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This explains the 35-day span where Easter can occur (March 22 - April 25, inclusive): the first Sunday after the first full moon after the spring equinox may occur as little as two or as many as 37 days from the equinox.

We owe this complicated formula, with its attention to both the sun (the equinox) and the moon (full phase), to a political compromise among Nicea's gathered factions. The Eastern Christians injected the irregular phases of the moon into the calculations—thus causing the "wandering" effect—because they wanted their lunar calendar to keep its historical (though problematic) role in determining important dates.

A portentous shift
Cumbersome though it was, the Nicean accord ruled the church's commemoration of Jesus' resurrection for next 900 to1000 years. But this was not the end of the story. Unfortunately, the Julian solar calendar contained a non-trivial flaw that reared its head as the centuries crept along. This flaw affected the celebration of Easter, and its correction wrought great strife and consternation among Christians.

In the mid-1200's, an English Friar named Roger Bacon observed that the date of Easter, in addition to its prescribed wandering, was drifting farther and farther into the spring season. Astronomers now knew that the length of the solar year was closer to 365.242 days than to the 365.250 days assumed in the Julian calendar year. In 1,000 years, the Julian calendar counted 365,250 days, while in actuality, 365,242 solar days had elapsed. Bacon realized that each Julian year "overflowed" slightly into the next solar year, and that any given date was farther along in real time than the calendar would imply. The man-made calendar might say one thing, but nature's seasons were not fooled! Though the small mismatch caused a shift of only 11 minutes per year, this had accumulated—from Julius Ceasar's to Roger Bacon's day—into a troubling 9 days. Bacon's petitions to correct the drift went unheeded.

By the mid-1500s, Pope Gregory XIII recognized the consequences of the drift and entrusted a solution to a Jesuit mathematician and astronomer, Christopher Clavius. The enlightened Pope endorsed Clavius's findings in 1563 at the Council of Trent, and 19 years later, on October 4, 1582, Gregory signed a papal bull promulgating the new calendar that bears his name—the Gregorian calendar.

Gregory's calendar inserted a correction to the Julian calendar from that time forward. Ingeniously, it removed eight of the 250 leap days (February 29) occurring in each 1,000 years of the Julian calendar, thereby approximating more accurately the average number of days in a year—namely, 365.242. The exact rule is that at the century boundaries, a leap day shall be observed only when the century number is wholly divisible by 400. In other words, observance of a leap day in 2,000 was a special event. It will not happen again at a century boundary until 2,400.

More trouble for Easter
While the Gregorian calendar solved the problem for future years, there remained the critical matter of correcting the older calendar's "slippage." By 1582, the cumulative mismatch of the Julian calendar year against the solar year totaled 10 days. The papal bull addressed this problem in a practical but provocative way: it advanced the Julian calendar by 10 days. The calendar days October 5—14, 1582 simply vanished!

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April (Web-only) 2004, Vol. 48