A HISTORY OF SCIENCE 



long after the death of Pliny. The exact dates of 

 Ptolemy's life are not known, but his recorded ob- 

 servations extend to the year 151 A.D. He was a 

 working astronomer, and he made at least one origi- 

 nal discovery of some significance namely, the obser- 

 vation of a hitherto unrecorded irregularity of the 

 moon's motion, which came to be spoken of as the 

 moon's evection. This consists of periodical aberra.- 

 tions from the moon's regular motion in its orbit, 

 which, as we now know, are due to the gravitation pull 

 of the sun, but which remained unexplained until the 

 time of Newton. Ptolemy also made original observa- 

 tions as to the motions of the planets. He is, therefore, 

 entitled to a respectable place as an observing astrono- 

 mer; but his chief fame rests on his writings. 



His great works have to do with geography and as- 

 tronomy. In the former field he makes an advance 

 upon Strabo, citing the latitude of no fewer than five 

 thousand places. In the field of astronomy, his great 

 service was to have made known to the world the labors 

 of Hipparchus. Ptolemy has been accused of taking 

 the star-chart of his great predecessor without due 

 credit, and indeed it seems difficult to clear him of this 

 charge. Yet it is at least open to doubt whether he 

 intended any impropriety, inasmuch as he all along is 

 sedulous in his references to his predecessor. Indeed, 

 his work might almost be called an exposition of the 

 astronomical doctrines of Hipparchus. No one pre- 

 tends that Ptolemy is to be compared with the Rho- 

 desian observer as an original investigator, but as a 

 popular expounder his superiority is evidenced in the 

 fact that the writings of Ptolemy became practically 



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