps 20° or even more.

It occasionally happens that spots are seen on Jupiter’s belts.
Sometimes these remain visible for a considerable period. They are
either dark or luminous, and their origin is unknown. Besides these
casual spots, which are always small in size, there was visible during
many years following 1878 a very remarkable and conspicuous large spot,
strongly red in colour for several years, though it afterwards became
much fainter. This spot exhibited an oval outline and was about 27,000
miles long and 8000 miles broad. For about 4 years it maintained its
intense red colour and its shape almost unaltered; but after 1882, the
shape remaining, the colour sensibly faded. The observations which were
made on this spot during 1886 by Professor Hough at Chicago, U. S., with
an 18-inch refractor, led him to the opinion that the persistence of the
red spot for so many years rendered untenable the generally accepted
theory that the phenomena seen on the surface of the planet are due to
atmospheric causes.

Some astronomers have thought that a relationship subsists between the
spots on the Sun and the spots on Jupiter. There certainly seems an
apparent identity in point of time between the two classes of spots, and
on the assumption that the spots on Jupiter are indicative of
disturbances on the planet, Ranyard broached the idea that both classes
of phenomena are dependent on some extraneous cosmical change; and are
not related as cause and effect. Browning suggested many years ago that
the red colour of the belts is a periodical phenomenon coinciding with
the epoch of the greatest display of sunspots, but this thought does not
appear to have been followed up by any one. Spots on Jupiter seem to
have been first recorded by Robert Hooke in 1664. In the following year
Cassini saw a spot which he found to be in motion, and by following it
attentively he inferred that the planet rotated on its axis in 9h. 56m.
It is a remarkable illustration of the great care bestowed by Cassini on
his astronomical work that the best modern determinations of Jupiter’s
rotation-period differ from Cassini’s estimate by only half a minute.

Bearing in mind the enormous size of Jupiter compared with the Earth,
whilst its period of rotation is considerably less than half the
Earth’s, it will be at once seen that the velocity of matter at the
planet’s equator is immensely great—466 miles per minute against the
Earth’s 17 miles per minute. One result of this is the great intensity
of the centrifugal force at the equator, and likewise the greatness of
the compression of the planet’s body at the poles. Hind has suggested
that the great velocity which thus evidently exists may have the effect,
by reason of the development of the heat which it gives rise to, of
compensating the planet for the small amount of heat which owing to its
distance it receives from the Sun.

On favourable occasions the brilliancy of Jupiter is very considerable;
so much so that it rivals Venus and Mars. And besides this, there
appears to be something special in the nature of Jupiter’s surface, for
not only does it seem to radiate a much larger proportion of the solar
light which falls on it than do the planets generally, but some
observers have expressed the opinion that it possesses inherent light of
its own. Speculations, however, such as this must always be received
with reserve, because of the evident difficulty of making sure of the
facts on which they must be based. One thing, however, seems less open
to doubt. Bearing in mind the small amount of heat which reaches Jupiter
from the Sun, there is reason to infer that the clouds which certainly
exist on Jupiter must owe their origin to the influence of some other
heat than solar heat; in other words that Jupiter possesses sources of
heat within itself.

Jupiter has satellites, 5 in number. The discovery of four of these, was
one of the first fruits of the invention of the telescope, for they were
found by Galileo in January, 1610. The 5th satellite is so small that it
escaped notice until as recently as 1892, having been discovered on
September 9 of that year by Professor Barnard, with the great Lick
telescope in California. It is, however, so minute that one can count on
one’s fingers the telescopes capable of showing it.

The four old satellites of Jupiter shine as stars of about the 7th
magnitude; in other words, they are sufficiently bright to be visible
with telescopes however small: indeed several instances are on record of
persons gifted with very good sight, having been able to see them with
the naked eye. For the study of their physical appearance very powerful
optical assistance is necessary, but their movements are so rapid, and
the phenomena which result from those movements are so interesting, that
these bodies may be considered to occupy the first place in the
stock-in-trade of every amateur astronomer, who lays himself out for
planet-gazing, with the object of profiting himself or his friends. The
phenomena here alluded to are known as eclipses, transits, and
occultations.

The four old satellites do not bear any names, but are numbered from the
innermost outwards, and are always alluded to by their numbers as I, II,
III and IV.

An eclipse of a Jovian satellite is identical in principle with an
eclipse of the Moon; that is to say, just as an eclipse of the Moon
happens when the Moon passes into and is lost in the Earth’s shadow, so
an eclipse of a Jovian satellite happens when such satellite becomes
lost in the shadow cast by the planet into space. The Ist IInd and IIIrd
satellites in consequence of the smallness of the inclination of their
orbits, undergo eclipse once in every revolution round their primary,
but the IVth is less often eclipsed, owing to the joint effect of its
considerable orbital inclination, and of the distance to which it
recedes from its primary.

An occultation of a Jovian satellite is akin in principle to an
occultation of a star by the Moon. As the Moon moving forwards suddenly
covers a star, so the planet, on occasions, suddenly covers one of its
satellites. If the satellite in question is the IVth, its disappearance
behind the planet and its reappearance from behind the planet will both
be visible in due succession. This is often true also of the IIIrd
satellite, but for reasons connected with the proximity to their primary
of the Ist and IInd satellites, only their disappearance _or_
reappearance (not both) can, as a rule, be observed on the same
occasion. The most interesting, by far, however, of the phenomena
connected with Jupiter’s satellites are their transits in front of, that
is across, the visible disc of the planet. Though these transits are of
frequent occurrence, yet they are always interesting because of the
diverse appearances which the satellites exhibit at different times, and
which cannot be said to be in accordance with any recognised laws.
Moreover, in observing the transit of a satellite, we may often see the
black shadow cast by the satellite on the planet’s disc; and this shadow
will sometimes precede and sometimes follow the satellite itself. From
the fact that the satellite generally appears as a bright spot on a
bright background whilst the shadow is black, or blackish, an
inexperienced observer is apt to look at the shadow and think he is
seeing the satellite.

Jupiter revolves round the Sun in not quite 12 years at a mean distance
of 483 millions of miles. Its apparent diameter varies between 50″ and
30″ according to its position with respect to the Earth. Its true
diameter is about 88,000 miles. Owing to its large size and rapid
rotation, as has already been mentioned, Jupiter is very much flattened
at the poles. The amount of this (the polar “compression” as it is
called) is about 1/16.




                               CHAPTER X.
                                SATURN.


Next beyond Jupiter, proceeding outwards from the Sun, we reach the
planet Saturn, which beyond any doubt is the most beautiful and most
interesting of all the planets. Nobody who has ever had a fairly good
chance of seeing it can have the least doubt that this is the case.
Briefly stated the three main features which constitute its claims
are:—(1) Its belts, (2) its rings, (3) its satellites.

The belts of Saturn resemble generally those of Jupiter, but they are
more faint and less changeable. Their physical cause, however, may be
assumed to be the same. Taking the planet as a whole, it may be said
that its ordinary colour is yellowish white, the belts inclining to
grayish white; though the dark belts have often been thought to exhibit
a greenish hue. Lassell considered that the south pole is generally
darker than the north pole and more blue in tinge.

[Illustration: Fig. 15.—Saturn, Jan. 26, 1889 (Antoniadi).]

There is one important particular in which the belts of Saturn differ
from those of Jupiter. Jupiter’s belts are straight, whereas Saturn’s
are sensibly curved. Supposing, as is probable, that Saturn’s belts are
parallel to the planet’s equator, then we must assume that the plane of
this equator makes a rather considerable angle with the ecliptic. Spots
on Saturn are very rare. Whether Saturn has an atmosphere seems
uncertain, or perhaps it may be said that one has not been proved to
exist but may exist. The question of polar snow is also uncertain, but
Sir W. Herschel thought he could trace changes of hue at the poles which
might be due to the melting of snow.

It is usual to speak of the planet itself under the name of the “Ball”
when it is not a question of referring to the whole Saturnian system
collectively. In consequence of its distance from the Sun, Saturn
undergoes no equivalent to a phase; or to be more exact, no phase can be
detected, though theoretically when the planet is in quadrature the disc
must undergo an infinitesimally small loss of light.

Though the point has now-a-days no scientific importance, it may perhaps
be desirable just to make a brief allusion to Sir W. Herschel’s curious
theory that Saturn was seen by him to be compressed not only at the
poles but at the equator, so that it resembled a parallelogram with the
corners rounded off. It is difficult to imagine what could have given
rise to this strange idea, though, of course, Herschel’s good faith in
advancing it cannot be called in question. I refer to it because it will
be found mentioned in so many books on astronomy, often under the name
of the “square-shouldered” figure of Saturn. As a theory it may be
regarded as quite exploded in consequence of accurate measures by
Bessel, Main and others having conclusively shown that the form of the
ball does not depart from that of a regular spheroid.

In referring to Saturn generally, we speak of its ring in the singular
number, but, in point of fact, there are several rings—three in
particular. The principal bright ring is really double, and within the
innermost bright ring there is a dusky one, perfect as a ring, but not
luminous as the outer rings are. By way of distinguishing one ring from
another, it is usual to adopt Struve’s nomenclature, whereby the
outermost bright ring is called A, the inner bright ring B, and the
dusky ring C.

A good engraving will convey more fully and more clearly an idea of what
the Saturnian system consists of than the fullest verbal description
will do. (See _Frontispiece_.)

To the earliest astronomers who possessed telescopes, Saturn proved a
great puzzle, because it seemed to undergo changes of shape which were
quite inexplicable on any principles then known. Galileo, when first he
saw it, thought it presented an oval outline which might be due to a
central planet having a smaller planet on each side of it, and
accordingly he announced to his friend, Kepler, that the most distant
planet was _tergeminum_ or tri-form. But greater magnifying power led
him to arrive at the conclusion that the planet was not a triple
combination of spheres, but one body, either oblong or oval in outline.
This conclusion, however, was soon found to be untenable, because the
two (supposed) tributary bodies gradually decreased in size until they
entirely disappeared. Galileo writing to his friend, Welser, in December
1612, thus expressed himself:—

“What is to be said concerning so strange a metamorphosis? Are the two
lesser stars consumed after the manner of the solar spots? Have they
vanished or suddenly fled? Has Saturn, perhaps, devoured his own
children? Or were the appearances indeed illusion or fraud, with which
the glasses have so long deceived me, as well as many others to whom I
have shewn them? Now, perhaps, is the time come to revive the well-nigh
withered hopes of those who, guided by more profound contemplations,
have discovered the fallacy of the new observations, and demonstrated
the utter impossibility of their existence. I do not know what to say in
a case so surprising, so unlooked for, and so novel. The shortness of
the time, the unexpected nature of the event, the weakness of my
understanding, and the fear of being mistaken have greatly confounded
me.”

[Illustration: Fig. 16.—General view of the Phases of Saturn’s Rings.]

Galileo seems to have become so out of heart in consequence of the
difficulty of determining what these changes really meant, that he gave
up altogether observing Saturn. In the course of time, but by very
gradual steps, astronomers came to realise what the facts were. The next
idea that was broached, was that the planet consisted of simply one
central ball, and that the excrescences which Galileo had been puzzled
by were merely handles as they were called, (_ansæ_) projecting like the
handles, say of a soup tureen, though why they should vary in size at
stated intervals remained as great a mystery as ever. It was not until
about 1656 that the true explanation was arrived at by a Dutchman, named
Christopher Huygens. It was the fashion in those days for scientific men
to intimate to the world discoveries which they had made by resort to
mysterious anagrams, which served in some degree the purpose which in
the present day is served by the law regulating copyright or patent
rights. Accordingly Huygens published the following singular
memorandum:—

aaaaaaa cccc d eeeee g h i iiiiii llll mm nnnnnnnnn oo oo pp q rr s
ttttt uuuuu.

