(Θέων). Of three of this name whose writings yet remain, two are mathematicians who are often confounded together. The first is Theon the elder, of Smyrna, best known as an arithmetician, who lived in the time of Hadrian. The second is Theon the younger, of Alexandria, the father of HYPATIA, best known as an astronomer and geometer, who lived in the time of Theodosius the elder. Both were heathens, a fact which the date of the second makes it desirable to state; and each held the Platonism of his period. The confusion would probably be avoided, if they were named after their leaders in science : they would then be called Theon the Pythagorean, and Theon the Ptolemaist.
The date of " Theon of Smyrna the philosopher," to quote in full the account which Suidas gives of him, depends upon the assumption (which there seems no reason to dispute) that he is the Theon whom Ptolemy and the younger Theon mention as having made astronomical observations in the time of Hadrian. Theon of Smyrna certainly wrote on astronomy. On the assumption just made, Ptolemy has preserved his observations of Mercury and Venus (A. D. 129-133). Bouillaud supposes that it is Theon of Smyrna to whom Proclus alludes as having written on the genealogies of Solon and Plato, and Plutarch as having written on the lunar spots. (See Bouillaud's preface, or the quotations in Fabric. Bibl. Graec. vol. iv. p. 35.)
Of the life of Theon of Alexandria, called the younger (described by Suidas as ὁ ἐκ τοῦ μουσείου), nothing is known except the melancholy history of his daughter HYPATIA. We shall now take the