JUPITER (ante), the largest planet in our solar system. having a mass in excess of all the other members by nearly three-fifths. Its orbit is about five and a half times as far from the sun as that of the earth, or at a mean distance of 475,692,000 m., and its eccentricity is considerable, the planet's greatest and least distances from the solar center being 498,639,000 and 452,475,000 m. respectively. The planet's mean distance from the earth, when in oppogition, is about 361,000,000 m., and it moves around the sun in 11 of our years and days, so that the interval between its returns to opposition has a mean value of 398,867 clays, and its orbit is inclined to the ecliptic about 1'18'40.3". The mean diameter is about 85,000 m., with a polar compression of according to measurements by Mr. Main, thus exceeding the earth in volume a little over 1:83 times. One of the distinguishing features of the planet is the belts, or stratified change able bands crossing time disk in a generally parallel direction with the plane of the orbit. The number of these belts varies, there sometimes being only one, while at times the whole disk is covered; but there are usually three prominent bands or zones. Months will sometimes pass without any remarkable change in their appearmice, when suddenly considerable alterations will take a few hours. The first. obeerver of these - - • bands was Ilnygmis who published an account of them in 1659, with his discovery of similar bands on Mars. and of the rings of Saturn. Cassini afterwards made extended observations upon these bands, and also upon matters relating to the planet. There are also certain spots observable upon Jupiter's disk, the first one being discovered by Hooke in 1664, and which he observed to travel from e. to tv. in the course of two hours over a space about equal to half the diameter of the planet. Cassini afterwards, at the Paris observatory, assigned a nearly correct rate of motion, by which he was enabled to determine very nearly the diurnal rotation of the planet, Oh. 56m. Airy, late astronomer royal of England, made an estimate of 9h. 55m. 25s., and Maedler another, which is regarded as the most correct, partly because of the number of observations which were made a basis of calculation. His estimate is 9h. 55m. 26.6s. The observations of Cassini, subsequent to 1666, indicated that the spots, besides rotating with the planet, have a certain degree of motion on its surface, and the elder Herschel established the correctness of these observations. These spots have at times been regarded as being per manent, and the one discovered by Hooke has sometimes been called the " old spot "; hut they are now regarded as changeable and the effect of cyclonic disturbances in a deep atmosphere, or beneath it, and as having somewhat the nature of sun spots; and the phenomena of the belts are also thought to be connected with causes resembling those in the solar atmosphere, or gaseous envelope.
The inclination of Jupiter's equator to the ecliptic is 3° 5' 30", which would fix the changes of the seasons within narrow limits, were the planet existing under other cir cumstances resembling those of the earth; but as the temperature of Jupiter is above redness (how far above is not known), the sun's rays, at its immense distance—five and a half times that of the earth, can hardly be taken as an element of its surface heat. Jupiter has four satellites or moons. The first moon has a mass compared to that of the planet of .0000173231, and revolves in an orbit having no sensible eccentricity at a mean distance of 6.04853 times the planet's equatorial radius, in ld. 18h. 28m., earth time. The second moon has a mass compared to that of the planet of .0000232355, and revolves in a similarly non-eccentric orbit at a mean distance of 9.62347 times the planet's equatorial radius, in M. 1311. 14m., earth time. The third moon's mass is comparatively:0000884972; it revolves in an orbit of. small but variable eccentricity at a mean distance of 15.35024 times the planet's equatorial radius, in 7d. 3h. 43m., earth time. The fourth moon's mass has a comparative value of .0000426591, revolving in an orbit of greater eccentricity than the third, at a mean distance of 26.9983591 times the planet's equatorial radius, in 16d. 1611. 32m., earth time. From the micrometric observations of Struve at the Dorpat observatory, the following are the estimated diameters of these satellites. In the order above given, first, 2,429 m. ; second, 2,180; third, 3,561; and the fourth, 3.046. Their densities must, therefore, differ, the second having nearly double the density of the first, and consid erably more than that of the third. Indeed, the density of the first satellite is only about one-fifth of that of the earth, and less than one-fifth more than water. The density of the second is only about two and one-fifth times that of water, but they all have a density greater than that of the planet, which is a little less than one-fourth of that of the earth. On account of the slight inclination of Jupiter's equator to the ecliptic, and the fact that the planes of the satellites' orbits vary but little from the plane of the equator, all of them except the first (which sometimes escapes), suffer an eclipse at every revolution. The mean duration of the eclipses are respectively 21 3i, and 4} hours. The eclipses of Jupiter's satellites have an interesting reference to the subject of the velocity of light, which was first estimated by means of observations on. tl:ese occultations by the banish astronomer Roemer (q.v.).