Greek astronomer and mathematician. In his celestial mechanics, the planets, Sun, Moon, and the sphere of fixed stars were all set at intervals congruent with an octave. His only surviving work, Expositio rerum mathematicarum ad legendum Platoneum utilium, is in two manuscripts, one on mathematics and one on astronomy and astrology.
Theon collated and organized discoveries made by his predecessors, and articulated the interrelationships between arithmetic, geometry, music, and astronomy. The section on mathematics deals with prime, geometrical, and other numbers in the Pythagorean pantheon; the section on music considers instrumental music, mathematical relations between musical intervals, and the harmony of the universe.
The astronomical section is by far the most important. Theon puts forward what was then known about conjunctions, eclipses, occultations, and transits. Other subjects covered include descriptions of eccentric and epicyclic orbits, and estimates of the greatest arcs of Mercury and Venus from the Sun.