CH. XIX. 



THE SUN'S DIAMETER. 



157 



distance from the earth, the two points will be 2\ times 

 7200 miles, that is 18,000 miles apart. Here, then, we 



FIG. 27. 



Venus as seen upon the sun by two observers, one at F/ and one at E. (Proctor.) 



s, The sun. v v', Appearance of Venus on the sun's face. Venus is travelling in the 



direction of the arrow. 



have a certain number of miles measured off on the sun's face. 

 But how are we to tell accurately what proportion this 

 interval between the spots bears to the whole diameter of 

 the sun ? 



By Halley's method the whole time that Venus takes in 

 crossing the sun is used as the means of measurement 

 The observer at each of the two stations notes exactly the 

 time when Venus begins to 

 cross the face of the sun, and 

 the moment when she passes 

 off it again, and so reckons 

 exactly how long she has taken 

 in making the whole transit. 



It was already known, from 

 the rate at which Venus 

 moves, exactly how long she 

 would take in crossing the 

 centre or widest part of the 

 sun. We will call this time 6 

 hours, so as to use whole num- 

 bers. Now it is clear that in 

 crossing a narrower part of 

 the disc she will take less time. Suppose, therefore, that 

 one man says she was exactly 5 hours crossing from A to 

 B, Fig. 28, and the other that she was 5^ hours crossing 



FIG. 28. 



Transit of Venus. 



s, Face of the sun. v, Venus. A B, 

 Transit observed so as to occupy five 

 hours. C D, Same transit observed so 

 as to occupy five-and-a-quarter hours. 



