78 LIGHT SCIENCE FOR LEISURE HOURS. 



select it would be impossible to form a clear con- 

 ception of tlie circumstances with which astronomers 

 are about to deal. There is, however, no real diffi- 

 culty about this part of the subject, and we shall only 

 ask of the reader to give his attention to it for a very 

 brief space of time. 



Suppose the whole of that hemisphere of the earth 

 on which the sun is shining when the transit is about 

 to begin were covered with observers waiting for the 

 event. As Yen us sweeps rapidly onward to the criti- 

 cal part of her path, it is clear that some of these ob- 

 servers will get an earlier view of the commencement 

 of the transit than others will ; just as at a boat-race, 

 persons variously placed round a projecting corner of 

 the course see the leading boat come into view at 

 different times. Some one observer on the outer rim 

 of the hemisphere would be absolutely the first to see 

 the transit begin. Then rapidly other observers would 

 see the phenomenon ; and in the course of a few 

 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. 



