Jupiter (planet), one of the planets of our solar system, is second in brilliancy to Venus, unlike which, however, it is a "superior" planet, having its orbit outside that of the earth. It is about five times as brilliant as Sirius, the brightest of the fixed stars. The orbit of Jupiter is but slightly inclined (1° 19') to the ecliptic. Its average distance from the sun is 483 million miles; its distance from the earth varies from 369 to 576 million miles. The length of its year or period of revolution around the sun is 11-86 terrestrial years; its apparent year or synodic period, the time between successive conjunctions of the planet with the sun, is 399 days. Its diameter from pole to pole is 83,000 miles; its equatorial diameter is 88,200miles, and the departure from perfect sphericity much more marked than in the case of the earth. Its density is 0-24 compared with that of water, and its mass 316 times that of the earth. The length of its day, or period of axial rotation, is about 9h. 55m. The planet is a beautiful object when viewed with a telescope; it is probable that the markings are entirely due to its atmosphere, and that the actual surface of the planet is rarely visible. Jupiter has hardly yet cooled from the condition of incandescence, and it is only slightly solidified. It possesses five satellites, four of which were discovered by Galileo when he applied the telescope first to the investigation of the heavens. The names given to these are Io, Europa, Ganymede, and Callisto; their distances from the planet vary from 262 to 1,169 thousand miles, and their orbits are almost circular. By means of these satellites the first observations of the velocity of light were made. A fifth was discovered in 1892 at the Lick Observatory; it is close to the planet, and performs one revolution in 11 hours 57 minutes.