THE SOLAR SYSTEM.
Exceptional Importance of the Sun and Moon--The Course to be
pursued--The Order of Distance--The Neighbouring Orbs--How are they
to be discriminated?--The Planets Venus and Jupiter attract Notice
by their Brilliancy--Sirius not a Neighbour--The Planets Saturn and
Mercury--Telescopic Planets--The Criterion as to whether a Body is
to be ranked as a Neighbour--Meaning of the word _Planet_--Uranus
and Neptune--Comets--The Planets are illuminated by the Sun--The
Stars are not--The Earth is really a Planet--The Four Inner
Planets, Mercury, Venus, the Earth, and Mars--Velocity of the
Earth--The Outer Planets, Jupiter, Saturn, Uranus, Neptune--Light
and Heat received by the Planets from the Sun--Comparative Sizes of
the Planets--The Minor Planets--The Planets all revolve in the same
Direction--The Solar System--An Island Group in Space.
In the two preceding chapters of this work we have endeavoured to
describe the heavenly bodies in the order of their relative importance
to mankind. Could we doubt for a moment as to which of the many orbs in
the universe should be the first to receive our attention? We do not now
allude to the intrinsic significance of the sun when compared with other
bodies or groups of bodies scattered through space. It may be that
numerous globes rival the sun in real splendour, in bulk, and in mass.
We shall, in fact, show later on in this volume that this is the case;
and we shall then be in a position to indicate the true rank of the sun
amid the countless hosts of heaven. But whatever may be the importance
of the sun, viewed merely as one of the bodies which teem through space,
there can be no hesitation in asserting how immeasurably his influence
on the earth surpasses that of all other bodies in the universe
together. It was therefore natural--indeed inevitable--that our first
examination of the orbs of heaven should be directed to that mighty body
which is the source of our life itself.
Nor could there be much hesitation as to the second step which ought to
be taken. The intrinsic importance of the moon, when compared with other
celestial bodies, may be small; it is, indeed, as we shall afterwards
see, almost infinitesimal. But in the economy of our earth the moon has
played, and still plays, a part second only in importance to that of the
sun himself. The moon is so close to us that her brilliant rays pale to
invisibility countless orbs of a size and an intrinsic splendour
incomparably greater than her own. The moon also occupies an exceptional
position in the history of astronomy; for the law of gravitation, the
greatest discovery that science has yet witnessed, was chiefly
accomplished by observations of the moon. It was therefore natural that
an early chapter in our Story of the Heavens should be devoted to a body
the interest of which approximated so closely to that of the sun
himself.
But the sun and the moon having been partly described (we shall
afterwards have to refer to them again), some hesitation is natural in
the choice of the next step. The two great luminaries being abstracted
from our view, there remains no other celestial body of such exceptional
interest and significance as to make it quite clear what course to
pursue; we desire to unfold the story of the heavens in the most natural
manner. If we made the attempt to describe the celestial bodies in the
order of their actual magnitude, our ignorance must at once pronounce
the task to be impossible. We cannot even make a conjecture as to which
body in the heavens is to stand first on the list. Even if that
mightiest body be within reach of our telescopes (in itself a highly
improbable supposition), we have not the least idea in what part of the
heavens it is to be sought. And even if this were possible--if we were
able to arrange all the visible bodies rank by rank in the order of
their magnitude and their splendour--still the scheme would be
impracticable, for of most of them we know little or nothing.
We are therefore compelled to adopt a different method of procedure, and
the simplest, as well as the most natural, will be to follow as far as
possible the order of distance of the different bodies. We have already
spoken of the moon as the nearest neighbour to the earth; we shall next
consider some of the other celestial bodies which are comparatively near
to us; then, as the subject unfolds, we shall discuss the objects
further and further away, until towards the close of the volume we shall
be engaged in considering the most distant bodies in the universe which
the telescope has yet revealed to us.
Even when we have decided on this principle, our course is still not
free from ambiguity. Many of the bodies in the heavens are in motion, so
that their relative distances from the earth are in continual change;
this is, however, a difficulty which need not detain us. We shall make
no attempt to adhere closely to the principle in all details. It will be
sufficient if we first describe those great bodies--not a very numerous
class--which are, comparatively speaking, in our vicinity, though still
at varied distances; and then we shall pass on to the uncounted bodies
which are separated from us by distances so vast that the imagination is
baffled in the attempt to realise them.
