THEORY OF THE EAIITH. 193 



their courses during the night ; but since what 

 period were they astronomers, and to what per- 

 fection did they carry the science ? Here rests the 

 question. It is generally allowed that Callisthe- 

 nes sent to Aristotle observations made by them, 

 and which referred to a period of 2200 years be- 

 fore Christ ; but this fact is related only by Sim- 

 plicius *, as stated upon the authority of Por- 

 phyry, and 600 years after Aristotle. Aristotle 

 himself says nothing on the subject, nor has any 

 creditable astronomer spoken of it. Ptolemy 

 mentions and makes use of ten observations of 

 eclipses really made by the Chaldeans ; but they do 

 not refer to an earlier period than that of Nabo- 

 nassar (721 years before Christ) ; they are inaccu- 

 rate also ; the time is expressed only in hours and 

 half-hours, and the shadow only in halves or 

 fourths of the diameter. Notwithstanding, as 

 they had fixed dates, the Chaldeans must have 

 had some knowledge of the true length of the 

 year, and some means of measuring time. They 

 appear to have known the period of eighteen 



* See M. Delambre, Hist, de T Astronomic, vol. i. p. 212. 

 See also his analysis of Geminus, ibid. p. 211. Compare 

 this with M. Ideler's Memoirs on the Astronomy of the 

 Chaldeans, in the fourth volume of M. Raima's Ptolemy, 

 p. 166. 



N 