These letters arranged in their proper order furnish the following Latin
sentence:—

_Annulo cingitur, tenui, plano, nusquam cohaerente, ad eclipticam
inclinato_; which Latin sentence becomes in the English tongue:—

“[The planet] is surrounded by a slender flat ring inclined to the
ecliptic, but which nowhere touches [the body of the planet.]”

Huygen’s discovery was not a mere piece of guesswork, for he spent
several years carefully observing the alterations of form which Saturn
underwent, before he came to the conclusion that it was only the
existence of a ring surrounding the planet which would explain the
various observed changes.

It was by way of guarding himself from being robbed of the fruits of his
discovery whilst he was accumulating the necessary proof of its truth,
that he buried his thoughts in the logogriph or anagram just quoted.
Having arrived at the conclusion which he did, he thought himself
sufficiently sure of his facts to predict that in July or August 1671,
the planet would again appear round, the ring becoming invisible. This
surmise proved practically correct, in so far, that in May 1671, or
within 2 months of the time predicted by Huygens, Cassini saw the planet
as a simple ball unaccompanied by any ring.

This is a convenient place at which to offer a brief explanation of the
changes of appearance as regards the ball and rings which Saturn
undergoes. These changes depend jointly on Saturn’s motion in its orbit
round the Sun, and on the corresponding motion of the Earth in its
orbit. Neither Saturn nor the Earth revolve round the Sun exactly in the
ecliptic, and this want of coincidence results in the fact, that twice
in the 29½ years occupied by Saturn in journeying round the Sun, the
plane of its ring is seen edgeways by us on the Earth; whilst at two
other periods intermediate but equi-distant the ring is seen opened out
to the widest possible extent; that is, so far as we on the Earth can by
any possibility have a chance of seeing it.

[Illustration: Fig. 17.—Phases of Saturn’s Rings at specified dates.]

The appearances presented by the rings when undergoing the
transformations to which they are subject, will be readily understood by
an inspection of the annexed engravings. Fig. 17, indicates the actual
appearances in the years specified, and these years may be considered as
carried forward and brought up to date by substituting 1877 for 1848,
1885 for 1855, 1891 for 1862, and 1898 for 1869.

Adverting to fig. 16, it will suffice to remark that the two central
phases of the rings, opened wide, are to be deemed co-related, or indeed
identical in a geometrical sense (so to speak) the difference being that
one of them is to be deemed to show the northern side of the ring (which
is now in view and will continue in view till 1907) whilst the other
represents the southern side, which was in view from 1877 till 1891. The
foregoing is a brief statement of the general principle involved in the
changes which take place, but the motions of the two planets introduce
certain technical complications into the details which would be seen by
an observer using a large telescope; with these, however, the ordinary
reader will not care to concern himself, and need not do so.

A great deal might be said with respect to the rings treated
descriptively. I will now mention a few matters of general interest.
Huygens regarded the appendage to Saturn, whose existence he
established, to be a single ring, but as far back as 1675, Cassini
determined that Huygen’s single ring was really made up of two, one
lying inside the other. Cassini in this conclusion outstepped not only
all the observers of his own century, but those of the succeeding
century, for Sir W. Herschel even 100 years after Cassini, was for a
long time unable to satisfy himself, even with his superior telescopes,
that the black streaks seen in the ring by Cassini, and regarded by him
as indicative of a severance of the ring into two parts, really implied
a severance. It is now, however, accepted as a fact that not only are
the rings which are known as A and B absolutely distinct, but that A
also is itself certainly duplex, that is, that it certainly consists of
two independent rings. In addition to this many competent observers
armed with powerful telescopes have obtained traces of other
sub-divisions, both in A and B; and though there is some want of harmony
in the details, as stated by the different observers, yet undoubtedly we
must speak of Saturn’s rings collectively as forming a _multiple_
system.

What the rings are is a highly debatable point, but the preponderating
idea is that they are not what they appear to be, namely solid masses of
matter, but are swarms of independent fragments of matter. Yet
“fragment” is not the best word to use, because it implies that
something has been broken up to make the fragments. Rather, perhaps, we
should say with Professor Young, that the rings are “composed of a swarm
of separate particles, each a little independent moon pursuing its own
path around the planet. The idea was suggested long ago, by J. Cassini
in 1715, and by Wright in 1750, but was lost sight of until Bond revived
it in connection with his discovery of the dusky ring. Professor
Benjamin Pierce soon afterwards demonstrated that the rings could not be
continuous solids; and Clerk Maxwell finally showed that they can be
neither solid nor liquid sheets, but that all the known conditions would
be answered by supposing them to consist of a flock of separate and
independent bodies, moving in orbits nearly circular, and in one
plane—in fact, a swarm of meteors.”

The thickness of the rings seen edgeways has been variously estimated.
Sir J. Herschel suggested 250 miles as an outside limit, which G. P.
Bond reduced to 40 miles. It is generally considered, however, that 100
miles is probably not far from the truth. Young has pointed out that if
a model of them were constructed on the scale of 1 inch to represent
10,000 miles, so that the outer ring of such a model would be nearly 17
inches in diameter, then the thickness of the ring would be represented
by that of an ordinary sheet of writing paper.

Considered as a system, the rings are distinctly more luminous than the
planet, and of the two bright rings, the inner one is brighter than the
outer one; and the inner one is less bright at its inner edge than
elsewhere. It is also to be noticed that when seen edgeways just about
the time of the Saturnian equinoxes, when the Sun is shifting over from
one side of the ring to the other, and the ring is dwindling down to a
narrow streak, its edges (forming the _ansæ_ as they are termed) do not
disappear and reappear at the same time, and are not always of the same
apparent extent. One ansa, indeed, is sometimes visible without the
other, and most commonly it is the Eastern one that is missing. To what
causes these various peculiarities are due is unknown.

Many physical peculiarities have been either noticed or suspected with
reference to the bright rings. For instance, on comparing one with
another, some persons have thought that their surfaces are convex, and
that they do not lie in the same plane. The existence of mountains on
their surface has more than once been suspected. Again, it has been
fancied that they are surrounded by an extensive atmosphere. It seems
hardly likely that the rings would have an atmosphere and not the ball
(or _vice versâ_), and, therefore, no wonder that we have no
observations which countenance the idea that the ball does really
possess an atmosphere. This, indeed, seems to flow from Trouvelot’s
observation, that the ball is less luminous at its circumference than at
its centre.

The circumstances of ring C, otherwise called the “Dusky” or “Crape”
ring are as curious historically, as they are mysterious physically. In
1838, Galle of Breslau, noticed what he thought to be a gradual shading
off of the interior bright ring towards the ball. Though he published a
statement of what he saw, the matter seems to have attracted little or
no notice. In 1850, G. P. Bond in America perceived something luminous
between the ring and the ball, and after repeated observations in
concert with his father, came to the conclusion that the luminous
appearance which he saw, was neither less nor more than an independent
and imperfectly illuminated ring lying within the old rings and
concentric with them. Before, however, tidings of Bond’s discovery
reached England, but a few days after the discovery in point of actual
date, Dawes suddenly noticed one evening as Bond had done, a luminous
shading within the bright rings, which he was not long in finding out to
be in reality a complete ring, except so far that a portion of it was of
course hidden from view behind the ball. He, and O. Struve likewise,
noticed that this new Dusky Ring was occasionally to be seen divided
into two or more rings. The Dusky Ring is transparent, though this fact
was not ascertained until 1852, or two years after Bond’s discovery of
the ring.

The Dusky Ring is now recognised as a permanent feature of Saturn, but
how far it used to be permanent, or how long it has been so, is a matter
wrapped in doubt. Recorded observations by Picard in 1673, and by Hadley
in 1723, made of course with telescopes infinitely less powerful than
those of the present day, seem to suggest that both the observers named
saw the Dusky Ring, without, however, being able to form a clear
conception that it was a ring. It is strange that during the long period
from 1723 to 1838, no one—not even Sir W. Herschel, with his various
telescopes—should have obtained or at least have recorded any suspicion
of its existence. There is, however, direct evidence that the Dusky Ring
is wider and less faint than formerly. This was directly confirmed by
Carpenter in 1863, who says he saw it “nearly as bright as the
illuminated ring, so much so, that it might easily have been mistaken
for a part of it.” In 1883, Davidson found a marked difference in the
brilliancy of the two ends (_ansæ_) of the ring.

In 1889 Barnard was fortunate enough to observe an eclipse of one of
Saturn’s satellites by the ring, but the eclipse, that is the
concealment of the satellite, was only effected when it passed behind
the bright rings; the dusky ring did not obliterate it, and hence there
was obtained a conclusive proof of the transparency of the dusky ring.
Barnard further concluded from his observations that there was no
separating space or division between the inner bright ring and the dusky
ring, as has frequently been represented in drawings. This transparency
of the Dusky Ring, as a matter of fact, is therefore undoubted; yet what
are we to consider to be the meaning of an observation by Wray in 1861,
that whilst looking at the dusky ring edgeways the impression was
conveyed to his eye that that ring was very much thicker than the bright
rings?

A very interesting question which has been much discussed has reference
to the stability of the rings. It is generally agreed that the
constituent particles of the rings must be in motion round the primary
or their equilibrium could not be maintained: almost equally certain is
it, and for the like reason, that the rings cannot be solid. Of actual
change in the rings as regards their dimensions, we have no satisfactory
proof, though authorities differ on the point, some thinking that the
rings are expanding inwards, so that ultimately they will come into
contact with the ball, whilst others consider no proof whatever of such
change can be obtained from any of the observations yet made in the way
of measurements.

We must now proceed to consider the satellites of Saturn. These are 8 in
number, 7 of which move in orbits whose planes coincide nearly with the
planet’s equator, whilst the remaining one is inclined about 12°
thereto. One consequence of this coincidence in the planes of these
satellites, which, it should be stated, are the 7 innermost, is that
they are always visible to the inhabitants of both hemispheres when they
are not actually undergoing eclipse in the shadow of Saturn. The
satellites are of various sizes, and succeed one another in the
following order, reckoning from the nearest, outwards:—Mimas, Enceladus,
Tethys, Dione, Rhea, Titan, Hyperion and Iapetus. Any good 2-inch
telescope will show Titan; a 3-inch will sometimes show Iapetus; a
4-inch will show Iapetus well, together with Rhea and Dione, but hardly
Tethys; all the others require large telescopes. If Saturn has any
inhabitants at all constituted like ourselves, which is highly
improbable, they will have a chance of seeing celestial phenomena of the
greatest interest. What with the rings surrounding the planet and 8
moons in constant motion, there will be an endless succession of
astronomical sights for them to study. The amount of light received from
the Sun cannot be much—barely 1/100th what the earth receives. The ring
and satellites will therefore be useful as supplementary sources of
light; yet the satellites will not furnish much, for it has been
calculated that the surface of the sky occupied by all the satellites
put together would to a dweller on Saturn only amount to 6 times the
area of the sky covered by our Moon; whilst the intrinsic brightness of
all put together would be no more than 1/16th part of the light which we
receive from our Moon.

