Theon of Smyrna: Book on Astronomy

Theon of Smyrna1 and Cleomedes2 say that Mercury departs from the Sun by a maximum of 20 degrees. Either, therefore, the author of that observation is not the same as the author of the Astronomy, or he wrote the Astronomy before he observed that fact. The former seems more probable; for our author ignores phenomena already discovered by others in many instances, for the sake of following philosophical opinions3: which, if it befits a philosopher, certainly does not befit a sky-observer.

It is therefore hardly understood how, having read the Astronomy of Theon of Smyrna—a copy of which he says was lent to him by the distinguished Mr. J. Bake, his teacher—the distinguished Mr. de Gelder4 could doubt whether the observations mentioned by Ptolemy are found in this work, or whether Ptolemy read them in some other book on astronomy by Theon of Smyrna: it is obvious that neither of these is true.

Nevertheless, there does not remain much doubt regarding the time when Theon of Smyrna lived5. For, in the first place, he is not prior

1 Astron., ch. XXXIII, fol. 22 b.
2 II, 7, p. 150 (Bake edition).
3 See Part II, ch. IV, § 11, 15, 16, and 21 of this dissertation.
4 Prefatory Matters, ch. II, § 5, pp. XXXVIII-XL.
5 It is agreed that he lived in the beginning of the second century after the birth of Christ by Bullialdus (To the Reader, p. 8); Saxius (Onomasticon I, p. 293); Heilbroner (History of Universal Mathematics, p. 333 et seq.); Weidler (op. cit.); Brücker (Critical History of Philosophy, vol. II, p. 164 et seq.); Fabricius (Greek Library, vol. IV, p. 35, Harles edition); Harles (ibid.); Montucla (op. cit.); Bailly (History of Modern Astronomy, vol. I, pp. 134 and 504); Delambre (op. cit.); Schœll (History of Greek Literature, vol. V, p. 232); and de Gelder (Prefatory Matters, ch. I). J. Blancanus (Mathematical Chronicle, p. 57), who thinks he should be relegated to the XI or XII century, need not even be refuted. Nor should G. J. Vossius (On the Mathematical Sciences XXXIII, 13, p. 162; and LXII, 14, pp. 359-360) be heeded, who thinks there were two Theons of Smyrna. Cf. Brücker, Critical History of Philosophy, 2nd ed., vol. II, pp. 164-165.