NEW CONCEPTIONS IN SCIENCE 



construct a right-angled triangle. The diagram be- 

 low should make his method clear: 



MOON STJN. 



Aristarchus's estimate of the angle marked a was 

 87, and he had then merely to measure the sides of 

 his triangle to find that the distance of the sun from 

 the earth was nineteen times that to the moon. Of 

 course his result was wrong ; we now know that the 

 distance is 400 times. What the inquiring Alex- 

 andrian lacked was accurate instruments. The 

 angle is actually 87 50', and although the error of 

 less than one degree seems small, when this is ap- 

 plied to a line approaching a right angle (90) the 

 resulting error is very great. But that does not 

 diminish one's respect for the kind of a mind which 

 could conceive such an idea. It will be noted that 

 Aristarchus understood well that the moon does 

 not shine by its own light. That the eclipses had 

 shown thousands of years before. 



The method employed by his successor, Eratos- 

 thenes, in his attempt to measure the size of the 

 earth, was equally ingenious. In some fashion he 

 learned that the ancient city of Syene, in Upper 



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