NEW WAYS OP MEASURING THE SUN'S DISTANCE. 59 



Accordingly the same observer cannot work at both ends of 

 the base-line, and they have to send out expeditions to 

 occupy each station. All the circumstances of temperature, 

 atmosphere, personal observing qualities, etc., are unlike at 

 the two ends of the base-line. The task of measuring the 

 sun's distance directly is, in fact, at present beyond the 

 power of observational astronomy, wonderfully through its 

 methods have developed in accuracy. 



We all know how, by observations of Venus in transit, 

 the difficulty has been so far reduced that trustworthy results 

 have been obtained. Such observations belong to the 

 surveying method, only Venus's distance is made the object 

 of measurement instead of the sun's. The sun serves simply 

 as a sort of dial-plate, Venus's position while in transit across 

 this celestial dial-plate being more easily measured than 

 when she is at large upon the sky. The devices by which 

 Halley and Delisle severally caused time to be the relation 

 observed, instead of position, do not affect the general 

 principle of the transit method. It remains dependent on 

 the determination of position. Precisely as by the change 

 of the position of the hands of a clock on the face we measure 

 time, so by the transit method, as Halley and Delisle respec- 

 tively suggested its use, we determine Venus's position on 

 the sun's face, by observing the difference of the time she 

 takes in crossing, or the difference of the time at which she 

 begins to cross, or passes off, his face. 



Besides the advantage of having a dial-face like the sun's 

 on which thus to determine positions, the transit method 

 deals with Venus when at her nearest, or about 25 million 

 miles from us, instead of the sun at his greater distance of 

 from 90^ to 93^ millions of miles. Yet we do not get the 

 entire advantage of this relative proximity of Venus. For 

 the dial-face the sun, that is changes its position too in 

 less degree than Venus changes hers, but still so much as 

 largely to reduce her seeming displacement The sun being 

 further away as '92 to 25, is less displaced as 25 to 92. 

 Venus's displacement is thus diminished by ffnds of its full 



