Anno Domini

When Pope John I in 525 asked Dionysius Exiguus to write a chronology to tabulate future dates of Easter, Dionysius looked at how years were numbered and wanted something more appropriate. Of course there was the Roman Empire numbering of AUC, which stood for abs urbe condita, "from the founding of the city." Dionysius wanted something a little more appropriate for a Christian world.

Other methods of keeping track of years included Olympiads or the regnal years of Caesars. In various countries, years were called by the regnal years of the king. When a new king was crowned, his records began with Year 1 of his reign. 

At the time, years in Rome were designated by the terms of the Roman consuls. Dionysius wrote that the "present year" was the "consulship of Probus Junior," which he also claimed was 525 years since the birth of Christ. (We assume he looked at various historical records and counted them up to determine that year.)

Christians were using the "Diocletian era," which was calculated since the last big persecution of Christians under Emperor Diocletian. Dionysius did not want to use a calendar based on someone who was hostile to Christians. There had been a previously calculated set of dates by Victorius of Aquitaine c.457 that Dionysius ignored as being "off" and developed his own system, laying down dates for the years 532-626.

His solution, therefore, to keep track of years was the phrase anno Domini, "year of [our] Lord." This divided time into two sections: everything that happened prior to the Incarnation, and then everything that happened starting at the Incarnation.

Problems arose, of course. One question was what defined the "Incarnation": was it birth, or conception? The Diocletian Year began on 1 September, but consulships began on 1 January, so were we tabulating years correctly? Lists of consuls were not always complete, nor were the dates of emperors.

We are not sure how Dionysius determined the year of the Incarnation. It may have been based on the Gospel of Luke, where he says Jesus was about 30 shortly after the 15th year of the reign of Tiberius Caesar.

The use of AD was not adopted universally. Tomorrow we'll look at other conventions and the slow rise of AD's popularity.