Theon of Alexandria

Theon of Alexandria was a Greek geometer, a mathematician and astronomer, flourished in Alexandria during the second part of the fourth century.

He lived in Alexandria in the times of Theodosius I (who reigned AD 379-395) and taught at the Museum.

His predictions and observances of the solar and lunar eclipses of 364, establish that he was an active scholar at that time; similarly, he is said to have reached maturity during the latter two decades of the 4th century.

Theon’s longest and chronologically earliest work is his commentary on Ptolemy’s Almagest, in thirteen books, written in parallel order to the great classic. The commentary survives almost intact except for Book 5, which is a fragment, and Book 11, which is lost.

He produced a commentary on Euclid's Optics and on his Data. Theon's commentary on the Data is written at a relatively advanced level and in it Theon tends to shorten Euclid's proofs rather than to amplify them. The Optics on the other hand is elementary and written in a totally different style and some historians conjecture that it is really a set of lecture notes by one of Theon's students.

The Islamic tradition inherited and was influenced by several of Theon’s works, especially the one on the astrolabe and his edition of Ptolemy’s Handy Tables. He is included in the Fihrist, the famous Arabic bibliographical list.

Theon of Alexandria was the last person definitely known to have been associated with the Museum. Because he recorded two eclipses (one of the sun and one of the moon) and because he is also credited with achievements during therein of Theodosius I, it is thought that he was at the height of his powers in the decade360-370. Theon may well have been the last “president” of the Museum.

Theon of Alexandria

Theon of Alexandria (AD 335 – 405)

He flourished about 370. The tenth-century encyclopedia known as the Suda says that Theon lived in the time of the Emperor Theodosius I (379-395).

Theon taught at the Museum of Alexandria, a cultural and intellectual gathering place that included a number of schools, celebrated library and public lecture halls.

He was not a mathematician of special note, but people indebted to him for an edition of Euclid’s Elements and a commentary of the Almagest; the latter gives a great deal of miscellaneous information about the numerical methods used by the Greeks.

Theon of Alexandria has given a geometrical explanation of the algorithm of the square roots in his Commentary on Ptolemy’s Almagest, while constructing for astronomical use a table of arc lengths, need to calculate the chord of an arc 36°.

Until a little over a century ago, his edition of Euclid’s Elements, on which his daughter Hypatia may have collaborated, was then only known Greek text of the Elements.

He reported that he had observed a solar eclipse at Alexandria  in June 16, 364.
Theon of Alexandria (AD 335 – 405)