68 LIGHT SCIENCE FOR LEISURE HOURS. 



minutes 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 we neglect the earth's rotation, of course, 

 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. 



