118 THE EVOLUTION OF SATELLITES. 



earth tlieii than it does now. May we not coujecture that the oscilla- 

 tion of the molten planet became so violent that, in cooperation with 

 the rapid rotation, it shook the planet to pieces, detaching huge frag- 

 ments, which nltimately were consolidated into the moon? There is 

 nothing to tell us whether this theory affords the true explanation of 

 the birth of the moon, and I say that it is only a wild speculation, 

 incapable of verification. 



But the truth or falsity of this speculation does not militate against 

 the acceptance of the general theory of tidal friction, which, standing 

 on the firm basis of mechanical necessity, throws much light on the 

 history of the earth and the moon, and correlates the lengths of our 

 present day and month. 



I have said above that the sequence of events has been stated with- 

 out reference to the scale of time. It is of the utmost importance, how- 

 ever, to gain some idea of the time requisite for all the changes in the 

 system. If millions of millions of years were necessary, the applica- 

 bility of the theory to the moon and the earth would have to be 

 rejected, because it is known from other lines of argument that there 

 is not an unlimited bank of time on which to draw. The uncertainty 

 as to the duration of the solar system is wide, yet we are sure that it 

 has not existed for an almost infinite past. 



JSTow, although the actual time scale is indeterminate, it is possible to 

 find the minimum time adequate for the transformation of the moon's 

 orbit from its supposed initial condition to its present shape. It may 

 be proved, in fact, that if tidal friction had always operated under the 

 conditions most favorable for producing rapid change, the sequence of 

 events from the beginninguntil to-day would have occupied a j^eriod of 

 between fifty and sixty millions of years. The actual period, of course, 

 must have been much greater. Various lines of argument as to the age 

 of the solar system have led to results which differ widely among them- 

 selves, yet I can not think that the applicability of the theory of tidal 

 friction is negatived by the magnitude of the period demanded. It may 

 be that science will have to reject the theory in its full extent, but it 

 seems improbable that the ultimate verdict will be adverse to the pre- 

 ponderating influence of the tide on the evolution of our planet. 



III. 



If this history be true of the earth and the moon, it should throw 

 light on many peculiarities of the solar system. In the first x>lace, a 

 corresponding series of changes must have taken place in the moon 

 herself. Once on a time she must have been molten, and the great 

 extinct volcanoes revealed by the telescope are evidences of her primi- 

 tive heat. The molten mass must have been semifluid, and the earth 

 must have raised in it enormous tides of molten lava. Doubtless the 

 moon once rotated rapidly on her axis, and the frictional resistance to 

 her tides must have impeded her rotation. She rotated then more and 



