Theon was an active scholar, he is known to have observed the solar eclipse that took place in Alexandria on 16 June 364 and the lunar eclipse on 25 November of the same year.
He was the author of commentaries on the Almagest (the Arabs' distorted name for Claudius Ptolemy's work, which is the greatest monument of astronomical knowledge) and on Ptolemy's astronomical tables, describing their use and construction.
His greatest work was the compilation of a version of the The Elements (a geometric and arithmetic treatise from the late 4th century BC, written by Euclid), which until the end of the 19th century was the only known Greek text of this book. It is also known that Theon compiled a list of Roman consuls up to the year 372.
Theon was the last director of the Library of Alexandria, which was closed in 391 by order of the Emperor Theodosius.
