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. Theon was the father of the mathematician and "pagan" martyr Hypatia.
Theon's most durable achievement may be his edition of Euclid's Elements, published around 364 and authoritative into the 19th century. 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.
Apparently he is the first author on the theory of "trepidation of the equinoxes", as an alternative to precession.
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