THE COMING TRANSIT OF VENUS. 21 



as she crosses that face, and so judge how long the 

 chord is which she has traversed. This shows how 

 nearly the chord approaches the sun s centre, and thus 

 gives a determination as satisfactory as an actual mea 

 surement. Of course, there are many details to be 

 taken into account : for instance, the apparent path of 

 Venus is not a straight line in reality, because the 

 observer s station is not at rest, but carried round the 

 axis of the rotating earth. But the mathematician 

 finds no difficulty in taking such considerations fully 

 into account. 



Secondly, Delisle proposed that astronomers should 

 note the actual moment (of absolute, not local time) 

 when Venus seems to enter or leave the sun s face, as 

 seen from different stations on the earth. It will be 

 manifest, on a moment s consideration of the actual 

 circumstances of the case, that the transit will not 

 seem to begin or end at the same instant, as seen from 

 different parts of the earth. There is the great globe 

 of the sun at one side, and the smaller globe of the 

 earth on the other ; and Venus passes between. Now, 

 in order to show more clearly what must happen, let 

 us take an illustrative case drawn from an event which 

 in a few weeks from the present time will interest a 

 large proportion of our population. Suppose that on 

 one side of the river Thames there is a long building 

 whose extremeties* we call A and B. Suppose that just 

 opposite there is a barge whose corresponding extreme- 

 ties we call a and b. Now suppose the winning boat to 

 be coming along so as to pass between the house and 



