JUPITER, the largest planet of the solar system, and the fifth (excluding the asteroids) in order of distance from the sun. Its mean diameter is about 86,500 miles; polar diameter about 83,000; its mean distance from the sun 483,300,000 miles; its period of revolution round it 11 years 10A months; its orbit is inclined to the ecliptic at the angle 1° 18' 40.3". The in clination of its axis is 3° 5' 30", so that changes in the seasons must be almost unknown; its volume is 1,309 times that of the earth, but its mass is only 317.7 times.
Jupiter's satellite system is the most in teresting of that of any of the planets. Alto gether, nine moons are known; the four bright est, which were discovered three centuries ago, are visible in a very small telescope and even in a good field glass; the others are far too faint to be seen in any but the very largest telescopes, and there is indeed one of them so faint that it cannot be viewed visually with any telescope now in existence: it can only be detected by the employment of the photographic plate. The four brightest satellites were dis covered by Galileo in 1610, and are sometimes referred to as the Galilean Stars. They are very large objects, their diameters ranging from 2,100 to 3,550 miles; the largest of them thus exceeds Mercury and approaches closely to Mars in size. These moons are compara tively very close to the planet, yet in 1892 Barnard, at the Lick Observatory, discovered a fifth, excessively faint, satellite whose orbit lies so far within all of them that this moon is but 69,000 miles from the planet's surface. The remaining four were discovered by photog raphy. The sixth, seventh and ninth were found at the Lick Observatory, the first two by Perrine and the last by Nicholson, while the eighth was discovered by Melotte at the Obser vastory of Greenwich. It is remarkable that no less than four of the Jovian satellites were thus discovered at a single observatory. The orbits of the four newest satellites differ greatly from those of the older ones. While the five inner moons form a compact system whose nearly circular orbits all lie very approximately in one plane, the outer ones move in highly ec centric orbits which are greatly inclined to the planes of the others, and at great distances from the planet. Thus the ninth satellite is nearly twenty times as far away as the outermost of the older satellites; moreover, the eighth and ninth satellites revolve about Jupiter in a retrograde direction. This
i last fact raises interesting questions as to the mode of development of the system and even suggests, the possibility that these fainter satel lites originally not have belonged to the system, but that they may be outriding members of the asteroid swarm, or some other bodies which approached so near to Jupiter that they were captured and forced to revolve around the planet. The following table contains the prin cipal data referring to the nine known satellites.
The disk of Jupiter is crossed in a direction parallel with the equator by three or four vividly marked bands or belts; other of these belts vary in density and distinctness. Spots also appear and remain for some time on its sur face. In particular, a large red spot of varying dimensions has been observed from time to time. Prof. George Washington Hough sums up the results of his 23 years of obser vations of Jupiter somewhat as follows: First, its equatorial belt changes both in size and position slowly and gradually. Second, the fainter belts also vary. Third, the circular white spots are very permanent in latitude, but are not fixed in position one with another. Fourth, the dark spots of the same size as the circular white spots are not so stable as they, and probably lie at the level of the equatorial belt. Fifth, the large, irregular white spots near the equator make one rotation in 9 hours 50 minutes. As to the constitution of the planet, Professor Hough concludes that the matter at the visible boundary of Jupiter has a density about half that of water (the average density of the planet is 1.37 times that of water). The medium at the boundary is in the nature of a liquid. In it the great red spot and the circular white spots are located. In such a medium all motions would be slow and gradual and the shape and size of an object would be very permanent. The equatorial and polar belts may be located on the surface or at a higher level than the red spot. In middle latitudes within 20° of the equator the higher atmosphere car ries a layer of dark matter. In this envelope are formed the openings that we call white spots, and by unequal distribution black spots. The belts may be assumed to be some sort of vapor of considerable density. The planet is at a high temperature, but not noticeably self luminous.