The only physical fact worth noting here in connection with the
satellites concerns Iapetus. Cassini two centuries ago with his
indifferent telescopes thought he had ascertained that this satellite
was subject to considerable variations of brilliancy. Sir W. Herschel
confirmed Cassini as to this. He found that it was much less brilliant
when traversing the eastern half of its orbit than at other times. Two
conclusions have been drawn from this fact. One is that the satellite
rotates once on its axis in the same time that it performs one
revolution round its primary; and that there are portions of its surface
which are almost entirely incapable of reflecting the rays of the Sun.
This last named supposition may perhaps be well founded, but the former
needs more proof than is as yet forthcoming. Iapetus on the whole may be
said to shine as a star of the 9th magnitude. To this it may be added
that Titan is of the 8th magnitude, but all the others much smaller.

[Illustration: Fig. 18.—Saturn with the shadow of Titan on it, March 11,
1892 (Terby).]

Saturn revolves round the Sun in a little under 29½ years at a mean
distance of 886 millions of miles. Its apparent diameter varies between
15″ and 20″; its true diameter may be put at 75,000 miles. The
flattening of the poles, or “polar compression” as it is called, is
greater than that of any other planet, but is usually less noticeable
than in the case of Jupiter, because the ring is apt to distract the
eye, except when near the edgeways phase. The compression may be taken
at 1/9.




                              CHAPTER XI.
                                URANUS.


To the Ancients Saturn was the outermost planet of the System, nothing
beyond it being known. Nor indeed was it to be assumed that any more
could possibly exist, because Mercury, Venus, the Earth, Mars, Jupiter,
and Saturn, with the Sun, made 7 celestial bodies of prime importance;
and 7 was the number of perfection; and there was thus provided one
celestial body to give a name to each of the days of the week.

But Science is not sentimental; and when men of Science come upon what
looks like a discovery they do their best to bring their discovery to a
successful issue, however much people’s prejudices may seem to stand in
the way at the moment.

On a certain evening in March, 1781, Sir William Herschel, then
gradually coming into notice as a practical astronomer, was engaged in
looking at different fields of stars in the constellation Gemini when he
lighted on one which at once attracted his special attention. Altering
his eyepiece, and substituting a higher magnifying power he found the
apparent size of the mysterious object enlarged, which conclusively
proved that it was not a star; for it is a well-known optical property
of all stars that whatever be the size of telescope employed on them,
and however high the magnifying power no definite disc of light can be
obtained when in focus. Herschel’s new find, therefore, was plainly not
a star, and no idea having in those days come into men’s minds of there
being any new planets awaiting discovery, he announced as a matter of
course that he had found a new comet, so soon as he ascertained that the
new body was in motion. The announcement was not made to the Royal
Society till April 26, more than six weeks after the date of the actual
discovery, an indication, by the way, of the dilatory circulation of
news a hundred years ago. The supposed comet was observed by Maskelyne,
the Astronomer Royal, four days after Herschel had first seen it, and
Maskelyne seems to have at once got the idea into his head that he was
looking at a planet and not at a comet. As soon as possible after the
discovery of a new comet the practice of astronomers is to endeavour to
determine what is the shape of the orbit which it is pursuing. All
attempts to carry out this in the case of Herschel’s supposed new comet
proved abortive, because it was found impossible to harmonise, except
for a short period of time, the movements of the new body with the form
of curve usually affected by most comets, namely, the parabola. It is
true, as we shall see later on in speaking of comets, that a certain
number of those bodies do revolve in the closed curve known as the
ellipse, but it is usual to calculate the parabolic form first of all,
because it is the easier to calculate; and to persevere with it until it
plainly appears that the parabola will not fit in with the observed
movements of the new object. This practice was carried out in the case
of Herschel’s new body, and it was eventually found that not only was
its orbit not parabolic; that not only was its orbit not an elongated
ellipse of the kind affected by comets; but that it was nearly a circle,
and as the body itself showed a defined disc the conclusion was
inevitable: it was in real truth a new planet. It has not taken long to
write this statement, and it will take still less time for the reader to
read what has been written, but the result just mentioned occupied the
attention of astronomers many months in working out, step by step, in
such a way as to make sure that no mistake had been made.

When it was once clearly determined that Herschel had added a new planet
to the list of known planets it became an interesting matter of inquiry
to find out whether it had ever been seen before; and to settle the name
it should bear. A little research soon showed that the new planet had
been seen and recorded as a fixed star by various observers on 20
previous occasions, beginning as far back as Dec. 13, 1690, when
Flamstead registered at Greenwich as a star. These various observations,
spread over a period of 91 years, and all recorded by observers of skill
and eminence materially helped astronomers in their efforts to calculate
accurately the shape and nature of the new planet’s orbit. One observer,
a Frenchman named Le Monnier, saw the planet no less than 12 times
between 1750 and 1771, and if he had had (which it is known he had not)
an orderly and methodical mind, the glory of this discovery would have
been lost to England and obtained by France. Arago has left it on record
that he was once shown one of these chance observations of Uranus, which
had been recorded by Le Monnier on an old paper bag in which hair powder
had been sold by a perfumer.

A long discussion took place on the question of a name for the new
planet. Bode’s suggestion of “Uranus” is now in universal use, but it is
within the recollection of many persons living that this planet bore
sometimes the name of the “Georgium Sidus” and sometimes the name of
“Herschel.” The former designation was proposed by Herschel himself in
compliment to his sovereign and patron George III. of England; whilst a
French astronomer suggested the latter name. However, neither of these
appellations was acceptable to the astronomers of the Continent, who
declared in favour of a mythological name, though it was a long time
before they agreed to accept Bode’s “Uranus.” The symbol commonly used
to represent the planet is formed of Herschel’s initial with a little
circle added below, though the Germans employ something else, “made in
Germany,” to quote a too familiar phrase.

The visible disc of Uranus is so small that none but telescopes of the
very largest size can make anything of it. A few sentences therefore
will dispose of this part of the subject. The disc is usually bluish in
tinge, and most observers who look at it consider it uniformly bright,
but there is satisfactory testimony to the effect that under the most
favourable circumstances of instrument and atmosphere two or more belts,
not unlike the belts of Jupiter, may be traced. From the position in
which these belts have been seen it is inferred that the satellites of
Uranus (presently to be mentioned) are unusually much inclined to the
planet’s equator, and revolve in a retrograde direction, contrary to
what is the ordinary rule of the planets and satellites. It is assumed
as the basis of these ideas, (and by analogy it is reasonable to do
this) that the belts are practically parallel to the planet’s equator,
and at right angles to the planet’s axis of rotation. To speak of the
planet’s axis of rotation is, in one sense, another assumption, because
available observations can scarcely be said to enable us to demonstrate
that the planet does rotate on its axis, yet we can have no moral doubt
about it. Taylor has suggested grounds for the opinion that “there can
be very little doubt that Uranus is to a very large extent
self-luminous, and that we do not see it wholly by reflected light.” To
this Gore adds the idea that there is “strong evidence in favour of the
existence of intrinsic heat in the planet.”

Uranus is attended by several satellites. It was once thought that there
were eight, of which six were due to Sir W. Herschel, the other two
being of modern discovery. Astronomers are, however, now agreed that no
more than four satellites can justly be recognised as known to exist,
and they are so minute in size that only the very largest telescopes
will show them; and therefore our knowledge of them is extremely
limited. Sir W. Herschel’s idea that he had seen six satellites appears
to have resulted from his having on some occasions mistaken some very
small stars for satellites. Two only of his six are thought to have been
real satellites. The other two recognised satellites were found both in
1847, one by Lassell, and the other by O. Struve.

Uranus revolves round the Sun in rather more than 84 years, at a mean
distance of 1781 millions of miles. Its apparent diameter, seen from the
Earth, does not vary much from 3½″ which corresponds to about 31,000
miles. It has been calculated that the light received from the Sun by
Uranus would be about the amount furnished by 300 full Moons seen by us
on the Earth, though another authority increases this to 1670 full
Moons. From Uranus Saturn can be seen, and perhaps Jupiter, both as
inferior planets, just as we see Venus and Mercury; but all the other
inner planets, including Mars and the Earth, would be hopelessly lost to
view, because perpetually too close to the Sun. Possibly, however, they
might, on rare occasions, be seen in transit across the Sun’s disc.
Neptune, of course, would be visible and be the only superior planet.
The Sun itself would appear to an observer on Uranus as a very bright
star, with a disc of 1¾′ of arc in diameter.




                              CHAPTER XII.
                                NEPTUNE.


We now come to the best known planet of the solar system, reckoning
outwards from the Sun, and though this planet itself, as an object to
look at, has no particular interest for the general public, yet the
history of its discovery is a matter of extreme interest. Moreover, it
is very closely mixed up with the history of the planet Uranus, which
has just been described. After Uranus had become fully recognised as a
regular member of the solar system, a French astronomer named Alexis
Bouvard set himself the task of exhaustively considering the movements
of Uranus with a view of determining its orbit with the utmost possible
exactness. His available materials ranged themselves in two groups:—the
modern observations between 1781 and 1820, and the early observations of
Flamsteed, Bradley, Mayer, and Le Monnier, extending from 1690 to 1771.
Bouvard found in substance that he could frame an orbit which would fit
in with each group of observations, but that he could not obtain an
orbit which would reconcile both sets of observations during the 130
years over which they jointly extended. He therefore rashly came to the
conclusion that the earlier observations, having been made when methods
and instruments were alike relatively imperfect, were probably
inaccurate or otherwise untrustworthy, and had better be rejected. This
seemed for awhile to solve the difficulty, and results which he
published in 1821 represented with all reasonable accuracy the then
movements of the planet. A very few years, however, sufficed to reveal
discordances between observation and theory, so marked and regular as to
make it perfectly clear that it was not Bouvard’s work which was faulty
but that Uranus itself had gone astray through the operation of definite
but as yet unknown causes. What these causes were could only be a matter
of surmise based upon the evident fact that there was some source of
disturbance which was evidently throwing Uranus out of its proper place
and regular course. First one and then another astronomer gave attention
and thought to the matter, and eventually the conclusion was arrived at
that there existed, more remote from the Sun than Uranus, an
undiscovered planet which was able to make its influence felt by
deranging the movements of Uranus in its ordinary journey round the Sun
every 84 years. This conclusion on the part of astronomers becoming
known, a young Cambridge student, then at St. John’s College, John Couch
Adams by name, resolved, in July 1841, to take up the subject, though it
was not until 1843 that he actually did so. The problem to be solved was
to suggest the precise place in the sky at a given time of an imaginary
planet massive enough to push, or pull, out of its normal place the
planet Uranus, which was evidently being pushed at one time and pulled
at another. It would also be part of the problem to predict the distance
from the Sun of the planet thus imagined to exist. Adams worked
patiently and silently at this very profound and difficult problem for
1¾ years when he found himself able to forward to Airy, who had become
Astronomer Royal after being a Cambridge Professor, some provisional
elements of an imaginary planet of a size, at a distance, and in a
position to meet the circumstances. It is greatly to be regretted, on
more grounds than one, that Airy did nothing but pigeon-hole Adams’s
papers. Had he done what might have been, and probably was, expected,
that is, had he made them public, or better still had he made telescopic
use of them, a long and unpleasant international controversy would have
been avoided, and Adams would not have been robbed in part of the
well-deserved fruits of his protracted labours.

