OBSERVATION AND EXPERIMENT 75 



fixed stars. Imagine a line drawn at the time of the vernal 

 equinox, say, from the center of the earth to the center of the 

 sun. This Hne prolonged will pierce the celestial sphere in two 

 points, and if either point can be located, the position of the 

 sun with reference to the stars becomes known. Hipparchus 

 fixed this position by noting the location among the stars of the 

 center of the shadow cast by the earth at the times of eclipses 

 of the moon. By a comparison of his own observations of such 

 eclipses with those made by his predecessors he was able to de- 

 termine the apparent motion of the sun with reference to the 

 stars, or what we now know to be the motion of the equinoxes 

 with reference to the stars. To establish this fact of precession 

 from such meager observations was a great step ; and it seems 

 not a little singular that a phenomenon so striking should not 

 have led to speedy investigations for its source. But about 

 eighteen centuries elapsed before Newton clearly visualized the 

 mechanical interpretation of this phenomenon, and it was only 

 after an additional half century that the interpretation was fully 

 worked out by d'Alembert. 



How rapidly the spirit of science dies out when its devotees 

 cease to observe and experiment is shown by the failure of the 

 " Divine School of Alexandria" to maintain the high standard 

 set by Hipparchus. His immediate successors became at best 

 only commentators. They wrote much but observed little ; 

 and it does not appear that any of them attempted even to ver- 

 ify the remarkable discoveries of Hipparchus during the two 

 hundred and fifty years which elapsed between the period of his 

 activity and the advent of his worthy disciple and expounder 

 Ptolemy. 



It is to the work of Ptolemy chiefly that we owe our knowl- 

 edge of the discoveries and theories of the Hipparchian epoch. 

 His treatise on the " Great Construction," the Megiste Syn- 

 taxis, or the Al Magisti and hence Almagest of the Arabians, 

 is the earliest of the great systematic treatises on astronomy. 

 It is in this work that the theory of eccentrics and epicycles of 

 Hipparchus is explained and elaborated, and it is this work 

 w^hich has given the name of Ptolemy rather than that of his 



