JUPITER

, <*>, one of the superior planets, remarkable for its brightness, being the brightest of all, except sometimes the planet Venus, and is much the largest of all the planets.

Jupiter is situated between Mars and Saturn, being the 5th in order of the primary planets from the sun. His diameter is more than 10 times the diameter of the earth, and therefore his magnitude more than 1000| times. His annual revolution about the sun, is performed in 11 years 314 days 12 hours 20 minutes 9 seconds, going at the rate of more than 25 thousand miles per hour; and he revolves about his own axis in the short space of 9 hours 56 minutes, by which his equatorial parts are carried round at the amazing rate of 26 thousand miles per hour, which is about 25 times faster than the like parts of our earth revolve.

Jupiter is surrounded by faint substances, called zones or belts, in which so many changes appear, that they are generally ascribed to clouds: for some of them have been first interrupted and broken, and then have vanished entirely. They have sometimes been observed of different breadths, and afterwards have all become nearly of the same breadth. Large spots have been seen in these belts; and when a belt vanishes, the contiguous spots disappear with it. The broken ends of some belts have often been observed to revolve in the same time with the spots: only those nearer the equator in somewhat less time than those nearer the poles; perhaps on account of the sun's greater heat near the equator, which is parallel to the belts and course of the spots. Several large spots, which appear round at one time, grow oblong by degrees, and then divide into two or three round spots. The periodical time of the spots near the equator is 9 hours 50 minutes, but of those near the poles 9 hours 56 minutes. See Dr. Smith's Optics, § 1105 and 1109.

The axis of Jupiter is so nearly perpendicular to his orbit, that he has no sensible change of seasons; which is a great advantage, and wisely ordered by the Author of Nature. For, if the axis of this planet were inclined any considerable number of degrees, just so many degrees round each pole would in their turn be almost 6 years together in darkness. And, as each degree of a great circle on Jupiter contains about 706 miles, it is easy to judge what vast tracts of land would be rendered uninhabitable by any considerable inclination of his axis.

The difference between the equatorial and polar diameters of Jupiter, is upwards of 6000 miles; the former being to the latter as 13 to 12: so that his poles are more than 3000 miles nearer his centre than the equator is. This happens from his quick motion round his axis; for the fluids, together with the light particles, which they can carry or wash away with them, recede from the poles which are at rest, towards the equator where the motion is quickest, until there be a sufficient number accumulated to make up the deficiency of gravity lost by the centrifugal force, which always arises from a quick motion round an axis: and when the deficiency of weight or gravity of the particles is made up by a sufficient accumulation, there is then an equilibrium, and the equatorial parts rise no higher.

Jupiter's orbit is 1° 20′ inclined to the ecliptic. The place of his aphelion 9° 10′ of <*>, the place of his ascending node 7° 29′ of <*>, and that of his south or descending node 7° 29′ of <*>. The excentricity of his orbit is 1/20 of his mean distance from the sun.

The sun appears to Jupiter but the 48th part so large as to us; and his light and heat are in the same small proportion, but compensated by the quick returns of them, and by 4 moons, some of them larger than our earth, which revolve about him; so that there is scarce-any part of this huge planet but what is, during the whole night, enlightened by one or more of these moons, except his poles, whence only the farthest moons can be seen, and where their light is not wanted, because the sun conslantly circulates in or near the horizon, and is very probably kept in view of both poles by the refraction of Jupiter's atmofphere, which, if it be like ours, has certainly refractive power enough for that purpose. This planet seen from its nearest moon, appears 1000 times as large as our moon does to us; increasing and waneing in all her monthly shapes, every 42 1/2 hours. The period<*>, distances, in semidiameters of Jupiter, and angles of the orbits of these moons, seen from the earth are as follow:

The three nearest moons of Jupiter fall into his shadow, and are eclipsed in every revolution: but the orbit of the 4th satellite is so much inclined, that it passeth by its opposition to Jupiter, without falling into his shadow, two years in every six. By these eclipses, astronomers have not only discovered that the sun's light takes up 8 minutes of time in coming to us; but have also by them determined the longitudes of places on this earth, with greater certainty and facility, than by any other method yet known. The outermost of these satellites will appear nearly as large as the moon does to us. See M. De la Place's Theory of Jupiter's Satellites, in the Memoires de l'Acad. and in the Connoissance des Temps for 1792, pa. 273.

Though there be 4 primary planets below Jupiter, yet an eye placed on his surface would never perceive any of them; unless perhaps as spots passing over the sun's disc, when they happen to come between the eye and the sun.—The parallax of the sun, viewed from Jupiter, will scarce be sensible, being not much above 20 seconds; and the sun's apparent diameter in Jupiter, but about 6 minutes.—Dr. Gregory adds, that an astronomer in Jupiter would easily distinguish two kinds of planets, four nearer him, viz his satellites, and two more remote, viz the sun and Saturn: the former however will fall vastly short of the sun in brightness, notwithstanding the great disproportion in the distances and apparent magnitude.