FUNDAMENTAL CONCEPTIONS 365 



ally exists. Herein astronomy ultimately accomplished a great serv- 

 ice to the development of knowledge in demonstrating the important 

 function of the intellectual process through which the direct impres- 

 sions of perceptions should be interpreted. 



Hipparchus, the first great investigator in astronomy, saw that 

 the solution of the problem of planetary motions must be approached 

 through the invention of simple geometrical conceptions by the 

 mathematical consequences of which the observed motions could be 

 represented. He found that the apparent motion of the sun in a 

 plane inclined to the celestial equator could be represented as arising 

 from a uniform circular motion of the sun around a centre outside 

 the earth. He deduced the nominal orbit of the sun in this way and 

 verified the result by showing that this hypothesis represented his 

 observed positions of the sun, as he saw it on the sky, within the 

 possible errors of his observations. This is modern scientific investi- 

 gation complete at every point. There is every reason to believe that 

 Hipparchus consciously employed his invention as an hypothesis 

 without insisting strongly on its objective reality. His discovery is 

 in the nature of an intellectual truth which was the seed from which 

 subsequent knowledge of the solar system developed. 



Acting on the idea suggested in the work of Hipparchus, Ptolemy 

 extended the hypothesis of uniform circular motion to include motion 

 in an epicycle; that is to say, he conceived a planet to be revolving 

 uniformly on the circumference of a circle, the centre of which was 

 also revolving uniformly on the circumference of another circle. 

 Though he added other mechanism to his scheme, this of uniform 

 motion in eccentrics and epicycles was the fundamental notion. We 

 know from Ptolemy's account of his labors, that he regarded his 

 hypothesis mainly as a computing device, a geometrical concep- 

 tion which could be successfully applied in the representation of 

 apparent planetary motions, as the astronomers of that age saw 

 them; he was perfectly aware of the fact that the sun could be em- 

 ployed as the centre of reference for planetary motion. But that idea 

 was repugnant to human experience; since it involved the conse- 

 quence that the massive earth must turn upside down every twenty- 

 four hours ; and that this giant body of matter, toward the centre of 

 which all bodies were believed to tend, must become tributary to the 

 sun, then supposed to be a body of fire, at that time classified as the 

 lightest of elements. The astonishing facts of the planetary system 

 already known at that time invested them with a degree of mystery 

 which rendered it next to impossible to regard the substance of which 

 they are composed, or the means by which their actual motions are 

 maintained, as having any credible analogies in terrestrial experience. 



In view of all the circumstances, it seems reasonable to agree with 

 the view of Delambre that Ptolemy, in the light of his time, would 



