SATURN, URANUS AND NEPTUNE.
SATURN is only a little smaller than Jupiter, and very light indeed in
weight. Not at all like our firm and solid Earth. He actually weighs
_less than water_; which means that if we could make a huge globe, all
of water, the same size as Saturn, this water-globe would be heavier
than Saturn.
This does not look as if Saturn were a very cold or solid globe, does
it? A solid globe would surely weigh a great deal more than water.
Saturn whirls round on his axis once in ten hours, like Jupiter. But
his year is much longer than Jupiter’s year: partly because he is twice
as far away from the Sun, which means a very much longer journey, and
partly because at that distance he goes much more slowly. So one year
of Saturn is as long as nearly thirty of our years. A man who on the
Earth is seventy would on Saturn be only a little over two years old.
In shape Saturn is very flat at the north and south poles, the same as
Jupiter. Also on Saturn can be seen dimly-colored bands and markings.
But these are much less clear than on Jupiter.
[Illustration: _Jupiter._]
[Illustration: _Saturn._]
However Saturn has something which Jupiter has not: Saturn has his
Rings.
Until telescopes were made these rings could not be seen; and when
first noticed they were a great puzzle.
They lie round the vast globe of Saturn, one outside another,
stretching far away up into Saturn’s sky. If you were on Saturn,
standing just underneath the rings, the most you could see would be
a narrow rim, or line, far over your head. But if you walked some
distance off, in the right direction, you would have a lovely view of
the rings, as wide bands, one above another, shining in the sunlight.
For the rings of Saturn, like the eight moons of Saturn, have no
brightness of their own. They shine when the Sun shines on them.
And the Sun, as seen from Saturn, is very far off, and very small,
compared with the big round orb which we see in our sky. Those rings
and moons must shine but dimly, compared with the shining of our bright
Moon.
Yet, since _we_ can see them and find them lovely, even across all this
great width of distance, they must surely be beautiful seen from Saturn.
But to talk of anybody walking about on Saturn, to gaze at the rings,
is really only nonsense.
For Saturn, like Jupiter, seems to be only a half-cooled world--in
fact, even less cooled, less solid, than Jupiter. Nobody could very
well walk across great masses of heated and seething clouds in a
perpetual turmoil of storms.
I think we may safely say that Saturn at present would not offer a very
comfortable home, at all events, for any such living creatures as we
know upon the Earth.
URANUS, the next planet outside Saturn, was seen first, rather more
than a hundred years ago, by a famous English astronomer named Herschel.
It takes Uranus 84 Earthly years to travel once round the Sun, at a
rate of about four miles each second. So a man of 84 on Earth would be
only just one year old on Uranus.
Four moons journey with Uranus; and some glimpses have been caught
of very faint band-markings on the planet, like those of Jupiter and
Saturn. Little can be seen or known of worlds so far away: but it
is most likely that Uranus and Neptune are both more or less in the
half-hot state of the two big twin planets. Both Uranus and Neptune are
light in make, weighing about the same as water.
NEPTUNE, the very farthest off world of all known to us, journeys
round the Sun at a distance of about 2,800 millions of miles, or
_eighty times as far off as Mercury_. It is not very easy to see in our
minds what this means. We must climb up to the thought, step by step.
Think first of a rope one hundred miles long. Perhaps you have gone in
the train from New York to Philadelphia. A rope one hundred miles in
length would reach all the way and ten miles farther.
Next, think of ten such ropes joined together, making a single rope one
thousand miles long.
Then think of twenty-five of those ropes joined into one rope, 25,000
miles long.
This rope would just about go round the Earth, lying on the equator
like a girdle.
It would take _ten_ such Earth-girdles to reach straight from the Earth
to the Moon.
But we have to get the thought of one million miles. Well, you would
need about _forty_ Earth-girdles--forty ropes, each one being 25,000
miles long--to make a rope one million miles in length.
And when we get so far it is still only one million. Mercury is
thirty-five millions of miles away from the Sun.
