A HISTORY OF SCIENCE 



ceived most careful attention from Hipparchus. Dom- 

 inated by his conception of the perfect spheres, he 

 could find but one explanation of the anomalous mo- 

 tions which he observed, and this was to assume that 

 the various heavenly bodies do not fly on in an un- 

 varying arc in their circuit about the earth, but de- 

 scribe minor circles as they go which can be likened 

 to nothing so tangibly as to a light attached to the 

 rim of a wagon-wheel in motion. If such an invisible 

 wheel be imagined as carrying the sun, for example, 

 on its rim, while its invisible hub follows unswervingly 

 the circle of the sun's mean orbit (this wheel, be it 

 understood, lying in the plane of the orbit, not at right- 

 angles to it), then it must be obvious that while the hub 

 remains always at the same distance from the earth, 

 the circling rim will carry the sun nearer the earth, 

 then farther away, and that while it is traversing that 

 portion of the arc which brings it towards the earth, 

 the actual forward progress of the sun will be retarded 

 notwithstanding the uniform motion of the hub, just 

 as it will be accelerated in the opposite arc. Now, if 

 we suppose our sun-bearing wheel to turn so slowly 

 that the sun revolves but once about its imaginary 

 hub while the wheel itself is making the entire circuit 

 of the orbit, we shall have accounted for the observed 

 fact that the sun passes more quickly through one-half 

 of the orbit than through the other. Moreover, if we 

 can visualize the process and imagine the sun to have 

 left a visible line of fire behind him throughout the 

 course, we shall see that in reality the two circular 

 motions involved have really resulted in producing an 

 elliptical orbit. 



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