VENUS ON THE SUN'S FACE. 79 



!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 afterward 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 possi- 

 ble astronomical problems to determine' over what pro- 

 portion of her orbit Yenus passed in the interval of 

 time which elapsed between these observations ; and 

 the observers would now have learned that that por- 

 tion of Yenus'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 

 Yenus'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. 



The two observers who saw the transit end earliest 

 and latest could do the like. 



Speaking generally, and neglecting all the com- 

 plexities which delight the soul of the astronomer, this 

 is Delisle's method of utilizing a transit. It has obvi- 

 ously one serious disadvantage as compared with the 

 other. An observer at one side of the earth has to 



