58 LIGHT SCIENCE FOR LEISURE HOURS. 



whatever effect an observer's change of place would 

 produce upon the sun would be more than trebled in 

 the case of Venus. But it must not be forgotten that 

 we are to judge the motions of Venus by means of the 

 dial-plate formed by the solar disc, and that dial-plate 

 is itself shifted as the observer shifts his place. Venus 

 is shifted three times as much, it is true ; but it is only 

 the balance of change that our astronomer can recog- 

 nise. That balance is, of course, rather more than twice 

 as great as the sun's change of place. 



So far, then, we have not gained much, since it haa 

 been already mentioned that the sun's change of place 

 is not measurable by any process of observation astro- 

 nomers can apply. 



It is to the fact that we have the sun's disc, where- 

 by to measure the change, that we chiefly trust ; and 

 even that would be insufficient were it not for the fact 

 that Venus 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 Venus cross the sun's 

 face at a certain distance from the centre, while an 

 observer at another place sees her follow a path slightly 

 farther from the centre, the transit clearly seems 

 longer to the former observer than to the latter. 



This artifice of exchanging a measurement of time 

 for one of space 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 appli- 

 cation in the way above described. It will be noticed 



