Olbers's Essay on Comets. 15] 



For the orbit of the comet, having obtained §' (§41), we must 

 determine the time and distance for the perihelium, and then 

 the true anomaly \P, at the time of the middle observation. 

 Hence, as we have already tp and x, the difference of the true 

 anomalies between the first and second observation being t, and 

 between the first and third <r, we have t=»//— ^ and cr=(p+x 

 -</'==%-T ; and then for the chord of the orbit of the comet 

 we have AD : DC=:r' sin t : r'" sin <7 : [tp being however here 

 the true anomaly at the time of thejirst observation]. 



[7b be continued.] 



vi. An easy Method of computing the Aberration of the Stars. 

 By the Rev. John Brinkley, D. D., Professor of Astronomy 

 iri the University of Dublin. 



Tables for finding the constant Arc and Maximum of Aberra- 

 tion in Declination for any Star. 



Rule. 

 Tab. I. X sin Delci. * + Tab. II. = a 

 Tab. III. X sin Decli. * ~ a! 



tan K (or Arc for Aberr. in Decli) = — 



a 



(K nearer C or 180°) m= — ^ 

 ' cosK 



(K nearer 90° or 270°) m = "' 

 sin K 



Aberration in N.P.D. = - w cos (K ^ © Long. 



When a is + & a'— K is in 2d Quadrant. 



a + «& a' + K is in 3d Quadrant. 



a — «& a'— K is in 1st Quadrant. 



a — & a' -I- K is in 4th Quadrant. 

 N. B. When Declination of Star is S. its sine is negative. 



