METHOD OF VARIATWNS. 63 



siderable presumption in favour of the identity of the 

 body apart from the similarity of the orbit. There is 

 nothing which so strongly fascinates the attention of men 

 as the recurrence time after time of some unusual event. 

 Things and appearances which remain ever the same, 

 like mountains and valleys, fail to excite the curiosity of 

 a primitive people. It has been remarked by Laplace 

 that even in his day the rising of Venus in its brightest 

 phase never failed to excite surprise and interest. So 

 there is little doubt that the first germ of physical 

 science arose in the attention given by Eastern people to 

 the changes of the moon and the motions of the planets. 

 One of the earliest astronomical discoveries must have 

 consisted in proving the identity of the morning and 

 evening stars, on the ground of their similarity of aspect 

 and invariable alternation 9. Periodical changes of a 

 somewhat complicated kind must have been understood 

 by the Chaldseans, because they were aware of the cycle 

 of 6585 days or 19 years which brings round the new 

 and full moon upon the same days, hours, and even 

 minutes of the year. The earliest efforts of scientific 

 prophecy were founded upon this knowledge, and if at 

 present we cannot help wondering at the precise antici- 

 pations of the nautical almanack, we may readily imagine 

 the wonder excited by such successful predictions in 

 early times. 



Combined Periodic Changes. 



We shall seldom or never find a body subject to a single 

 periodic variation, and free from any other disturbances. 

 As a general rule we may expect the periodic variation 

 itself to undergo variation, which may possibly be secular 

 or incapable of repetition, but is more likely to prove 



( i Laplace, ' System of the World/ vol. i. pp. 50, 54, &c. 



