Leon the Mathematician
Leon the Mathematician or Philosopher (circa 790-circa 870) was archbishop of Thessalonike (840-43) and a Byzantine sage at the time of the first Byzantine renaissance of letters and the sciences in the 9th century. He was born probably in Constantinople where he studied grammar; he later learnt philosophy, rhetoric, and arithmetic in Andros.

From 835 as a private tutor in Constantinople he taught arithmetic, geography, music, and astronomy. His fame was so great that Al Mamoun, caliph of Baghdad, invited him to teach in his capital, promising lavish remuneration to him (2,000 lbs of gold) and permanent peace with the Byzantine state. The emperor Theophilos refused the offer and named Leon "teacher of the state".

In 855 Leon was appointed director and mathematical sciences teacher at the Magnavra, one of the schools of the University of Constantinople. Some of his pupils themselves acquired fame, such as the geometrician Theodoros, the sage Arethas, Photios, patriarch of Constantinople, Cyril, missionary to the Slavs, and the philosopher Constantine Sikeliotis, who later accused him of idolatry (Leon referred to himself as a Hellene).

In 840 Leon's relative the iconomach patriarch John VII the Grammarian consecrated him archbishop of Thessalonike. Following the restoration of icons in 843 he was forced to withdraw from his archbishopric, despite having won the respect of his flock. Leon made a notable contribution to the preservation and study of the writings of ancient Greek mathematicians and astronomers.

Among other achievements, he introduced the use of the letters of the Greek alphabet for the denotation of ordinary numbers and constructed an optical telegraph device by means of which a chain of watch-towers could transmit not one but twelve different messages by using two accurately synchronized chronometers.