78 LIGHT SCIENCE FOR LEISURE HOURS. 



to the angle which a hair's breadth subtends when 

 seen from a distance of 125 feet. 



The error was first detected when other modes of 

 determining the sun's distance were applied by the 

 skilful astronomers and physicists of our own day. 

 We have no space to describe as fully as they deserve 

 the ingenious processes by which the great problem 

 has been attacked without aid from Venus. Indeed, 

 we can but barely mention the principles on which 

 those methods depend. But to the reader who takes 

 interest in astronomy, we can recommend no subject 

 as better worth studying than the masterly researches 

 of Foucault, Leverrier, Stone, and Hansen upon the 

 problem of the sun's distance. 



The problem has been attacked in four several ways. 

 First, the tremendous velocity of light has been 

 measured by an ingenious arrangement of revolving 

 mirrors; the result combined with the known time 

 occupied by light in travelling across the earth's orbit 

 immediately gives the sun's distance. Secondly, a 

 certain irregularity in the moon's motion, due to the 

 fact that she is most disturbed by the sun when 

 traversing that half of her path which is nearest to 

 him, was pressed into the service with similar results. 

 Thirdly, an irregularity in the earth's motion, due to 

 the fact that she circles around the common centre of 

 gravity of her own mass and the moon's, was made a 

 means of attacking the problem. Lastly, Mars, a 

 planet which, as we have already mentioned, approaches 



