408 A Short History of Astronojny [CH. xm. 



the internal distribution of density in the sun, which is 

 uncertain, but making any reasonable assumption as to this 

 we find that the amount, of shrinking required to supply 

 the sun's expenditure of heat would only diminish the 

 diameter by a few hundred feet annually, and would 

 therefore be imperceptible with our present telescopic 

 power for centuries, while no earlier records of the sun's 

 size are accurate enough to shew it. It is easy to calculate 

 on the same principles the amount of energy liberated by a 

 body like the sun in shrinking from an indefinitely diffused 

 condition to its present state, and from its present state to 

 one of assigned greater density ; the result being that we 

 can in this way account for an expenditure of sun-heat at 

 the present rate for a period to be counted in millions of 

 years in either past or future" time, while if the rate of 

 expenditure was less in the remote past or becomes less 

 in the future the time is extended to a corresponding 

 extent. 



No other cause that has been suggested is competent 

 to account for more than a small fraction of the actual 

 heat-expenditure of the sun ; the gravitational theory 

 satisfies all the requirements of astronomy proper, and goes 

 at any rate some way towards meeting the demands of 

 biology and geology. 



If then we accept it as provisionally established, we 

 are led to the conclusion that the sun was in the past 

 larger and less condensed than now, and by going suffi- 

 ciently far back into the past we find it in a condition not 

 unlike the primitive nebula which Laplace presupposed, 

 with the exception that it need not have been hot. 



320. A new light has been thrown on the possible 

 development of the earth and moon by Professor G. H. 

 Darwin's study of the effects of tidal friction (cf. 287 and 

 292, 293). Since the tides increase the length of the 

 day and month and gradually repel the moon from the 

 earth, it follows that in the past the moon was nearer to 

 the earth than now, and that tidal action was consequently 

 much greater. Following out this clue, Professor Darwin 

 found, by a series of elaborate calculations published in 

 1879-81, strong evidence of a past time when the moon 

 was close to the earth, revolving round it in the same time 



