One never feels so impressed with the power of the sun as when one contemplates it in relation to Jupiter. Great Jupiter, he may well be called, nearly five hundred million miles out in space, almost a sun himself, the center of a system containing bodies larger than the sun’s nearest planet, Mercury; and yet just Jupiter, one of the planets, held firmly in leash like the others by the sun’s overwhelming force of gravity, forever compelled to revolve about that parent body with the rest of its offspring, to stay at home within the bounds of the sun’s domain, to keep within certain limits in his own orbit, forced to hasten on when he comes nearest the power that controls him, and unable to keep up the same rate of speed when he is farther away. One may well wonder at the immensity beyond comprehension of the stars, among which our sun is but a very small one, when one considers how even this small one can thus swing huge Jupiter about. For Jupiter is, after the sun itself, the mammoth member of our system. In volume he is larger than all the other planets put together, and in mass he is more than double as large as the combined mass of all the others. He is about equal to the sun in density, and about one-fourth as dense as the earth.

There is less difference in size between Jupiter and the sun than there is between Jupiter and the earth. His diameter is eleven times greater than that of the earth. The sun’s diameter is only ten times greater than Jupiter’s. His surface is one hundred and sixteen times that of the earth; the sun’s own surface is only a hundred times larger than his. Jupiter weighs more than three hundred times as much as the earth; the sun weighs only six times more than Jupiter. At the equator his diameter is about ninety thousand miles; but, as the planet is much flattened at the poles, the diameter from pole to pole is only a little more than eighty-four thousand miles. This flattening is due to the very rapid spinning of the planet on its axis, a motion that will always cause a plastic body to bulge at the equator, and thus flatten at the poles.

The force of gravity on Jupiter is about two and one-half times greater than on the earth. A fairy-like figure weighing here only a hundred pounds would be held to the surface of Jupiter with a force equal to two hundred and sixty pounds. This tremendous power makes Jupiter the greatest disturbing body among all the planets. He gives Saturn a mighty pull when the two planets come near each other; he draws some of the little asteroids five or six degrees out of their course when it carries them into the field of his influence; and there are as many as thirty comets that have become permanent members of the solar system, because through his great power of attraction he has made them captive.

Jupiter is so much farther from the sun than we are that his orbit is about five times larger than that of the earth. In consequence also of his greater distance from the sun, he moves much more slowly than the earth. His average velocity is about eight miles a second. It requires more than four thousand days, or nearly twelve of our years, for him to make one revolution around the sun, and he thus consumes more than ten thousand of his own days. He travels through about one sign of the zodiac each year, and is thus not very difficult to keep trace of, since the signs and the constellations of the zodiac so nearly coincide. His synodic period, or the period from one opposition to another, is a fraction less than three hundred and ninety-nine days, or about one year and a little more than a month. His daily motion in the skies is almost too small for us to detect it without observation for more than a day. It is in one day about equal to one-sixth of the apparent diameter of the moon; but in a month he has moved a distance about half as great as that between the two pointers in the Big Dipper, as can be easily seen by comparison with the stars near him.

JUPITER’S PLACE IN THE SKY

Jupiter is now (1912) in the constellation Scorpio, and he will be in this region, and thus a summer star, for several years to come. In 1913 he will be in opposition early in July, and will then be in Sagittarius, not far from the little “milk dipper,” and will be a gloriously beautiful object during all the summer. He will be in opposition about August 10, 1914, in Capricornus, and will again be the most brilliant object in the summer sky. In 1915 he will be in opposition a little after the middle of September, and will then be situated on or near the eastern edge of Aquarius, where he will be a very distinguished star during all the charming evenings of late summer and the autumn. He always seems particularly splendid when in this season of the year he reaches opposition. The insistent brilliancy of his disc brings him then into view before the sun is fairly down; and he hangs, placid and alone, in the southeastern sky during the autumn twilight, and later in the evening shows to advantage his dominating beauty, with Antares on the west of him and Fomalhaut below him, no less charming in their own way, but far less brilliant than this splendid planet.

