CHAPTER XVII.

JUPITER.

Passing at one leap over the belt of tiny asteroids, about which we know little beyond their general movement, and the size and weight of a few among them, we reach at once the giant planet Jupiter: Mighty Jupiter, hurrying ever onward, with a speed, not indeed equal to that of Mercury or of our earth, yet eighty times as rapid as the speed of a cannon-ball! Think of a huge body, equal in bulk to twelve hundred earths, equal in weight to three hundred earths, rushing ceaselessly through space, at the rate of seven hundred thousand miles a day!

Jupiter’s shape is greatly flattened at the poles. He spins rapidly on his axis, once in nearly ten hours, and has therefore a five hours’ day and a five hours’ night. As the slope of his axis is exceedingly slight, he can boast little or no changes of season. The climate near the poles has never much of the sun’s heat. In fact, all the year round the sun must shine upon Jupiter much as he shines on the earth at the equinoxes.

But the amount of light and heat received by Jupiter from the sun is only about one twenty-fifth part of that which we receive on earth; and the sun, as seen from Jupiter, can have but a small, round surface, not even one-quarter the diameter of the sun we see in the sky.

When looked at with magnifying power, the bright, starlike Jupiter grows into a broad, softly-shining disk or plate, with flattened top and bottom, and five tiny, bright moons close at hand. Sometimes one moon is on one side, and four are on the other; sometimes two are one side and three on the other; sometimes one or more are either hidden behind Jupiter or passing in front of him. Jupiter has also curious markings on his surface, visible through a telescope. These markings often undergo changes; for Jupiter is no chill, fixed, dead world, such as the moon seems to be.

[Illustration: GENERAL ASPECT OF JUPITER--SATELLITE AND ITS SHADOW.]

There are dark belts and bright belts, usually running in a line with the equator, from east to west. Across the regions of the equator lies commonly a band of pearly white, with a dark band on either side of “coppery, ruddy, or even purplish” hue. Light and dark belts follow one after another, up to the north pole and down to the south pole.

When we talk of “north and south poles” in the other planets, we merely mean those poles which point towards those portions of the starry heavens which we have chosen to call “northern” and “southern.” You know that all the chief planets travel round the sun in very nearly the same _plane_ or flat surface that we do ourselves. That plane is called the “plane of the ecliptic.” Suppose that you had an enormous sheet of cardboard, and that in the middle of this cardboard the sun were fixed, half his body being above and half below. At a little distance, fixed in like manner in the card, would be the small body of the earth, half above and half below, her axis being in a slanting position. The piece of cardboard represents what is called in the heavens the _plane of the ecliptic_--an imaginary flat surface, cutting exactly through the middle of the sun and of the earth.

If the planets all traveled in the same precise plane, they would all be fixed in the cardboard just like the earth, half the body of each above and half below. As they do not so travel, some would have to be placed a little higher, some a little lower, according to what part of their orbits they were on. This supposed cardboard “plane of the ecliptic” would divide the heavens into two halves. One half, containing the constellations of the Great Bear, the Little Bear, Cepheus, Draco, and others, would be called the Northern Heavens. One end of the earth’s axis, pointing just now nearly to the Polar Star, we name the North Pole; and all poles of planets pointing towards this northern half of the heavens, are in like manner named by us their north poles.

With regard to west and east, lay in imagination upon this cardboard plane a watch, with its face upwards; remembering that all the planets and nearly all the moons of the Solar System are said both to spin on their axes, and to travel in their orbits round the sun, _from west to east_. Note how the hands of your watch would move in such a position. The “west to east” motions of planets and moons would be in exactly the opposite direction from what the motions of the watch-hands would be.

To return to Jupiter. It is believed that these bands of color are owing to a heavy, dense atmosphere, loaded with vast masses of cloudy vapor. By the “size” of Jupiter, we really mean the size of this outside envelope of clouds. How large the solid body within may be, or whether there is any such solid body at all, we do not know. The extreme lightness of Jupiter, as compared with his great size, has caused strong doubts on this head.

The white belts are supposed to be the outer side of cloud-masses shining in the sunlight. Travelers in the Alps have seen such cloud-masses, spreading over the whole country beneath their feet, white as driven snow, and shining in the sunbeams which they were hiding from villages below; or looking like soft masses of cotton-wool, from which the mountain-peaks rose sharply here and there.

