A HISTORY OF SCIENCE 



against the critic himself ; he was an astronomer pure 

 and simple. His gift was the gift of accurate obser- 

 vation rather than the gift of imagination. No scien- 

 tific progress is possible without scientific guessing, 

 but Hipparchus belonged to that class of observers 

 with whom hypothesis is held rigidly subservient to 

 fact. It was not to be expected that his mind would 

 be attracted by the heliocentric theory of Aristarchus. 

 He used the facts and observations gathered by his 

 great predecessor of Samos, but he declined to accept 

 his theories. For him the world was central ; his prob- 

 lem was to explain, if he could, the irregularities of 

 motion which sun, moon, and planets showed in their 

 seeming circuits about the earth. Hipparchus had 

 the gnomon of Eratosthenes doubtless in a perfected 

 form to aid him, and he soon proved himself a master 

 in its use. For him, as we have said, accuracy was 

 everything ; this was the one element that led to all his 

 great successes. 



Perhaps his greatest feat was to demonstrate the 

 eccentricity of the sun's seeming orbit. We of to-day, 

 thanks to Keppler and his followers, know that the 

 earth and the other planetary bodies in their circuit 

 about the sun describe an ellipse and not a circle. But 

 in the day of Hipparchus, though the ellipse was recog- 

 nized as a geometrical figure (it had been described and 

 named along with the parabola and hyperbola by 

 Apollonius of Perga, the pupil of Euclid), yet it would 

 have been the rankest heresy to suggest an elliptical 

 course for any heavenly body. A metaphysical theory, 

 as propounded perhaps by the Pythagoreans but ar- 

 dently supported by Aristotle, declared that the circle 



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