In 1916, when opposition will occur not far from Hallowe’en, Jupiter will be about on the eastern border of the constellation Pisces, and, rising then just as the sun sets, will enliven the evening view for the rest of that year. He will appear at his very best at this time, for he will be at about his nearest to the sun; and all that this situation can do for him in the way of enhancing his brilliancy may then be seen.

In 1917 he will be in opposition to the sun about the first of December, in Taurus; and for the next few years he will be a winter star, moving majestically along his path in the zodiac, never more than one and a half degrees from the ecliptic, and passing in turn the Pleiades, Aldebaran, Castor and Pollux, and the little Bee-hive in Cancer. There will be no opposition in 1918; but one will occur early in January, 1919, when Jupiter is in the eastern half of Gemini; and toward the middle of February, 1920, another will take place, when the planet is in Cancer, with Castor and Pollux, the sparkling twin stars in Gemini, to the west of him.

During part of 1920 and all of the next three years Jupiter will be journeying across Leo, Virgo, Libra, and Scorpio. He will be opposite the sun in 1921, a little after the middle of March; in 1922, in the latter half of April; and in 1923, toward the very last of May. He will pass near Regulus, the sparkling star in the handle of the Sickle, in the summer of 1920; near Spica in 1921; and he will not be far from Antares in 1923.

In 1924 Jupiter’s cycle of twelve years will be completed, and he will be in opposition again early in July, and situated near the western edge of Sagittarius, not far from where he was in 1912.

These cycles do not repeat themselves exactly; but the planet lacks only four days of having been in opposition eleven times during twelve of our years, so that it is not difficult to keep track of him through a long series of years. For exact dates, such as one needs in a very close study of the planet, an almanac must be consulted; but this is not necessary for mere recognition, which is all that is needed to enjoy the acquaintance of great Jupiter.

Every year Jupiter is an evening star for more than six months. For two months before opposition he rises somewhat after sundown; at opposition he appears exactly at the setting of the sun; and thereafter he is found in the evening sky, appearing farther toward the west each evening, until, when nearing conjunction, he is lost to our view for a time. He is a morning star for an equal length of time, and for about three months can be seen between midnight and six in the morning; but much of the rest of the time he is obscured by the daylight.

Jupiter retrogrades in his motion for about two months before and after each opposition; but, since he changes his place to the extent of only two and a half degrees a month, the whole apparently backward movement amounts only to ten degrees a year. Still, it is very interesting to watch him swing back and forth over this ten degrees before he starts out on each yearly journey.

DISTANCE, LIGHT, AND HEAT

Jupiter is nearly five times farther from the sun than we are. His mean distance from that orb is four hundred and eighty-three millions of miles. His orbit is not so eccentric as that of Mercury or of Mars, but the eccentricity is sufficient to make his distance vary by as much as forty-two millions of miles. His distance is five hundred and four millions of miles when he is farthest from the sun, and four hundred and sixty-two millions when he is nearest to it. On account of his orbit being outside of ours, we are at times nearer to him and at others farther from him than the sun ever is. At his best situation when in opposition, we are three hundred and sixty-nine million miles from him. This is more than ten times farther than we are from Mars at that planet’s most favorable oppositions, and yet Jupiter is much brighter at such times than Mars ever appears to be. At the times of conjunction he is five hundred and ninety-six millions of miles from us, but is still always brighter than a first-magnitude star like Capella or Vega.

Although the distance of Jupiter from us varies thus two hundred and twenty-seven million miles, there is never in him the marked difference in brilliancy that we see in Mars. He is at all times so far away that the variation in distance does not count for as much, though we can see the effect of it plainly enough, even with the naked eye. Light, with all its marvelous speed, consumes more than fifty-three minutes in its journey from Jupiter to the earth when we are most widely separated from him. When we are nearest to him light comes to us from the planet in twenty minutes less time. At his average distance from the sun it requires about forty-three minutes for light to pass from the sun to Jupiter.

