266 A Short History of Astronomy [CH. x. 



accurate and careful than Bradley. But in his paper on 

 aberration (1729) we find him writing : 



" I have likewise met with some small varieties in the declina- 

 tion of other stars in different years which do not seem to 

 proceed from the same cause. . . . But whether these small 

 alterations proceed from a regular cause, or are occasioned by 

 any change in the materials, etc., of my instrument, I am not yet 

 able fully to determine." 



The slender clue thus obtained was carefully followed 

 up and led to a second striking discovery, which affords 

 one of the most beautiful illustrations of the important 

 results which can be deduced from the study of " residual 

 phenomena." Aberration causes a star to go through a 

 cyclical series of changes in the course of a year ; if there- 

 fore at the end of a year a star is found not to have 

 returned to its original place, some other explanation of 

 the motion has to be sought. Precession was one known 

 cause of such an alteration ; but Bradley found, at the end 

 of his first year's set of observations at Wansted, that the 

 alterations in the positions of various stars differed by a 

 minute amount (not exceeding 2") from those which would 

 have resulted from the usual estimate of precession ; and 

 that, although an alteration in the value of precession would 

 account for the observed motions of some of these stars, 

 it would have increased the discrepancy in the case of 

 others. A nutation or nodding of the earth's axis had, 

 as we have seen ( 207), already presented itself to him 

 as a possibility ; and although it had been shewn to be 

 incapable of accounting for the main phenomenon due to 

 aberration it might prove to be a satisfactory explanation 

 of the much smaller residual motions. It soon occurred 

 to Bradley that such a nutation might be due to the action 

 of the moon, as both observation and the Newtonian 

 explanation of precession indicated : 



" I suspected that the moon's action upon the equatorial parts 

 of the earth might produce these effects : for if the precession 

 of the equinox be, according to Sir Isaac Newton's principles, 

 caused by the actions of the sun and moon upon those parts, 

 the plane of the moon's orbit being at one time above ten 

 degrees more inclined to the plane of the equator than at 

 another, it was reasonable to conclude, that the part of the 



