JUPITER. The largest planet in our solar system. having a mass nearly three times as great as the combined 11141SSeS of all the other planets. Its orbit is about five and two-tenths times as far front the sun as that of the earth, or at a mean distance of 4S3.3 millions of miles, and its eccentricity is considerable, the planet's greatest and least distances from the solar centre varying between 4112 and 504 millions of miles. The planet's mean distance from the earth when in opposition is about 390 million miles, and it moves around the son in eleven of our years and 314.54 days, so that the interval between its returns to opposition has a mean value of 399 days. and its orbit is inclined to the ecliptic about 1° IS' 41". The mean diameter is about it(3,500 miles, with a polar compression of about one-seventeenth. thus eyeeeding the earth in vol ume a little over 1279 times. Onc of the dis tinguishing features of the planet is the belt cr stratified changeable baud crossine. the disk in a parallel dire•tion with the of the orbit. The bert varies greatly. being at times narrow, while sonic-times almost the whole disk is covered. Months will sometimes pass with out any remarkable change in the telescopic appearance of Jupiter's surfaee, and then sud denly considerable alterations will take place in a few hours. !n addition to these changeable bands, more permanent spots sometimes appear. The 'Great Red Spot,' first seen in ISIS, is the most important of these. It is still visible. 'hit the rotation period of the planet en inlet be fixed very accurately from observations of the spots. because none of them retains its position with sufficient permanence. The rotation is known. however, to take place in about !I hours 55 min utes. There can be no doubt hut that atmosphere currmits on :limiter materially affect his appear 1111Ce in our telescopes.
The inelination of .Ittpiter's equator to the plane of his orbit is 3° 5'. which would fix the changes of the seasons within narrow limits, were the planet existing 'under eiremnstanees resembling those of the earth: but as the tem perattire of is above redness (how far above is not known). the sun's rays, at his im mense distance. can hardly be taken as an ele ment of the surface heal.
Jupiter has five satellites or moons, as follows: 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 most distant one (which sometimes escapes) suffer an eclipse at every revolution. These eclipses of Jupiter's satellites are of interest in connection with the subject of the velocity of light, which was first estimated by means of observations of these eclipses by the Danish astronomer Roemer (q.v.) in 1675. Another most interesting phenomenon of the satellites is that of their 'shadow transits.' When a satellite passes between Jupiter and the sun, a shadow falls upon the planet. This shadow is analogous to that cast on the earth by our moon in total solar eclipses. ( See EcuesFs. I To an observer at the telescope these satellite shadows appear as tiny dark dots moving across Jupiter's disk. The satellites themselves can also he observed (though with difficulty) projected against the disk of the planet, and transiting across. The satellites are also at times hidden or occulted behind the disk. See PLANETS: SOLAR SYSTEM.