Notwithstanding the sun’s great power over Jupiter in shaping his course, it does not give him much in return for his subserviency. So far as light and brilliancy are concerned, it is to Jupiter a very small sun indeed. To an observer on Jupiter the sun would not appear to be more than one-fifth as large as it seems to us. The light it furnishes to Jupiter is twenty-five times less than we receive; and if the planet depended entirely upon the sun for heat, his temperature would be more than two hundred degrees below zero, Fahrenheit. There is every reason to believe that the little heat the sun gives to this mighty planet does not count for much one way or the other at the planet’s present stage of development. Jupiter does not need the nourishing that the smaller terrestrial planets must have, or die. He is probably almost a sun himself. We are not at all certain that the planet is even so far cooled as to have a solid surface. If it has, there is reason to think that the surface is at least red hot, and gives to the planet a temperature higher than anything we have any comprehension of. Jupiter’s atmosphere, too, is extremely thick and dense, so that the planet is probably so protected that it gets very little heat from the sun and loses very little of its own.

It is certain, however, that this great planet is not so much of a sun as to shine by its own light. The light we receive, though it is very brilliant, is reflected sunlight. This is shown by the fact that the planet does not furnish light for its own satellites. When they pass into its shadow the sunlight is shut off from them; and if they receive any light from Jupiter, it is too dusky to be perceptible to us. That the planet may have a red glow, though, is also suggested by the action of the satellites. When they pass between us and Jupiter they sometimes cast less of a shadow on his surface than would be expected, thus indicating that the surface is not altogether dark, though it may only dully glow rather than shine.

DAY AND NIGHT, SEASONS, AND ATMOSPHERE

Jupiter accomplishes one rotation in a little less than ten hours; but, curiously enough, all parts of the planet do not rotate in the same length of time. A day at the equator is nine hours and fifty minutes in length. In some of the higher latitudes it is nine hours and fifty-five minutes, and this notwithstanding the equator is so much larger in circumference than any other part and any one point on it has farther to go in a revolution. As many as eight different rates of rotation have been observed; and even in the same zones some parts seem to lag behind others, taking a little more time to complete the rotation than other parts surrounding them. This is another indication that Jupiter is not a solid body. The surface features are none of them permanent, though some of them remain practically the same for years. It is through this occasional stability of them that it has been possible to mark the planet’s time of rotation.

In the matter of seasons Jupiter has very little variety. The axis of the planet is inclined but little more than three degrees to its orbit, so that whatever amount of heat the sun’s radiance affords must be very nearly uniform during the entire Jovian year. Its distance, too, is at all times so great that there would be no appreciable change in temperature between its perihelion and aphelion positions.

There is every indication that Jupiter has an extraordinarily dense and deep atmosphere. It has been sometimes estimated to be as much as a thousand miles in depth, and the spectroscope shows it to be heavily laden with vapor. But beyond these very general facts not much is definitely known about it. It is certain, though, that it is very different from our atmosphere. The spectroscope shows in it elements, or compounds of elements, which are not familiar to us. The enormous gravitative power of Jupiter would enable him to hold gases rarer than the earth, or the smaller planets like the earth, ever acquired. A molecule of gas would have to move more rapidly than thirty-seven miles a second to escape from Jupiter. The earth, as we have seen, cannot hold any gases moving faster than seven miles a second. So there are many gases which may forever remain in Jupiter’s atmosphere and yet have never had a place in ours.

SURFACE FEATURES

Seen through a telescope, Jupiter shows the loveliest variety of colors, with the reddish ones always most conspicuous. The slightly pink-tinted steady light that we get from the planet with the naked eye in no way suggests the turbulent, flame-like aspect that a telescopic view opens to us. The telescope also reveals very clearly that flattening at the poles which has already been spoken of.

With so dense an atmosphere as Jupiter most likely has, it is sometimes doubtful whether his surface can be seen by us at all. But it is certain that we see something apparently much more dense and stable than an atmosphere is supposed to be; and hence it is thought that, in spite of its thickness, the atmosphere may be only partially opaque, and that it may be in some places even more or less transparent.