Let us, then, scan the heavens to discover those orbs which lie in our
neighbourhood. The sun has set, the moon has not risen; a cloudless sky
discloses a heaven glittering with countless gems of light. Some are
grouped together into well-marked constellations; others seem scattered
promiscuously, with every degree of lustre, from the very brightest down
to the faintest point that the eye can just glimpse. Amid all this host
of objects, how are we to identify those which lie nearest to the earth?
Look to the west: and there, over the spot where the departing sunbeams
still linger, we often see the lovely evening star shining forth. This
is the planet Venus--a beauteous orb, twin-sister to the earth. The
brilliancy of this planet, its rapid changes both in position and in
lustre, would suggest at once that it was much nearer to the earth than
other star-like objects. This presumption has been amply confirmed by
careful measurements, and therefore Venus is to be included in the list
of the orbs which constitute our neighbours.
Another conspicuous planet--almost rivalling Venus in lustre, and
vastly surpassing Venus in the magnificence of its proportions and its
retinue--has borne from antiquity the majestic name of Jupiter. No doubt
Jupiter is much more distant from us than Venus. Indeed, he is always at
least twice as far, and sometimes as much as ten times. But still we
must include Jupiter among our neighbours. Compared with the host of
stars which glitter on the heavens, Jupiter must be regarded as quite
contiguous. The distance of the great planet requires, it is true,
hundreds of millions of miles for its expression; yet, vast as is that
distance, it would have to be multiplied by tens of thousands, or
hundreds of thousands, before it would be long enough to span the abyss
which intervenes between the earth and the nearest of the stars.
Venus and Jupiter have invited our attention by their exceptional
brilliancy. We should, however, fall into error if we assumed generally
that the brightest objects were those nearest to the earth. An observer
unacquainted with astronomy might not improbably point to the Dog
Star--or Sirius, as astronomers more generally know it--as an object
whose exceptional lustre showed it to be one of our neighbours. This,
however, would be a mistake. We shall afterwards have occasion to refer
more particularly to this gem of our southern skies, and then it will
appear that Sirius is a mighty globe far transcending our own sun in
size as well as in splendour, but plunged into the depths of space to
such an appalling distance that his enfeebled rays, when they reach the
earth, give us the impression, not of a mighty sun, but only of a
brilliant star.
The principle of selection, by which the earth's neighbours can be
discriminated, will be explained presently; in the meantime, it will be
sufficient to observe that our list is to be augmented first by the
addition of the unique object known as Saturn, though its brightness is
far surpassed by that of Sirius, as well as by a few other stars. Then
we add Mars, an object which occasionally approaches so close to the
earth that it shines with a fiery radiance which would hardly prepare
us for the truth that this planet is intrinsically one of the smallest
of the celestial bodies. Besides the objects we have mentioned, the
ancient astronomers had detected a fifth, known as Mercury--a planet
which is usually invisible amid the light surrounding the sun. Mercury,
however, occasionally wanders far enough from our luminary to be seen
before sunrise or after sunset. These five--Mercury, Venus, Mars,
Jupiter, and Saturn--comprised the planets known from remote antiquity.
We can, however, now extend the list somewhat further by adding to it
the telescopic objects which have in modern times been found to be among
our neighbours. Here we must no longer postpone the introduction of the
criterion by which we can detect whether a body is near the earth or
not. The brighter planets can be recognised by the steady radiance of
their light as contrasted with the incessant twinkling of the stars. A
little attention devoted to any of the bodies we have named will,
however, point out a more definite contrast between the planets and the
stars.
