Page:The Solar System - Six Lectures - Lowell.djvu/63

Rh On the other hand, the Earth's rotation and revolution do not coincide; and the relative effects of Sun and Moon on it are:—

Assuming, therefore, that the retardation began synchronously for all, Mercury, upon whom the effect was 43, should have reached the isochronous condition.

We may note incidentally that Venus on this assumption falls in the debatable ground, since the effect on it is 6.60.

But we do not know either the time of the birth of the Moon nor the relative age of the Earth and Venus. It is quite possible, for aught we know, that Venus may have been subjected to the process practically much longer than the Earth.

It is certainly significant that isochronism ceases just where a first approximation would put it.

Since the date of the detection of Mercury's isochronism by Schiaparelli, the third and fourth satellites of Jupiter, Ganymede and Callisto, have been added to the isochronous list by Mr. Douglass at Flagstaff. These, then, agree with theory. We may safely predict that all the other satellites