The Outline of Science 



rotating faster. There was a period, a long time ago, when the 

 day comprised only twenty hours. Going farther back still we 

 come to a day of ten hours, until, inconceivable ages ago, the 

 earth must have been rotating on its axis in a period of from three 

 to four hours. 



At this point let us stop and inquire what was happening to 

 the moon. We have seen that at present the moon is getting 

 farther and farther away. It follows, therefore, that when the 

 clay was shorter the moon was nearer. As we go farther back in 

 time we find the moon nearer and nearer to an earth rotating 

 faster and faster. When we reach the period we have already 

 mentioned, the period when the earth completed a revolution in 

 three or four hours, we find that the moon was so near as to be 

 almost grazing the earth. This fact is very remarkable. 

 Everybody knows that there is a critical velocity for a rotating 

 flywheel, a velocity beyond which the flywheel would fly into 

 pieces because the centrifugal force developed is so great as to 

 overcome the cohesion of the molecules of the flywheel. We have 

 already likened our earth to a flywheel, and we have traced its 

 history back to the point where it was rotating with immense 

 velocity. We have also seen that, at that moment, the moon was 

 barely separated from the earth. The conclusion is irresistible. 

 In an age more remote the earth did fly in pieces, and one of those 

 pieces is the moon. Such, in brief outline, is the tidal theory of 

 the origin of the earth-moon system. 



The Day Becoming Longer 



At the beginning, when the moon split off from the earth, it 

 obviously must have shared the earth's rotation. It flew round 

 the earth in the same time that the earth rotated, that is to say, 

 the month and the day were of equal length. As the moon began to 

 get farther from the earth, the month, because the moon took 

 longer to rotate round the earth, began to get correspondingly 

 longer. The day also became longer, because the earth was slow- 



