The most reliable determination of the time of rotatiou is due to Mr. Airy, whose observations of the planet were made at Cambridge in the years 1834-5. Having watched the motion of a spot on the apparent lower belt from December 16, 1834, to March 19, 1835, during which period the planet had effected 225 complete rotations, he hence concluded that the time of rotation is 55.r 21'3'. The inclination of Jupiter's equator to the ecliptic amounts to 3° 5' 30", The telescopic appearance of Jupiter seems to indicate that the body of the planet is enveloped in a cloudy atmosphere of great density. This opinion is strengthened by the variable appearance of the spots on his surface. Sir William Herschel supposed that the spots repre sent dense masses of clouds. He ascribes their variable motion to the prevalence of winds in the atmosphere of the planet, which blow periodically in the same direction. He also endeavours to explain the gradual increase observable in the motion of a spot after its first appearance, by the circumstance, that some period of time must necessarily elapse before the spot can acquire the full velocity of the wind by which it is impelled. From his observations of the spots, he inferred that in some instances the winds travel at the rate of some what more than 200 miles in an hour. This, however, does not exceed the velocity of occasional winds on the earth's surface. Mr. De La lbw has executed an admirable delineation of the physical features of the planet.
The discovery of the important fact that Jupiter is accompanied by four satellites is due to Galileo. Having examined the planet with a new telescope on the 7th of January, 1610, his attentiou was drawn to three small but very bright stars which appeared in the vicinity of the planet, two on the east side of his disc, and one on the west side. On the following evening he again perceived the stars, but their arrangement with respect to the planet was not the same as it was previously. On the 13th he perceived four stars in the vicinity of the planet. By continuing his observations he established beyond all doubt the fact that the four stars seen by him revolved regularly around the planet, and he even arrived at a rough approximation of their times of revolution.
The four satellites of Jupiter appear to revolve on axes, each in the time of its revolution round the planet, in the same manner as our moon. The elements of their orbits are as follows (Bally, Adron. Tables and Formulas ') : the units of time, length, and mass being the earth's mean solar day, the planet's equatorial semidiameter, and the planet's mass.
The first satellite has no sensible excentricity, and its orbit is very nearly indeed in the plane of Jupiter's equator. The second has no
sensible excentricity ; the inclination of its orbit to the planet's equator is under half a minute, and its nodes have a retrograde revo lution of about thirty years. The third has a small but very variable excentricity, and the liue of apsides has a direct but variable motion. The inclination of its orbit is under a quarter of a minute, and the nodes make a retrograde revolution in about 142 years. The fourth has a greater excentricity, and the direct mean motion of its apsides is nearly three-quarters of a degree per annum. The nodes made by its orbit with that of the planet have a direct motion of 41 minutes per annum, and the inclination of the orbit to that of Jupiter is 3° nearly.
The apparent diameters of the satellites were determined by W. Struve with a micrometric apparatus attached to the great refractor of the Dorpat Observatory. The following are the apparent diameters which they would subtend if viewed at the mean distance of the planet from the earth These measures indicate the absolute diameters of the satellites, counting outwards from the planet, to be respectively, 2429 miles," 2180 miles, 3561 miles, and 3046 miles, In consequence of the smallness of the angle by which Jupiter's equator is inclined to the ecliptic, and of the nearness of the orbits of the satellites to the plane of the equator, all the satellites, except the fourth, which sometimes escapee, undergo one eclipse once in every revolution. Since [ECLIPSE] this is caused by the entry of the satel lite into Jupiter's shadow, the eclipse is independent of the earth's position, and the observation can certainly be made, unless Jupiter be too near to the sun. It is found that an immersion or emersion is not visible, generally speaking, unless Jupiter be as much as above the horizon, and the sun as much as 8° below it. It never happens that both the immersion and emersion can be observed, in the case of the first satellite, and rarely in that of the second; though it is other wise with the third and fourth. The reason is, that the planet itself aleses bides a part of ha own shadow, and the first satellite is so near the planet that it most either enter or leave the shadow behind the *wet; which also happens, for the moat part, ID the case of the mead satellite. Before the planet is in opposition Its shadow is on the weeders side of the planet, and after opposition on the eastern; while brier. the wreathes immersions only are visible, and after opposition senerssee., in the cam of the first and second satellites. The eclipses of the fair were) satellites last about 21, 21, 31, awl 41 hours, one tam with another. For the use of these eclipses in determining Losarecia, see that vmrd.