THE EARTH'S BROTHERS AND SISTER
The earth is not the only world that, poised in space, swings around the
sun. It is one of a family called the Solar System, which means the
system controlled and governed by the sun. When we look up at the
glorious sky, star-studded night by night, it might seem to us that the
stars move only by reason of the earth's rotation; but when men first
began to study the heavens attentively--and this is so long ago that the
record of it is not to be found--they noticed that, while every shining
object in the sky was apparently moving round us, there were a few which
also had another movement, a proper motion of their own, like the moon.
These curious stars, which appeared to wander about among the other
stars, they called planets, or wanderers. And the reason, which was
presently discovered, of our being able to see these movements was that
these planets are very much nearer to us than any of the real stars,
and in fact form part of our own solar system, while the stars are at
immeasurable distances away. Of all the objects in the heavens the
planets are the most intensely interesting to us; for though removed
from us by millions of miles, the far-reaching telescope brings some of
them within such range that we can see their surfaces and discover their
movements in a way quite impossible with the stars. And here, if
anywhere, might we expect to find traces of other living beings like
ourselves; for, after all the earth is but a planet, not a very large
nor a very small one, and in no very striking position compared with the
other planets; and thus, arguing by what seems common-sense, we say, If
this one planet has living beings on its surface, may not the other
planets prove to be homes for living beings also? Counting our own
earth, there are eight of these worlds in our solar system, and also a
number of tiny planets, called asteroids; these likewise go round the
sun, but are very much smaller than any of the first eight, and stand in
a class by themselves, so that when the planets are mentioned it is
generally the eight large well-known planets which are referred to.
If we go back for a moment to the illustration of the large lamp
representing our sun, we shall now be able to fill in the picture with
much more detail. The orbits of the planets, as their paths round the
sun are called, lie like great circles one outside another at various
distances, and do not touch or cut each other. Where do you suppose our
own place to be? Will it be the nearest to the sun or the furthest away
from him? As a matter of fact, it is neither, we come third in order
from the sun, for two smaller planets, one very small and the other
nearly as large as the earth, circle round and round the sun in orbits
lying inside ours. Now if we want to place objects around our lamp-sun
which will represent these planets in size, and to put them in places
corresponding to their real positions, we should find no room large
enough to give us the space we ought to have. We must take the lamp out
into a great open field, where we shall not be limited by walls. Then
the smallest planet, named Mercury, which lies nearest of all to the
sun, would have to be represented by a pea comparatively close to the
sun; Venus, the next, would be a greengage plum, and would be about
twice as far away; then would come the earth, a slightly larger plum,
about half as far again as Venus. After this there would be a lesser
planet, called Mars, like a marble. These are the first four, all
comparatively small; beyond them there is a vast gap, in which we find
the asteroids, and after this we come to four larger planets, mighty
indeed as regards ourselves, for if our earth were a greengage plum,
the first of these, Jupiter, would have to be the size of a football at
least, and the next, Saturn, a smaller football, while Uranus and
Neptune, the two furthest out, would be about the size of the toy
balloons children play with. The outermost one, Neptune, would be thirty
times as far from the sun as we are.
[Illustration: COMPARATIVE SIZES OF THE PLANETS.]
This is the solar system, and in it the only thing that shines by its
own light is the sun; all the rest, the planets and their moons, shine
only because the rays of light from the sun strike on their surfaces and
are reflected off again. Our earth shines like that, and from the nearer
planets must appear as a brilliant star. The little solar system is
separated by distances beyond the realm of thought from the rest of the
universe. Vast as are the intervals between ourselves and our planetary
neighbours, they are as nothing to the space that separates us from the
nearest of the steady shining fixed stars. Why, removed as far from us
as the stars, the sun himself would have sunk to a point of light; and
as for the planets, the largest of them, Jupiter, could not possibly be
seen. Thus, when we look at those stars across the great gulf of space,
we know that though we see them they cannot see us, and that to them our
sun must seem only a star; consequently we argue that perhaps these
stars themselves are suns with families of planets attached to them; and
though there are reasons for thinking that this is not the case with
all, it may be with some. Now if, after learning this, we look again at
the sky, we do so with very different eyes, for we realize that some of
these shining bodies are like ourselves in many things, and are shining
only with a light borrowed from the sun, while others are mighty glowing
suns themselves, shining by their own light, some greater and brighter,
some less than our sun. The next thing to do is to learn which are stars
and which are planets.
Of the planets you will soon learn to pick out one or two, and will
recognize them even if they do change their places--for instance, Venus
is at times very conspicuous, shining as an evening star in the west
soon after the sun goes down, or us a morning star before he gets up,
though you are not so likely to see her then; anyway, she is never found
very far from the sun. Jupiter is the only other planet that compares
with her in brilliancy, and he shines most beautifully. He is, of
course, much further away from us than Venus, but so much larger that he
rivals her in brightness. Saturn can be quite easily seen as a
conspicuous object, too, if you know where to look for him, and Mars is
sometimes very bright with a reddish glow. The others you would not be
able to distinguish.
