ABERRATION of the STARS, S:c, ^7 



The aberration in declination is negative, when a point 3 

 behind the Sun's longitude, falls on the fame fide of a diameter 

 at ri^ht angles to the meridian of the ftar, with the flar's right 

 afcenfion or point M ; but the contrary is to be obferved when 

 the point falls on the oppofite fide. The foregoing equations 

 wheu tabled will ftand as below. 



Sun's Long, c + i .96 — 6 Sun's Long. 



1 — 2. 98 + 7 



2 7.12 8 



3 9 -35 9 



4 9 .07 10 



5 6.36 II 



6 I .96 o 



Of Nutatio?!. 



THE nutation or libratory motion of the Earth's axis is occa- 

 iloned by the inclination of the Moon's orbit .to the ecliptic, 

 and the retrograde revolution of her nodes ; which is performed 

 in about 18 years and 7 months. On which account the action 

 of the Moon on the equatorial, or longer diameter of the Earth, 

 is not uniform, and muft therefore from the principles of gra- 

 vity produce a motion in the Earth's axis, which will be ap- 

 parently in the ftars. For the completion of this difcovery, 

 we are indebted to the very laborious, and ingenious Dr. Brad- 

 ley.* This elTeft of the Moon has been fettled by a feries of 

 accurate obfervations, and therefore not to be confidered as a 

 fpeculative argument in favour of the Newtonian Philofophy ; 

 but an abfolute confirmation of it. 



It mull: be evident from the theory, that the poles of the 

 equator will complete a retrograde revolution about the mean 

 poles, in the fame period which completes a revolution of the 

 Moon's nodes : But as the aftion of 'he Moon on ihe equatorial 

 diameter of the Earth, will be fomewhat varied in di'ierent 

 lituations of her nodes, this revolution of the poles will not be 

 performed in a circle, but a fmall ellipfe, with the tranfverfe 

 diameter lying in the folilitial colure, and amounting to 19 . i , 



* Vide his piper upon this fubjeil in Vol. 45, N® i. of the Tranfaiflioris 

 cf tile Royal ijov-iety. 



and 