So, for the distance of Mercury, you would need--first, forty
Earth-girdles joined into a one-million mile rope, and then
thirty-five of those million-mile ropes, to stretch all the great way
from the Sun to his nearest planet, Mercury.
When you have in mind that enormously long rope, reaching from the Sun
to Mercury, the rest is easier.
_Two_ such ropes would about reach from the Sun to Venus. _Three_ such
ropes would about reach from the Sun to our Earth. _Four_ such ropes
would about reach from the Sun to Mars.
But to reach from the Sun to Jupiter no less than _fourteen_ such ropes
would be needed.
And to reach all the way to the distant Neptune, from the Sun, _eighty_
such ropes would be needed!
There indeed we find ourselves in a region of dimness and fearful
cold. We can hardly fancy any human beings like ourselves living at so
enormous a distance from the storehouse of light and heat.
Our bright and glorious Sun, seen from Neptune, looks no larger than
the planet Venus looks to us here. You and I on Earth have _nine
hundred times_ as much light, and _nine hundred times_ as much heat,
from the Sun, as a man on Neptune would have. Of course, if Neptune is
only partly cooled, there may be plenty of heat from the planet itself.
However, you must not think that the Sun even there looks only like
Venus or Jupiter in our sky. Though small in size, he shines dazzlingly
still. But after what we enjoy on Earth Neptune would indeed to us be a
world of darkness.
Travelling at the rate of three miles in a second, Neptune gets once
round the Sun in 165 of our years.
This planet was not discovered by accident, but through careful
searching. Some day you will read with interest the story of how and
why it was hunted for in the sky--and found.[2]
[2] See “Sun, Moon and Stars,” pp. 227-234.
Speaking of the distance of one planet from another we mean usually
their _nearest_ distances, when they are both on one side of the Sun
together. When they are on opposite sides of the Sun they are very much
farther apart.
The moons belonging to these planets are really like planets, or
worlds, travelling with the bigger worlds. Some of them are not so
very little, either. Mars’ moons are most tiny; but one of Jupiter’s
moons, as you heard, is larger than Mercury. Mercury, however, being
the nearest planet to the Sun, is a much more important world than a
far-off moon of Jupiter can be.
Each moon, whether of Jupiter, of Saturn, or of any other planet,
travels, like our Moon, in a pathway of its own round the Sun. And as
it goes it curves backwards and forwards, so as to face in turn each
side of the large world with which it journeys.
The pull of a great body like Jupiter is very strong; and the moons
in consequence travel very fast round Jupiter--the nearest going most
rapidly, the farthest off most slowly. It is the same again with
Saturn’s eight moons.
QUESTIONS.
1. What is the size of Saturn?
Almost as large as Jupiter.
2. Does Saturn spin on his axis?
Yes, in about ten hours, like Jupiter.
3. How long is Saturn’s year?
Nearly thirty of our years.
4. Is Saturn like Jupiter in make?
Saturn is very light, even lighter than Jupiter; not so heavy as water.
Saturn, too, has faint bands of color.
5. What is Saturn’s state believed to be?
Half-cooled, and very stormy, with great masses of cloud.
6. How many moons has Saturn?
Eight moons, and also three rings.
7. How do the rings shine?
Like the moons, on one side, by reflected sunlight.
8. When was Uranus discovered?
About one hundred years ago.
9. By whom?
By Herschel.
10. How long is the year of Uranus?
Eighty-four of our years.
11. How many moons has Uranus?
Four moons are known.
12. What size are these two outer planets, Uranus and Neptune?
Much larger than Venus or the Earth, much smaller than Jupiter or
Saturn.
13. How far is Neptune from the Sun?
Eighty times the distance of Mercury, or twenty-eight hundred millions
of miles.
14. How fast does Neptune travel?
Some three miles each second.
15. What is the length of Neptune’s year?
About one hundred and sixty-five of our years.
16. How many moons has Neptune?
Only one has been seen.
17. Are Uranus and Neptune light or heavy in make?
About as light in make as water.