It is to our earth's family of these eight large planets going steadily
round the same sun that we must give our attention first, before going
on to the distant stars. Many of the planets are accompanied by
satellites or moons, which circle round them. We may say that the sun is
our parent--father, mother, what you will--and that the planets are the
family of children, and that the moons are _their_ children. Our earth,
you see, has only one child, but that a very fine one, of which she may
well be proud.
When I say that the planets go round the sun in circles I am only
speaking generally; as a matter of fact, the orbits of the planets are
not perfect circles, though some are more circular than others. Instead
of this they are as a circle might look if it were pressed in from two
sides, and this is called an ellipse. The path of our own earth round
the sun is one of the most nearly circular of them all, and yet even in
her orbit she is a good deal nearer to the sun at one time than another.
Would you be surprised to hear that she is nearer in our winter and
further away in our summer? Yet that is the case. And for the first
moment it seems absurd; for what then makes the summer hotter than the
winter? That is due to an altogether different cause; it depends on the
position of the earth's axis. If that axis were quite straight up and
down in reference to the earth's path round the sun we should have equal
days and nights all the year round, but it is not; it leans over a
little, so that at one time the North Pole points towards the sun and at
another time away from it, while the South Pole is pointing first away
from it and then toward it in exactly the reverse way. When the North
Pole points to the sun we in the Northern Hemisphere have our summer. To
understand this you must look at the picture, which will make it much
clearer than any words of mine can do. The dark part is the night, and
the light part the day. When we are having summer any particular spot on
the Northern Hemisphere has quite a long way to travel in the light, and
only a very short bit in the dark, and the further north you go the
longer the day and shorter the night, until right up near the North
Pole, within the Arctic Circle, it is daylight all the time. You have,
perhaps, heard of the 'midnight sun' that people go to see in the North,
and what the expression means is that at what should be midnight the
sun is still there. He seems just to circle round the horizon, never
very far above, but never dipping below it.
When the sun is high overhead, his rays strike down with much more force
than when he is low. It is, for instance, hotter at mid-day than in the
evening. Now, when the North Pole is bowed toward the sun, the sun
appears to us to be higher in the sky. In the British Isles he never
climbs quite to the zenith, as we call the point straight above our
heads; he always keeps on the southern side of that, so that our shadows
are thrown northward at mid-day, but yet he gets nearer to it than he
does in winter. Look at the picture of the earth as it is in winter.
Then we have long nights and short days, and the sun never appears to
climb very high, because we are turned away from him. During the short
days we do not receive a great deal of heat, and during the long night
the heat we have received has time to evaporate to a great extent. These
two reasons--the greater or less height of the sun in the sky and the
length of the days--are quite enough to account for the difference
between our summer and winter. There is one rather interesting point to
remember, and that is that in the Northern Hemisphere, whether it is
winter or summer, the sun is south at mid-day, so that you can always
find the north then, for your shadow will point northwards.
[Illustration: THE ENGLISH SUMMER (LEFT) AND WINTER (RIGHT).]
New Zealand and Australia and other countries placed in the Southern
Hemisphere, as we are in the Northern, have their summer while we have
winter, and winter while we have summer, and their summer is warmer than
ours, because it comes when the earth in its journey is three million
miles nearer to the sun than in our summer.
All this seems to refer to the earth alone, and this chapter should be
about the planets; but, after all, what applies to one planet applies to
another in some degree, and we can turn to the others with much more
interest now to see if their axes are bowed toward the sun as ours is.
It is believed that in the case of Mercury, in regard to its path round
the sun, the axis is straight up and down; if it is the changes of the
seasons must depend on the nearness of Mercury to the sun and nothing
else, and as he is a great deal nearer at one time than another, this
might make a very considerable difference. Some of the planets are like
the earth in regard to the position of their axes, but the two outermost
ones, Uranus and Neptune, are very peculiar, for one pole is turned
right toward the sun and the other right away from it, so that in one
hemisphere there is continuous day all the summer, in the other there is
continuous night, and then the process is reversed. But these little
peculiarities we shall have to note more particularly in the account of
the planets separately.
