436 HISTORY OF PHYSICAL ASTRONOMY. 



Saros. This period was anciently used for predicting the eclipses oi 

 the sun and moon ; for those eclipses which happen during this period, 

 are repeated again in the same order, and with nearly the same cir- 

 cumstances, after the expiration of one such period and the commence- 

 ment 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 irreg- 

 ularities of the moon's motion, however complex they may be, would 

 recur after such an interval ; and that, therefore, if the requisite cor- 

 rections were determined by observation for one such period, we might 

 by means of them give accuracy to the Tables for all succeeding 

 periods. This idea occurred to him before he was acquainted with 

 Newton's views. 8 After the lunar theory of the Principia had ap- 

 peared, he could not help seeing that the idea was confirmed ; for the 

 inequalities 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. 



Halley announced, in 1G91, 9 his intention of following this idea into 

 practice ; in a paper in which he corrected the text of three passages 

 in Pliny, in which this period is mentioned, and from which it is 

 sometimes called the Plinian period. In 1710, in the preface to a 

 new edition to Street's Caroline Tables, he stated that he had already 

 confirmed it to a considerable extent. 10 And even after Newton's 

 theory had been applied, he still resolved to use his cycle as a means 

 of obtaining further accuracy. On succeeding to the Observatory at 

 Greenwich in 1720, he was further delayed by finding that the 

 instruments had belonged to Flamsteed, and were removed by his 

 executors. " And this," he says, 11 '* was the more grievous to me, on 

 account of my advanced age, being then in my sixty-fourth year; 

 which put me past all hopes of ever living to see a complete period of 

 eighteen years' observation. But, thanks to God, he has been pleased 

 hitherto (in 1731) to afford me sufficient health and strength to exe- 

 cute my office, in all its parts, with my own hands and eyes, without 

 any assistance or interruption, during one whole period of the moon's 



8 Phil Trans. 1731, p. 133. Ib. p. 536. 



" Ib. 1731, p. 187. Ib. p. 193. 