It does not seem probable that the markings on Jupiter are wholly atmospheric. Some of them indicate that the substance we see has considerably more consistency than a mere gas. The whole surface of the planet is covered with belts and spots of various colors and varying shapes. The belted appearance is particularly marked. It has been noticed for more than two hundred years, and can be seen with a comparatively small telescope. Sometimes as many as twenty or thirty belts have been seen at one time. All of them are parallel with the equator.

Two broad red belts on each side of the equator, called the tropical belts, are very distinct, and sometimes retain the same shape and color for months at a time, though sometimes they change rapidly in both color and outline. Between them is the equatorial belt, which is also a semi-permanent feature, remaining often for a considerable period unchanged. These belts, and the spots that sometimes appear on and near them, have been closely watched, because about the equator, and especially just south of it, is the region of greatest activity on Jupiter’s surface.

One feature that more nearly suggests solidity and permanency than anything else on Jupiter is the famous great red spot which lies in the southern hemisphere just below the southern tropical belt. It appeared about thirty-five years ago, in July, 1878, as a pale pink spot, grew brighter for two or three years, and then faded, until, at the end of two or three more years, it was almost invisible. In another year it came again, and increased in brightness for five or six years. Then it grew a little fainter, and has since remained a rather faint red spot, but plainly visible.

In shape the great red spot is an immense oval as much as thirty thousand miles from east to west and seven thousand miles from north to south, which gives it a surface four or five times as large as the land area on the entire earth, and larger even than the whole surface of the earth including the oceans. Although retaining its own shape, it seems to drift about among its surroundings, showing that it is not attached to any solid surface; and yet it has a suggestion of solidity in itself, which was shown when it and another smaller spot were seen to be drifting toward each other, and then finally to meet. Instead of colliding or going over or under, they calmly drifted to one side and went around each other.

Appearances such as this have suggested the idea that the great spot might be a continent in process of formation. Such an idea is at best a speculation; but it would be interesting if it should prove that we are witnessing on Jupiter the process through which our own earth must at one time have passed when its crust began to solidify in patches, as one of the steps in the long period of evolution which has prepared it for our uses. It is not at all certain that Jupiter will ever be just like the earth. The differences between its atmosphere and ours may have some influence in its development that we have little knowledge of at present, and there are some other fundamental differences between the two planets which may in some way effect a difference in development. But in a general way we know that the planet will in time become more condensed than it now is and will finally solidify. Whether the processes will be carried on in just the same way in which they have been here on the earth is not so certain.

JUPITER’S SYSTEM OF SATELLITES

Jupiter is the center of a superb system of satellites, eight in number. Four of them were first seen in 1610, and have the honor to be the first heavenly bodies discovered by means of the telescope. The fifth one was not discovered until 1892. The sixth was first seen in 1904, and the seventh in 1905. After three years an eighth was discovered (in 1908).

When the first four satellites were discovered they were named respectively, in the order of their distances from Jupiter, Io, Europa, Ganymede, and Callisto. Ganymede is not only the largest of the four, but is also the largest satellite in the solar system. It is larger than Mercury, and not much smaller than Mars. Callisto is next to Ganymede in size, and is about the size of Mercury. Io is about the size of our moon, and Europa is not much smaller. Under very favorable conditions Ganymede and Callisto can be seen by the naked eye; but a good many persons think they see the moons of Jupiter when they see only some small stars in that region. They are invisible to most people, but probably could be seen oftener if it were not for the glaring light of the planet, which more or less obscures anything so near it.

After the discovery of Jupiter’s fifth satellite, astronomers seem to have become possessed with that dull spirit of orderliness such as is sometimes exhibited by city councils in substituting numbers for historic and beautiful names in designating streets. No more of Jupiter’s satellites were given names such as might be appropriate for members of this Jovian family; but all were given numbers—the first four in order of their distance from Jupiter, the others in order of their discovery. Io, Europa, Ganymede, and Callisto are now designated, respectively, I, II, III, and IV, while V, VI, VII, and VIII have never had any designation other than these numbers.