Observe, for instance, Jupiter, on any clear night when the heavens can
be well seen, and note his position with regard to the constellations in
his neighbourhood--how he is to the right of this star, or to the left
of that; directly between this pair, or directly pointed to by that. We
then mark down the place of Jupiter on a celestial map, or we make a
sketch of the stars in the neighbourhood showing the position of the
planet. After a month or two, when the observations are repeated, the
place of Jupiter is to be compared again with those stars by which it
was defined. It will be found that, while the stars have preserved their
relative positions, the place of Jupiter has changed. Hence this body is
with propriety called a _planet_, or a wanderer, because it is
incessantly moving from one part of the starry heavens to another. By
similar comparisons it can be shown that the other bodies we have
mentioned--Venus and Mercury, Saturn and Mars--are also wanderers, and
belong to that group of heavenly bodies known as planets. Here, then, we
have the simple criterion by which the earth's neighbours are readily to
be discriminated from the stars. Each of the bodies near the earth is a
planet, or a wanderer, and the mere fact that a body is a wanderer is
alone sufficient to prove it to be one of the class which we are now
studying.
Provided with this test, we can at once make an addition to our list of
neighbours. Amid the myriad orbs which the telescope reveals, we
occasionally find one which is a wanderer. Two other mighty planets,
known as Uranus and Neptune, must thus be added to the five already
mentioned, making in all a group of seven great planets. A vastly
greater number may also be reckoned when we admit to our view bodies
which not only seem to be minute telescopic objects, but really are
small globes when compared with the mighty bulk of our earth. These
lesser planets, to the number of more than four hundred, are also among
the earth's neighbours.
We should remark that another class of heavenly bodies widely differing
from the planets must also be included in our system. These are the
comets, and, indeed, it may happen that one of these erratic bodies will
sometimes draw nearer to the earth than even the closest approach ever
made by a planet. These mysterious visitors will necessarily engage a
good deal of our attention later on. For the present we confine our
attention to those more substantial globes, whether large or small,
which are always termed planets.
Imagine for a moment that some opaque covering could be clasped around
our sun so that all his beams were extinguished. That our earth would be
plunged into the darkness of midnight is of course an obvious
consequence. A moment's consideration will show that the moon, shining
as it does by the reflected rays of the sun, would become totally
invisible. But would this extinction of the sunlight have any other
effect? Would it influence the countless brilliant points that stud the
heavens at midnight? Such an obscuration of the sun would indeed produce
a remarkable effect on the sky at night, which a little attention would
disclose. The stars, no doubt, would not exhibit the slightest change in
brilliancy. Each star shines by its own light and is not indebted to the
sun. The constellations would thus twinkle on as before, but a
wonderful change would come over the planets. Were the sun to be
obscured, the planets would also disappear from view. The midnight sky
would thus experience the effacement of the planets one by one, while
the stars would remain unaltered. It may seem difficult to realise how
the brilliancy of Venus or the lustre of Jupiter have their origin
solely in the beams which fall upon these bodies from the distant sun.
The evidence is, however, conclusive on the question; and it will be
placed before the reader more fully when we come to discuss the several
planets in detail.
Suppose that we are looking at Jupiter high in mid-heavens on a winter's
night, it might be contended that, as the earth lies between Jupiter and
the sun, it must be impossible for the rays of the sun to fall upon the
planet. This is, perhaps, not an unnatural view for an inhabitant of
this earth to adopt until he has become acquainted with the relative
sizes of the various bodies concerned, and with the distances by which
those bodies are separated. But the question would appear in a widely
different form to an inhabitant of the planet Jupiter. If such a being
were asked whether he suffered much inconvenience by the intrusion of
the earth between himself and the sun, his answer would be something of
this kind:--"No doubt such an event as the passage of the earth between
me and the sun is possible, and has occurred on rare occasions separated
by long intervals; but so far from the transit being the cause of any
inconvenience, the whole earth, of which you think so much, is really so
minute, that when it did come in front of the sun it was merely seen as
a small telescopic point, and the amount of sunlight which it
intercepted was quite inappreciable."
The fact that the planets shine by the sun's light points at once to the
similarity between them and our earth. We are thus led to regard our sun
as a central fervid globe associated with a number of much smaller
bodies, each of which, being dark itself, is indebted to the sun both
for light and for heat.
