At the Council of Nicaea a lot of things were agreed upon. One thing the western and eastern halves of the Roman Empire could not agree on was when Easter should be celebrated. Each half—western and eastern—celebrated on a different date. This went on for a long time until finally in ad 525 an expert was called in. Dionysius Exiguus (his name means ‘Little Dennis’) was a scholarly monk who got the job of figuring out exactly when the Christian holy day of Easter should occur every year.
Dionysius decided to go back and find when the first Easter occurred. Jesus’ resurrection happened during the Jewish Passover—Pesach. The Jewish calendar relies on the motions of the Moon and Earth’s orbit around the Sun. Using some incredibly complicated astronomical calculations, he was able to arrive at the original date.
Dionysius realized once he’d found the date of Jesus’ resurrection, he could then figure out when Jesus was born. Jesus was 33 years old when He was crucified, so Dionysius counted back 33 years from the first Easter to get the year of Jesus’ birth.
In the past, years had been named after whoever was the imperial consul at the time. Dionysius decided it was time to change that. He named the years after Jesus, the Christian Savior. So the years beginning with Jesus’ birth are numbered and called Anno Domini (ad for short)—Latin for ‘the year of our Lord.’
By the way, Dionysius reckoned that Easter should occur on the first Sunday following the 14th day of the lunar cycle—the full moon—that falls on or after the spring equinox.
Back to the beginning of The Western Civ User’s Guide to Time & Space.