Close — Sipparclius, and the Precession of the Equinoxes. 451 



points inyolyes a retrograde oblique turning of the plane of the Earth's 

 equator, that is, a turning of the Earth itself. Ptolemy and his 

 successors chose the former alternative, the progression of the stars. "We, 

 of course, know that we must abide by the latter, the retro gradation of 

 the equinoctial points. 



But which of these alternatives did Hipparchus himself adopt ? 

 It is the object of this paper to obtain, if possible, an answer to this 

 question. This is an interesting and not Tinimportant point in the 

 history of astronomy. Besides, our reverential regard for the Founder 

 of mathematical astronomy, that ' ' laboui'-loving and truth-loving man," 

 as Ptolemy calls him, must make us desirous, for personal reasons, to 

 find out, ii we can, what may have been his decision on this matter. 



It might be said — But surely this question, if it can be settled, 

 must have been settled long ago by writers on astronomy or on its 

 liistory. Let us, then, in the first place, appeal to some of those 

 writers for information on this point. 



However, when we do so, we find, at once, a remarkable difference 

 of opinion existing amongst them. We have collected into Grroup A, 

 below, the names of some astronomers who tell us that Hipparchus 

 selected the progression of the fixed stars ; and into Group B the 

 names of some others who tell us the opposite, i.e.^ that Hipparchus 

 selected the retrogression of the equinoctial points ; the names being 

 arranged in each grouxD in approximately chronological order. 



Ptolemy. 2 



Copernicus. 



Eiccioli.* 



Gregory.^ 



Weidler.'' 



Geoup a. 



Laplace.' 



Lalande.® 



Long.^ 



Eothman.io 



IS'arrien.^^ 



Arago.^2 

 Hoefer.^^ 

 Flammarion.^* 

 Wolf, Eudolf.15 



2 Almagest, Bk. iii., ch. i, and Bk. vii., 



chs. I, 2, 3. 

 .3 De Revolutionibus Orbium Caelestium, 



Bk. III., ch. I. 



4 AlmagestumNo\mm, vol. i., Ft. i., p. 164, 



col. 2. 



5 Elem. of Astron., vol. i., p. 314. 



6 Hist. Astronomias, ch. vi., p. 142. 



7 Mec. Cel., Bk. xiv., ch. i., and Syst. du 



Monde, Bk. v., ch. 11. 



('^) Flammarion, in Ms Vie de Copernic, p. 135, foot-note, quoting the preface 

 to Copernicus' De Mevolutionihis, mentions a letter from Lysis to Hipparchus, 

 whom Flammarion calls ancien Fythagoricien. But this Hipparchus, being a con- 

 temporary of Lysis, lived about 100 years before the great astronomer of that name. 



8 Astronomic, vol. i., p. 406. 



9 Astronom}', Bk. v., pp. 677 and 680 (1785) 



10 Hist, of Astron., in Libr. of Useful KhqwI 



pp. 25 and 27. 



11 Hist, of Astron., pp. 223 and 252. 



12 Astron. Populaire, vol. IV., p, 95. 



13 Hist, de I'Astron., p. 172. 



14 Vie de Copernic, p. 10. 



15 Geschichte der Astron., p. 158. 