There is a curious fact in regard to the distances of the planets from
the sun. Each one, after the first, is, very roughly, about double the
distance from the sun of the one inside it. This holds good for all the
first four, then there is a great gap where we might expect to find
another planet, after which follow the four large planets. Now, this gap
puzzled astronomers greatly; for though there seemed to be no reason why
the planets should be at regular distances one outside the other, yet
there the fact was, and that the series should be broken by a missing
planet was annoying. So very careful search was made, and a thrill of
excitement went all through the scientific world when it was known that
a tiny planet had been discovered in the right place. But this was not
the end of it, for within a few years three or four more tiny planets
were observed not far from the first one, and, as years rolled on, one
after another was discovered until now the number amounts to over six
hundred and others are perpetually being added to the list! Here was a
new feature in the solar system, a band of tiny planets not one of which
was to be compared in size with the least of those already known. The
largest may be about as large as Europe, and others perhaps about the
size of Wales, while there may be many that have only a few square miles
of surface altogether, and are too small for us to see. To account for
this strange discovery many theories were advanced.
One was that there had been a planet--it might be about the size of
Mars--which had burst up in a great explosion, and that these were the
pieces--a very interesting and exciting idea, but one which proved to be
impossible. The explanation now generally accepted is a little
complicated, and to understand it we must go back for a bit.
When we were talking of the earth and the moon we realized that once
long ago the moon must have been a part of the earth, at a time when the
earth was much larger and softer than she now is; to put it in the
correct way, we should say when she was less dense. There is no need to
explain the word 'dense,' for in its ordinary sense we use it every day,
but in an astronomical sense it does not mean exactly the same thing.
Everything is made up of minute particles or atoms, and when these
atoms are not very close together the body they compose is loose in
texture, while if they are closer together the body is firmer. For
instance, air is less dense than water, and water than earth, and earth
than steel. You see at once by this that the more density a thing has
the heavier it is; for as a body is attracted to another body by every
atom or particle in it, so if it has more particles it will be more
strongly attracted. Thus on the earth the denser things are really
heavier. But 'weight' is only a word we use in connection with the
earth; it means the earth's pulling power toward any particular thing at
the surface, and if we were right out in space away from the earth, the
pulling power of the earth would be less, and so the weight would be
less; and as it would be impossible always to state just how far away a
thing was from the earth, astronomers talk about density, which means
the number of particles a body contains in proportion to other bodies.
Thus the planet Jupiter is very much larger than the earth, but his
density is less. That does not mean to say that if Jupiter were in one
scale and the earth in the other he would weigh less, because he is so
very much bigger he would outweigh the earth still; his total _mass_
would be greater than that of the earth, but it means that a piece of
Jupiter the same size as a piece of the earth would weigh less under the
same conditions.
Now, before there were any planets at all or any sun, in the place of
our solar system was a vast gaseous cloud called a nebula, which slowly
rotated, and this rotation was the first impulse or force which God gave
it. It was not at all dense, and as it rotated a part broke off, and
inheriting the first impulse, went on rotating too. The impulse would
have sent it off in a straight line, but the pull of gravity from the
nebula held it in place, and it circled round; then the nebula, as it
rotated, contracted a little, and occupied less space and grew denser,
and presently a second piece was thrown off, to become in time another
planet. The same process was repeated with Saturn, and then with the
huge Jupiter. The nebula was always rotating and always contracting. And
as it behaved, so did the planets in their turn; they spun round and
cooled and contracted, and the moons were flung off from them, just as
they--the planets--had been flung off from the parent nebula.
Now, after the original nebula had parted with the mighty mass of
Jupiter, it never again made an effort so great, and for a long time
the fragments that were detached were so small as hardly to be worth
calling planets; they were the asteroids, little lumps and fragments
that the nebula left behind. But as it still contracted in time there
came Mars; and having recovered a little, the nebula with more energy
got rid of the earth, and next Venus, and lastly little Mercury, the
smallest of the eight planets. Then it contracted further, and perhaps
you can guess what the remainder of it is--the sun; and by spinning in a
plastic state the sun, like the earth, has become a globe, round and
comparatively smooth; and its density is now too great to allow of its
losing any more fragments, so, as far as we can see, the solar system is
complete.
This theory of the origin of the planets is called the nebula theory. We
cannot prove it, but there are so many facts that can only be explained
by it, we have strong reason for believing that something of the kind
must have happened. When we come to speak of the starry heavens we shall
see that there are many masses of glowing gas which are nebulae of the
same sort, and which form an object-lesson in our own history.
We have spoken rather lightly of the nebula rotating and throwing off
planets; but we must not think of all this as having happened in a
short time. It is almost as impossible for the human mind to conceive
the ages required for such slow changes as to grasp the great gulfs of
space that separate us from the stars. We can only do it by comparison.
You know what a second is, and how the seconds race past without ceasing
day and night. It makes one giddy to picture the seconds there are in a
year; yet if each one of those seconds was a year in itself, what then?
That seems a stupendous time, but it is nothing compared with the time
needed to form a nebula into a planetary system. If we had five thousand
of such years, with every second in them a year, we should then only
have counted one billion real years, and billions must have passed since
the sun was a gaseous nebula filling the outermost bounds of our
system!