OTHER WORLDS.
Now for the Planets, or Worlds, which journey as our Earth journeys,
round and round the Sun, each in its own particular pathway. And--to
begin with--a few words as to what keeps them in their pathways.
Two things working together do this. There is an inward pull, and there
is also an outward pull.
The inward pull is the pull of Attraction, known also as Gravitation.
You have heard a little about Attraction before. You know how the Earth
pulls, with a steady downward drag, everything upon her surface. And in
just the very same manner the Sun pulls towards himself all the worlds,
little or big, which float around him in the sky.
When you try to jump up from the Earth you drop back. It is impossible
for you to get right away, merely by jumping, because of the Earth’s
strong pull.
And if our Earth tries to get away from the Sun, she cannot do so
either; because of the Sun’s strong pull. In fact she is always trying
and never succeeds.
She is always trying to get away because she is always on the rush;
always hastening at a great speed, and struggling to go straight
forward in her rush, while the pull of the Sun keeps drawing her out of
a straight line and making her travel in a bent path round the Sun.
If it were not for the Earth’s rapid onward movement she would soon
fall down upon the Sun; and if it were not for the Sun’s pull she
would soon wander away from him. These two things--the inward pull of
the Sun’s attraction and the outward pull of our Earth’s own quick
rush--keep her at her present distance from the Sun.
It is the same with the other Planets. They too journey round the Sun
in oval pathways. Those worlds which are nearer to him are pulled more
strongly; and they have to fly along at a great speed, to escape from
falling down upon his fiery surface. Those which are farther off are
pulled more feebly; and they move at a much slower pace.
When “attraction” is spoken of, remember that it is always a pull on
both sides. The Earth attracts the Sun, as well as the Sun attracting
the Earth; and all the Planets attract one another. But the pull of the
Sun is so powerful that other pullings seem small by comparison.
Our world is only one little planet in the great Kingdom of the Sun.
That kingdom is commonly called “THE SOLAR SYSTEM.”
A “system” means something which is arranged, or which is made up of
different parts put together in an orderly manner. The word “solar” is
from the Latin for “Sun.” So, by the Solar System we mean that great
System or Arrangement of Worlds which is governed by the Sun.
No two worlds are at the same distance from the Sun; but all the larger
planets travel on very much the same _plane_--that is, on the same
level, or the same _flat_, in the sky.
Also, they all go the same way. They journey round the Sun from west to
east; not from east to west.
Astronomers have sometimes fancied that they could catch a glimpse of
one small world very near to the Sun, which they named VULCAN. But it
is very doubtful whether there really is any such planet at all. If
there is, he is almost lost in the glare of the Sun.
The nearest to the Sun of which we know positively is named MERCURY.
He is much smaller than our world, though larger than our Moon; and he
whirls along at a dizzy speed.
Outside the pathway of Mercury, like a large hoop round a little one,
only at a good distance off, lies the oval-shaped pathway of VENUS.
Though we often speak carelessly of this lovely world as “The Evening
Star,” Venus is no star, but a planet like the Earth, shining only in
the Sun’s light. And, although perhaps not really brighter in herself
than all other worlds, Venus is by far the brightest in our sky.
Mercury’s pathway lies too close to the Sun to give us often very good
views of him. Besides, Mercury is not only much smaller than Venus, but
much farther away from us.
That is to say, Mercury at his nearest is farther off from us than
Venus at her nearest. When Mercury happens to be between the Sun and
us, while Venus happens to be far away on the other side of the Sun,
just then, of course, Mercury for a little while is the closer to us of
the two.
If you have three hoops of different sizes you will be able to see
quite easily how this comes about.
Lay the hoops on the floor, one within another, and place a ball in
the middle for the Sun. Then lay one marble, for the Earth, close
to the outermost and largest hoop, and another marble for Mercury,
close to the innermost and smallest hoop, on the _same side_ as the
Earth-marble. Then put a marble, for Venus, close to the middle-sized
hoop, still on the same side.
So the three worlds are all together on the same side, as near as they
ever can come one to another. And you will see that Mercury can never
get so close to the Earth as Venus can.
