76 LIGHT SCIENCE FOR LEISURE HOURS. 



to measure the change that we must chiefly trust ; and 

 even that would be insufficient were it not for the fact 

 that Yenus is not at rest, but travels athwart the great 

 solar dial-plate. We are thus enabled to make a time 

 measurement take the place of a measurement of space. 

 If an observer in one place sees Yenus cross the sun's 

 face at a certain distance from the centre, while an 

 observer at another place sees her follow a path slight- 

 ly farther from the centre, the transit will clearly seem 

 longer to the former observer than to the latter. 



This artifice of exchanging a measurement of time 

 for one of &pace or vice versa is a very common 

 one among astronomers. It was Edmund Halley, the 

 friend and pupil of Sir Isaac Newton, who suggested 

 its application in the way above described. It will be 

 noticed that what is required for the successful appli- 

 cation of the method is that one set of observers should 

 be as far to the north as possible, another as far to the 

 south, so that the path of Yenus may be shifted as 

 much as possible. Clearly the northern observers will 

 see her path shifted as much to the south as it can pos- 

 sibly be, while the southern observers will see the path 

 shifted as far as possible toward the north. 



One thing, however, is to be remembered. A 

 transit lasts several hours, and our observers must be 

 so placed that the sun will not set during these hours. 

 This consideration sometimes involves a difficulty. 

 For our earth does not supply observing room all over 



