64 LIGHT SCIENCE FOR LEISURE HOURS. 



natural dial of the heavens, just as the division-marks 

 on a clock-face serve to indicate the position of the 

 hands. 



Now we can at once see why Venus holds so im- 

 portant a position in this sort of inquiry. Venus is 

 our nearest neighbour among the planets. She comes 

 several millions of miles nearer to us than Mars, our 

 next neighbour on the other side. That is the primary 

 reason of her being so much considered by astronomers. 

 But there is another of equal importance. Venus 

 travels nearer than our earth to the sun. And thus 

 there are occasions when she gets directly between the 

 earth and the sun. At those times she is seen upon 

 his face, and his face serves as a dial-plate by which to 

 measure her movements. When an observer at one 

 part of the earth sees her on one part of the sun's face, 

 another observer at some other part of the earth will 

 see her on another, and the difference of position, if 

 accurately measured, would at once indicate the sun's 

 distance. As a matter of fact, other modes of reading 

 off the indications of the great dial-plate have to be 

 adopted. Before proceeding to consider those modes, 

 however, we must deal with one or two facts about 

 Venus's movements which largely affect the question 

 at issue. 



Let us first see what we gain by considering the 

 distance of Venus rather than that of the sun. 



At the time of a transit Venus is of course on a line 

 between the earth and the sun, and she is at somewhat 

 less than a third of the sun's distance from us. Thus 



