ASTRONOMY. 15$ 



like that of the moon five days before or after the 

 conjunction. LA LANDE, 1147. 



157. After the superior conjunction, the orb 

 of Venus increases in magnitude ; but the en- 

 lightened part diminishes, just reversing the; 

 former order, till she arrive at her greatest 

 eastern elongation ; after which the phenome- 

 na are repeated, as already mentioned ; and the 

 period which circumscribes all those changes, 

 or the time from one conjunction to the next 

 conjunction of the same sort, is, in its mean 

 quantity, 584 days. 



a. This is called the Synodical Revolution of Venus. 

 She is retrograde with respect to the fixed stars 

 during forty-two days of that period. 



158. Hence it is evident, that the orbit of 

 Venus surrounds the Sun, but excludes the 

 Earth ; and that her motion, with respect to the 

 sun, may be equable, notwithstanding the va- 

 riety of appearances it puts on to a spectator 

 in the earth. 



a. If a be the distance of the earth from the sun, and 

 x that of. Venus from the sun, a -f x will be its 

 distance from the earth, at the superior conjunc- 

 tion, and a x at the inferior; and by 156. 

 a -f- x : a x : : 6 : 1 ; so that 2 a : 2 x : : 7 : 5, or 

 as 10 to 7. If the earth's distance from the sun 

 be called 10, that of Venus is 7 ; and this serves as 



a 