That was, indeed, a grand step in astronomy which demonstrated the
nature of the solar system. The discovery that our earth must be a
globe isolated in space was in itself a mighty exertion of human
intellect; but when it came to be recognised that this globe was but one
of a whole group of similar objects, some smaller, no doubt, but others
very much larger, and when it was further ascertained that these bodies
were subordinated to the supreme control of the sun, we have a chain of
discoveries that wrought a fundamental transformation in human
knowledge.
We thus see that the sun presides over a numerous family. The members of
that family are dependent upon the sun, and their dimensions are
suitably proportioned to their subordinate position. Even Jupiter, the
largest member of that family, does not contain one-thousandth part of
the material which forms the vast bulk of the sun. Yet the bulk of
Jupiter alone would exceed that of the rest of the planets were they all
rolled together.
Around the central luminary in Fig. 31 we have drawn four circles in
dotted lines which sufficiently illustrate the orbits in which the
different bodies move. The innermost of these four paths represents the
orbit of the planet Mercury. The planet moves around the sun in this
path, and regains the place from which it started in eighty-eight days.
The next orbit, proceeding outwards from the sun, is that of the planet
Venus, which we have already referred to as the well-known Evening Star.
Venus completes the circuit of its path in 225 days. One step further
from the sun and we come to the orbit of another planet. This body is
almost the same size as Venus, and is therefore much larger than
Mercury. The planet now under consideration accomplishes each revolution
in 365 days. This period sounds familiar to our ears. It is the length
of the year; and the planet is the earth on which we stand. There is an
impressive way in which to realise the length of the road along which
the earth has to travel in each annual journey. The circumference of a
circle is about three and one-seventh times the diameter of the same
figure; so that taking the distance from the earth to the centre of the
sun as 92,900,000 miles, the diameter of the circle which the earth
describes around the sun will be 185,800,000 miles, and consequently
the circumference of the mighty circle in which the earth moves round
the sun is fully 583,000,000 miles. The earth has to travel this
distance every year. It is merely a sum in division to find how far we
have to move each second in order to accomplish this long journey in a
twelvemonth. It will appear that the earth must actually complete
eighteen miles every second, as otherwise it would not finish its
journey within the allotted time.
[Illustration: Fig. 31.--The Orbits of the Four Interior Planets.]
Pause for a moment to think what a velocity of eighteen miles a second
really implies. Can we realise a speed so tremendous? Let us compare it
with our ordinary types of rapid movement. Look at that express train
how it crashes under the bridge, how, in another moment, it is lost to
view! Can any velocity be greater than that? Let us try it by figures.
The train moves a mile a minute; multiply that velocity by eighteen and
it becomes eighteen miles a _minute_, but we must further multiply it
by sixty to make it eighteen miles a _second_. The velocity of the
express train is not even the thousandth part of the velocity of the
earth. Let us take another illustration. We stand at the rifle ranges to
see a rifle fired at a target 1,000 feet away, and we find that a second
or two is sufficient to carry the bullet over that distance. The earth
moves nearly one hundred times as fast as the rifle bullet.
[Illustration: Fig. 32.--The Earth's Movement.]
Viewed in another way, the stupendous speed of the earth does not seem
immoderate. The earth is a mighty globe, so great indeed that even when
moving at this speed it takes almost eight minutes to pass over its own
diameter. If a steamer required eight minutes to traverse a distance
equal to its own length, its pace would be less than a mile an hour. To
illustrate this method of considering the subject, we show here a view
of the progress made by the earth (Fig. 32). The distance between the
centres of these circles is about six times the diameter; and,
accordingly, if they be taken to represent the earth, the time required
to pass from one position to the other is about forty-eight minutes.
Outside the path of the earth, we come to the orbit of the fourth
planet, Mars, which requires 687 days, or nearly two years, to complete
its circuit round the sun. With our arrival at Mars we have gained the
limit to the inner portion of the solar system.
The four planets we have mentioned form a group in themselves,
distinguished by their comparative nearness to the sun. They are all
bodies of moderate dimensions. Venus and the Earth are globes of about
the same size. Mercury and Mars are both smaller objects which lie, so
far as bulk is concerned, between the earth and the moon. The four
planets which come nearest to the sun are vastly surpassed in bulk and
weight by the giant bodies of our system--the stately group of Jupiter
and Saturn, Uranus and Neptune.
