74 OUTLINES OF NATURAL PHILOSOPHY. 



ently referred to it. When a star is seen in the 

 zenith of any place, it is seen in the same position 

 as if it were viewed from the centre. 



75. The parallax of a body at a given dis- 

 tance from the centre of the earth, is greatest 

 when the body is seen in the horizon. This is 

 called the Horizontal Parallax ; and the paral- 

 lax at any given altitude, or the quantity by 

 which the true altitude is diminished, is to the 

 horizontal parallax as the cosine of the altitude 

 to the radius. 



If P be the horizontal parallax, p the parallax at the 

 altitude , p == P X cos a. 



If r be the radius of the earth, supposing it spherical, 



r 



and d the distance of the body, - = sin P. 







When P is very small, P = -%, P being expressed? 

 not in degrees, but in parts of the radius ; to have 

 it in degrees, -j must be multiplied by m, the 

 number of degrees in an arch equal to the radius. 



If the horizontal parallax is known, the distance d 



is known ; for d = - =:. 

 sin P 



The 