We must now turn to consider something that was happening in France. In
the summer of 1845, just before Adams had finished his work, and one and
a half years after he commenced it a young Frenchman, who afterwards
rose to great eminence, U. J. J. Le Verrier, turned his attention to the
movements of Uranus with a view of ascertaining the cause of their
recognised irregularity. In November 1845 he made public the conclusion
that those irregularities did not exclusively depend upon Jupiter or
Saturn. He followed this up in June 1846 by a second memoir to prove
that an unknown exterior planet was the cause of all the trouble, and he
assigned evidence as to its position very much as Adams had done 8
months previously. Airy on receiving a copy of Le Verrier’s memoir seems
so far, at last, to have been roused that he took the trouble to compare
Le Verrier’s conclusions with those of Adams so long in his possession
neglected. Finding that a remarkably close accord existed between the
conclusions of the two men, he came to realise that both must be of
value, and he wrote a fortnight later to suggest to Professor Challis
the desirability of his instituting a search for the suspected planet.
Challis began within two days, but was handicapped by not having in his
possession any map of the stars in the neighbourhood suggested as the
_locale_ of the planet. He lost no time however in making such a map,
but, of course, the doing so caused an appreciable delay, and it was not
until September 29, 1846, that he found an object which excited his
suspicions and eventually proved to be the planet sought for. It was
subsequently ascertained that the planet had been recorded as a star on
August 4 and 12, and that the star of August 12 was missing from the
zone observed on July 30. The discovery of the planet was therefore just
missed on August 12 because the results of each evening’s work were not
adequately compared with what had gone before.

Meanwhile things had not been standing still in France. In August 1846,
Le Verrier published a third memoir intended to develope information
respecting the probable position of the planet in the heavens. In
September 23 a summary of this third memoir was received by Encke at
Berlin, accompanied by the request that he would cooperate
instrumentally in the search for it. Encke at once directed two of his
assistants named D’Arrest and Galle to do this, and they were
fortunately well circumstanced for the task. Unlike Challis, who, as we
have seen, could do nothing until he had made a map for himself, the
Berlin observers had one ready to hand, which by good chance had just
been published by the Berlin Academy for the part of the heavens which
both Adams and Le Verrier assigned as the probable locality in which the
anxiously desired planet would be found. Galle called out the visible
stars one by one whilst D’Arrest checked them by the map, and suddenly
he came upon an unmarked object which at the moment looked like an 8th
magnitude star. The following night showed that the suspicious object
was in motion, and it was soon ascertained to be the trans-Uranian
planet which was being searched for. The discovery when announced
excited the liveliest interest all over the world. It did more; it
created a bitter feeling of resentment on the part of French astronomers
that the laurels claimed by them should have been also claimed in an
equal share by a young and unknown Englishman, and accordingly the old
cry of “_perfide Albion_” arose on all sides. I have been particular in
stating the various dates which belong to this narrative, in order to
make as clear as possible the facts of the case. This is even now
necessary, because though the astronomers of England and Germany are
willing to give Adams and Le Verrier each their fair share of this great
discovery, the same impartial spirit is not to be found in France, for
nothing is more common, even in the present day, in looking at French
books of astronomy, than to find Adams’s name either glossed over or
absolutely suppressed altogether when the planet Neptune is under
discussion.

How remarkable a discovery this was, will perhaps be realized, when it
is stated that Adams was only 2½° out in assigning the position of the
new planet, whilst Le Verrier was even nearer, being barely 1° out.

We know practically nothing respecting the physical appearance of
Neptune, owing to its immense distance from us, and for the like reason
the Neptunian astronomers, if there are any, will know absolutely
nothing about the Earth; indeed, their knowledge of the Solar System
will be restricted to Uranus, Saturn, and the Sun. Even the Sun will
only have an apparent diameter of about 1′ of arc, and, therefore, will
only seem to be a very bright star, yielding light equal in amount,
according to Zöllner, to about 700 full moons. There is one satellite
belonging to Neptune, and as this has been calculated to exhibit a disc
10° in diameter, a certain amount of light will no doubt be afforded by
it especially if, as is not unlikely, Neptune is itself possessed of
some inherent luminosity independently of the Sun.

The fact that Neptune seems destitute of visible spots or belts, results
in our being ignorant of the period of its axial rotation, though it
should be stated that in 1883, Maxwell Hall in Jamaica, observed
periodical fluctuations in its light, which he thought implied that the
planet rotated on its axis in rather less than 8 hours. Several
observers thought 20 or 30 years ago, that they had noticed indications
of Neptune being surrounded by a ring like Saturn’s ring, but the
evidence as to this is very inconclusive. It is quite certain that none
but the very largest telescopes in the world would show any such
appendage, and this planet seems to have been neglected of late years,
by the possessors of such telescopes. Moreover, if a ring existed it
would only open out to its full extent once in every 82 years, being the
half of the period of the planet’s revolution round the Sun (just as
Saturn’s ring only opens out to the fullest extent every 14½ years), so
that, obviously, supposing suspicions of a ring dating back 30 or 40
years were well founded, it might well be that another 30 or 40 years
might need to elapse, before astronomers would be in a position to see
their suspicions revive.

Neptune revolves round the Sun in 164½ years, at a mean distance of 2791
millions of miles. Its apparent diameter scarcely varies from 2¾″. Its
true diameter is about 37,000 miles. No compression of the Poles is
perceptible. Its one satellite revolves round Neptune in 5¾ days, and in
a retrograde direction, at a mean distance of 223,000 miles, and shines
as a star of the 14th magnitude. This is a peculiarity which it only
shares with the satellites of Uranus, so far as it regards the planetary
members of the Solar System, though there are many retrograde Comets.

The question has often been mooted, whether there exists, and belonging
to the Solar System, a planet farther off than Neptune. There does seem
some evidence of this, as we shall better understand, when we come to
the subject of long-period Comets, though it cannot be said that much
progress has yet been made in arriving at a solution of the problem.

Unless there does exist a trans-Neptunian planet, a Neptunian astronomer
will know very little about planets, for Uranus and Saturn will alone be
visible to him. Both will of course be what we call “inferior planets,”
and under the best of circumstances will cut a poor figure in the
Neptunian sky.




                             CHAPTER XIII.
                                COMETS.


I suppose that it is the experience of all those who happen to be in any
sense, however humble, specialists in a certain branch of science, that
from time to time, they are beset with questions on the part of their
friends respecting those particular matters which it is known that they
have specially studied. There is no fault to be found with this thirst
for information, always supposing that it is kept within due bounds; but
my motive for alluding to it here, is to see whether any well-marked
conclusion can be drawn from it, within my own knowledge as regards
astronomical facts or events. Now in the case of the science of
astronomy (for which in this connection I, for the moment, will venture
to speak), there is certainly no one department which so unfailingly, at
all times and in all places, seems to evoke such popular sympathy and
interest as the department which deals with Comets.

Sun-spots may come and go; bright planets may shine more brightly; the
Sun or Moon may be obscured by eclipses; temporary stars may burst
forth,—all these things are within the ken of the general public by
means of newspapers or almanacs, but it is a comet which evokes more
questionings and conversations than all the other matters just referred
to put together. When a new and bright comet appears, or even when any
comet not very bright gets talked about, the old question is still fresh
and verdant—“Is there any danger to the Earth to be apprehended from
collision with a Comet?” followed by “What is a Comet?” “What is it made
of?” “Has it ever appeared before?” “Will it come back again?” and so
on. Questions in this strain have more often than I can tell of been put
to me. They seem the stock questions of all who will condescend to
replace for five minutes in the day the newest novel or the pending
parliamentary election.

It may be taken as a fact (though in no proper sense a rule) that a
bright and conspicuous comet comes about once in 10 years, and a very
remarkable comet every 30 years. Thus we have had during the present
century bright comets in 1811, 1825, 1835, 1843, 1858, 1861, 1874 and
1882, whereof those of 1811, 1843, and 1858 were specially celebrated.
Tested then by either standard of words “bright and conspicuous,” or
“specially celebrated,” it may be affirmed that a good comet is now due,
so let us prepare for it by getting up the subject in advance.

I will not attempt to answer in regular order or in any set form the
questions which I have just mentioned as being stock questions, but they
will be answered in substance as we go along. There is one matter in
connection with comets which has deeply impressed itself upon the public
mind, and that is the presence or absence of a “tail.” It is not too
much to say that the generality of people regard the tail of a comet as
_the_ comet; and that though an object may be a true comet from an
astronomer’s point of view, yet if it has no tail its claims go for
nought with the mass of mankind. We have here probably a remnant of
ancient thought, especially of that line of thought which in bygone
times associated Comets universally with the idea that they were
especially sent to be portents of national disasters of one kind or
another. This is brought out by numberless ancient authors, and by none
more forcibly than Shakespeare. Hence we have such passages as the
following in _Julius Cæsar_ (Act ii., sc. 2):—

  “When beggars die there are no comets seen,
  The Heavens themselves blaze forth the death of princes.”

In _Henry VI._ (Part I., Act i., sc. 1) we find the well-known passage:—

  “Comets importing change of times and states
  Brandish your crystal tresses in the sky,
  And with them scourge the bad revolting stars
  That have consented unto Henry’s death.”

There are in point of fact two distinct ideas evolved here: (1) that
comets are prophetic of evil, and (2) stars potential for evil.

There is another passage in _Henry VI_. (Part I., Act iii., sc. 3) even
more pronounced:—

  “Now shine it like a Comet of revenge,
  A prophet to the fall of all our foes.”

Again; in _Hamlet_ (Act i., sc. 1) we find:—

  “As stars with trains of fire, and dews of blood,
  Disasters in the Sun.”

Once more; in the _Taming of the Shrew_ (Act iii., sc. 2) we have the
more general, but still emphatic enough, idea expressed by the simple
words of reference to—

  “Some Comet or unusual prodigy.”

Shakespeare may be said to have lived at the epoch when astrology was in
high favour, and it may be that he only gave utterance to the current
opinion prevalent among all classes in those still somewhat “Dark Ages”
(so called). This, however, can hardly be said of the author of my next
quotation—John Milton (_Paradise Lost_, bk. II.):—

                            “Satan stood
  Unterrified, and like a Comet burned,
  That fires the length of Ophiuchus huge
  In th’ Arctic sky, and from its horrid hair
  Shakes pestilence and war.”

Jumping over a century we find the ancient theory still in vogue, or
Thomson (_Seasons_, Summer) would never have written:—

                  “Amid the radiant orbs
  That more than deck, that animate the sky,
  The life-infusing suns of other worlds;
  Lo! from the dread immensity of space,
  Returning with accelerated course,
  The rushing comet to the sun descends;
  And, as he sinks below the shading earth,
  With awful train projected o’er the heavens,
  The guilty nations tremble.”

Even Napoleon I. had servile flatterers who, as late as 1808, tried to
extract astrological influence out of a comet by way of bolstering up
“Old Bony.” But enough of poetry and fiction, let us hasten back to
prosaic fact.

[Illustration: Fig. 19.—Telescope Comet with a nucleus.]

