208 THE GREEK ASTRONOMY. 



Among these we may notice the researches 

 which were made concerning the Parallax of the 

 heavenly bodies, that is, their apparent displace- 

 ment by the alteration of position of the observer 

 from one part of the earth's surface to the other. 

 This subject is treated of at length by Ptolemy; and 

 there can be no doubt that it was well examined 

 by Hipparchus, who invented a parallatic instru- 

 ment for that purpose. The idea of parallax, as 

 a geometrical possibility, was indeed too obvious 

 to be overlooked by geometers at any time; and 

 when the doctrine of the sphere was established, 

 it must have appeared strange to the student, that 

 every place on the earth's surface might alike be 

 considered as the center of the celestial motions. 

 But if this was true with respect to the motions 

 of the fixed stars, was it also true with regard to 

 those of the sun and moon? The displacement 

 of the sun by parallax is so small that the best 

 observers among the ancients could never be sure 

 of its existence ; but with respect to the moon, the 

 case is different. She may be displaced by this 

 cause to the amount of twice her own breadth, a 

 quantity easily noticed by the rudest process of in- 

 strumental observation. The law of the displace- 

 ment thus produced is easily obtained by theory, 

 the globular form of the earth being supposed 

 known; but the amount of the displacement de- 

 pends upon the distance of the moon from the 

 earth, and requires at least one good observation 



