166 HISTORY OF PHYSICAL ASTRONOMY. 



discovery; namely, that very Nutation which he had formerly rejected. 

 This may appear strange, but it is easily explained. The aberration 

 is an annual change, and is detected by observing a star at differ- 

 ent seasons of the year : the Nutation is a change of which the cycle 

 is eighteen \ears; and which, therefore, though it does not much 

 change the place of a star in one year, is discoverable in the altera- 

 tions of several successive years. A very few years' observations showed 

 Bradley the effect of this change ; 9 and long before the half cycle of 

 nine years had elapsed, he had connected it in his mind with the true 

 cause, the motion of the moon's nodes. Machin was then Secretary to 

 the Royal Society, 10 and was " employed in considering the theory of 

 gravity, and its consequences with regard to the celestial motions :" to 

 him Bradley communicated his conjectures ; from him he soon received 

 a Table containing the results of his calculations ; and the law was 

 found to be the same in the Table and in observation, though the 

 quantities were somewhat different. It appeared by both, that the 

 earth's pole, besides the motion which the precession of the equinoxes 

 gives it, moves, in eighteen years, through a small circle; or rather, 

 as was afterwards found by Bradley, an ellipse, of which the axes are 

 nineteen and fourteen seconds. 11 



For the rigorous establishment of the mechanical theory of that effect 

 of the moon's attraction from which the phenomena of Nutation flow, 

 Bradley rightly and prudently invited the assistance of the great mathe- 

 maticians of his time. D'Alembert, Thomas Simpson, Euler, and others, 

 answered this call, and the result was, as we have already said in the 

 last chapter (Sect. 7), that this investigation added another to the rec 

 ondite and profound evidences of the doctrine of universal gravitation. 



It has been said 12 that Bradley's discoveries " assure him the most 

 distinguished place among astronomers after Hipparchus and Kepler." 

 If his discoveries had been made before Newton's, there could have 

 been no hesitation as to placing him on a level with those great men. 

 The existence of such suggestions as the Newtonian theory offered on 

 all astronomical subjects, may perhaps dim, in our eyes, the brilliance 

 of Bradley's achievements; but this circumstance cannot place any 

 other person above the author of such discoveries, and therefore we 

 may consider Delambre's adjudication of precedence as well warranted, 

 and deserving to be permanent. 



' fiigaud, Ixiv. i" Ib. 25. " Ib. Ixvi. 



12 Delambre, Ast. du IS Siec. p. 420. Eigaud, xxxvii. 



