James Bradley, Nevile Maskelyne 63 



for this residual effect. He thus discovered the " nutation " 

 of the earth's axis, which is caused by an attractive effect 

 of the sun on the equatorial protuberance of the earth, 

 which is not an exact sphere, but a spheroid with a larger 

 equatorial than polar diameter. 



When it is considered that every measurement of a star's 

 position has to be corrected so as to eliminate the effects of 

 aberration and nutation before its true position is ascer- 

 tained, Delambre's judgment that the accuracy of astro- 

 nomical observations owes everything to Bradley cannot 

 be gainsaid, and we shall also probably agree with the same 

 author 1 that " ce double service assure a son auteur la place 

 la plus distinguee apres celle de Hipparque et de Kepler, et 

 au-dessus des plus grands astronomes de tousles ages et de 

 tous les pays." 



After Bradley's death, Nathaniel Bliss, Savilian Professor 

 of Geometry at Oxford, was appointed Astronomer Royal, 

 but he only held the position for two years. Nevile 

 Maskelyne (1732-1811), a man of much greater ability, 

 next had charge of Greenwich Observatory. He graduated 

 as seventh wrangler at Cambridge in 1754, and twelve years 

 later was appointed to the post of Astronomer Royal, the 

 duties of which he discharged successfully during forty- 

 six years. His mind was first turned to astronomy as 

 a boy of sixteen by watching a solar eclipse. During a 

 voyage undertaken to observe the Transit of Venus, in 

 1761, he became interested in a process for determining 

 longitudes by measuring the distances of selected stars 

 from the moon, and he ultimately succeeded in introducing 

 this method as a regular practice in navigation. The im- 

 portance of the procedure consisted in its being independent 

 of timekeepers, and it consequently retained its place until 

 recently, when the construction of chronometers improved 

 so much that it lost its practical value. 



In order to make the tabulations of the position of the 

 moon and of the selected stars readily accessible to navi- 

 gators, Maskelyne persuaded the Government to issue an 

 annual publication. This was the origin of the Nautical 



1 Delambre, " Histoire de I'Astronomie au dix huiti&ne si&ele." 