But now, leaving the Earth and Mercury alone, move the Venus-marble
half-way round its hoop, to just the opposite side of the Sun. You
will then understand how sometimes, for a little while, Venus may be
actually much farther off than Mercury from our Earth.
These worlds all travel on different pathways at different speeds, and
the pathways are not of the same length. So the worlds never keep long
side by side. For a little while they may journey in company; then one
gets ahead and the other drops behind. By-and-by they are on opposite
sides of the Sun; and then in time they draw near one to another again.
Both Mercury and Venus have _phases_, or seeming changes of shape, like
our Moon. They shine only on one side, that side which is towards the
Sun; and sometimes we see only part of the bright side, not the whole
of it. But the changes are too small at such a distance to be seen
without a telescope.
Venus is very nearly the same size as our Earth. She lies farther from
the Sun than Mercury, and nearer the Earth. This means that she has
more light and more heat than we have, but less light and less heat
than Mercury has. From Venus the Sun looks very much larger and more
brilliant than we see him, yet much smaller and less brilliant than as
seen from Mercury.
Also, the Sun pulls Venus more strongly than he pulls the Earth, but
less strongly than he pulls Mercury. Venus does not journey so fast as
Mercury, but she goes farther than our Earth goes.
You see how perfectly these things are all planned, so as just to fit
in one with another. We may well talk of our Sun’s kingdom in the sky
as a _System_, when we find its wonderful arrangements and note the
order and beauty of the whole.
The two inner worlds, Mercury and Venus, are called “Inferior Planets,”
because they lie between the Earth and the Sun. All other worlds,
having pathways outside our Earth’s pathway, are called “Superior
Planets.”
The next oval hoop which surrounds the pathway of Venus is that of THE
EARTH.
Outside the pathway of our Earth lies that of MARS: another world, a
good deal smaller than the Earth or Venus, but larger than Mercury.
Both Mercury and Venus can only be seen in the sky near to the Sun,
either a short time before he rises in the morning, or not long after
he sets in the evening--either towards the east in the morning, or
towards the west in the evening. But Mars and all the other outer
planets may be seen in various parts of the sky at different times.
Of these four small Worlds our Earth is the largest, being nearly 8,000
miles straight through from side to side.
Venus is the next in size, being nearly as large as Earth.
Mars is the next, being about 4,000 miles through.
Mercury is the smallest, being less than 3,000 miles through.
And our Moon, as you know, is smaller still, being only 2,000 miles
through.
Suppose we look upon these worlds, as we have done earlier, in a
lessened size; letting one inch stand for 2,000 miles.
Then to picture our Moon we should want a very large grape, or a small
walnut, one inch through.
Our Earth would be a very big apple, or a small cocoa-nut, four inches
through.
Venus would be another big apple, almost as large as the Earth.
Mars might be a small orange, two inches through.
Mercury might be a crab-apple, only one inch and a half straight
through.
If you can manage to find five balls of the right sizes, and put them
all in a row, you will get a very fair idea of the sizes of these
worlds, as _compared_ one with another.
Try also to fix the names firmly in your memory, by saying them often
over and over--“Mercury, Venus, Earth, Mars.”
Remember that “Earth” is the name of our world, just as “Venus” is the
name of another world. All the planets are “worlds,” but only one of
them is “Earth.”
After Mars comes a wide space in the heavens, which for a long while
was thought to be quite empty of worlds. But it is not empty. Instead
of one big planet a great many tiny ones are there, journeying round
the Sun in company. Nearly three hundred and fifty are known to us, and
fresh ones are still often found.
When first discovered, about one hundred years ago, these small
worlds were named ASTEROIDS, or Little Stars. Now they are known as
PLANETOIDS, or Little Planets. This is the right name for them, since
they are not stars but planets, or worlds.
Only they are very, very tiny worlds. The biggest of them all is under
400 miles through; and most of them are much less. So if the Moon is
pictured by a large grape, a pea would be far too big for most of the
Planetoids.
