A.D. (or AD) - How Christianity Underlies our Calendars

AD (or A.D.) is an abbreviation for the Latin expression Anno Domini, which translates to the Year of Our Lord, and equivalent to C.E. (the Common Era). Anno Domini refers to the years which followed the supposed birth year of the philosopher and founder of Christianity, Jesus Christ. For the purposes of proper grammar, the format is properly with the A.D. before the number of the year, so A.D. 2018 means The Year of Our Lord 2018, although it is sometimes placed  before the year as well, paralleling the use of B.C. The choice of starting a calendar with the birth year of Christ was first suggested by a few Christian bishops including Clemens of Alexandria in C.E. 190 and Bishop Eusebius at Antioch, C.E. 314–325. These men labored to discover what year Christ would have been born by using available chronologies, astronomical calculations, and astrological speculation. Dionysius and Dating Christ In 525 C.E., the Scythian monk Dionysius Exiguus used the earlier computations, plus additional stories from religious elders, to form a timeline for Christs life. Dionysius is the one credited with the selection of the AD 1 birth date that we use today—although it turns out he was off by some four years. That wasnt really his purpose, but Dionysius called the years that occurred after Christs supposed birth The years of our Lord Jesus Christ or Anno Domini. Dionysiuss real purpose was trying to pin down the day of the year on which it would be proper for Christians to celebrate Easter. (see the article by Teres for a detailed description of Dionysius efforts). Nearly a thousand years later, the struggle to figure out when to celebrate Easter led to the reformation of the original Roman calendar called the Julian Calendar into the one most of the west uses today--the Gregorian calendar. The Gregorian Reform The Gregorian reform was established in October of 1582  when Pope Gregory XIII published his papal bull Inter Gravissimas. That bull noted that the existing Julian calendar in place since 46 B.C.E. had drifted 12 days off-course. The reason the Julian calendar had drifted so far is detailed in the article on B.C.: but briefly, calculating the exact number of days in a solar year was nearly impossible prior to modern technology, and Julius Caesars astrologists got it wrong by about 11 minutes a year. Eleven minutes isnt too bad for 46 B.C.E., but it was a twelve-day lag after 1,600 years. However, in reality, the main reasons for the Gregorian change to the Julian calendar were political and religious ones. Arguably, the highest holy day in the Christian calendar is Easter, the date of the ascension, when the Christ was said to have been  resurrected from the dead.  The Christian church felt that it had to have a separate celebration day for Easter  than the one originally used by the founding church fathers, at the start of the Jewish Passover.   The Political Heart of Reform The founders of the early Christian church were, of course, Jewish, and they celebrated Christs ascension on the 14th day of Nisan, the date of Passover in the Hebrew calendar, albeit adding a special significance to the traditional sacrifice to the Paschal lamb. But as Christianity gained non-Jewish adherents, some of the communities agitated for separating out Easter from Passover. In 325 C.E., the Council of Christian bishops at Nicea set the annual date of Easter to fluctuate, to fall on the first Sunday after the first full moon occurring on or next after the first day of spring (vernal equinox). That was intentionally complex  because to avoid ever falling on the Jewish Sabbath, Easters date had to be based on the human week (Sunday), the lunar cycle (full moon) and the solar cycle (vernal equinox). The lunar cycle used by the Nicean council was the Metonic cycle, established in the 5th century B.C.E., that showed that new moons appear on the same calendar dates every 19 years. By the sixth century, the ecclesiastical calendar of the Roman church followed that Nicean rule, and indeed, it is still the way the church determines Easter each year. But that meant that the Julian calendar, which had no reference to lunar motions, had to be revised. Reform and Resistance To correct the Julian calendars date slippage, Gregorys astronomers said they had to deduct 11 days out of the year. People were told they were to go to sleep on the day they called September 4th and when they woke up the next day, they should call it September 15th. People did object, of course, but this was only one of numerous controversies slowing acceptance of the Gregorian reform. Competing astronomers argued over the details; almanac publishers took years to adapt—the first was in Dublin 1587. In Dublin, people debated what to do about contracts and leases (do I have to pay for the full month of September?). Many people rejected the papal bull out of hand—Henry VIIIs revolutionary  English reformation had taken place only fifty years earlier. See Prescott for an amusing paper on the problems this momentous change caused everyday people. The Gregorian calendar was better at counting time than the Julian, but most of Europe held off accepting the Gregorian reforms until 1752. For better or worse, the Gregorian calendar with its embedded Christian timeline and mythology is (essentially) what is used in the western world today. Other Common Calendar Designations Islamic: A.H. or AH, meaning Anno Hegirae or in the year of the HijraHebrew: AM or A.M., meaning Year After CreationWestern: BCE or B.C.E., meaning Before the Common EraWestern: CE or C.E., meaning the Common EraChristian-Based Western: BC or B.C., meaning Before ChristScientific: AA or A.A., meaning the Atomic AgeScientific: RCYBP, meaning Radiocarbon Years Before the PresentScientific: BP or B.P., meaning Before the PresentScientific: cal BP, meaning Calibrated Years Before the Present or Calendar Years Before the Present Sources Macey SL. 1990. The Concept of Time in Ancient Rome. International Social Science Review 65(2):72-79.Peters JD. 2009. Calendar, clock, tower. MIT6 Stone and Papyrus: Storage and Transmission. Cambridge: Massachusetts Institute of Technology.Prescott AL. 2006. Refusing Translation: The Gregorian Calendar and Early Modern English Writers. The Yearbook of English Studies 36(1):1-11.Taylor T. 2008. Prehistory vs. Archaeology: Terms of Engagement. Journal of World Prehistory 21:1–18.Teres G. 1984. Time computations and Dionysius Exiguus. Journal for the History of Astronomy 15(3):177-188.