CHANGES IN THE LENGTH OF THE DAY 



By Ernest W. Brown 

 Yale Observatory 



Even to the professional astronomer it comes perhaps as a shock 

 when he first learns that the moon must be regarded as our ultimate 

 exact measure of time. He is so accustomed to the apparent vagaries 

 of its motion, to seeing it all over the sky and to being bothered by 

 its presence that it is difficult to realize the exactness with which every 

 detail of its motion can be followed. On the other hand, the stars 

 follow a regular and nearly uniform motion due to the rotation of 

 the earth, so that the latter seems to be the natural standard. He 

 knows, of course, that the length of the day is very slowly increasing 

 because the friction caused by the tides is slowing down the rotating 

 earth, but he knows that its amount is very small and supposes that 

 it is known. 



However, our observational knowledge of this gradual increase in 

 the length of the day comes chiefly from the moon, because it depends 

 almost entirely on the study of ancient eclipses of the sun by the 

 moon. The place of occurrence of these eclipses is not the same as 

 calculation shows it should be if the revolving earth had maintained 

 its rate. The earth moves so fast on its axis that if the moon is not 

 at the appointed place at the given moment the earth can slip round 

 an appreciable amount before the meeting with the sun occurs, so 

 that a total obscuration occurs elsewhere than in the place pre- 

 dicted. The same thing occurs if the earth has been going round 

 more slowly than the calculated value. 



Several workers have examined these old records. The question 

 is not a simple one, chiefly because the historians who wrote them 

 were not expert in observing the phenomena or in gathermg the 

 particular kind of information that is needed by the astronomer. 

 We have to judge from the details which may be given that the all- 

 important question whether the eclipse was seen as total or partial 

 is properly answered. The latest and, I think, the best of all these 

 investigations was made by Dr. J. K. Fotheringham, of Oxford, 

 England, who died recently. Dr. Fotheringham was a notable 

 classical scholar who knew the languages in which the accounts were 

 originally written and the habits of thought and expression of the men 

 who wrote them, and so was able to appraise with some degree of 



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