260 A Short History of Astronomy [CH. x. 



motion. It was, however, at once evident to Bradley that 

 this motion was not the parallactic motion of which he 

 was in search, for the position of the star was such that 

 parallax would have made it appear farthest south in 

 December and farthest north in June, or in each case three 

 months earlier than was the case in the actual observations. 

 Another explanation which suggested itself was that the 

 earth's axis might have a to-and-fro oscillatory motion or 

 nutation which would alter the position of the celestial pole 

 and hence produce a corresponding alteration in the position 

 of the star. Such a motion of the celestial pole would 

 evidently produce opposite effects on two stars situated on 

 opposite sides of it, as any motion which brought the pole 

 nearer to one star of such a pair would necessarily move 

 it away from the other. Within a fortnight of the decisive 

 observation made on January i st a star * had already been 

 selected for the application of this test, with the result which 

 can best be given in Bradley's own words : 



" A nutation of the earth's axis was one of the firSv things that 

 offered itself upon this occasion, but it was soon found to be 

 insufficient ; for though it might have accounted for the change 

 of declination in y Draconis, yet it would not at the same time 

 agree with the phaenomena in other stars ; particularly in a small 

 one almost opposite in right ascension to y Draconis, at about 

 the same distance from the north pole of the equator : for though 

 this star seemed to move the same way as a nutation of the 

 earth's axis would have made it, yet, it changing its declination 

 but about half as much as y Draconis in the same time, (as 

 appeared upon comparing the observations of both made upon 

 the same days, at different seasons of the year,) this plainly 

 proved that the apparent motion of the stars was not occasioned 

 by a real nutation, since, if that had been the cause, the altera- 

 tion in both stars would have been near equal." 



One or two other explanations were tested and found 

 insufficient, and as the result of a series of observations 

 extending over about two years, the phenomenon in ques- 

 tion, although amply established, still remained quite 

 unexplained. 



By this time Bradley had mounted an instrument of his 



* A telescopic star named 37 Camelopardi in Flamsteed's 

 catalogue. 



