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.