Jupiter is by far the largest of the planets, it is 87,030 miles in diameter, and placed at the enormous distance of 494,000,000 miles from the sun. This great planet takes 12 years and 52 days to perform its circuit; it turns upon its axis in 9 hours 55 minutes, a surprisingly short time considering the immensity of its bulk. As a result of this rapid motion Jupiter is very far removed from the form of a true sphere, for the oblate form of heavenly bodies is caused by their rotatory motion, and the centrifugal force set up by it. In fig. 16 is an outline of the earth and Jupiter, showing their relative size. It has no phases, like those planets which are nearer to the sun than the earth, its great distance preventing this, as may be seen in fig. 17, in which the earth (e e) is placed at the two widest lateral positions of its orbit, but the earth is too near the sun, in proportion to the great distance of Jupiter, to allow any part of the latter to come within the range of vision, except that which is illuminated by the sun. This planet is high up in the heavens the greater number of nights in the year, and is therefore a very conspicuous object. It also presents a most beautiful appearance through a good telescope, its vast size causing it to look larger than those which are much nearer; it has several shadowy belts across it, which are supposed to be openings in the strata of clouds which surround it, drawn into ring-like forms by the rotation of the planet, these are shown in fig. 18. It being probable that this great planet is surrounded by strata of the densest clouds which only open in the tropical region, its inhabitants therefore, (if there be any) get but a glimpse of the firmanent and its stars through them, in those situations at or near to the planet's equator. These dense clouds serve a very useful purpose in regions so very far removed from the source of heat, for if radiation were permitted to go on freely from the surface of the planet the sun's rays would be too feeble to compensate for it, and the cold would be intense; but the clouds reflect back the heat radiated from the surface and keep in what little heat is received at that great distance. Jupiter has four satellites or moons, which revolve round it as our moon does round this earth.
FIG. 18.
FIG. 19.
Far beyond Jupiter rolls another stupendous orb called Saturn, not so large as Jupiter, but still immense, being 847 times the bulk of the earth. It is placed at the distance of 906 millions of miles from the sun. An idea of this may be formed from the fact that light, which travels at the rate of nearly 200,000 miles a second or 12,000,000 a minute, takes about an hour and a quarter to pass from the sun to Saturn. It performs its journey round the sun only in 29½ years, which are therefore but as one year, yet all this time it is moving at the rate of nearly 22,000 miles an hour, so immense is the orbit it has to traverse, but it revolves on its axis in about 10½ hours, so that the nights and days are extremely short while the years are prodigiously long. There is every reason to believe that it has changes of seasons and variation of climate similar to those in our world, but, being so far from the sun, they must be altogether more severe than ours. The most extraordinary part of this great globe is its possession of three (perhaps more) great flattened rings, which surround it, one within the other; these rings are of immense size and width, but very thin, the great breadth through all from the inner to the outer edges being about 30,000 miles, while their thickness cannot exceed 250. These rings are placed at a right angle to the planet's axis of rotation and revolve with it, so that when the planet is at the equinox, the edge of these rings is turned towards the sun, they can then be seen only by the most powerful telescopes, forming a faint streak on each side of the orb of the planet (fig. 19), but as they become inclined they appear as a very long ellipse, the ends of which project in loop-like forms on either side, giving rise to the notion of the planet having two handles (fig. 20). This ellipse becomes broader and broader as the plane of the rings forms a greater angle with the line of vision. For a short time before and after the equinoxes of Saturn, the rings become invisible, owing to the earth and sun being on opposite sides of them, as may be seen in fig. 21, so that the darkened side is turned towards the earth and the edge, which is the only part illuminated, is towards the sun.
FIG. 20.
FIG. 21.