JUPITER, the name of one of the old planets, the laCzest of all the bodies, except the sun, in the solar system. The astronomical history of this planet (or of any other, except the newly-discovered small planets) is so completely entangled in that of the progress of astro nomy in general and pure mathematics, that it would be useless to attempt any separation. In the article GRAVITATION will be found the most remarkable points of what an astronomer would call the theory of Jupiter, meaning the explanation of his motions by means of the law of gravitation. We shall here confine ourselves to the statement of the elements of the planet's motion, and that of the satellites.
The figure of Jupiter is sufficiently oblate to appear of a sensibly spheroidal form in a moderate telescope. According to the recent researches of Mr. Main, the axis of revolution is to the equatorial diameter in the proportion of 15'84 to 16'84. The apparent equa torial diameter varies between half and three-quarters of a minute, being when the planet is at its mean distance from the earth. The real equatorial diameter is 11'46 times that of the earth, or upwards of 91,000 miles. Its mean density is nearly the same as that of the sun or about one-quarter of that of the earth.
The mass of Jupiter, from the time of Newton to the present, was supposed to be to that of the sun in the proportion of 2 to 2141 ; but the recent observations of various astronomers, concluding with those of Mr. Airy, contained in the ' Memoirs of the Astronomical Society, make it somewhat greater. Mr. Airy's result is that the mass of Jupiter is to that of the sun in the proportion of 1 to 104617, or 2 to 2094 nearly.
Element.t of the Orbit of Jupiter.
Epoch 1799, December 31, 12" mean astronomical time at Paris. Semiaxis major 5/0115524, that of the earth being assumed as the unit.
Excentricity •0481621; its secular increase (or increase in 100 years) '0001594.
Inclination of the orbit to the ecliptic 18' 52"; its secular diminution 23".
Longitudes from the mean equinox of the epoch (1) of the ascending node 98° 25' 44"•90; its secular increase (combined with the precession) 3430" ; (2) of the perihelion 11* 7' 38"•26 ; its secular increase (combined with the precession) 5710"; (3) of the planet (mean) 81' 52' 19".33.
Mean sidereal motion in 365} days 109256"•b9; sidereal revolution 4332'5848212 menu solar days.
When observed with a telescope of sufficient power, the surface of this planet is perceived to be diversified by faint belts extending in parallel directions across hfsrdise. These belts are observed to vary in magnitude and number ; bid. in general three belts are conspicuous, forming a zone across the body of the planet, and extending in a direction nearly parallel to the plane of his orbit. Jupiter, like several of the other planets, is observed to revolve on a fixed axis. The earliest aurmise of this fact is due to Kepler, who inferred, from the rapid motion of the satellites of the planet compared with the slower motion of the moon around the earth, that Jupiter rotates more rapidly then the earth does. The subsequent observations of astro nomers with the telescope have confirmed this interesting conjecture. Hooke appears to have first discovered the rotation of the'planet. In the month of May, 1664, while engaged in a telescope survey of the planet he discovered a small spot on the largest of the three belts, which, in the course of two hours, travelled from east to west over a space equal to about half the diameter of the planet. It was reserved for Cassini to assign the period of rotation with some degree of pre cision. Having found that the planet generally presented a very altered appearance after the lapse of five hours, he concluded that the time of rotation must be somewhere about ten hours. From observations of a spot near, the southern belt of the planet, he found that in Oh it returned to the same position on the disc. During this interval the planet must have effected twenty-nine com plete rotations. It hence resulted that the time of rotation amounted to 9" 56o. The subsequent observations of Cassini indicated that the spots, besides participating in the rotation of the planet, have a proper motion on his surface. The elder Herschel established the existence of this fact beyond all doubt. Ile found that the time of rotation as indicated by the return of the spots to the same position on the disc fluctuated between 9h 55"' 40' and 50"' 48'.