SUN S DISTANCE FROM Till-: KARTH. 7 



accuracy to be used as the basis of so nice a calculation. 



It will not, therefore, be supposed that either of these 

 methods is the one employed. The correct result is obtained 

 by a most complicated process of trial and error based upon 

 the difference of times of which so much has been said, and in 

 which much more exact cognizance of that difference is taken 

 than in this little treatise. What has been said, however, is 

 sufficient to show that the principles which lie at the base of 

 the calculation are neither complicated nor abstruse. 



The astronomer proceeds solely by what is called the 

 method of Parallax. 



" Parallax is the angular variation in the position of an 

 object caused by the eccentric situation of the observer with 

 respect to a certain point of reference." 



In somewhat less learned language : Parallax is the appa- 

 rent change of place which bodies undergo by being viewed 

 from different points. The centre of the sun, for instance, 

 would be referred to a lower point upon the celestial sphere b}^ 

 an observer standing upon the surface of the Earth than by 

 one at the centre of the P^arth. Since, then, a heavenly body 

 is liable to be referred to different positions upon the celestial 

 vault and some confusion is thus created in the determination 

 of points upon the star sphere, astronomers have agreed to 

 consider the true place of a heavenly body as that where it 

 would appear if seen from the centre of the Earth. 



The doctrine of Parallax teaches how to reduce observa- 

 tions made at any place upon the surface of the Earth to such 

 as they would be if seen from the centre. 



Suppose now two observers, one at the centre of the Earth 

 and one at the surface; both looking at the sun. The two 

 visual rays intercept at that point and form equal vertical 

 angles. One of these angles is the angular variation in the 

 position of the sun's centre, caused by the different situations 

 of the two observers ; the other is evidently the angle sub- 

 tended at the sun's centre by the P^arth's semi-diameter. The 

 latter is the one selected for use, and is called the sun's paral- 



