298

THE PLANET JUPITER.

of Mars , as we have drawn it, and the semi-circle in questionthere would remain a vacant space, which, although only justdistinguishable in the figure, is really about 500 miles across.

Jupiter pursues his journey round the Sun * at a meandistance very nearly equal to 51- times that of the Earth . Ifthe Earth ’s mean distance be estimated at 93,000,000 miles,Jupiter ’s will therefore be about 484,000,000 miles.

But this mean distance is subject to considerable variationowing to the ovalness of the planet’s orbit, which, although onlyabout one-half of that of the orbit of Mars , and less than one-fourth of that of Mercury, is three times that of the orbit of theEarth . Such an eccentricity, or ovalness, is in fact measured

* We should more accurately say (see Lecture V., p. 121), round thecommon centre of gravity of the Sun and Jupiter . For a similar lawto that which we explained in the case of the revolution of the Earth and the Moon around their common centre of gravity (see Lecture III.,p. 53) holds for any two mutually attracting bodies; and it is worthy ofnotice, that, so far as the Sun and Jupiter are concerned, the commoncentre of gravity about which they would revolve is not within the Sun ,but even when Jupiter is at its nearest to the Sun , and their centreof gravity consequently at its closest approach to the centre of thelatter, it is some thousands of miles outside the Sun ’s surface.

This may be roughly proved as follows. Let s in Fig . LXY. be the

Fig . LXV.—The common centre o i gravity of the bun and of Jupiter lies outside the Sun .

centre of the Sun ; ,j of Jupiter ; g their common centre of gravity.Then the weight of Jupiter being about a xA^th part of that of the Sun ,(ij must be about 1048 times as, or gs a rAsth part of sj. Thesmallest value of sj being about 461,000,000 miles, we have for theminimum value of so, the quotient of this by 104!), or about 438,500miles ; which is about 6000 miles further from s than the Sun ’s surface,if we take 432,500 miles to be the most probable value of the Solar semi-diameter. It is therefore hardly accurate to state, as is sometimes done,that the weight of the Sun is so enormously preponderant, that the centreof gravity of it and of all the planets lies very close to the Sun ’s centre.