THE SATELLITES OF JUPITER.

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viz., 2,160 miles, which belongs to the diameter of our ownMoon ; and with 3,000 miles, the probable diameter of theplanet Mercury . Such a comparison shows that the smallestof the four Moons of Jupiter is almost exactly of the same sizeas our own, while the other three are decidedly larger. Itmay be calculated that their united bulk (inasmuch as thevolumes of spheres vary as the cubes of their diameters) isbetween nine and ten times that of our Moon , and between 4thand £th of that of the Earth . It also follows that the volumeof the largest is about two-thirds as large again as that ofMercury; so that this Satellite affords an example of asecondary body in the Solar System which is much larger thananother primary body.

It is probable that this is also true of the largest of theSatellites of Saturn , and still more so of that of Neptune.But, so far as we have as yet gone in our outward journey fromthe Sun, this is the first instance of the kind with which wehave met. We certainly cannot consider such a Moon to bein any sense unimportant; especially if we further notice thatits diameter is about T r T ths, or not much less than one-half ofthat of the Earth , and about -£ths of that of Mars ; while itssurface is more than frds of the area of Mars . If there beno other reason against the habitability of such a globe, noobjection can be raised upon the score of its size.

The diameters of the orbits of the Satellites , which are nearlycircular in form, next demand our attention. They may easilybe deduced by Kepler’s 3rd law (see Lecture V., p. 122), fromthe periodic times in which the orbits are described. By thatlaw they must be such, that the squares of those times are inthe same ratios as the cubes of the respective distances of theSatellites from Jupiter . The resulting distances from thecentre of the planet are found to be about 262,000 miles;417,000 miles ; 666,000 miles ; 1,171,000 miles ; respectively :i.e. they vary from about a quarter of a million to about 11millions of miles.

From this last statement a very interesting illustration maybe obtained of the vast power of the telescope, when used toview the heavenly bodies, in neutralizing the effect of distance