A HISTORY OF SCIENCE 



portant observation, and because of its accuracy it has 

 served modern astronomers well for comparison in 

 measuring the trifling change due to our earth's slow, 

 swinging wobble. For the earth, be it understood, 

 like a great top spinning through space, holds its po- 

 sition with relative but not quite absolute fixity. 

 It must not be supposed, however, that the experi- 

 ment in question was quite new with Eratosthenes. 

 His merit consists rather in the accuracy with which 

 he made his observation than in the novelty of the 

 conception ; for it is recorded that Eudoxus, a full cen- 

 tury earlier, had remarked the obliquity of the ecliptic. 

 That observer had said that the obliquity corresponded 

 to the side of a pentadecagon, or fifteen-sided figure, 

 which is equivalent in modern phraseology to twenty- 

 four degrees of arc. But so little is known regarding 

 the way in which Eudoxus reached his estimate that 

 the measurement of Eratosthenes is usually spoken of 

 as if it were the first effort of the kind. 



Much more striking, at least in its appeal to the pop- 

 ular imagination, was that other great feat which 

 Eratosthenes performed with the aid of his perfected 

 gnomon the measurement of the earth itself. When 

 we reflect that at this period the portion of the earth 

 open to observation extended only from the Straits 

 of Gibraltar on the west to India on the east, and from 

 the North Sea to Upper Egypt, it certainly seems enig- 

 matical at first thought almost miraculous that an 

 observer should have been able to measure the entire 

 globe. That he should have accomplished this through 

 observation of nothing more than a tiny bit of Egyp- 

 tian territory and a glimpse of the sun's shadow makes 



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