NEW WA YS OF MEASURING THE SUN'S DISTANCE. 63 



Venus being observed on the sun, all the disturbing in- 

 fluences due to the sun's heat are at work in rendering the 

 observation difficult The air between us and the sun at 

 such a time is disturbed by undulations due in no small 

 degree to the sun's action. It is true that we have not, in 

 the case of Mars, any means of substituting time measures 

 or time determinations for measures of position, as we have 

 in Venus's case, both with Bailey's and Delisle's methods. 

 But to say the truth, the advantage of substituting these time 

 observations has not proved so great as was expected. 

 Venus's unfortunate deformity of figure when crossing the 

 sun's edge renders the determination of the exact moments of 

 her entry on the sun's face and of her departure from it by no 

 means so trustworthy as astronomers could wish. On the 

 whole, Mars would probably have the advantage even with- 

 out that point in his favour which has now to be indicated. 



Two methods of observing Mars for determining the 

 sun's distance are available, both of which, as they can be 

 employed in applying one of the new methods, may 

 conveniently be described at this point. 



An observer far to the north of the earth's equator sees 

 Mars at midnight, when the planet is in opposition, displaced 

 somewhat to the south of his true position that is, of the 

 position he would have as supposed to be seen from the 

 centre of the earth. On the other hand, an observer far to 

 the south of the equator sees Mars displaced somewhat to 

 the north of his true position. The difference may be com- 

 pared to different views of a distant steeple (projected, let 

 us suppose, against a much more remote hill), from the 

 uppermost and lowermost windows of a house corresponding 

 to the northerly and southerly stations on the earth, and 

 from a window on the middle story corresponding to a view 

 of Mars from the earth's centre. By ascertaining the dis- 

 placement of the two views of Mars obtained from a station 

 far to the north and another station far to the south, the 

 astronomer can infer the distance of the planet, and thence 

 the dimensions of the solar system. The displacement is 



