SEQUEL TO THE EPOCH OF HIPPARCHUS. 209 



to determine it. Ptolemy has given a table of the 

 effects of parallax, calculated according to the 

 apparent altitude of the moon, assuming certain 

 supposed distances ; these distances, however, do 

 not follow the real law of the moon's distances, in 

 consequence of their being founded upon the Hypo-. 

 thesis of the Eccentric and Epicycle. 



In fact this Hypothesis, though a very close 

 representation of the truth, so far as the positions 

 of the luminaries are concerned, fails altogether 

 when we apply it to their distances. The radius 

 of the epicycle, or the eccentricity of the eccentric, 

 are determined so as to satisfy the observations of 

 the apparent motions of the bodies : but, inasmuch 

 as the hypothetical motions are different altogether 

 from the real motions, the Hypothesis does not, at 

 the same time, satisfy the observations of the dis- 

 tances of the bodies, if we are able to make any 

 such observations. 



Parallax is one method by which the distances 

 of the moon, at different times, may be compared ; 

 her Apparent Diameters afford another method. 

 Neither of these modes, however, is easily capable 

 of such accuracy as to overturn at once the Hypo- 

 thesis of epicycles; and, accordingly, the Hypothesis 

 continued to be entertained in spite of such mea- 

 sures; the measures being, indeed, in some degree 

 falsified in consequence of the reigning opinion. 

 In fact, however, the imperfection of the methods 

 of measuring parallax and magnitude, which were 

 VOL. i. P 



