326 Biographical Account of [May, 



of the tropics, then the two semidiameters of the earth exposed 

 to the sun, being no longer equal, the equator is more strongly 

 attracted than the rest of the globe, which -alters a little its posi- 

 tion and its inclination to the plane of the ecliptic. And as the 

 part of the orbit, included between the autumnal and vernal equi- 

 nox is smaller than that included between the vernal and the 

 autumnal equinox, it follows that the derangement caused by the 

 sun, while it passes through the northern signs, is not entirely 

 compensated by that produced while it passes through the 

 southern signs; and that the parallehsm of the terrestrial axi» 

 and its i«chnation with the ecliptic remain a little altered. 

 Hitherto we see nothing which has any relation to the period of 

 nine years. We shall see immediately what produces this period. 



What the sun operates upon the earth by its attraction, the 

 moon operates also, and it acts with the greater effect the more 

 it deviates from the equator. But when its nodes concur with 

 the equinoxial points, its greatest latitude is added to the greatest 

 obhquity of the ecliptic. This then is the time of its greatest 

 action to derange the position of the terrestrial axis. And the 

 revolution of the nodes of the moon occupying a period of 18 

 years, it is clear, that during that period the nodes will be twice 

 in the equinoxial points ; consequently, during that period, the 

 axis of the earth will be the most deranged possible two several 

 times. Thus the axis will be the most deranged possible once 

 every nine years ; or, which comes to the same thing, it will 

 have a vibration, the period of which will be nine years, as Dr. 

 Bradley had observed. This vibration is what he termed the 

 nutation of the terrestrial axis. He published an account of it in 

 1737. Thus within 10 years he communicated to the pubhctwo 

 of the greatest discoveries in modern astronomy — discoveries 

 which will always mark a memorable epocha in the history of the 

 science. 



Dr. Bradley had always enjoyed the esteem and the friendship 

 of Dr. Halley, at that time Astronomer Royal, but in a very 

 advanced period of life, and unable to contribute as usual to the 

 promotion of his favourite science. He conceived that he could 

 not confer a greater favour on it than by endeavouring to procure 

 Dr. Bradley for his successor. With this view he wrote to Dr. 

 Bradley several letters, which were found among that gentle- 

 man's papers after his death, requesting permission to solicit for 

 him the reversion of his office, and even offering, if necessary,'to 

 resign in his favour. But Dr. Halley died before he was able to 

 accomplish this desirable object. The Earl of Macclesfield, 

 however, well known for his attachment to astronomy, and 

 afterwards President of the Royal Society, had sufficient interest 

 to secure him the situation of Astronomer Royal. As soon as 

 the nomination was publicly known, the University of Oxford 

 enrolled him as one of their own body by creating him Doctor 

 in Divinity. . 