Comets as objects to look at may be classed under three forms, though
the same comet may undergo such changes as will at different epochs in
its career cause it to put on each variety of form in succession. Thus
the comet of 1825 seen during that year as a brilliant naked-eye object,
after being lost in the sun’s rays, was again found on April 2, 1826 by
Pons. Lamentable were his cries at the miserable plight it was in. He
described it as totally destroyed: without tail, beard, coma or nucleus,
a mere spectre. The simplest form of comet is a mere nebulous mass,
almost always circular, or perhaps a little oval, in outline. It may
maintain this appearance throughout its visibility; or, growing brighter
may become a comet of the second class, with a central condensation,
which developing becomes a “nucleus” or head. It may retain this feature
for the rest of its career, or may pass into the third class and throw
out a “coma” or beard, which will perhaps develop into a tail or tails.
Doing this it will not unfrequently grow bright enough and large enough
to become visible to the naked eye. In exceptional cases the nucleus
will become as bright as a 2nd or even 1st magnitude star, and the tail
may acquire a length of several or many degrees. In the last named case
of all the comet becomes, _par excellence_ according to the popular
sentiment, “a comet.” It will now be readily inferred that the
astronomer in his observatory has to do with many comets which the
public at large never hear of, or if they do hear of, treat with
contempt, because they are destitute of tails.

[Illustration: Fig. 20.—Wells’s Comet of 1882, seen in full daylight
near the Sun on Sept. 18.]

[Illustration: Fig. 21.—Quenisset’s Comet, July 9, 1893 (Quenisset).]

The tails of comets exhibit very great varieties not only of size but of
form; some are long and slender; some are long and much spread out
towards their ends, like quill pens, for instance; some are short and
stumpy, mere tufts or excrescences rather than tails. Not unfrequently a
tail seems to consist of two parallel rays with no cometary matter, or
it may be only a very slight amount of cometary matter traceable in the
interspace; some have one main tail consisting of a pair of rays such as
just described, together with one or more subsidiary or off-shoot tails.
The comet of 1825 had five tails and the comet of 1744 had six tails. It
might be inferred from all this that the tails of comets are so
exceedingly irregular, uncertain and casual as to be amenable to no
laws. This was long considered to be the case; but a Russian observer
named Bredichin, as the result of much study and research, has arrived
at the conclusion that all comet tails may be brought under one or other
of three types; and that each type is indicative of certain distinct
differences of origin and condition which he considers himself able to
define. The first type comprises tails which are long and straight;
“they are formed” (to quote Young’s statement of Bredichin’s views) “of
matter upon which the Sun’s repulsive action is from twelve to fifteen
times as great as the gravitational attraction, so that the particles
leave the comet with a relative velocity of at least four or five miles
a second; and this velocity is continually increased as they recede,
until at last it becomes enormous, the particles travelling several
millions of miles in a day. The straight rays which are seen in the
figure of the tail of Donati’s Comet, tangential to the tail, are
streamers of this first type; as also was the enormous tail of the comet
of 1861. The second type is the curved plume-like train, like the
principal tail of Donati’s Comet. In this type the repulsive force
varies from 2.2 times gravity (for the particles on the convex edge of
the tail) to half that amount for those which form the inner edge. This
is by far the most common type of cometary train. A few comets show
tails of the third type—short, stubby, brushes violently curved, and due
to matter of which the repulsive force is only a fraction of
gravity—from 1/10 to ½.”

Bredichin wishes it to be inferred that the tails of the 1st type are
probably composed of hydrogen; those of the 2nd type of some
hydro-carbon gas; and those of the 3rd of the vapour of iron, probably
with some admixture of sodium and other substances. Bredichin, as a
reason for these conclusions, supposes that the force which generates
the tails of comets is a repulsive force, with a surface action the same
for equal surfaces of any kind of matter; the effective accelerating
force therefore measured by the velocity which it would produce would
depend upon the ratio of surface to mass in the particles acted upon,
and so, in his view, should be inversely proportional to their molecular
weights. Now it happens that the molecular weights of hydrogen, of
hydro-carbon gases, and of the vapour of iron bear to each other just
about the required proportion.

I am here stating the views and opinions of others without definitely
professing to be satisfied with them, but as they have met with some
acceptance, it is proper to chronicle them, though we know nothing of
the nature of the repulsive force here talked about. It might be
electric, it might be anything. The spectroscope certainly lends some
countenance to Bredichin’s views, but we need far more knowledge and
study of comets before we shall be justly entitled to dogmatise on the
subject.

[Illustration: Fig. 22.—Holmes’s Comet, Nov. 9, 1892 (Denning).]

[Illustration: Fig. 23.—Holmes’s Comet, Nov. 16, 1892 (Denning).]

This has been rather a digression. I go back now to prosaic matters of
fact, of which a vast and interesting array present themselves for
consideration in connection with comets. Let us consider a little in
detail what they are, to look at. We have seen that a well-developed
comet of the normal type usually comprises a nucleus, a head or coma,
and a tail. Comets which have no tails generally exhibit heads of very
simple structure; and if there is a nucleus, the nucleus is little else
than a stellar point of light. But in the case of the larger comets,
which are almost or quite visible to the naked eye, the head often
exhibits a very complex structure, which in not a few cases seems to
convey very definite indications of the operations going on at the time.
Figs. 22 and 23 may be taken as samples of a complex cometary head,
though no two comets resemble one another exactly in details. Fig. 24
forcibly conveys the idea that we are looking at a process of
development analogous to an uprush of water from a fountain, or perhaps
I might better say, from a burst waterpipe. There is a distinct idea of
a jet. This self-same idea, in another form, presents itself in the case
of those comets which exhibit what astronomers are in the habit of
calling “luminous envelopes.” The jet in this case is not strictly a jet
because it is not a continuous outflow, or overflow, of matter; the idea
rather suggests itself of an intermittent overflow resulting in
accumulated layers, or strata, of matter becoming visible. But with this
we come to a standstill; we cannot tell where the matter comes from, and
still less, where it goes to; we can only record what our eyes, assisted
by telescopes, tell us. There can, however, I think, be no doubt that
the matter of a comet becomes displayed to our senses as the result of a
process of expulsion, or repulsion, from the nucleus; and then, having
become launched into space, it comes under the influence, also
repulsive, of the Sun. All these things are visible facts. As to causes,
we suggest little, because we know so little. Anyone who has seen a
comet and has watched the displays of jets and luminous envelopes, such
as I have endeavoured to set forth, will realise at once how impossible
it is to describe these things in words. They must be seen either in
actual being or in picture. Some further allusions to this branch of the
subject may perhaps be more advantageously made after we have considered
the movements and orbits of comets.

[Illustration: Fig. 24.—Comet III. of 1862, on Aug 22, showing jet of
luminous matter (Challis).]

There is often a slight general resemblance between a planet and a
comet, as regards the path which each class of body pursues. Probably
the least reflective person likely to be following me here understands
the bare fact, that all the planets revolve round the Sun, and are held
to defined orbits by the Sun’s influence, or attraction, as it is
called. Perhaps, it is not equally realised, that in a somewhat similar,
but not quite the same way, comets are influenced and controlled by the
Sun.

Comets must be considered as regards their motions to be divisible into
two classes:—(1) Those which belong to the Solar System; and (2) those
which do not. Each of these two classes must again be sub-divided, if we
would really obtain a just conception of how things stand.

By the Comets which belong to the Sun, I mean those which revolve round
the Sun in closed orbits;[5] and are, or may be, seen again and again at
recurring intervals. There are 2 or 3 dozen comets which present
themselves to our gaze at stated intervals, varying from about 3 to 70
years. There are again other comets which without any doubt
(mathematically) are revolving round the Sun in closed orbits, but in
orbits so large and with periods of revolution so long (often many
centuries), that though they will return again to the sight of the
inhabitants of the earth some day, yet no second return having been
actually recorded, the astronomer’s prediction that they will return,
remains at present a prediction based on mathematics but nothing more.

There is another class of Comet of which we see examples from time to
time, and having seen them once shall never see again. This is because
these Comets move in orbits which are not closed, and which are known as
parabolic or hyperbolic orbits respectively, because derived from those
sections of a cone which are called the Parabola and the Hyperbola. It
must be understood that what I am now referring to is purely a matter of
orbit, and that no relationship subsists between the size and physical
features of a Comet and the path it pursues in space. The only sort of
reservation, perhaps, to be made to this statement is, that the comets
celebrated for their size and brilliancy, are often found to be
revolving in elliptic orbits of great eccentricity, which means that
their periods may amount to many centuries.

It may be well to say something now as to what is the ordinary career of
a comet, so far as visibility to us, the inhabitants of the Earth, is
concerned. Though this might be illustrated by reference to the history
of many comets, perhaps there is no one more suitable for the purpose
than Donati’s Comet of 1858. In former times, when telescopes were few
or non-existent, brilliant comets often appeared very suddenly, just as
a carriage or a man does, as you turn the corner of a street. Such
things even happen still: for instance, the great comet of 1861 burst
upon us all at once at a day’s notice. Usually, however, now in
consequence of the large size of the telescopes in use, and the great
number of observers who are incessantly on the watch, comets are
discovered when they are very small, because remote both from the Earth
and Sun, and many weeks, or even months, it may be, before they shine
forth in their ultimate splendour. Now, let us see how these statements
are supported by the history of Donati’s comet in 1858. On June 2 in
that year, it was first seen by Donati at Florence, as a faint
nebulosity, slowly journeying northwards. June passed away, and July,
and August, the comet all the while remaining invisible to the naked
eye; that is to say, it first became perceptible to the naked eye on
August 29, having put forth a faint tail about August 20. After the
beginning of September its brilliancy rapidly increased. On September
17, the head equalled in brightness a 2nd magnitude star, the tail being
4° long. Passing its point of nearest approach to the Sun on September
29, it came nearest to the Earth on October 10; though, perhaps, its
appearance a few days previously, namely on October 5, is the thing best
remembered by those who saw it, because it was on that night that the
comet passed over the 1st magnitude star Arcturus. For several days
about this time, the comet was an object of striking beauty in the
Western Heavens, during the hours immediately after sun-set. After
October 10, it rapidly passed away to the Southern hemisphere,
diminishing in brightness, as it did so, because receding from the Earth
and the Sun. It continued its career through the winter; became
invisible to the naked eye; and finally invisible altogether in March
1859. It remained in view, therefore, for more than nine months, not to
return again till about the year 3158 A.D., for its period of revolution
was found to be about 2000 years.

I have been particular in sketching somewhat fully the history of this
comet so far as we are concerned, because, as I have already said, it is
typical of the visible career of many comets. Halley’s comet in 1835 and
1836, went through a somewhat similar series of changes. This comet—a
well-known periodical one of great historic interest and brilliancy—may
be commended to the younger members of the rising generation, because it
is due to return again to these parts of space a few years hence, or in
1910.

[Illustration: Fig. 25.—Sawerthal’s Comet, June 4, 1888 (Charlois).]

What is a comet made of? Men of Science equally with the general public
would like to be able to answer this question, but they cannot do so
with satisfactory certainty. A great many years ago Sir John Herschel
wrote thus:—“It seems impossible to avoid the following conclusion, that
the matter of the nucleus of a comet is powerfully excited and dilated
into a vaporous state by the action of the Sun’s rays escaping in
streams and jets at those points of its surface which oppose the least
resistance, and in all probability throwing that surface or the nucleus
itself into irregular motions by its reaction in the act of so escaping,
and thus altering its direction.” This passage was written of course
before the spectroscope had been brought to bear on the observations of
comets, but so far as Sir John Herschel’s remark implies the presence of
vapour, that is gas, in a comet, the surmise has been amply borne out by
later discoveries. The fact that as a comet approaches the Sun some
forces, no doubt of solar origin, come into operation to vaporise and
therefore expand the matter composing the comet is sufficiently shown by
the great developement which takes place as we have seen in the tails of
comets, but in regard to the heads of comets we are face to face with a
strange enigma. Though the tails expand the heads contract as the comet
approaches its position of greatest proximity to the Sun. Having passed
this point the head expands again. This curious circumstance, first
pointed out by Kepler in 1618, has often been noticed since, and noticed
indeed not as the result of mere eye impressions, but after careful
micrometrical measurement with suitable instruments. I think the
confession must be made that we are hopelessly ignorant of the nature of
comet’s except that gases are largely concerned in their constitution.

