74 WOODWARD 



plane. He was an observer and a theorist of the highest type, 

 being able at once to collect facts and to interpret their relations, 

 and he deserves to be ranked among the great astronomers of 

 all times. He was the first to clearly appreciate the value of a 

 catalogue of the fixed stars and constructed one giving the rel- 

 ative positions of i,o8o stars. He observed with surprising 

 precision the interval of the tropical year ; he made the first 

 tables of the sun and moon ; he discovered the remarkable fact 

 of the precession of the equinoxes ; and he thus early led the 

 way to the great advances of modern times. 



The peculiar merit of the work of Hipparchus lies not alone 

 in the fact that he saw how the apparent motions of the heav- 

 enly bodies may be determined by observations, but also in the 

 fact that he saw how these motions may be determined by a 

 very small number of appropriate observations. Thus, for ex- 

 ample, the interval from the vernal equinox to the summer sol- 

 stice and the interval from the latter to the autumnal equinox 

 sufficed to give him a close approximation to the apparent mo- 

 tion of the sun ; while the records of a few eclipses of the moon 

 enabled him to deduce a closely correct value of the precession 

 of the equinoxes, that shifting of the line of intersection of the 

 equator and the ecliptic which goes on so slowly that an inter- 

 val of nearly 26,000 years is required for a complete circuit. 



Hipparchus may be called the founder of the geocentric the- 

 ory, since he demonstrated the accordance of the phenomena 

 known to him with that theory. The fact that this theory is 

 false detracts little from his merits ; for the sole requisites of a 

 good theory are simplicity of statement and conformity with ob- 

 servation. We now know, indeed, that mechanical phenomena 

 are, in general, susceptible of multiple interpretations, and that 

 observation must decide which of them is to be preferred. 



The method which Hipparchus used to measure the sun's 

 apparent motion among the fixed stars is very noteworthy, 

 especially when we consider the utter lack of effective instru- 

 ments in his time. If the sun moves regularly about the earth, 

 as first supposed by Hipparchus, it ought to return at any 

 epoch, as that of an equinox, to the same position among the 



