THE CENTURY'S PROGRESS IN ASTRONOMY 



This highly satisfactory conclusion was an orthodox 

 belief of celestial mechanics until 1853, when Professor 

 Adams of Xeptunian fame, with whom complex analyses 

 were a pastime, reviewed Laplace's calculation, and dis- 

 covered an error, which, when corrected, left about half 

 the moon's acceleration unaccounted for. This was a 

 momentous discrepancy, which at first no one could 

 explain. But presently Professor Helmholtz, the great 

 German physicist, suggested that a key might be found 

 in tidal friction, which, acting as a perpetual 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 account- 

 ed 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 esti- 

 mated stability of our system, instead of being re-estab- 

 lished, was quite upset. For the tidal retardation is not 

 an oscillatory change which will presently correct itself, 

 like the orbital wobble, but a perpetual change, acting 

 always in one direction. Unless 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 



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