Dionysius Exiguus

From 500 he lived as monk and friend of Cassiodorus in Rome, where, as an abbott and learned member of the Vatican's Curia, he translated from Greek into Latin 401 ecclesiastical canons, including the apostolical canons and the decrees of the councils of Nicaea, Constantinople, Chalcedon and Sardis, and also a collection of the decretals of the popes from Siricius (385) to Anastasius II. (498). These collections had great authority in the West and still guide church administrations. He was known for his mathmatical skill and for being versed in astronomy, as well.

Pope John I requested that Dionysius devise a method for finding the dates of Easter. DE produced a matrix for finding the date that is no longer used. In addition, Dionysius proposed to base the year numbering from the incarnation of Jesus Christ instead of from the accession of Roman Emperor Diocletian (September 29, 284), as was the custom since the council of Nicea (325). Dionysius calculated Christ's incarnation as March 753 AUC and his birth as December 25, 754 ab urbe condita (from the founding of Rome). Skipping the year 0, Jesus would actually have been born "BC".

Mediated through the Venerable Bede, the system of Dionysis was adopted, with adjustments, in the Middle Ages by Pope Gregory. Since England adopted it in 1752, the "Gregorian calendar" is now the world standard. Bede himself seems to have instituted "BC" and "AD" year naming.

DE's definition of Easter date (per Council of Nicea): Easter is the Sunday following the first Luna XIV (the 14th day of the moon) that occurs on or after XII Kalendas Aprilis (21 March). (kalendas means counting days backward starting with 1 on the first day of the given month, which is according to the Roman calendar, and the change from 15 Nisan to Luna 14 probably has to do with the fact that on the Hebrew calendar days start at sunset, while in the Christian A.D. calendar (which was also introduced by Dionysius) days start at midnight. DE's method of counting Easter is called the Julian method.

External sources

• Cassiodorus knew Dionysius Exiguus and wrote about him in Institutiones.
• Dionysius Exiguus apparently proposed his year numbering system in a letter to a Bishop Petronius.