The fifth satellite, discovered in 1892, is the nearest to Jupiter, and the smallest of all his satellites. Its diameter is probably not more than one hundred and twenty miles, but its exact size can be estimated only by the amount of light it reflects. It is too small to show a measurable disc, and cannot even be seen when it makes a transit across the planet. It would seem then a mere speck, if we could see it at all. It makes one revolution about Jupiter in less than twelve hours (eleven hours and fifty-seven minutes), and is only a little more than twenty-two thousand miles from the surface of the planet at the equator. It appears to us as a star of about the thirteenth magnitude, and cannot be seen except with a large telescope. Owing to the great curvature of the planet, and to the satellite’s being so near him, it cannot be seen from the surface of Jupiter beyond sixty-five degrees of latitude. It moves faster than any other satellite in the solar system, going at the rate of sixteen and a half miles a second. It does not make a revolution in as short a time as Phobos, the little satellite of Mars, does, but it has a much longer distance to travel and goes at a faster rate. The fact that Jupiter rotates in ten hours and the satellite makes a revolution around him in twelve hours results in the satellite’s taking five of Jupiter’s days to cross from the eastern horizon to the western. It would go through all its phases four times during that period if it were not that, being so near the planet, his huge form cuts off the sunlight from the little satellite for nearly one-fifth of the time, and it is never seen “full.”

This satellite is very difficult for us to see on account of its diminutive size and its nearness to the shining disc of Jupiter; yet it was discovered by means of the telescope, and not by photography, as so many small bodies are discovered nowadays, and by a man who thus far has not been able to see the fine line markings on Mars, which some other astronomers think they can see—a fact that is very interesting as showing the difference between observers even of great keenness of vision. From this satellite Jupiter would seem an enormous body, nearly eighty-five times larger than our sun appears to us, and, no doubt, a splendid object. But the little satellite pays rather dearly for the view by suffering numerous and long-continued eclipses.

The sixth and seventh satellites are also very minute bodies, measuring probably less than one hundred miles in diameter. They circle about Jupiter at a distance nearly thirty times more remote than our moon is from us. They are about seven million miles from the planet, and probably not more than barely visible from it. It takes them two hundred and sixty-five days to make one revolution, which is more than five hundred times as long as the period of Jupiter’s nearest satellite. These two satellites are so nearly of one size and revolve so nearly in the same time and at the same distance from Jupiter that they are thought to have had a common origin. Just what their relation is has not yet been determined.

The eighth satellite, discovered in January, 1908, is certainly no larger, and is perhaps still more tiny, than the sixth and the seventh, though it is a little brighter than either one of them. It is about three times farther away from Jupiter than the seventh satellite, and with eyes such as ours would not be visible from Jupiter. It shows to us as about a seventeenth-magnitude star, which is almost at the limit of our vision with even the largest telescope. It seems to revolve about Jupiter in a direction exactly opposite to that of the other satellites—a retrograde motion that appears in the solar system in only two or three other cases and has not yet been entirely accounted for.

Jupiter’s satellites have played an important part in astronomical discoveries and investigations. It was through observation of their transits that it was discovered that light occupied time in passing through space. When Jupiter was near us in his orbit, the eclipses occurred too soon for their calculated time; when he was farther away, they occurred too late. It was found that these irregularities were due to the fact that light is not transmitted through space instantaneously, and further investigation showed that it travels at the rate of 186,400 miles a second. The eclipses of Jupiter’s moons are carefully computed and recorded in the Nautical Almanac, and it is through observations of them that chronometers are corrected at sea.

Ganymede and Callisto have been found to keep always the same face toward the planet, as our moon keeps always the same face toward us; and it is thought that all of Jupiter’s satellites probably do this.

The symbol of Jupiter is ?, a hieroglyph for the eagle, which was the bird of Jove.