sun's distance from the earth. , 9 



V". In the triangle SYV" the outer angle CVS is equal to 

 the sum of the inward and opposite angles VSV" and VV"S. 

 But CVS is the horizontal parallax of Venus, or the angle 

 under which the Earth's semi-diameter is viewed from Venus ; 

 and VV'S is the sun's horizontal parallax, or the angle 

 under which the Earth's semi-diameter is viewed from the 

 sun ; and VSV" is the angle under which the displacement of 

 Venus on the sun is viewed from the Earth. It is called the 

 horizontal parallax of Venus from the sun. Now as the dis- 

 tance of the Earth from the sun is to the distance of Venus 

 from the sun as 25 : 7, the angle CVS will be nearly enough 

 twenty-five-sevenths of the angle VV'S, and VSV" will be 

 geometrically the difference between the two horizontal paral- 

 laxes of Venus and the sun ; as is right and natural, for the 

 displacement of Venus upon the sun is due to the difference 

 of parallaxes. 



Now VSV" is known, for it can be measured, as will be 

 shown below. Representing then by Z the sun's horizontal 

 parallax, unknown,— (SVC = VV"S) ; by Y the horizontal 

 parallax of Venus (CVS), likewise unknown, and by M the 

 measured distance between these two parallaxes, we shall have 

 two independent e(|uations : — 



_ 25 Z 



7 

 Y = Z + M 



From which 



Y 



7 M 



18 



25 M 



18 



Both in terms of M. ., 



Now to measure M, or VSV". 



It will be well here to remark that VV" is the chord of an 

 arc which measures an angular opening. This arc is an ali- 



