TIDAL EVOLUTION. n 



her axial rotation; the earth conversely reacts upon the moon 

 by pushing her further and further away. In this manner the 

 reaction exhibits itself. At first thought there does not seem 

 to be much importance attached to this phenomenon, but a little 

 reasoning will soon show that in this reaction we are 

 dealing with a problem which for far-reaching conclusions is 

 perhaps unparalled in the whole solar system. 



At present the distance separating the two bodies is about 

 240,000 miles, and the length of the day is about 24 hours. 

 If the theory of tidal friction be true, then both the distance 

 of the moon and the position of the earth must have been 

 shorter yesterday than today, shorter the day before yesterday 

 than yesterday, and so on. Looking backward a million years, 

 the revolution was perhaps only some minutes shorter than it 

 is today, and the radius of the moon 's orbit which of course 

 is the distance of the satellite from the earth was correspond- 

 ingly shorter. 



Before arriving at the critical result of this analysis, there 

 are two other dynamical elements which must be taken into 

 consideration and thoroughly understood. First, that in 

 accordance with Keppler's second law, that the radius vector 

 always describes equal areas in equal times, it follows that the 

 moon in past ages must have completed her lunation in shorter 

 periodic time than at present. Second, that the quicker the 

 earth turns on her axis the more centrifugal force becomes dis- 

 played and that if the rotation becomes too quick the centrif- 

 ugal force will become so pronounced that it will overcome 

 the cohesion of the particles of the earth and the gravitation 

 which attracts them toward the center. The result w T ould be 

 that the globe would fly to pieces, or, which is still more 

 likely, would leave a ring behind from off its outward bulged 

 equatorial zone. 



Thus then by retracing our steps backward in time in 

 accordance with tidal evolution we arrive at a critical 

 time, when the moon must have been grazing the surface of 

 the earth, and when one revolution of the earth on its axis 

 must have corresponded to one lunation of the moon. It 

 would appear that the time required was about three hours. 



