DOUBLE STARS 145 



presented with the remarkable fact that the speed of the 

 Earth's rotation is being reduced in consequence of the 

 tides. The period of the Earth's rotation determines 

 the day, and consequently the day must be increasing in 

 length. No doubt the rate of increase is now very slight, 

 but there can be little doubt that this has not always been 

 the case. The Earth was at one time a mass of molten 

 rock, in which bodily tides must have been formed, while 

 friction itself must have been far greater in the case 

 of such a viscous mass than in water. Further, as we 

 shall see, the Moon must have been nearer the Earth 

 than it is now, and its tide-producing power consequently 

 more intense. Under these conditions we can well 

 imagine that the loss of rotation proceeded at a com- 

 paratively rapid pace, and that the day was formerly far 

 shorter than it is at the present time. 



The slackening of the Earth's rotation is not, however, 

 the only result of tidal friction. A reaction upon the 

 Moon is inevitable, and it appears that, as a necessary 

 consequence, the Moon must recede from the Earth, its 

 orbital speed decreasing at the same time. Its period of 

 revolution round the Earth, which we may define as the 

 month, is therefore increasing, so that in consequence of 

 the tides, the day and the month are both becoming 

 longer. It follows, however, from simple considerations 

 that this cannot continue indefinitely. The day is in- 

 creasing, and so also is the month, but there must come 

 a time when the day must increase more rapidly than the 

 month 1 , and it must ultimately overtake it. The length 

 of each will then be fifty-five of our present days. The 

 Earth, then rotating in the same period as that of the 

 Moon's revolution round it, will continually present the 



1 This time has already been passed in the case of the Earth-Moon 

 system. 



