Theon (c. 335 - c. 405 AD) was a scholar and the last director of the Library of Alexandria in the Museion, until it was closed by the patriarch Theophilus on order of the emperor Theodosius in 391 AD. He wrote some commentaries on important works by his hellenistic predecessors, notably the Almagest and the "Handy Tables" by Ptolemy. Apparently he is the first author on the theory of "trepidation of the equinoxes", as an alternative to precession.

His most lasting achievement was his edition of Euclids Elements which was published in ~364. This edition remained in publication in some form until 1814 or an astonishing ~1450 years.

The bulk of Theon's work, however, consisted of commentaries on important works by his hellenistic predecessors. These included a "conferences" (Synousiai) on Euclid, and commentaries (Exegeseis) on the "Manual Tables" and Almagest of Ptolemy, and on the technical poet Aratus.

Theon was the father of the martyr Hypatia.

Theon of Alexandria

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