It seems impossible to doubt that some tails of comets are hollow
cylinders or hollow cones. Such a theory would account for the fact, so
often noticed, that single tails are usually much brighter at their two
edges than at the centre. This is the natural effect of looking
transversely at any translucent cylinder of measureable thickness.

It was long a moot point whether comets are self-luminous, or whether
they shine by reflected light; but it is now generally admitted that
whilst a part of the light of a comet may be derived by reflection from
the Sun yet as a rule they must be regarded as shining by their own
intrinsic light.

It should be stated here by way of caution that the observations on this
subject are not so consistent as one could wish, and it seems necessary
to assume that all comets are not constituted alike, and that therefore
what is true of one does not necessarily apply to another.

To those who possess telescopes (not necessarily large ones)
opportunities for the study of comets have much multiplied during the
last few years, for we are now acquainted with a group of small comets
which are constantly coming into view at short intervals of time. The
comets have now become so numerous that seldom a year passes without one
or more of them coming into view. Whilst that known as Encke’s revolves
round the Sun in 3¼ years, Tuttle’s doing the same in 13½ years, there
are four others whose periods average about 5½ years, 5 which average 6½
years, together with one of 7½ years and one of 8 years. It is thus
evident that there is a constant succession of these objects available
for study, and that very few months can ever elapse that some one or
more of them are not on view. They bear the names of the astronomers who
either discovered them originally, or who, by studying their orbits,
discovered their periodicity. The names run as follows, beginning with
the shortest in period and ending with the longest:—

  Encke’s.                        Winnecke’s.
  Temple’s Second (1873, II.)     Brorsen’s.
                                  Temple’s First (1867, II.)
  Swift’s (1880, V.)              Wolf’s (1884, III.)
  Barnard’s (1884, II.)           Faye’s.
  D’Arrest’s.                     Denning’s.
  Finlay’s.                       Tuttle’s.

I cannot stay to dwell upon either the history or description of these
comets separately, but must content myself by saying generally that
whilst as a rule they are not visible to the naked eye, yet several of
them may occasionally become so visible when they return to perihelion
under circumstances which bring them more near than usual to the earth.

Several other comets are on record which it was supposed at one time
would certainly have been entitled to a place in the above list, but
three of them in particular have, under very mysterious circumstances,
entirely disappeared from the Heavens.

Chief amongst the mysterious comets must be ranked that which goes by
the name of Biela. This comet, first seen in 1772, was afterwards found
to have a period of about 6¾ years, and on numerous occasions it
reappeared at intervals of that length down to 1845, when the mysterious
part of its career seems to have commenced. In December of that year
this comet threw off a fragment of nearly the same shape as itself, and
the two portions travelled together side by side for four months, the
distance between the fragments slowly increasing. At the end of the four
months in question the comet passed out of sight owing to the distance
from the earth to which it had attained. The comet returned again to
perihelion in 1852, remaining visible for three weeks. The two portions
of the comet noticed in 1846 retained their individuality in 1852, but
the distance between them had increased to about eight times the
greatest distance noticed in 1846. As a comet Biela’s Comet has never
been seen since 1852, and it must now be regarded as having permanently
disappeared. But what seems to have happened is this, that Biela’s Comet
has become broken up into a mass of meteors. On November 27, 1872, and
again in November 1885, when the earth in travelling along its own orbit
reached a certain point where its orbit intersected the former orbit of
Biela’s Comet the Earth encountered, instead of the comet which ought to
have been there, a wonderful mass of meteors; and it is now generally
accepted that these meteors, which apparently are keeping more or less
together as a fairly compact swarm, are nought else than the
disintegrated materials of what once was Biela’s Comet.

[Illustration: Fig. 26.—Biela’s Comet, February 19, 1846.]

It is extremely probable that as time goes on we shall be able to say
that an intimate connection subsists between particular comets which
have been and particular meteoric swarms. We already possess proof that
other comets which once came within our view were at that time revolving
round the Sun in orbits so comparatively small that they should have
reappeared at intervals of half-a-dozen or so years, yet they have not
reappeared. The question therefore suggests itself, Have they been
subject to some great internal disaster which has led to their
disintegration? It may be said without doubt that this is in the highest
degree probable; but short of this, that is short of total
disintegration into small fragments, we have several cases on record of
what I may, for the moment, call ordinary comets breaking up into two or
three fragments. For a long while astronomers were naturally loath to
believe that this was possible, and therefore they discredited the
statements to that effect which had been made. Though it would occupy
too much space to give the particulars of these comets in full it may
yet be worth while just to mention the names of some of them, presumed
to be of short period, which seemed nevertheless to have eluded our
grasp. I would here specially mention Liais’s Comet of 1860 and the
second comet of 1881 as seemingly having undergone some sort of
disruption akin to what happened in the case of Biela’s Comet.

There is another group of periodical comets to be mentioned. These are
six in number and seem to have periods of 70 years or a little more. Of
these three have not yet given us the chance of seeing them again; two
have paid us a second visit, and therefore their periods are not open to
doubt; whilst the most famous of this group, “Halley’s,” has been
recorded to have shown itself to the Earth no less than 25 times,
beginning with the year 11 B. C. It was Halley’s comet which shone over
Europe in April 1066, and was considered the forerunner of the conquest
of England by William of Normandy. It figures in the famous Bayeux
tapestry as a hairy star of strange shape.

It would seem that there exists in some inscrutable manner a connection
between each of the three great exterior planets and certain groups of
comets. In the case of Jupiter the association is so very pronounced as
long ago to have attracted notice; but the French astronomer,
Flammarion, has brought forward some suggestions that Saturn has one
comet (and perhaps two) with which it is associated; Uranus, two (and
perhaps three); and Neptune, six; whilst farther off than Neptune the
fact that there are two comets, supposed periodical, without a known
planet to run with them has inspired Flammarion to look with a friendly
eye on the idea (often mooted) that outside of Neptune there exists
another undiscovered planet revolving round the sun in a period of about
300 years.

The Jupiter group of comets deserves a few additional words. There are
certainly nine, and perhaps twelve comets revolving round the Sun in
orbits of such dimensions that they either reach up to or slightly
overreach the orbit of Jupiter. The effect of this condition of things
is that on occasions Jupiter and each of the comets may come into such
proximity that the superior mass of Jupiter may exercise a very
seriously disturbing influence over a flimsy and ethereal body like a
comet. There is reason to suppose that some of the comets now belonging
to the Jupiter group have not done so for any great length of time, but
having been wandering about, either in elliptic orbits of great extent,
or even in parabolic orbits, have accidentally come within reach of
Jupiter, and so have been, as it were, captured by him. Hence, the
origin of the term, the “capture theory,” as applied to these family
groups of comets which I have just stated to exist, each presided over,
as it were, by a great planet. It may be that at some future time this
theory will help us to a clue to the fact that besides the comets of
Lexell of 1770, Blainpain of 1819, and Di Vico of 1844, short period
comets unaccountably missing, there are several others presumed to have
been revolving in short period orbits when discovered, and as to which
it is very strange that they should not have been seen before their only
recorded visit to us, and equally strange that they should never have
been seen since.

Is there any reason to fear the results of a collision between a comet
and the Earth? None whatever. However vague may be, and in a certain
sense must be, our answer to the question, “What is a comet?” certain is
it that every comet is a very imponderable body—a sort of airy nothing,
a mass of gas or vapour.[6] At the same time it always has been and
perhaps still is difficult to persuade the public that whatever might be
the effect on a comet if it were to strike the Earth, the effect on the
Earth, were it to be struck by a comet, would be _nil_. This is not
altogether a matter of speculation, for according to a calculation by
Hind, on June 30, 1861, the Earth passed into and through the tail of
the great comet of that year at about two-thirds of its distance from
the nucleus. Assuredly there was no dynamical result; but it seems,
however, not unlikely that there was an optical result; at any rate,
traces of something of this sort were noted. Hind himself, in Middlesex,
observed a peculiar phosphorescence or illumination of the sky which he
attributed at the time to an auroral glare. Lowe, in Nottinghamshire
confirmed Hind’s statement of the appearance of the heavens on the same
day. The sky had a yellow auroral glare-like look, and the Sun, though
shining, gave but feeble light. The comet was plainly visible at 7.45 p.
m. (during sunshine), and had a much more hazy appearance than on any
subsequent evening. Lowe adds that his Vicar had the pulpit candles
lighted in the Parish Church at 7 o’clock (it was a Sunday), though only
five days had elapsed since Midsummer day, which itself proves that some
sensation of darkness was felt even while the Sun was shining.

So far as I remember there has been no such thing as a comet panic
during the present generation, at any rate in civilised countries, but
it is on record that there was a very considerable panic in 1832 in
connection with the return of Biela’s Comet in the winter of that year.
Olbers as the result of a careful study in advance of the comet’s
movements found that the comet’s centre would pass only 20,000 miles
within the Earth’s orbit, and that as the nebulosity of the comet had in
1805 been more than 20,000 miles in diameter, it was certain, unless its
dimensions had diminished in the 27 years, that some of the comet’s
matter would overlap the Earth’s orbit; in other words would envelop the
Earth itself, if the Earth happened to be there. This conclusion when it
became public was quite enough to create a panic and make people talk
about the forthcoming destruction of our globe. It was nothing to the
point (in the public mind) that astronomers were able to predict that
the Earth would not reach the place where the comet would cross the
Earth’s orbit until four weeks after the comet had come and gone.
However, we now know that nothing happened, and I am justified in adding
that even if there had been contact, Earth meeting comet face to face,
nothing (serious) would have occurred so far as the Earth was concerned.

This seems a convenient place for referring to a matter which when it
was first broached excited a great deal of interest, but about which one
does not hear much now-a-days. The period of the small comet known as
Encke’s (which, revolving as it does round the Sun in a little more than
three years, has the shortest period of any of the periodical comets)
was found many years ago to be diminishing at each successive return.
That is to say, it always attained its nearest distance from the Sun at
each apparition 2½ hours sooner than it ought to have done. In order to
account for this gradual diminution in the comet’s period Encke
conjectured the existence of a thin ethereal medium sufficiently dense
to affect a light flimsy body like a comet, but incapable of obstructing
a planet. It has been remarked by Hind that “this contraction of the
orbit must be continually progressing, if we suppose the existence of
such a medium; and we are naturally led to inquire, What will be the
final consequence of this resistance? Though the catastrophe may be
averted for many ages by the powerful attraction of the larger planets,
especially Jupiter, will not the comet be at last precipitated on the
Sun? The question is full of interest, though altogether open to
conjecture.”

