trating the earth's condition hundreds of millions of years hence. Accordingly, there is nothing absolutely incredible in the theory that during the remote ages when the moon had seas the tidal wave which traversed them, continually retarding the moon's motion of rotation, gradually coerced it into absolute agreement with her motion of revolution around the earth. Still it must be admitted that the theory is not very easily to be accepted as it stands. The seas of the moon were probably less in relative extent, even when at their largest, than those of Mars now are, and such seas could have no tidal waves which even in thousands of millions of years could reduce the moon's rate of rotation in any considerable degree; and, as we shall presently see, the duration of the era when the moon had seas can hardly have been measured by periods so vast. On the whole, while we may admit the probability that at some very distant time in the past the earth may have exerted influences on lunar seas resembling those which the moon now exerts on our seas, it does not appear to us probable that the peculiar feature we are now considering can be attributed either wholly or in very large degree to the retarding influence of tidal waves upon the moon.
One other theory remains which seems to have more in its favor than either of those hitherto considered. Before the moon became a separate planet her frame, then vaporous, must have been enwrapped in the vaporous frame of the earth. While this continued the moon was necessarily compelled to move as a portion of the earth's outer envelope, and therefore, of course, turned upon her axis in the same time that that exterior portion of the earth revolved. So soon as the contraction of the earth's vaporous frame left the moon outside, she was free if she could to change her rate of rotation; that is to say, the earth's enwrapping vapor-masses no longer prevented the moon from changing her rotation rate. And there were two causes at work, either of which, if in action alone, would have markedly changed the moon's rate of turning on her axis. One was the gradual contraction of the moon's frame in cooling. This would have made her turn more quickly on her axis. The other was the continually gathering in of meteoric matter from without, which was a process taking place probably far more rapidly then than now, seeing that the meteoric systems now remaining are the merest residue of a residue compared with those existing hundreds of millions of years ago. This process would tend to make the moon turn more slowly upon her axis. However, the former process would probably operate far more effectively, and thus the moon would on the whole have acquired a more rapid rate of rotation, and the coincidence between rotation and revolution existing when she first had separate existence would have disappeared. But there was all the time a force at work to check the tendency to change in this respect. The earth was there, exerting that very force which we have already described in considering another theory, a force competent, we may infer, to check the tendency to a slow increase in the moon's rate of rotation, and to preserve that relation which existed when the moon was first formed. We say that the competence of this force may be inferred — meaning that the observed coincidence between the moon's rate of turning round upon her axis, and her rate of revolution around the earth, shows that the force was sufficient for that purpose. A similar force exerted by the sun upon the earth since she was first separately formed has not proved competent, as we know, to make the earth turn on her axis in the same time exactly that she travels round the sun; that is, in a year. Nor have any of the planets been forced to behave in this way. But we can readily understand that a great difference should exist between the formation of a planet which, having an enormously high temperature when first formed, would have an enormous amount of contraction to undergo; and the formation of a subordinate orb like the moon, which, though no doubt intensely hot when first thrown off[1] by the contracting earth, cannot have been nearly so hot as a planet at the corresponding stage of its existence. On the whole, there are (so it seems to us) good reasons for believing that that peculiar law of the moon's motion which causes the same lunar hemisphere to be constantly turned earthwards had its origin during the birth itself of our satellite. We may, indeed, find in that peculiarity one of the strongest arguments in favor of the theory that our solar system reached its present condition by a process of development, since on no other theory can a satisfactory solution be obtained of the most striking peculiarity of the moon's motions.
But the inhabitants of earth are more
- ↑ We here use the words "thrown off" as equivalent to "left behind." The theory that the moon was thrown off by the earth, or the earth by the sun, is altogether inconsistent with mechanical possibilities.