This belt of Planetoids comes after a broad gap of space, between it
and Mars; and it is followed by another wide gap.
Then we get to the pathway of JUPITER.
Here indeed is a contrast. The Planetoids are the smallest worlds in
the whole Polar System, and Jupiter is the largest. He is very, very
far away; yet, as he shines in our sky, he is often the most splendid
object we can see there, second only to Venus.
Venus is very much _smaller_ than Jupiter; but Venus is also very much
_nearer_ than Jupiter.
Not one of all the other planets is as big as Jupiter. While our Earth
is only eight thousand miles through, Jupiter is eighty-five thousand
miles through. This makes a wonderful difference. Jupiter is very small
beside the Sun, but he is very huge beside our little Earth.
Jupiter’s speed is far slower than that of the inner planets. At his
vast distance the pull of the Sun is much weaker, and so he does not go
so fast.
If Jupiter whirled round the Sun as fast as Mars does the Sun could not
hold him in, and he would wander away and be lost. But if Mercury were
to journey at Jupiter’s pace he could not keep away from the Sun, and
he would, most likely, soon be destroyed.
Jupiter does not travel alone. He has a family of moons; not one only,
like our Earth, but five moons, the nearest of which has been quite
lately found. These moons all journey with Jupiter round the Sun; and
they also go round Jupiter as our Moon goes round the Earth.
Beyond Jupiter, at a great distance, is another giant world, SATURN.
Not quite so big as Jupiter, but not very far behind him in size.
Saturn too has a family, not of five moons only but of eight moons.
He also has three very wonderful rings, which shine in the sunlight.
Neither rings nor moons can be seen without a very good telescope.
Outside the pathway of Saturn lies that of URANUS, another huge world,
though a good deal smaller than Saturn.
Outside the pathway of Uranus travels the dim and distant NEPTUNE--so
far as we know, the outermost world of the whole Solar System. Neptune
is rather larger than Uranus.
So there are first the four smaller or Lesser Planets--Mercury,
Venus, Earth, Mars; then the Planetoids; and then the four big Outer
Planets--Jupiter, Saturn, Uranus, Neptune.
Now look again at your little balls which picture the sizes of the
smaller planets. The biggest of them is our Earth--a ball or a
cocoa-nut four inches through.
But when we turn to Jupiter, still letting one inch stand for 2,000
miles, we shall want a ball or globe no less than _three feet and a
half_ through, from side to side.
And for Saturn we must find a globe _three feet_ through.
And for Uranus a globe less than _one foot and a half_ through.
And for Neptune a globe quite _one foot and a half_ through.
Then, to finish up, we shall want a big balloon, for the Sun,
_thirty-five feet_ through.
QUESTIONS.
1. What is the Solar System?
The Sun’s Kingdom of Worlds in the Sky.
2. What is a System?
An orderly arrangement.
3. What is a Planet?
A planet is a world which shines by borrowed light.
4. What is an Orbit?
A planet’s pathway.
5. How do the pathways of the planets lie round the Sun?
One outside another, and all of them very nearly on the same level.
6. Which are the four Lesser Planets--nearest to the Sun?
Mercury, Venus, Earth, Mars.
7. What comes next?
The belt of tiny Planetoids.
8. Which are the four Outer Planets?
Jupiter, Saturn, Uranus, Neptune.
9. Which is the largest planet of all?
Jupiter.
10. If our Earth were only four inches through how big should Jupiter
be?
Three feet and a half through.
11. How many moons has Jupiter?
Five moons.
12. How many moons has Saturn?
Eight moons, and three rings.
13. Which planet is nearest to the Sun?
Mercury.
14. Is this quite sure?
An inner planet, Vulcan, may be there; but this is very uncertain.
15. Which is the farthest off planet known to us?
Neptune.
16. How many Planetoids do we know of?
Nearly 350; and new ones are often found.
19. Which planets travel fastest?
Those nearest to the Sun.
20. Why do they travel faster?
Because the pull of the Sun is so much stronger.
21. In what direction do the planets travel?
All of them from west to east.
22. Which way do they spin?
All of them from west to east.