500?-560?

Scythian Theologian, Mathematician, and Astronomer

Dionysius Exiguus, a Roman theologian, mathematician, and astronomer, is known for inventing a Christian calendar that was later incorporated into the currently used Gregorian calendar. In addition to providing calculations for determining the date of Easter, Dionysius's calendar is notable for establishing its starting point from the birth of Jesus Christ, thus introducing the designations B.C. and A.D.

Little is known of Dionysius's early life. It is documented, however, that he arrived in Rome about the time of the death, in 496, of Pope Gelasius I, who had summoned him to organize the official archives of the church hierarchy. The Roman Empire was in ruins, and the city of Rome itself was dilapidated and nearly deserted.

A trained mathematician and astronomer, as well as a master theologian, Dionysius spent his days working at a complex now known as the...