CHAPTER XV.. The Planet Jupiter.

THE PLANET JUPITER.

Now we pass on to JUPITER, chief in size of all the worlds in the kingdom of the Sun.

The four inner planets are all small together. The four outer planets are all large together, Jupiter and Saturn being the twin giants of the Solar System.

You now know that the distance of Mercury from the Sun is about 35 millions of miles, and that the distance of Mars is about four times that of Mercury. But the distance of Jupiter from the Sun is nearly fourteen times that of Mercury. Think what an enormous gap this means between the pathway of Mars and the pathway of Jupiter.

And, distant as Jupiter is from the Sun, he is quite as far from his next neighbor on the other side, Saturn. So Jupiter lies just about half-way between the Sun and Saturn.

Yet Saturn is nearer to Jupiter than to his other neighbor, Uranus. The gap between the pathway of Saturn and the pathway of Uranus is twice as broad as the gap between Jupiter and Saturn.

Outside Uranus stretches another vast empty space: and then we get to the last known planet, far-away Neptune!

Jupiter whirls with such speed upon his axis, that it takes him less than ten hours to spin once round. A day of only five hours, and a night of only five hours! How should we like that?

But with the short day he has a very long year. Jupiter gets once round the Sun in twelve of our earthly years. So a man who on Earth is nearly forty years old would on Jupiter be just over three years old: and an old Earthly gentleman of seventy would there be under the age of six. Our little boys and girls would hardly like only one birthday in twelve years.

We have seen how, with greater distance from the Sun, each planet goes more and more slowly, as the Sun’s pulling becomes weaker. Jupiter rolls through the skies at a rate of only about eight miles each second.

A beautiful world is Jupiter, looked upon from the Earth: the brightest in our sky after Venus. No other planet, except Venus, and no Star in the heavens can outshine Jupiter. This is because of two things--his great size and his nearness to us. Not nearness compared with that of the smaller worlds, but nearness compared with that of Saturn and Uranus and Neptune.

Saturn, though almost as big as Jupiter, is very much farther off. And while Jupiter can hardly be so bright actually as Mars, because very much farther from the Sun, yet his huge size makes him greatly outshine Mars, which is so much nearer to us than he is.

Seen through a pretty good telescope, Jupiter grows into a broad, soft, moon-like world, very flat at the north and south poles, with colored bands round him, on and near his equator. Four small bright moons are also to be noticed. Sometimes all four can be seen at once; sometimes one or two are hidden behind him, or the shadow of one creeps like a black dot over his face. The fifth little moon, found lately, can seldom be seen.

Through a bigger telescope, Jupiter shows exquisite colors--rich reds, and browns, and greens, and purples. But these markings do not mean continents and oceans, as they perhaps mean on Mars. They are believed to belong to a very stormy Cloudland.

Jupiter seems to be wrapped in thick masses of clouds; and these clouds are ever on the move, always changing their shapes. It may be that we now and then get a tiny glimpse through them of the more solid world within, but this we cannot be sure of. It may be that the clouds never part so far as to let us see through. It may be that there is nothing solid within at all.

Anyhow, the solid part is very much smaller than the size of the Jupiter we see. For, like other planets, Jupiter has been weighed, and he is found to be very light in make. He is not nearly so heavy as one would expect with a globe of that size.

The inner part may or may not be solid; some say it is most likely not. At any rate, it is enfolded by an enormous thickness of heated and tempestuous clouds.

When you look up into the sky from the Earth you see the clouds moving and changing their shapes slowly. But if you could go quite near you would find their changes to be really very quick.

And just so--only very much more so--at the vast distance of Jupiter we see movements which to us seem tiny and slow, yet which we believe to mean there, on the spot, the wildest rushings of heated clouds hither and thither. No storms on the Earth can be spoken of in the same breath with the terrific storms on Jupiter.

And the question is--what brings this about? Our earthly tempests are caused by the heat of the Sun, but the Sun is so very far from Jupiter and yet the storms there are much more violent than any here.

