A HISTORY OF SCIENCE 



years, it is perhaps not too much to say that Hippar- 

 chus, " the lover of truth," missed one of the greatest 

 opportunities for the promulgation of truth ever vouch- 

 safed to a devotee of pure science. 



But all this, of course, detracts nothing from the 

 merits of Hipparchus as an observing astronomer. 

 A few words more must be said as to his specific dis- 

 coveries in this field. According to his measurement, 

 the tropic year consists of 365 days, 5 hours, and 49 

 minutes, varying thus only 12 seconds from the true 

 year, as the modern astronomer estimates it. Yet 

 more remarkable, because of the greater difficulties 

 involved, was Hipparchus' s attempt to measure the 

 actual distance of the moon. Aristarchus had made 

 a similar attempt before him. Hipparchus based his 

 computations on studies of the moon in eclipse, and he 

 reached the conclusion that the distance of the moon is 

 equal to 59 radii of the earth (in reality it is 60.27 

 radii). Here, then, was the measure of the base-line 

 of that famous triangle with which Aristarchus had 

 measured the distance of the sun. Hipparchus must 

 have known of that measurement, since he quotes the 

 work of Aristarchus in other fields. Had he now but 

 repeated the experiment of Aristarchus, with his per- 

 fected instruments and his perhaps greater observa- 

 tional skill, he was in position to compute the actual 

 distance of the sun in terms not merely of the moon's 

 distance but of the earth's radius. And now there 

 was the experiment of Eratosthenes to give the length 

 of that radius in precise terms. In other words, Hip- 

 parchus might have measured the distance of the sun 

 in stadia. But if he had made the attempt and, in- 



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