30 Astronomy. [Lecture 19. 



in different parts of the world, and the sun's 

 horizontal parallax was determined to be about 



V W, which when viewed from the earth, is equal to the 

 known chord of the sun CD; the heliocentric value or 

 length of the arc V W may likewise be readily found. 

 Suppose then an observer at A on the earth's surface to 

 view the planet Venus at V, it will appear just entered 

 within the sun's disc at C, and passing in the arc V W, 

 will appear to describe the line CD, arriving at D at the 

 end of the computed time. But during this time the ob- 

 server will, by the earth's diurnal revolution, be carried 

 from A towards P ; and arriving at P at the same instant 

 that Venus arrives at U, will behold the transit just finish- 

 ing atD : consequently it will be of a duration proportion- 

 ally as much shorter than the computed time, as the helio- 

 centric arc V U is shorter than V W. The arc V W is 

 known by cqmputation, therefore, since Venus's motion 

 may in very small arcs be reckoned uniform, 

 " As the computed time 



Is to the computed arc V W, 



So is the observed time 



To the arc V Uj 



which being taken from V W, leaves the aic U W, that 

 subtends the angle U D V. This last angle is the parallax 

 of the base A P; and the base. A P is found by the analogy 

 " As one day or 24 hours 



Is to the circumference of the earth (or 

 parallel of latitude) 



So is the observed time 



To the arc A P, whose chord is the base. 

 " But because the minutest errors in a business of this na- 

 ture are of very great consequence, and because the length 

 of the arc VW, depending on the sun's diameter, can 

 scarcely be obtained by calculation to that extreme degree 

 of exactness, which is requisite, it is advisable to take an- 

 other observation on a place so situated on the earth, that, 



