Page:The Solar System - Six Lectures - Lowell.djvu/52



isochronism. This question raises a wholly new set of problems in celestial mechanics from those in which celestial mechanicians were wont to engage. Until recently, mathematical astronomy dealt almost entirely with solids,—entirely so outside the consideration of the Earth. But no solid is absolutely rigid, and the action of one body upon another must cause mutual deformation of figure and give rise to tides in the two masses. Darwin has shown that this tidal action is an important cosmic factor, one which has played as constructive a part in the evolution of things as gravitation itself.

Not only were the planets not rigid in the past ; they are not rigid to-day. So far as we can judge, all the planets behave as plastic bodies at the present moment. So great are the masses that, even in the case of the denser and cooler ones, deformation of figure seems to be what fluidity and rotary conditions would require. They are, therefore, fit subjects for tidal action.

Owing to the great importance of the subject, and to the fact that the explanation given of it in almost all the text-books is erroneous, I shall  present it to you with some pains, the more so  that the action may, I think, be outlined quite