230 A Short History of Astronomy [CH. ix. 



motion of the planets is one of extreme difficulty (cf. 

 chapter xi., 228), and Newton was unable to solve it with 

 anything like completeness, yet he was able to point out 

 certain general effects which must result from the mutual 

 action of the planets, the most interesting being the slow 

 forward motion of the apses of the earth's orbit, which had 

 long ago been noticed by observing astronomers (chapter in., 

 59). Newton also pointed out that Jupiter, on account 

 of its great mass, must produce a considerable perturbation 

 in the motion of its neighbour Saturn, and thus gave some 

 explanation of an irregularity first noted by Horrocks 

 (chapter vin., 156). 



184. The motion of the moon presents special difficulties, 

 but Newton, who was evidently much interested in the 

 problems of lunar theory, succeeded in overcoming them 

 much more completely than the corresponding ones 

 connected with the planets. 



The moon's motion round the earth is primarily due to 

 the attraction of the earth; the perturbations due to the 

 other planets are insignificant ; but the sun, which though 

 at a very great distance has an enormously greater mass 

 than the earth, produces a very sensible disturbing effect 

 on the moon's motion. Certain irregularities, as we have 

 seen (chapter n., 40, 48 ; chapter v., in), had already 

 been discovered by observation. Newton was able to 

 shew that the disturbing action of the sun would neces- 

 sarily produce perturbations of the same general character 

 as those thus recognised, and in the case of the motion of 

 the moon's nodes and of her apogee he was able to get a 

 very fairly accurate numerical result;* and he also dis- 

 covered a number of other irregularities, for the most part 

 very small, which had not hitherto been noticed. He 

 indicated also the existence of certain irregularities in the 

 motions of Jupiter's and Saturn's moons analogous to those 

 which occur in the case of our moon. 



* It is commonly stated that Newton's value of the motion of the 

 moon's apses was only about half the true value. In a scholium 

 of the Principia to prop. 35 of the third book, given in the first 

 edition but afterwards omitted, he estimated the annual motion at 

 40, the observed value being about 41. In one of his unpublished 

 papers, contained in the Portsmouth collection, he arrived at 39 by 

 a process which he evidently regarded as not altogether satisfactory. 



