56 LIGHT SCIENCE FOR LEISURE HOURS. 



be of the least use to him, since these bodies are for the 

 most part farther off than the sun, and even those which 

 approach nearest to us are still far beyond the limits of 

 distance within which the simple plan followed by sur- 

 veyors could be of any service. And besides, it might 

 be supposed that information about the distance of one 

 celestial body could be of no particular service towards 

 the determination of the distance of another. 



But two things aid the astronomer at this point. 

 First of all, he has discovered the law which associates 

 together the distances of all the planets from the sun ; 

 so that if he can determine the distance of any one 

 planet, he learns immediately the distances of all. Se- 

 condly, the planets in their motion travel occasionally 

 into such positions that they become mighty indices, 

 tracing out on a natural dial-plate the significant lesson 

 from which the astronomer hopes to learn so much. To 

 take an instance from the motions of another planet 

 than the one we are dealing with. Mars comes some- 

 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 



