ASTRONOMY AND GEOLOGY EDDINGTON 199 



that the geologist may claim anything up to 10,000 million years 

 without provoking a murmur from astronomers. Although there 

 may still be some difficulties about the exact source from which the 

 vast heat energy the stars pour out into space is derived, it is now 

 clear that the Helmholtz contraction theory is inadequate to give the 

 necessary supply. The astronomer has no such precise means of 

 measuring geological time as the physicist has now discovered by the 

 analysis of radioactive minerals; but he can add his contributory 

 evidence that the sun, and presumably therefore the earth, is much 

 older than Lord Kelvin allowed- In the Cepheid variable stars it 

 seems possible to measure the actual rate at which evolution is pro- 

 ceeding — the rate at which the star is condensing from a diffused 

 state to a denser state. The star is believed to be pulsating, and as 

 it expands and contracts the light varies in quantity and character. 

 In a pulsating gravitating mass the period is proportional to the in- 

 verse square root of the density, so that by observing the rate at 

 which the period is changing we can deduce the rate at which the 

 density is changing. I may add that the law that the period depends 

 on the inverse square root of the density is very closely confirmed by 

 comparing the values for the various Cepheids. In this way we 

 find that for the best observed of these stars, 8 Cephei, the density is 

 changing 500 times slower than the contraction hypothesis assumes. 

 It would, of course, be risky to assume that the same proportion 

 holds at all stages of the evolution of a star; but it suggests that 

 Lord Kelvin's estimate of 20 million years for the age of the sun 

 might well be multiplied by 500 to give 10,000 million years. At 

 any rate, the Cepheid observations show that the stars must have 

 some other source of energy besides contraction. 



I suppose it must be a matter of interest to geologists whether the 

 intensity of the sun's heat has been constant or whether it was at one 

 time hotter than now. I think we can say fairly definitely that the 

 sun was formerly much hotter. 2 There must have been a time when 

 the sun's heat was from 20 to 50 times more intense than it is now. 

 That would no doubt have made a great difference to many geolog- 

 ical processes. Unfortunately, I can not say whether it occurred in 

 known geological epochs. It must have occurred after the earth 

 had begun to exist as a separate planet ; but whether it was before or 

 after the sequence of geological strata began to be laid down I have no 

 idea. It would not be unreasonable, however, to expect that in the early 

 geological times the sun was several times hotter than it is at present. 



After the evolution of the solar system, we naturally turn to con- 

 sider the evolution of the earth-moon system. My impression is that 



2 New facts have emerged since this was written. I think we can now say " fairly defi- 

 nitely " that the sun's heat has not altered appreciably during the last 10,000 million 

 years. (November, 1924.) 



