Zero - Charles Seife [22]
Calculating the date of Easter was no mean feat, thanks to a clash of calendars. The seat of the church was Rome, and Christians used the Roman solar calendar that was 365 days (and change) long. But Jesus was a Jew, and he used the Jewish lunar calendar that was only 354 days (and change) long. The big events in Jesus’ life were marked with reference to the moon, while everyday life was ruled by the sun. The two calendars drifted with respect to each other, making it very difficult to predict when a holiday was due. Easter was just such a drifting holiday, so every few generations a monk was drafted to calculate the dates when Easter would fall for the next few hundred years.
Dionysius Exiguus was one of these monks. In the sixth century the pope, John I, asked him to extend the Easter tables. While translating and recalculating the tables, Dionysius did a little research on the side; he realized that he could figure out just when Jesus Christ was born. After chugging through a bit of math, he decided that the current year was the 525th year since the birth of Christ. Dionysius decided that the year of Christ’s birth should, thenceforth, be the year 1 anno Domini, or the first year of Our Lord. (Technically, Dionysius said that Christ’s birth happened on December 25 the year before, and he started his calendar on January 1 to match the Roman year.) The next year after that was 2 AD, and the next 3 AD, and so forth, replacing the two dating systems then most commonly in use.* But there was a problem. Make that two.
For one thing, Dionysius got the date of Christ’s birth wrong. The sources agree that Mary and Joseph fled the wrath of King Herod, since Herod had heard a prophecy about a newborn Messiah. But Herod died in 3 BC, years before the supposed birth of Christ. Dionysius was clearly wrong; today most scholars believe that the birth of Christ was in 4 BC. Dionysius was a few years off.
In truth, this mistake was not so terrible. When choosing the first year of a calendar, it really doesn’t matter which year is chosen, so long as everything is consistent after that. A four-year error is inconsequential if everyone agrees to make the same mistake, as, indeed, we have. But there was a more serious problem with Dionysius’s calendar: zero.
There was no year zero. Normally this would be no big deal; most calendars of that day started with the year one, not the year zero. Dionysius didn’t even have a choice; he didn’t know about zero. He was brought up after the decline of the Roman Empire. Even during the heyday of Rome, the Romans were not exactly math whizzes. In the year 525, at the start of the Dark Ages, all Westerners clung to the clunky Roman style of numbers, and there was no zero in that counting system. To Dionysius, the first year of Our Lord was, naturally, the year I. The next year was year II, and Dionysius came to this conclusion in the year DXXV. In most circumstances this would not have caused any trouble, especially since Dionysius’s calendar did not catch on immediately. In 525 there was serious trouble for the intellectuals in the Roman court. Pope John died, and in the ensuing power shift all the philosophers and mathematicians like Dionysius were kicked out of office. They were lucky to escape with their lives. (Others were not so lucky. Anicius Boethius was a powerful courtier who was among the finest medieval Western mathematicians, which makes him worth noting. At about the same time that Dionysius was kicked out of office, Boethius, too, fell from power and was imprisoned. Boethius is not remembered for his math but for his Consolation of Philosophy, a tract in which he comforts himself with