Theon of Alexandria

Little is known about the life of Theon. He made predictions and observations of solar and lunar eclipses in 364 which show he was active at that time, and he is said to have lived during the reign of Theodosius I (379–395). The Suda, a tenth-century Byzantine encyclopedia, calls Theon a "man of the Mouseion". However, both the Library of Alexandria and the original Mouseion saw decline and eventually closed in the third century AD and according to classical historian Edward J. Watts, Theon was probably the head of a school called the "Mouseion", which was named in emulation of the Hellenistic Mouseion that had once included the Library of Alexandria, but which had little other connection to it. Theon's school was exclusive, highly prestigious, and doctrinally conservative. Theon dedicated his commentary on the Almagest to a boy named Epiphanius, who may have been his son. Also, in his commentary on the Almagest he states that his daughter Hypatia contributed to Book III of the Almagest stating "the edition having been prepared by the philosopher, my daughter Hypatia." A lunar crater, Theon Junior, now bears Theon's name. == Works ==

Edited works It is known that Theon edited the Elements of Euclid. He may also have edited some other works by Euclid and Ptolemy, although here the evidence is less certain. The editions ascribed to Theon are: • ''Euclid's Elements''. Theon's edition of the Elements was the only known version until François Peyrard discovered an older copy of the Elements in the Vatican Library in 1808. Comparison of the two versions show that Theon's edition attempts to remove difficulties that might be felt by learners in studying the text. Hence he amplified Euclid's text whenever he thought that an argument was too brief; attempted to standardise the way that Euclid wrote; and he corrected mistakes in the text, although occasionally he introduced his own errors. • Ptolemy's Handy Tables. A collection of astronomical tables originally compiled by Ptolemy. It has often been claimed in modern times that Theon edited this text. However, none of the surviving manuscripts mention Theon, and the evidence suggests that the surviving tables must be very similar to the tables Ptolemy provided. Commentaries Of his commentaries, those which are extant are: • Commentary on the Data of Euclid. This work is written at a relatively advanced level as Theon tends to shorten Euclid's proofs rather than amplify them. It is also useful for Theon's account of the Greek method of operating with the sexagesimal system as it was applied to calculations. Theon describes but did not endorse this theory. This idea inspired Thābit ibn Qurra in the 9th century to create the theory of trepidation to explain a variation which he (incorrectly) believed was affecting the rate of precession. • Catoptrics. The authorship of this treatise, ascribed to Euclid, is disputed. It has been argued that Theon wrote or compiled it. The Catoptrics concerns the reflection of light and the formation of images by mirrors. Among Theon's lost works, the Suda mentions On Signs and Observation of Birds and the Sound of Crows; On the Rising of the Dog[-Star]; and On the Inundation of the Nile. ==See also==