218 HISTORY OF PHYSICAL ASTRONOMY. 



veries in astronomy ; the period of 223 lunations, 

 or eighteen years and eleven days, the Chaldean 

 Saros. This period was anciently used for predict- 

 ing the eclipses of the sun and moon ; for those 

 eclipses which happen during this period, are re- 

 peated again in the same order, and with nearly 

 the same circumstances, after the expiration of one 

 such period and the commencement of a second. 

 The reason of this is, that at the end of such a 

 cycle, the moon is in nearly the same position with 

 respect to the sun, her nodes, and her apogee, as 

 she was at first ; and is only a few degrees distant 

 from the same part of the heavens. But on the 

 strength of this consideration, Halley conjectured 

 that all the irregularities of the moon's motion, 

 however complex they may be, would recur after 

 such an interval; and that therefore, if the requisite 

 corrections were determined by observation for one 

 such period, we might by means of them give ac- 

 curacy to the tables for all succeeding periods. 

 This idea occurred to him before he was acquainted 

 with Newton's views 7 . After the lunar theory of 

 the Principia had appeared, he could not help 

 seeing that the idea was confirmed ; for the inequa- 

 lities of the moon's motion, which arise from the 

 attraction of the sun, will depend on her positions 

 with regard to the sun, the apogee, and the node ; 

 and therefore, however numerous, will recur when 

 these positions recur. 



7 Phil. Trans. 1731, p, 188. 