Astronomers are not altogether agreed as to the propriety of this
explanation. One argument against it is that with _perhaps_ one
exception none of the other short-period comets (all of them small and
presumably deficient in density) seem affected as Encke’s is. On the
other hand Sir John Herschel favoured the explanation just given, as
also does Hind who is the highest living authority on comets. A German
mathematician, Von Asten, who devoted immense labour to the study of the
orbit of Encke’s Comet, thought there should be no hesitation in
accepting the idea of a resisting medium, subject to the limitation that
it does not extend beyond the orbit of Mercury. Von Asten’s allusion to
Mercury touches a subject which belongs more directly to the question of
Mercury’s orbit and to that other very interesting question, “Are there
any planets, not at present known, revolving round the Sun within the
orbit of Mercury.”

Which is the largest and most magnificent comet recorded in history? It
is virtually impossible to answer this question, because of the
extravagant and inflated language made use of by ancient and medieval (I
had almost added, and modern) writers. There is no doubt that the comet
of 1680, studied by Sir I. Newton, the tail of which was curved, and
from 70° to 90° long, must have been one of the finest on record, as it
was also the one which came nearest to the Sun, for it almost grazed the
Sun’s surface.

The comet of 1744, visible as it was in broad daylight, was, no doubt,
the finest comet of the 18th century, though in size it has been
surpassed; yet its six tails must have made it a most remarkable object.
So far as the 19th century is concerned, our choice lies between the
comets of 1811, 1843, 1858, and 1861. The comet of 1811 is spoken of by
Hind as “perhaps the most famous of modern times. Independently of its
great magnitude, the position of the orbit and epoch of perihelion
passage, were such as to render it a very splendid circumpolar object
for some months.” The tail as regards its length was not so very
remarkable, for at its best, in October 1811, it was only about 25°
long, its breadth, however, was very considerable; at one time 6°, the
real length of the tail, about the middle of October, was more than
100,000,000 of miles, and its breadth about 15,000,000 of miles. The
visibility of this comet was coincident with those events which proved
to be the turning-point in the career of Napoleon I., and there were not
wanting those who regarded the comet as a presage of his disastrous
failure in Russia. Owing to the long period (17 months), during which
this comet was visible, it was possible to determine its orbit with
unusual precision. Argelander found its period to be 3065 years, with no
greater uncertainty than 43 years. The great dimensions of its orbit
will be realised when it is stated that this comet recedes from the Sun
to a distance of 14 times that of the planet Neptune.

[Illustration: Fig. 27.—The Great Comet of 1811.]

Donati’s comet of 1858, has already received a good deal of notice at my
hands, but the question remains, what are its claims, to be regarded as
the comet of the century, compared with that of 1843? It is not a little
strange that though there must have been many persons who saw both, yet
it was only quite recently that I came across, for the first time, a
description of both these comets from the same pen. It ought, however,
to be mentioned by way of explanation, that the inhabitants of Europe
only saw the comet of 1843, when its brilliancy and the extent of its
tail had materially diminished, about a fortnight after it was at its
best.

The description of these two comets to which I have alluded, will be
found in General J. A. Ewart’s “_Story of a Soldier’s Life_,” published
in 1881. Writing first of all of the comet of 1843, General Ewart says:—

“It was during our passage from the Cape of Good Hope to the Equator,
and when not far from St. Helena, that we first came in sight of the
great comet of 1843. In the first instance a small portion of the tail
only was visible, at right angles to the horizon; but night after night
as we sailed along, it gradually became larger and larger, till at last
up came the head, or nucleus, as I ought properly to call it. It was a
grand and wonderful sight, for the comet now extended the extraordinary
distance of one-third of the heavens, the nucleus being, perhaps, about
the size of the planet Venus.”—(Vol. i., p. 75.)

[Illustration: Fig. 28.—The Great Comet of 1882, on October 19 (Artus).]

As regards Donati’s comet of 1858, what the General says is:—

“A very large comet made its appearance about this time, and continued
for several weeks to be a magnificent object at night; it was, however,
_nothing to the one I had seen in the year 1843_, when on the other side
of the equator.”—(Vol. ii., p. 205.)

Passing over the great comet of 1861, on which I have already said a
good deal, I must quit the subject of famous comets by a few words about
that of 1882, which, though by no means one of the largest, was, in some
respects, one of the most remarkable of modern times. It was visible for
the long period of nine months, and was conspicuously prominent to the
naked eye during September, but these facts, though note-worthy, would
not have called for particular remark, had not the comet exhibited some
special peculiarities which distinguished it from all others. In the
first place, it seems to have undergone certain disruptive changes, in
virtue of which the nucleus became split up into four independent
nuclei. Then the tail may have been tubular, its extremity being not
only bifid, but totally unsymmetrical with respect to the main part. The
tubular character of the tail was suggested by Tempel. To other
observers, this feature gave the idea of the comet, properly so-called,
being enclosed in a cylindrical envelope, which completely surrounded
the comet, and overlapped it for a considerable distance at both ends.
Finally (and in this resembling Biela’s comet) the comet of 1882 seems
to have thrown off a fragment which became an independent body.

What has gone before, will, I think, have served abundantly to establish
the position with which I started, namely, that comets occupy (and
deservedly so) the front rank amongst those astronomical objects in
which, on occasions, the general public takes an interest.


I have thus completed, so far as the space at my disposal has permitted,
a popular descriptive Survey of the Solar System. Those who have perused
the preceding pages, however slight may have been their previous
acquaintance with the Science of Astronomy taken as a whole, will have
no difficulty in realising that what I have said bears but a small
proportion to what I have left unsaid. They will equally, I hope, be
able to see, without indeed the necessity of a suggestion, that all
those wondrous orbs which we call the planets could neither have come
into existence nor have been maintained in their allotted places for so
many thousands of years, except as the result of Design emanating from
an All-powerful Creator.




                               FOOTNOTES.


[1]The remark in the text applies to all the major planets and to a
    large number of the minor planets, but certain of the minor planets
    travel in orbits which are considerably inclined to the ecliptic,
    and therefore to all the other planets.

[2]Given in full in my _Handbook of Astronomy_, 4th ed., vol. i., p. 26.

[3]“_Recollections of Past Life_,” 2nd ed., p. 305.

[4]For some information respecting these Secchi “Types” of Stars, see my
    “_Story of the Stars_,” 2nd ed., p. 140.

[5]The circle and the ellipse are what are called “closed” curves.

[6]It is not a little singular that the Chinese in bygone centuries have
    often alluded to comets under the name of vapours; _e.g._, the comet
    of 1618 is recorded as having been “a white vapour 20 cubits long.”




                 APPENDIX.—TABLES OF THE SOLAR SYSTEM.


                          THE SUN AND PLANETS.

 Name.        Sidereal  Mean      Diameter.  Surface.  Volume.    Mass.     Density.  Axial      Force of  Velocity
              period.   distance  Miles.     Earth=1.  Earth=1.   Earth=1.  Earth=1.  Rotation.  gravity.  in orbit.
                        from                                                          d. h. m.   Fall:     Miles per
                        Sun.                                                                     Ft. in 1  hour
                        Millions                                                                 sec.
                        of
                        miles.
 SUN          .....     .....     866,200    11,946    1,305,000  332,000   0.25      25 7 48    461
                                                                                      h. m. s.
 MERCURY      88 days.  36        3,008      0.144     0.055      0.066     1.26      24 5 30    7         107,000
 VENUS        225 ″     67        7,480      0.891     0.841      0.782     0.92      23 21 23   14        78,000
 EARTH        365 ″     93        7,926      1.000     1.000      1.000     1.00      23 56 4    16        66,000
 MARS         687 ″     141       5,000      0.398     0.251      0.107     0.45      24 37 23   4         53,000
 Minor Planets
 Eros (433)   1.76      .....     18         Eros is the nearest to the Sun of the Minor Planets, part
              years.                         of its orbit falling between the Earth and Mars.
 Vesta (4)    3.6 ″     219       214        Vesta is the largest of the Minor Planets.
 Thule (279)  8.8 ″     396       .....      Thule is the most distant from the Sun of the Minor Planets.
 JUPITER      11.8 ″    483       88,439     124       1,389      317       0.23      9 55 21    41        29,000
 SATURN       29.4 ″    886       75,036     89        848        94        0.11      10 29 17   18        21,000
 URANUS       84.0 ″    1782      30,875     15        59         14        0.25      ?          13        15,000
 NEPTUNE      164.6 ″   2792      37,205     21        103        17        0.17      ?          12        12,000


                        SATELLITE OF THE EARTH.

 Name.        Sidereal  Distance  Diameter   Surface.  Volume.    Mass.     Density.  Axial      Force of  Velocity
              period.   from      Miles.     Earth=1.  Earth=1.   Earth=1.  Earth=1.  Rotation.  gravity.  in orbit.
              d. h. m.  Earth.                                                        d. h. m.   Fall:     Miles per
                        Miles.                                                                   Feet in   hour.
                                                                                                 1 sec.
  MOON        27 7 43   237,300   2,160      0.074     0.02034    0.0128    0.63      27 7 43    2.48      2,273


                          SATELLITES OF MARS.

 Name.      Discoverer.      Mean      Sidereal  Diameter.  Maximum      Apparent
                             distance  period.   Miles.     elongation.  star
                             from      d. h. m.             ″            magnitude.
                             Mars.
                             Miles.
 1. PHOBOS  A. Hall, August  6,000     0 7 39    7          12           11½
            17, 1877.
 2. DEIMOS  A. Hall, August  15,000    1 6 18    6          32           13½
            11, 1877.


                         SATELLITES OF JUPITER.

 Name.        Discoverer.       Mean       Sidereal  Diameter.  Apparent    Apparent
                                distance   period.   Miles.     diameter    star
                                from       d. h. m.             of Jupiter  magnitude.
                                Jupiter.                        seen from
                                Miles.                          satellite.
                                                                ° ″
 5.           Barnard           .......    0 11 49   ?          .....       ....
 1. IO        ...............   267,000    1 18 29   2,390      19 49       7
 2. EUROPA    Galileo, January  425,000    3 13 13   2,120      12 25       7
              7-13, 1610.
 3. GANYMEDE  ...............   678,000    7 4 0     3,980      7 47        6
 4. CALLISTO  ...............   1,192,000  16 18 5   2,970      4 25        7


                         SATELLITES OF SATURN.

 Name.         Discoverer.        Mean        Sidereal  Diameter.  Apparent    Apparent
                                  distance    period.   Miles.     diameter    star
                                  from        d. h. m.             of Saturn   magnitude.
                                  Saturn.                          seen from
                                  Miles.                           satellite.
                                                                   °
 1. MIMAS      Sir W. Herschel,   115,000     0 22 37   1,000      33          17
               Sept, 17, 1789
 2. ENCELADUS  ″ ″ Aug. 28, 1789  147,000     1 8 53    ?          26          15
 3. TETHYS     J. D. Cassini,     183,000     1 21 18   500        21          13
               March, 1684
 4. DIONE      ″ ″ March, 1684    234,000     2 17 41   500        16          12
 5. RHEA       ″ ″ Dec. 23, 1672  327,000     4 12 25   1,200      12          10
 6. TITAN      Huygens, Mar. 25,  758,000     15 22 41  3,300      5           8
               1653
 7. HYPERION   W. Bond &          916,000     21 7 7    ?          4           17
               Lassell, Sept,
               19, 1848
 8. IAPETUS    J. D. Cassini,     2,221,000   79 7 53   1,800      2           9
               Oct. 25, 1671


                         SATELLITES OF URANUS.

