ASTRONOMY, 79 



nation and of right ascension. These changes 

 are called, the parallaxes in right ascension and 

 declination. 



. In fig. 8. if PZO be the meridian, P the pole, Z the 

 zenith, S the true place of the star, depressed by its 

 parallax to S', in the direction of the vertical circle 

 ZSS'; let PS be a circle of decimation passing 

 through the true place of the star, and PS' another 

 such circle, passing through the apparent place. 

 If SM be drawn perpendicular to PS', MS' is the 

 parallax in declination, and the angle SPM, the 

 parrllax in right ascension. The triangle SS'M 

 may be considered as rectilineal ; and if we make 

 the angle at S, or what is called the angle of posi- 

 tion, = j/, the parallax in altitude, or SS', being 

 = p 9 we have MS' = p X cosy = parallax in de- 

 clination ; also SM = p X sinj/; and hence the 



angle SPM = P sm ^ = the parallax in right 

 cos Dec. 



ascension. 



b. Hence it may be shown, that when the horizontal 

 parallax is given, the parallax in right ascension is 

 proportional to the sine of the hour angle ZPS. 



As the parallax in right ascension is nothing when a 

 body is on the meridian, and increases on either 

 side, it may be found by comparing the difference 

 between the times when the body and a star near 

 it, come to the meridian, with the difference be- 

 tween the times when they come to the same hour 

 circle, near the horizon ; and thence the horizon- 

 tal 



