ABSTRACT
The critical case of the earth, between planets with no satellites and planets with more than one, is quite unique, for our solitary moon is far bigger in proportion than any other satellite in the solar system. Laplace's theory of portions of rotating nebula successively breaking off from the main body, owing to the too great velocity required by gradual condensation. But the direct effect of tidal friction on a rotating satellite is proved to be to retard the rotation until its period becomes equal to that of revolution. This effect, familiar in the case of our own moon and recognised in several other instances, supplies a reason for some of the diversities of the planetary systems. It is supported on geological grounds by Professor Chamberlin, who sees in it a plausible explanation of such refractory problems as the distribution of land and water and the carboniferous era.