 Name.       Discoverer.            Mean distance  Sidereal  Maximum
                                    from Uranus.   period.   elongation.
                                    Miles.         d. h. m.  ″
 1. ARIEL    Lassell, September     124,000        2 12 28   12
             14, 1847
 2. UMBRIEL  O. Struve, October 8,  173,000        4 3 27    15
             1847
 3. TITANIA  Sir W. Herschel,       285,000        8 16 55   33
             January 11, 1787
 4. OBERON   ″ ″                    381,000        13 11 6   44


                         SATELLITE OF NEPTUNE.

  Discoverer.        Mean distance      Sidereal  Maximum      Apparent
                     from Neptune.      period.   elongation.  star
                     Miles.             d. h. m.  ″            magnitude.
  Lassell, October   223,000            5 21 8    18           14
  10, 1846




                             GENERAL INDEX.


                                   A.
  Adams, J. C., 145.
  Airy, Sir G. B., 145.
  Anagram on Venus, 68;
      on Saturn, 127.
  Aphelion, 11, 102.
  Apparent movements of the Planets, 12, 14.
  Arago, D. J. F., 99, 124, 140.
  Arctic regions of the Earth, 75.
  Argelander, F. G. A., 178.
  Aristarchus of Samos, 71.
  Aristarchus (Lunar Mountain), 93, 94.
  Asteroids, 110.
  Aurora Borealis and spots on the Sun, 55;
      and twinkling, 87.
  Autumnal Equinox, 74.
  Axial rotation of the Planets, 17, 183.

                                   B.
  Barnard. E. E., 120, 134.
  Barnard’s Comet, 168.
  Bayeux Tapestry, 171.
  Beer and Mädler;
      their map of the Moon, 96.
  Belts on Jupiter, 115, 123;
      on Saturn, 122.
  Bergeron’s Experiment, 93.
  Bessel, W., 124.
  Biela’s Comet, 168.
  Bode’s so-called “Law,” 110.
  Bond, G. P., 132, 133.
  Bouvard, A., 143.
  Bradley, Rev. J., 144.
  Bredechin’s theory of Tails of Comets, 157.
  Brorsen’s Comet, 167.

                                   C.
  “Canals” on Mars, 106.
  Carpenter, J., 134.
  Carrington, R. C., 46, 49.
  Cassini, J. D., 36, 118, 128, 130, 136.
  Ceres (Minor Planet), 112.
  Challis, Rev. J., 146.
  Charts, Celestial, 112, 147.
  Chinese observations, 25, 172.
  Clouds influenced by the Moon, 98.
  Coggia’s Comet, 1874, 151.
  Comets, 150;
      periodic, 162, 170;
      remarkable, 176.
  Comparative sizes of the Planets, 11.
  Comparative size of the Sun from the Planets, 17.
  Compression of the Planets, 70, 122, 137.
  Conjunction of the Planets, 14, 15, 66.
  Copernican System, 72.
  Copernicus, 72, 73, 76.

                                   D.
  D’Arrest, 147.
  D’Arrest’s Comet, 168.
  Dawes, W. R., 106, 133.
  Day and night, 12.
  Day, length of, 74.
  De La Rue, W., 35, 36, 49, 52.
  Denning’s Comet, 168.
  Density of the Planets, 17, 183.
  Diameter of the Sun and Planets, 183.
  Di Vico, 62, 63.
  Di Vico’s Comet, 172.
  Distance of the Planets from the Sun, 20, 183.
  Donati’s Comet of 1858, 157, 163, 178.
  Dufour on Twinkling, 84.

                                   E.
  Earth, annual motion of, 75;
      figure of, 70.
  Earthshine, 66, 93.
  Eccentricity of Planetary orbits, 10, 60, 75.
  Eclipses of the Sun, 56, 57;
      of the Moon, 120.
  Ecliptic, plane of, 9, 10, 74.
  Egyptian System, 72.
  Elliptic Orbits, 10.
  Elongation of Planets, 14.
  Encke, J. F., 147.
  Encke’s Comet, 167, 174, 175.
  Equinoxes, 74.
  Ewart, Gen. J. A., 178.

                                   F.
  Faculæ on the Sun, 42, 43.
  Faye’s Comet, 168.
  Finlay’s Comet, 168.
  Flammarion, C., 62, 66, 171.
  Flamsteed, Rev. J., 140, 144.

                                   G.
  Galileo, 89, 101, 120, 125.
  Galle, J. G., 147.
  Gassendi, 76.
  Geodesy, 70.
  Georgium Sidus, 141.
  Goldschmidt, H., 4.
  Granules, Solar, 38.
  Gravity on the Sun and Planets, 183.

                                   H.
  Hall, A., 109.
  Halley’s Comet, 164, 171.
  Heat rays of the Sun, 20;
      of the Moon, 96.
  Herschel, Sir W., 34, 40, 44, 58, 100, 103, 134, 137, 138, 142.
  Herschel, Sir J. F. W., 24, 96, 99, 103, 132, 175.
  Hind, J. R., 119, 173, 175.
  Holden, E. S., 106.
  Holland, Sir H., 78.
  Holmes’s Comet, 1892, 159.
  Homer’s _Iliad_, cited, 68.
  Howlett, Rev. F., 47.
  Huggins, W., 40.
  Huygens, C., 127, 130.
  Hyperbola, 162.
  Hyperbolic Comets, 162.

                                   I.
  Inclination of planetary orbits, 9.
  Inferior Planets, 9, 12.

                                   J.
  Janssen J., 47.
  Juno (Minor Planet), 112.
  Jupiter: its influence on Sun-spots doubtful, 51, 53;
      on Comets, 171;
      its light, 7.

                                   K.
  Kepler, 55, 125, 166.
  Kew Observatory, 47.

                                   L.
  La Hire, 63.
  Lalande, 34.
  Lassell, W., 123, 142, 185.
  Ledger, Rev. E., 84.
  Le Monnier, 140, 144.
  Le Verrier, U. J. J., 147.
  Lexell’s Comet, 172.
  Liais’s Comet, 170.
  Logogriphs as to Venus, 68;
      as to Saturn, 127.

                                   M.
  Mädler, J. H., 63, 96.
  Magnetism, Terrestrial, 55.
  Maps, Astronomical, 112, 147.
  Mars, 100.
  Maskelyne, Rev. N., 139.
  Mass of the Sun, and of the Planets, 20, 183.
  Medium, Resisting, 175.
  Mercury, 57;
      phases of, 15;
      its influence on Sun-spots, 51;
      its luminosity, 67.
  Milton, J., _Paradise Lost_, cited, 153.
  Minor Planets, 110.
  Montigny on Twinkling, 84.
  Moon, 89.
  Moonlight, Brightness of, 96.
  Motions of the Planets, 10.
  Mountains of the Moon, 90.

                                   N.
  Napoleon I., 154, 176.
  Nasmyth, J., 40.
  Needle, Magnetic, 55.
  Neptune, 143;
      its influence on Uranus, 143.
  Newton, Sir I., 176.
  Nubian after-glow, 83.
  Nucleus of a Sun-spot, 22.

                                   O.
  Obliquity of the ecliptic, 74.
  Occultations of Jupiter’s Satellites, 121.
  Olbers, W., 112, 174.
  Opposition of Planets, 13.
  Orbits of the Planets, 9, 10;
      of Comets, 161.

                                   P.
  Pallas (Minor Planet), 112, 113, 114.
  Parabola, 162.
  Penumbra of a Sun-spot, 22.
  Perihelion, 11, 102.
  Periodic Comets, 162, 170.
  Perturbations of Uranus by Neptune, 145.
  Phases of an inferior Planet, 15;
      of a superior Planet, 16;
      of Mars, 100;
      of Jupiter, 115.
  Piazzi, G., 111.
  Planets, classification of, 7;
      comparative sizes of, 11;
      movements of, 10.
  Plutarch, 71.
  Poles of Mars, snow at, 104.
  Primary Planets, 7.
  Ptolemaic System, 72.

                                   Q.
  Quadrature of a Planet, 16.
  Quenisset’s Comet, 1893, 156.

                                   R.
  Rays of the Sun, 20.
  Refraction, 77;
      effect of, 77.
  Red spot on Jupiter, 118.
  Resisting Medium, 175.
  Retrograde motion of a Planet, 15.
  “Rice grains” on the Sun, 39, 40.
  Rings of Saturn, 128.
  Rosse, Earl of, 93.
  Rotation of the Sun, 24, 28;
      of the planets, 17, 183.
  Rotundity of the Earth, 76.

                                   S.
  Satellites of Mars, 16, 184;
      of Jupiter, 16, 184;
      of Saturn, 16, 185;
      of Uranus, 16, 185;
      of Neptune, 16, 185.
  Saturn, 122.
  Sawerthal’s Comet, 165.
  Schiaparelli, J. V., 59, 62, 63, 106.
  Schmidt, J. F. F., 96.
  Schröter, J. J., 58, 62, 63.
  Schwabe’s observations of Sun spots, 46.
  Seas, Lunar, 94.
  Seasons on the Earth, 54, 74.
  Secchi, A., 28, 35, 42, 50, 84, 86, 106.
  Secondary Planets, 8.
  Shakespeare, citations from, 152.
  Smyth, C. P., 92.
  Snow on Venus, 66;
      on Mars, 104;
      doubtful on Saturn, 123.
  Solstices, 74.
  Spherical form of the Earth, 70.
  Spörer, 46, 50.
  Spots on the Sun, 21, 118;
      on Venus, 61;
      on Jupiter, 117;
      on Saturn, 123.
  Stewart, B., 52, 56.
  Struve, O., 134, 142.
  Sun, 18, 183.
  Superior Planets, 8;
      movements of, 15.
  Surfaces of the Sun and Planets, 183.
  Swift’s Comet, 168.
  Systems of the Universe, 72.

                                   T.
  Tables of the Major Planets, 183.
  Tails of Comets, 154.
  Temple’s Periodical Comets, 167.
  Terminator of the Moon, 90.
  Thomson’s _Seasons_ cited, 153.
  Tides, 98.
  Total Eclipses of the Sun, 57.
  Transits of inferior Planets, 13, 25, 65.
  Trouvelot, 133.
  Turtle’s Periodical Comet, 167, 168.
  Twilight, 80.
  Twinkling, 83.
  Tycho (Lunar Mountains), 94.
  Tycho Brahe, 73.
  Tychonic System, 73.

                                   U.
  Umbra of a Sun-spot, 22.
  Uranus, 138;
      the influence of Neptune on, 143.

                                   V.
  Velocity of the planets, 183.
  Venus, 61;
      phases of, 15;
      its influence on Sun-spots, 51, 52, 53.
  Vernal Equinox, 74.
  Vesta (minor planet), 112, 113.
  Virgil, citation from, 50.
  Volume of the Sun, 20;
      of the Planets, 183.

                                   W.
  Weather influences ascribed to the Sun, 44;
      to the Moon, 98.
  “Willow leaves” on the Sun, 39, 40.
  Wilson’s theory of Sun-spots, 32.
  Winnecke’s Periodical Comet, 167.
  Wolf, R., observer of Sun-spots, 45, 48.
  Wolf’s Periodical Comet, 168.

                                   Y.
  Young, C. A., 131.

                                   Z.
  Zodiac, movement of Planets in, 15.


                                THE END.




                          Transcriber’s Notes


--Some palpable typographical errors were corrected.

--Copyright and publisher’s information was included from the printed
  copy: this eBook is public domain in the country of publication.