252 



KNOWLE 



the other of the place it would have as seen from the 



centre of the earth. This shift gives the means of 



deducing the planet's distance from the earth in miles 



and the distance of the sun then follows from Keplei 



laws. Now, a minor planet, which looks like a ste' 



point in the telescope, can he measured with much gr 



precision than can a large, bright, unequally illun- 



disc like that of Mars. Dr. Gill's recent determir 



the sun's distance (ninety-two million, eight hn 



seventy-four thousand miles), which is probabl 



accurate yet made, was based on heliometer 



of Iris, Victoria, and Sappho, whose leastdis* 



earth (in astronomical units) are 0-81. 



respectively. It will be seen at once wb^ 



will be effected when DQ is observed at 



or only one-sixth of that of the abov 



fact, any uncertainty in the sun's di 



to one-sixth of its present amount. 



the autumn of 1900, though f 



favourable possible, should 



heliometer measures, for its 



than one-third of that of tl 



Dr. Gill. 



Another way in which 

 knowledge of the sun's 

 which the earth prod 

 siderable, and whii ' 

 of the earth's mas 

 the latter' s distant 

 able for many yi 

 give very accuri 



It might at " 

 closer approj 

 this is not " 

 so much 

 positio] 

 (in lo: 

 nearl 



:YGm. 



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