The Primary Planets. 301 



to say, that the transits in 1761 and 1769 were 

 carefully observed by very eminent astronomers 



ject, the following extract from Mr. Nicholson's Astro- 

 nomy will be satisfactory. 



*' The planet Venus passes the sun twice in revolving 

 from any position of elongation to the same position again. 

 At those times this planet is said to be in conjunction 

 with the sun. 



" When the planet Venus is situated in a line between 

 the sun and the earth, it is said to be in its inferior con- 

 junction ; and when it is in the opposite part of its orbit, 

 the sun being in a line between it and the earth, it is 

 said to be in its superior conjunction. If the orbits of 

 the earth and Venus were in the same plane, it is evident 

 that Venus would pass behind the sun with a direct 

 motion every superior conjunction, and would pass over 

 its disc, or before it, with a retrograde motion every in- 

 ferior conjunction. But as Venus's orbit is inclined to 

 the ecliptic in an angle of about 3? degrees, this planet 

 will, in general, pass to the northward or southward of 

 the sun, and will only be visible on its disc when the in- 

 ferior conjunction happens at or near one of the nodes. 

 This happens but once (or sometimes twice at an interval 

 of about 8 years) in more than 120 years. 



" To show how this transit is applied to the purpose of 

 finding the sun's distance, we shall pass over those ele- 

 ments that enter into the computation previous or subse- 

 quent to actual observation, and shall only explain the ge- 

 neral principles on which the method is founded. 



" Let s (PL XXVIII. fig. 117.) represent the Sun, E 

 the earth, V, U, W, the planet Venus in different posi- 

 tions, the arc L N a part of the earth's orbit, and the arc 

 O M a part of the orbit of Venus. Then, because the an- 

 gular velocities of Venus and the earth are known, as also 

 their proportional distances, it will be easy to compute 

 the time Venus will employ in passing through the arc 



