VENUS ON THE SUN'S FACE. 73 



have to be compared with an observation which the 

 other has made in winter. 



Thus, we can understand that astronomers should 

 have failed totally when they attempted to determine 

 the sun's distance without aid from the other celestial 

 bodies. 



It may seem at first sight as though nothing the 

 other celestial bodies could tell the astronomer would 

 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 fol- 

 lowed by surveyors could be of any service. And 

 besides, it might be supposed that information about 

 the distance of one celestial body could be of no partic- 

 ular service toward 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. Sec- 

 ondly, the planets in their motion travel occasionally 

 into such positions that they become mighty indices, 

 tracing out on a natural dial-plate the significant les- 

 son 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- 



4 