Do you remember being told that once upon a time, long, long ago, our Earth, now so cold and quiet a globe, was most likely a dazzling little Sun, and that she slowly cooled down from a Sun to a world?

When she was a Sun she was fiercely hot and glowing gases played over her; and instead of solid ground and liquid seas there were only raging vapors, bright with their own heat. The Earth was larger then than now, for gases take up much more room than water and rocks and earth.

Between those days and these our Earth must have passed through a half-way stage.

Suppose you have a lump of ice, and you wish to turn it into hot steam--how can you do it? Of course you must heat the ice, and then it will melt--not into steam, but into water. And when you have the water you can heat that again till it boils and goes off in steam--or, as we say, “it boils away.”

Again, if you had steam and wished to turn it into ice, it would have to go through being water between the steam-state and the ice-state.

So the water is a kind of half-way stage between ice and steam--between great cold and great heat.

No doubt, our Earth, as she cooled, passed through a “half-way stage” too. She did not all at once become firm and cool. First she was a bright Sun, made of glowing gases. Then she was a half-sun, half-world: no longer shining, yet very hot indeed; no longer made of gases, but by no means solid. Then lastly she cooled down, as we now see her.

These are, we suppose, three chief parts in the story or life of a heavenly body. Our Sun is in the early part--made of gases, exceedingly hot and bright. Our Earth is in the later part, cold and firm, and not shining!

But Jupiter seems to be still in the middle part, in the half-way stage. He is very, very hot, yet not so hot as to give forth light of his own, for he shines by the Sun’s light. He is not any longer a great mass of gases, yet he seems to be very far from being solid and firm. The clouds which cover Jupiter, though not like the fiercely-glowing Sun-clouds, are yet very unlike our cool Earthly mists, and perhaps they may be at least as hot as the steam which pours from a boiling kettle.

So the furious hurricanes on Jupiter are brought about, partly, at all events, by Jupiter’s own heat, and not by the Sun’s power alone.

On the whole, we can hardly look upon Jupiter as a nice and fit place for either animals or men to live in. That does not mean that he can never become nice and fit. Our Earth was a very, very long time being made ready to serve as a home for men. Perhaps Jupiter is being made ready also for some such use. As he is so large he cannot cool down nearly so fast as our Earth.

Jupiter’s moons all shine as our moon shines, by borrowed sunlight.

The smallest of his four chief moons--which can easily be seen from the Earth--is about the same size as our Moon, and the biggest is larger than Mercury.

QUESTIONS.

1. Which is the largest of the planets?

Jupiter.

2. How far is Jupiter from the Sun?

Nearly fourteen times as far as Mercury is.

3. How much farther still is Saturn?

Saturn is as far from Jupiter as Jupiter is from the Sun. So the distance of Saturn is twice the distance of Jupiter.

4. How far is Uranus?

Uranus is twice as far from Saturn as Saturn is from Jupiter.

5. Does Jupiter spin on his axis?

Yes, in less than ten of our hours.

6. How long is Jupiter’s year?

About twelve of our years in length.

7. How fast does Jupiter travel?

About eight miles each second.

8. Does Jupiter shine in our sky as brightly as Venus?

No, but he is the next brightest planet in our sky after Venus.

9. How many moons has Jupiter?

Five moons, four of which can be seen easily. The fifth was only discovered a little while ago.

10. Has Jupiter any markings?

He has bands and beautiful coloring when seen in a telescope.

11. Is Jupiter light or heavy?

Very light in make; so light that he is thought to be far from solid, and to be wrapped in very thick masses of clouds.

12. Is Jupiter a cooled world like the Earth?

Jupiter seems to be only a half-cooled world.

13. Is he hot enough to shine?

Jupiter is too cool to shine with his own light; but he seems to be in a very heated and stormy state.

14. How do Jupiter’s moons shine?

Like Jupiter himself, by reflected sunlight.