THE PLANET JUPITER.

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by a fraction which is rather less than J^th, so that the valueof its greatest distance from the Sun is rather less than a ^jtlipart (or about 23,000,000 miles) more, than the mean value,which we have already stated to be about 484,000,000 miles.Jupiter ’s greatest distance from the Sun consequently amountsto about 507,000,000 miles. In like manner its least distanceis 23,000,000 miles less than its mean distance, or very nearly461,000,000 miles.

If only approximate numbers be desired, such as may easilybe carried in the memory, it may be said that the planet’sdistance from the Sun varies from about 460,000,000 to about510,000,000 miles; its mean distance being somewhat overfive times that of the Earth . From this it would also followby Kepler’s third law (see p. 122) that, if we cube the number5 (which gives 125), and then take the square root of this latternumber (which is somewhat greater than 11), Jupiter ’s year,or the period of its orbital rotation, must be somewhat morethan 11 times as long as the Earth ’s year.

Its more accurate value is found to be very nearly 4,332|days, i.e., about lli years, or about 50 days less than 12 years.

Jupiter travels in an orbit, the plane of which is inclined atless than li( 0 to that of the ecliptic, or to the plane of theEarth ’s orbit; an inclination smaller than for any other planet,except Uranus . While travelling in such a plane it keeps itspolar axis very nearly perpendicular to it, the deviation froman upright position being only about 3°, instead of nearly234°, as in the case of the Earth .

Jupiter ’s velocity in its orbit, owing to its greater distancefrom the Sun , is, of course, much less than that of the Earth .While the latter moves at the rate of more than 18 miles persecond, the speed of the former is only about 8 miles persecond. This, however, amounts to about 29,000 miles perhour. On the other hand, the huge bulk of the planet appearsto turn on its polar axis nearly 21 times as rapidly as the Earth .

With regard to the exact period of its axial rotation, as weshall presently see, considerable doubt exists, owing to the factthat somewhat different results are given by the observation ofthe times in which spots upon different parts of its surface are