A HISTORY OF SCIENCE 



sun is really not merely eighteen times but more than 

 two hundred times the distance of the moon, as Wen- 

 delein discovered on repeating the experiment of Aris- 

 tarchus about two thousand years later. Yet this dis- 

 crepancy does not in the least take away from the 

 validity of the method which Aristarchus employed. 

 Moreover, his conclusion, stated in general terms, 

 was perfectly correct: the sun is many times more 

 distant than the moon and vastly larger than that 

 body. Granted, then, that the moon is, as Aristarchus 

 correctly believed, considerably less in size than the 

 earth, the sun must be enormously larger than the 

 earth ; and this is the vital inference which, more than 

 any other, must have seemed to Aristarchus to con- 

 firm the suspicion that the sun and not the earth is the 

 centre of the planetary system. It seemed to him in- 

 herently improbable that an enormously large body 

 like the sun should revolve about a small one such as 

 the earth. And again, it seemed inconceivable that a 

 body so distant as the sun should whirl through space 

 so rapidly as to make the circuit of its orbit in twenty- 

 four hours. But, on the other hand, that a small body 

 like the earth should revolve about the gigantic sun 

 seemed inherently probable. This proposition granted, 

 the rotation of the earth on its axis follows as a neces- 

 sary consequence in explanation of the seeming mo- 

 tion of the stars. Here, then, was the heliocentric 

 doctrine reduced to a virtual demonstration by Aris- 

 tarchus of Samos, somewhere about the middle of the 

 third century B.C. 



It must be understood that in following out the 

 steps of reasoning by which we suppose Aristarchus 



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