VENUS ON THE SUN'S FACE. 6 1 



some one observer on the outer rim of the hemisphere 

 almost exactly opposite the first would be absolutely 

 the last to see the transit begin. From that time the 

 transit would be seen by all for several hours I 

 neglect the earth's rotation, for the moment but the 

 end of the transit, like the beginning, would not be seen 

 simultaneously by the observers. First one would see 

 it, then in succession the rest, and last of all an observer 

 almost exactly opposite the first. 



Now, here we have had to consider four observers 

 who occupy exceptional positions. There is (1) the 

 observer who sees the transit begin earliest, (2) the one 

 who sees it begin latest, (3) the one who sees it end 

 earliest, and (4) the one who sees it end latest. Let 

 us consider the first two only. Suppose these two 

 observers afterwards compared notes, and found out 

 what was the exact difference of time between their 

 respective observations. Is it not clear that the result 

 would at once afford the means of determining the 

 sun's distance ? It would be the simplest of all possible 

 astronomical problems to determine over what pro- 

 portion of her orbit Venus passed in the interval of 

 time which elapsed between these observations; and 

 the observers would now have learned that that portion 

 of Venus's orbit is so many miles long, for they know 

 what distance separated them, and it would be easy to 

 calculate how much less that portion of Venus's orbit is. 

 Thus they would learn what the length"of her whole 

 orbit is, thence her distance from the sun, and thence 

 the sun's distance from us. 