The dark spaces between seem to be rifts or breaks in the clouds. Whether, when we look at those dark spaces, we are looking at the body of Jupiter, or only at lower layers of clouds, is not known. But sometimes blacker spots show upon the dark cloud-belts, and this seems rather as if they were only lower layers of clouds, the black spots giving us peeps down into still lower and deeper layers, or else perhaps to the planet itself. These appearances remind one strongly of the sun-spots, each with its penumbra, umbra, and nucleus. Occasionally bright white spots show, instead of dark ones. It is thought that they may be caused by a violent upward rush of dense clouds of white vapor. The white spots again recall the sun and his _faculæ_.

Jupiter’s bands are not fixed. Great changes go on constantly among them. Sometimes a white band will turn dark-colored, or a dark band will turn white. Sometimes few and sometimes many belts are to be seen. Sometimes a dark belt will lie slanting across the others, nearly from north to south. Once, in a single hour, an entirely new belt was seen to come into shape. Another time, two whole belts vanished in one day. The bands, in which such rapid movements are seen, are often thousands of miles in breadth. Sometimes these wide zones of clouds will remain for weeks the same. At another time a break or rift in them will be seen to journey swiftly over the surface of the planet.

The winds on earth are often destructive. A hurricane, moving at the rate of ninety miles an hour, will carry away whole buildings and level entire plantations. Such hurricanes rarely, if ever, last more than a few hours. But winds in Jupiter, judging from the movements of the clouds, often travel at the rate of one hundred and fifty miles an hour; and that, not for hours only, but for many weeks together. What manner of living beings could stand such weather may well be questioned.

Another difficulty which arises is as to the cause of these tremendous disturbances on Jupiter. Our earthly storms are brought about by the heat of the sun acting on our atmosphere. But the sun-heat which reaches Jupiter seems very far from enough to raise such vast clouds of vapor, and to bring about such prolonged and tremendous hurricanes of wind.

What if there is another cause? What if Jupiter is _not_ a cooled body like our earth, but a liquid, seething, bubbling mass of fiery heat--just as we believe our earth was once upon a time, in long past ages, before her outside crust became cold enough for men and animals to live thereon? _Then_, indeed, we could understand how, instead of oceans lying on his surface, all the water of Jupiter would be driven aloft to hang in masses of steam or be condensed into vast cloud-layers. _Then_ we could understand why a perpetual stir of rushing winds should disturb the planet’s atmosphere.

In that case would Jupiter be a planet at all? Certainly--in the sense of obeying the sun’s control. Our earth was once, we believe, a globe of melted matter, glowing with heat--and farther back still, possibly, a globe of gas. Some people are very positive about these past changes; but it is wise not to be over-positive where we can not know to a certainty what has taken place. However, Jupiter _may_ have cooled down only to the liquid state, and if he goes on cooling he may, by and by, gain a solid crust like the earth.

This idea about Jupiter’s hot and molten state belongs quite to late years. Certain other matters seem to bear it out, though of actual proof we have none. It is thought, for instance, that the dull, coppery red light, showing often in the dark bands, may be a red glow from the heated body within. Also it has been calculated that Jupiter gives out much more light than our earth would do, if increased to his size and moved to his place--more, in fact, than we could reasonably expect him to give out. If so, whence does he obtain the extra brightness? If he does not shine by reflected light alone, he probably shines also in some additional degree by his own light.

But what about Jupiter being inhabited? Would it in such a case be quite impossible? “Impossible” is not a word for us to use about matters where we are ignorant. We can only say that it is impossible for us to _imagine_ any kind of living creatures finding a home there, if our present notions about the present state of Jupiter are correct. Then is the chief planet of the Solar System a huge, useless monument of God’s power to create? Not so fast. Even as merely such a monument, he could not be useless. And even if he were put to no present use at all, it might be merely because this is a time of preparation for the future. God has his times of long and slow preparation, alike with worlds, with nations, and with individuals.

