PROGRESS OF MODERN ASTRONOMY 



This was a momentous discrepancy, which at first no 

 one could explain. But presently Professor Helm- 

 holtz, the great German physicist, suggested that a key 

 might be found in tidal friction, which, acting as a per- 

 petual brake on the earth's rotation, and affecting not 

 merely the waters but the entire substance of our 

 planet, must in the long sweep of time have changed its 

 rate of rotation. Thus the seeming acceleration of the 

 moon might be accounted for as actual retardation of 

 the earth's rotation a lengthening of the day instead 

 of a shortening of the month. 



Again the earth was shown to be at fault, but this 

 time the moon could not be exonerated, while the 

 estimated stability of our system, instead of being 

 re-established, was quite upset. For the tidal retarda- 

 tion is not an oscillatory change which will pres- 

 ently correct itself, like the orbital wobble, but a 

 perpetual change, acting always in one direction. Un- 

 less fully counteracted by some opposing reaction, 

 therefore (as it seems not to be), the effect must be 

 cumulative, the ultimate consequences disastrous. 

 The exact character of these consequences was first 

 estimated by Professor G. H. Darwin in 1879. He 

 showed that tidal friction, in retarding the earth, must 

 also push the moon out from the parent planet on a 

 spiral orbit. Plainly, then, the moon must formerly 

 have been nearer the earth than at present. At some 

 very remote period it must have actually touched the 

 earth ; must, in other words, have been thrown off from 

 the then plastic mass of the earth, as a polyp buds out 

 from its parent polyp. At that time the earth was spin- 

 ning about in a day of from two to four hours. 



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