$ 207] Bradley; Aberration 259 



on the minds of observing astronomers, many of whom had 

 tried to detect the motion in question, and some of whom 

 (including the " universal claimant " Hooke) professed to 

 have succeeded. Actually, however, all previous attempts 

 had been failures, and Bradley was no more successful than 

 his predecessors in this particular undertaking, but was 

 able to deduce from his observations two results of great 

 interest and of an entirely unexpected character. 



The problem which Bradley set himself was to examine 

 whether any star could be seen to have in the course of the 

 year a slight motion relative to others or relative to fixed 

 points on the celestial sphere such as the pole. It was 

 known that such a motion, if it existed, must be very 

 small, and it was therefore evident that extreme delicacy 

 in instrumental adjustments and the greatest care in obser- 

 vation would have to be employed. Bradley worked at first 

 in conjunction with his friend Samuel Molyneux (1689-1728), 

 who had erected a telescope at Kew. In accordance with the 

 method adopted in a similar investigation by Hooke, whose 

 results it was desired to test, the telescope was fixed in a 

 nearly vertical position, so chosen that a particular star in 

 the Dragon (y Draconis) would be visible through it when 

 it crossed the meridian, and the telescope was mounted 

 with great care so as to maintain an invariable position 

 throughout the year. If then the star in question were to 

 undergo any motion which altered its distance from the 

 pole, there would be a corresponding alteration in the posi- 

 tion in which it would be seen in the field of view of 

 the telescope. The first observations were taken on 

 December i4th, 1725 (N.S.), and by December 28th 

 Bradley believed that he had already noticed a slight dis- 

 placement of the star towards the south. This motion 

 was clearly verified on January ist, and was then observed 

 to continue ; in the following March the star reached its 

 extreme southern position, and then began to move north- 

 wards again. In September it once more altered its 

 direction of motion, and by the end of the year had 

 completed the 'cycle of its changes and returned to its 

 original position ; the greatest change in position amounting 

 to nearly 40' . 



The star was thus observed to go through some annual 