But now as to the five moons circling round Jupiter. There used to be some very pretty ideas afloat about the wonderful beauty of the moons, as seen from Jupiter, their united brilliancy so far surpassing the shining of our one poor satellite, and making up for the dim light of Jupiter’s sun.

A certain little difficulty was not quite enough considered. If anybody were living on the surface of Jupiter, he would have, one is inclined to think, small chance of often seeing the moons through the cloud-laden atmosphere.

[Illustration: SATELLITES OF JUPITER COMPARED WITH THE EARTH AND MOON.]

The nearest of the four larger moons to Jupiter would, it is true, appear--when visible at all--rather bigger than ours does to us; while the two next would be almost half as large, and the farthest about a quarter as large--supposing inhabitants of Jupiter to have our powers of vision.

All taken together they would cover a considerably larger space in the sky than does our moon. But it must be remembered that Jupiter’s moons, like ours, shine merely by reflected sunlight. And so dim is the sunshine at that distance compared with what it is at our distance, that all the five moons together, even if full at the same time, could only give about one-sixteenth part of the light which we obtain from our one full moon.

Besides, they never are full together, seen from any one part of Jupiter. The four inner moons are never to be seen “full” at all; for just when they might be so, they are eclipsed or shaded by Jupiter’s shadow. The fourth sometimes escapes this eclipse, from being so much farther away.

A thought has been lately put forward, which may or may not have truth in it. What if--instead of Jupiter being a world, inhabited by animals and people, as is often supposed, with a small distant sun and five dim moons to give them light--what if Jupiter is himself in some sort a second sun to his moons, and what if those “moons” are really inhabited planets? It may be so. That is all we can say. The idea is not an impossible one.

The so-called “moons” are certainly small. But they are by no means too small for such a purpose. Jupiter would in that case, with his five moons circling round him, enjoying his light and warmth, be a small picture of the sun, with his four inner planets and the asteroids circling round him, basking in a more lavish amount of the same.

Picturing the moons as giving light to Jupiter, we find them seemingly dim and weak for such a purpose--though, of course, we may here make a grand mistake in supposing the eyesight of living creatures in Jupiter to be no better than our own eyesight. Even upon earth a cat can see plainly where a man has to grope his way in darkness.

But by picturing the moons as inhabited, and Jupiter as giving out some measure of heat and light to make up for the lessened amount of light and heat received from the sun, the matter becomes more easy to our understanding.

[Illustration: THE SYSTEM OF JUPITER.]

The nearest moon has indeed a magnificent view of Jupiter as a huge bright disk in its sky, no less than three thousand times as large as our moon appears to us, shining brightly with reflected sunlight, and it may be glowing with a red light of his own in addition. Even the farthest off of the five sees him with a face sixty-five times the size of our moon. And the varying colors and stormy changes in the cloud-belts, viewed thus near at hand, must afford marvelously beautiful effects.

Just as Mercury, Venus, Earth, and Mars travel round the sun, at different distances, nearly in the same plane, so Jupiter’s five moons travel round him, at different distances, nearly in the same plane. Jupiter’s moons are always to be seen in a line, not one high and another low, one near his pole and another near his equator.

The moon nearest to Jupiter, discovered in September, 1892, by Professor Barnard, now of the Yerkes Observatory of the Chicago University, has a probable diameter of one hundred miles, and revolves round its primary in about twelve hours. The second satellite, named Io, is said to be over two thousand miles in diameter, travels round Jupiter in less than two of our days, and is eclipsed by Jupiter’s shadow once in every forty-two hours.

The third moon, Europa, is rather smaller, takes over three days to its journey, and suffers eclipse once in every eighty-five hours.

The fourth moon, Ganymede, is believed to be considerably larger than Mercury, journeys round Jupiter once a week, and is eclipsed once every hundred and seventy-one hours.

The fifth moon, Callisto, is also said to be slightly larger than Mercury, performs its journey in something more than sixteen days, and from its greater distance suffers eclipse less often than the other four.

The distance of the nearest is more than one hundred thousand miles from Jupiter; that of the farthest, more than one million miles.

