ASTRONOMY. 



increase of velocity which they produce, because 

 their action does not change the quantity of the 

 aberration. 



216. The aberration considered above, is that 

 of a fixed star, or of an immoveable body. In 

 the planets, there is another source of aberra- 

 tion, in the motion of the planet itself, equal to 

 the variation of its geocentric place, in the time 

 that light takes to move from the planet to the 

 earth. 



The simplest way of making allowance for this varia- 

 tion, is to compute the place of the planet for an 

 instant preceding the given instant, by the time 

 that light takes to move from the planet to the 

 earth. This w$ the method used by Dr MASKE- 



LYNE. VlNCE, Ast. vol. I. 527. 



No attention has been paid here to the velocity which 

 places on the Earth's surface derive from the mo- 

 tion of the Earth on its axis. This is too small 

 to produce a sensible effect, as is shewn by DE 

 LAMBRE, Astron. legon 19. J 23. 



Nutation of the Earth 9 s Axis. 



217. A small inequality, which has been ob- 

 served in the precession of the equinoxes, and 

 in the mean obliquity of the ecliptic, is known 

 by the name of the Nutation. 



This 



