266 HISTORY OF PHYSICAL ASTRONOMY. 



rules, which he had already discovered, and, in 

 1729, he gave his discovery to the Royal Society. 

 His paper is a very happy narrative of his labours 

 and his thoughts. His theory was so sound that 

 no astronomer ever contested it ; and his observa- 

 tions were so accurate, that the quantity which he 

 assigned as the greatest amount of the change (one 

 ninetieth of a degree) has hardly been corrected 

 by more recent astronomers. It must be noticed, 

 however, that he considered the effects in decli- 

 nation only ; the effects in right ascension required 

 a different mode of observation, and a consummate 

 goodness in the machinery of clocks, which at that 

 time was hardly attained. 



Sect. 4. Discovery of Nutation. 



WHEN Bradley went to Greenwich as Astronomer 

 Royal, he continued with perseverance observa- 

 tions of the same kind as those by which he had 

 detected Aberration. The result of these was an- 

 other 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 different seasons of the year ; the Nutation is a 

 change of which the cycle is eighteen years ; and 

 which, therefore, though it does not much change 

 the place of a star in one year, is discoverable in 

 the alterations of several successive years. A very 