[Illustration: Fig. 33.--The Orbits of the Four Giant Planets.]
These giant planets enjoy the sun's guidance equally with their weaker
brethren. In the diagram on this page (Fig. 33) parts of the orbits of
the great outer planets are represented. The sun, as before, presides at
the centre, but the inner planets would on this scale be so close to the
sun that it is only possible to represent the orbit of Mars. After the
orbit of Mars comes a considerable interval, not, however, devoid of
planetary activity, and then follow the orbits of Jupiter and Saturn;
further still, we have Uranus, a great globe on the verge of unassisted
vision; and, lastly, the whole system is bounded by the grand orbit of
Neptune--a planet of which we shall have a marvellous story to narrate.
The various circles in Fig. 34 show the apparent sizes of the sun as
seen from the different planets. Taking the circle corresponding to the
earth to represent the amount of heat and light which the earth derives
from the sun then the other circles indicate the heat and the light
enjoyed by the corresponding planets. The next outer planet to the earth
is Mars, whose share of solar blessings is not so very inferior to that
of the earth; but we fail to see how bodies so remote as Jupiter or
Saturn can enjoy climates at all comparable with those of the planets
which are more favourably situated.
[Illustration: Fig. 34.--Comparative Apparent Size of the Sun as seen
from the Various Planets.]
Fig. 35 shows a picture of the whole family of planets surrounding the
sun--represented on the same scale, so as to exhibit their comparative
sizes. Measured by bulk, Jupiter is more than 1,200 times as great as
the earth, so that it would take at least 1,200 earths rolled into one
to form a globe equal to the globe of Jupiter. Measured by weight, the
disparity between the earth and Jupiter, though still enormous, is not
quite so great; but this is a matter to be discussed more fully in a
later chapter.
[Illustration: Fig. 35.--Comparative Sizes of the Planets.]
Even in this preliminary survey of the solar system we must not omit to
refer to the planets which attract our attention, not by their bulk, but
by their multitude. In the ample zone bounded on the inside by the orbit
of Mars and on the outside by the orbit of Jupiter it was thought at one
time that no planet revolved. Modern research has shown that this region
is tenanted, not by one planet, but by hundreds. The discovery of these
planets is a charge which has been undertaken by various diligent
astronomers of the present day, while the discussion of their movements
affords labour to other men of science. We shall find something to learn
from the study of these tiny bodies, and especially from another small
planet called Eros, which lies nearer to the earth than the limit above
indicated. A chapter will be devoted to these objects.
But we do not propose to enter deeply into the mere statistics of the
planetary system at present. Were such our intention, the tables at the
end of the volume would show that ample materials are available.
Astronomers have taken an inventory of each of the planets. They have
measured their distances, the shapes of their orbits and the positions
of those orbits, their times of revolution, and, in the case of all the
larger planets, their sizes and their weights. Such results are of
interest for many purposes. It is, however, the more general features of
the science which at present claim our attention.
Let us, in conclusion, note one or two important truths with reference
to our planetary system. We have seen that all the planets revolve in
nearly circular paths around the sun. We have now to add another fact
possessing much significance. Each of the planets pursues its path in
the same direction. It thus happens that one such body may overtake
another, but it can never happen that two planets pass by each other as
do the trains on adjacent lines of railway. We shall subsequently find
that the whole welfare of our system, nay, its continuous existence, is
dependent upon this remarkable uniformity taken in conjunction with
other features of the system.
Such is our solar system; a mighty organised group of planets
circulating under the control of the sun, and completely isolated from
all external interference. No star, no constellation, has any
appreciable influence on our solar system. We constitute a little island
group, separated from the nearest stars by the most amazing distances.
It may be that as the other stars are suns, so they too may have systems
of planets circulating around them; but of this we know nothing. Of the
stars we can only say that they appear to us as points of light, and any
planets they may possess must for ever remain invisible to us, even if
they were many times larger than Jupiter.
We need not repine at this limitation to our possible knowledge, for
just as we find in the solar system all that is necessary for our daily
bodily wants, so shall we find ample occupation for whatever faculties
we may possess in endeavouring to understand those mysteries of the
heavens which lie within our reach.