74 LIGHT SCIENCE FOR LEISURE HOURS. 



times so near the earth that the distance separating us 

 from him is little more than one-third of that which 

 separates us from the sun. Suppose that, at such a 

 time, he is seen quite close to a fixed star. That star 

 gives the astronomer powerful aid in determining the 

 planet's distance. For, to observers in some parts of 

 the earth, the planet will seem nearer to the star than 

 he will to observers elsewhere. A careful comparison 

 of the effects thus exhibited will give significant evi- 

 dence respecting the distance of Mars. And we see 

 that the star has served as a fixed mark upon the vast 

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

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

 hands. 



JSTow, we can at once see why Venus holds so im- 

 portant a position in this sort of inquiry. Yen us is 

 our nearest neighbor among the planets. She comes 

 several millions of miles nearer to us than Mars, our 

 next neighbor on the other side. That is the primary 

 reason of her being so much considered by astronomers. 

 But there is another of equal importance. Yenus 

 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 



