20 OUTLINES OF NATURAL PHILOSOPHY. 



29. Another purpose for which spherical tri- 

 gonometry is employed, is, when the position 

 of a star relatively to one circle of the sphere 

 is given, to find its position relatively to ano- 

 ther which is given in position with respect to 

 the first. 



The position of the fixed stars, by the foregoing ob- 

 servations, are found relatively to the equator; 

 that is, the perpendicular arc from the star to the 

 equator, is given, and also the distance of that 

 perpendicular from a given point in the circum- 

 ference of the equator ; but there is another cir- 

 cle, called the Ecliptic, to which it becomes neces- 

 sary to refer the stars. This circle cuts the equa- 

 tor at the points of the vernal and autumnal equi- 

 noxes, and is inclined to it at an angle known 

 from observations which will be afterwards ex- 

 plained, to be nearly 23. 27'. 30". 



The distance of a star from this circle is called the 

 Latitude of the star, and is reckoned north or 

 south according as the star is on the same side of 

 the ecliptic with the North Pole, or the South. 



The arc of the ecliptic intercepted between the per- 

 pendicular from the star, and the point of the ver- 

 nal equinox, is called the Longitude of the star. 



Let the angle formed by the equator and ecliptic, or 

 the Obliquity of the Ecliptic be =0. 



30* Suppose 



