ALEXANDRIAN OR HELLENISTIC PERIOD 



is the perfect figure, and pronounced it inconceivable 

 that the motions of the spheres should be other than 

 circular. This thought dominated the mind of Hip- 

 parchus, and so when his careful measurements led him 

 to the discovery that the northward and southward 

 journey ings of the sun did not divide the year into four 

 equal parts, there was nothing open to him but to either 

 assume that the earth does not lie precisely at the cen- 

 tre of the sun's circular orbit or to find some alternative 

 hypothesis. 



In point of fact, the sun (reversing the point of view 

 in accordance with modern discoveries) does lie at one 

 focus of the earth's elliptical orbit, and therefore away 

 from the physical centre of that orbit ; in other words, 

 the observations of Hipparchus were absolutely ac- 

 curate. He was quite correct in finding that the sun 

 spends more time on one side of the equator than on the 

 other. When, therefore, he estimated the relative dis- 

 tance of the earth from the geometrical centre of the 

 sun's supposed circular orbit, and spoke of this as the 

 measure of the sun's eccentricity, he propounded a 

 theory in which true data of observation were curious- 

 ly mingled with a positively inverted theory." That the 

 theory of Hipparchus was absolutely consistent with 

 all the facts of this particular observation is the best 

 evidence that could be given of the difficulties that 

 stood in the way of a true explanation of the mechan- 

 ism of the heavens. 



But it is not merely the sun which was observed to 

 vary in the speed of its orbital progress ; the moon and 

 the planets also show curious accelerations and re- 

 tardations of motion. The moon in particular re- 



235 