The following table presents more minutely the results of the latest discoveries concerning these satellites:

+-----------------------+----------------+------------+-----------+ | | DISTANCE FROM | | PERIOD OF | | SATELLITES. | CENTER OF | DIAMETER. |REVOLUTION.| | | JUPITER. | | | +-----------------------+----------------+------------+-----------+ | | | |D. H. M. S.| |1. Barnard’s Satellite,| 112,500 miles.| 100 miles.| 11 57 23| |2. Io, | 266,000 ” |2,356 ” | 1 18 27 33| |3. Europa, | 424,000 ” |2,046 ” | 3 13 13 42| |4. Ganymede, | 676,000 ” |3,596 ” | 7 3 42 33| |5. Callisto, |1,189,000 ” |2,728 ” |16 15 32 11| +-----------------------+----------------+------------+-----------+

The fact of these eclipses, and of the shadow thrown by Jupiter’s body, shows plainly that though he may give out some measure of light, as has been suggested, yet that light can not be strong, or it would prevent any shadow from being thrown by the sunlight. Also, the dense masses of cloud around him, though reflecting sunlight brightly, would shut in much of his own light. Possibly it is chiefly as heat-giver and as sunlight-reflector that Jupiter serves his five satellites.

As with our own moon, so with Jupiter’s moons, the real center of their orbit is the sun, and not Jupiter. They accompany Jupiter in his journey, controlled by the sun, and immensely influenced by Jupiter.

At night the spectacle of the sky seen from Jupiter is, with reference to the constellations, the same as that which we see from the earth. There, as here, shine Orion, the Great Bear, Pegasus, Andromeda, Gemini, and all the other constellations, as well as the diamonds of our sky: Sirius, Vega, Capella, Procyon, Rigel, and their rivals. The 390,000,000 of miles which separate us from Jupiter _in no way_ alter the celestial perspectives. But the most curious character of this sky is unquestionably the spectacle of the five moons, each of which shows a different motion. The second moves in the firmament with an enormous velocity, and Barnard’s satellite still faster, and produce almost every day total eclipses of the sun in the equatorial regions. The four inner moons are eclipsed at each revolution, just at the hours when they are at their “full.” The fifth alone attains the full phase.

Contrary to the generally received opinion, these bodies do not give to Jupiter all the light which is supposed. We might think, in fact, as has been so often stated, that these five moons illuminate the nights five times better relatively than our single moon does in this respect, and that they supplement in some measure the feebleness of the light received from the sun. This result would be, assuredly, very agreeable, but nature has not so arranged it. The five satellites cover, it is true, an area of the sky greater than our moon, but they reflect the light of a sun twenty-seven times smaller than ours; indeed, the total light reflected is only equal to a sixteenth of that of our full moon, even supposing the soil of these satellites to be as white as it appears to be, especially the fifth satellite.

Jupiter appears to be a world still in process of formation, which lately--some thousands of centuries ago--served as a sun to his own system of five or perhaps more worlds. If the central body is not at present inhabited, his satellites may be. In this case, the magnificence of the spectacle presented by Jupiter himself to the inhabitants of the satellites is worthy of our attention. Seen from Barnard’s satellite, Jupiter’s disk has a diameter of 47°, or more than half the distance from the horizon to the zenith. Seen from the second satellite, the Jovian globe presents an immense disk of twenty degrees in diameter, or 1,400 times larger than the full moon! What a body! What a picture, with its belts, its cloud motions, and its glowing coloration, seen from so near! What a nocturnal sun!--still warm, perhaps. Add to this the aspect of the satellites themselves seen from each other, and you have a spectacle of which no terrestrial night can give an idea.

Such is the world of Jupiter from the double point of view of its vital organization and of the spectacle of external nature, seen from this immense observatory.

The attraction of the planets has always played an important part in the motion of comets and the form of their orbits. The enormous size of Jupiter gives it more influence than any other planet, and we are not surprised that it should have seriously interfered with some of the comets that belong to the Solar System. We have already seen that Biela’s comet was captured by Jupiter, and that it was probably dissolved into meteoric dust. We show in the cut the orbits of eight others that circle around the sun, but retreat no farther away than Jupiter. These are the orbits as now determined, but they vary from age to age on account of disturbances of other planets. But it is not likely that the comets themselves will ever escape from the control of Jupiter.

[Illustration: ORBITS OF NINE COMETS CAPTURED BY JUPITER.]