$ 2i 3 ] Nutation 267 



whole annual precession, which arises from her action, would 

 in different years be varied in its quantity ; whereas the plane 

 of the ecliptic, wherein the sun appears, keeping always nearly 

 the same inclination to the equator, that part of the precession 

 which is owing to the sun's action may be the same every year; 

 and from hence it would follow, that although the mean annual 

 precession, proceeding from the joint actions of the sun and 

 moon, were 50", yet the apparent annual precession might 

 sometimes exceed and sometimes fall short of that mean 

 quantity, according to the various situations of the nodes of 

 the moon's orbit." 



in his discussion of precession (chapter ix., 188; 

 Book III., proposition 21) had pointed out 

 the existence of a small irregularity with a period of six 

 months. But it is evident, on looking at this discussion 

 of the effect of the solar and lunar attractions on the 

 protuberant parts of the earth, that the various alterations 

 in the positions of the sun and moon relative to the earth 

 might be expected to produce irregularities, and that the 

 uniform precessional motion known from observation and 

 deduced from gravitation by Newton was, as it were, only 

 a smoothing out of a motion of a much more complicated 

 character. Except for the allusion referred to, Newton 

 made no attempt to discuss these irregularities, and none 

 of them had as yet been detected by observation. 



Of the numerous irregularities of this class which are now 

 known, and which may be referred to generally as nutation, 

 that indicated by Bradley in the passage just quoted is 

 by far the most important. As soon as the idea ot an 

 irregularity depending on the position of the moon's nodes 

 occurred to him, he saw that it would be desirable to watch 

 the motions of several stars during the whole period (about 

 19 years) occupied by the moon's nodes in performing the 

 circuit of the ecliptic and returning to the same position. 

 This inquiry was successfully carried out between 1727 and 

 1747 with the telescope mounted at Wansted. When the 

 moon's nodes had performed half their revolution, i.e. 

 after about nine years, the correspondence between the 

 displacements of the stars and the changes in the moon's 

 orbit was so close that Bradley was satisfied with the general 

 correctness of his theory, and in 1737 he communicated the 

 result privately to Maupertuis ( 221), with whom he had 



