VENUS ON THE SUN'S FACE. 75 



see her on another, and the difference of position, if 

 accurately measured, would at once indicate the sun's 

 distance. As a matter of fact, other modes of reading 

 off the indications of the great dial-plate have to be 

 adopted. Before proceeding to consider those modes, 

 however, we must deal with one or two facts about 

 Yenus's movements which largely affect the question 

 at issue. 



Let us first see what we gain by considering the 

 distance of Venus rather than that of the sun. 



At the time of a transit Yenus is of course on a line 

 between the earth and the sun, and she is at somewhat 

 less than a third of the sun's distance from us. Thus 

 whatever effect an observer's change of place would 

 produce upon the sun would be more than trebled in 

 the case of Yenus. But it must not be forgotten that 

 w^e are to judge the motions of Yenus by means of the 

 dial- plate formed by the solar disk, arid that dial-plate 

 is itself shifted as the observer shifts his place. Yenus 

 is shifted three times as much, it is true ; but it is only 

 the balance of change that our astronomer can rec- 

 ognize. That balance is, of course, rather more than 

 twice as great as the sun's change of place. 



So far, then, we have not gained much, since it has 

 been already mentioned that the sun's change of place 

 is not measurable by any process of observation astron- 

 omers can apply. 



It is to the fact that we have the sun's disk whereby 



