How did Dionysius determine
which year was AD 1?


See also: Christian era.

In attempting to fix the year of Christ’s birth, Dionysius relied on the following clues from the New Testament and later traditions:

1. Christ was born on the 25th of December when Herod the Great was king of Judea. Augustus was Emperor of Rome, and it was a census year.

2. The Resurrection occurred on Sunday the 25th of March 30 years later (Luke iii, 23). The moon was full. Pontius Pilate was governor of Judea.

Enough was known about the chronology of the Roman Empire that, given the information about Augustus, Dionysius could be sure that the Nativity was roughly 500 to 550 years before his own time.

He then applied astronomical rather than historical knowledge, setting out to find all the March 25ths in his own time that were Sundays and that were also the date of a full moon, and then checking to see if the 25th of December, 30 years earlier, was a Sunday. He discovered that the pair of years we now call ad 533 and ad 563 fit the bill.

Dionysius then made use of the fact that, in the Julian calendar, in any year the days of the month will fall on the same weekdays, and the phases of the moon on the same dates, as they did in the year 532 years before. (See Great Paschal period.) Subtracting the 532 years from what we call ad 533, he arrived at what we call ad 1 as the year of Christ's birth.

Within a couple of centuries other scholars (Bede in the 8th century and German monk Regino of Prüm in the 9th century) suspected that Christ could not have been born in ad 1. Historians now think Herod died in 4 bc and the census took place in 6 bc.

Dionysius also erred in another way, one familiar to beginning computer programmers using arrays: he called the first year 1 instead of 0. As a result, 2000, for example, was the last year of the old century and millennium instead of the first year of the new.

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