ASTRONOMY. 21 



SO. Suppose than the right ascension and de- 

 clination of a star are given, to find its longi- 

 tude and latitude. Find an arc #, such that, 



cot x = sin z x.cot Dec. 



The declination, if north, is reckoned positive, if 

 south, negative, and x has the same sign with it. 



Let y = x <p, (p being reckoned positive for that 

 half of the ecliptic which is north of the equa- 

 tor, and negative for the opposite, or which 

 comes to the same, positive when the right ascen- 

 sion is less than a semicircle, negative when it is 

 greater. 



Then tan long. = cosj/Xtan Righ^Asc^ 



COS X 



and tan Lat. = sin Long. X tan y. 



If tan Long, come out negative, the longitude is 

 greater than a semicircle ; if tan Lat. is negative, 

 the latitude is south. 



This rule, which is Dr MASKELYNE'S, is quite free 

 from ambiguity ; it is as simple as the nature of 

 the case will allow, and is deduced from fig. 2., 

 in which A is the intersection of the equator 

 AQ, and the ecliptic AE ; S the place of a star, 

 SB an arc of the meridian equal to the declina- 

 tion ; AB the right ascension, SD the latitude, ^ nd 

 AD the longitude of the sfcr. See VINCE'S Ast. 

 vol. i. p. 39. LA LANDE computes the angle BAS, 



and 



