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. [Illustration: FIG. 18.] [Illustration: FIG. 19.]