Naturalist's Almanac

Imagine that you went to bed the night of October 4 and woke up the next morning to find that it’s October 15. That’s exactly what happened in 1582, thanks to Pope Gregory XIII’s reform of the calendar that now governs our daily lives.

Pope Gregory XIII was faced with three major problems: Julius Caesar’s faulty leap year formula, the church’s decree that March 21 would always be the date of the spring equinox, and the perpetual challenge of determining when Easter would be.

The leap year problem began back in 46 B.C., when Caesar’s astronomer, Sosigenes, told him that a solar year had 365.25 days. That figure was 11 minutes 14 seconds too long, and the regular addition of an extra leap day every four years caused Caesar's calendar to drift away from the solar seasons.

At first the extra days didn’t make much difference. But by A.D. 325, when church leaders met at Nicea, there were observable problems. The spring equinox, which occurred on March 25 in Caesar’s day, had drifted to March 21.

Instead of solving the leap year problem, the Council of Nicea merely decreed that henceforth the spring equinox would always occur on March 21. So the extra leap days kept accumulating, and by 1582, the real spring equinox had drifted all the way to March 11.

Pope Gregory decided to address himself to the leap year-equinox-Easter problem once and for all. His astronomers, Aloysius Lilius and Christopher Clavius, had estimated that a solar year actually lasts only 365.2425 days. Therefore, they decided the calendar should omit three leap years every 400 years to stay in sync with the sun.

But first Pope Gregory had to get rid of the 10 extra days that had accumulated since the Council of Nicea. He decided to drop the 10 days between October 4 and 15 because that block of days was conveniently free of church holy days.

Then he had to correct the leap year formula, which he did by omitting leap years in the century years that cannot be divided by 400. So 1600 was a leap year, 1700, 1800, and 1900 were not, and 2000 was again.

Modern astronomers have determined that Pope Gregory’s reformed calendar is still based on an imprecision of about 26 seconds a year. Taking into account the additional reality that the solar year is decreasing in length, these astronomers estimate that a new adjustment will be necessary in about A.D. 3719.

By then, maybe the world will be ready for another calendar reform — or maybe just a special day-with-no-date to keep Pope Gregory’s 1582 calendar aligned with the sun.

MORE INFORMATION

Catholic Encyclopedia
http://www.newadvent.org/cathen/03168a.htm

This basic article includes all the important details of what Pope Gregory was up against and what he did about it.

Calendar Reform
http://personal.ecu.edu/mccartyr/calendar-reform.html

This site offers a history of calendars and calendar reform plus current proposals for new reforms, including the World Calendar, which would have equal-length quarters and be the same every year. It offers numerous links to other information on calendars and calendar reform.

Gregorian Calendar - Wikipedia
http://en.wikipedia.org/wiki/Gregorian_calendar

The volunteer author of this entry seems to know about as much about calendars and calendar reform as anyone else I’ve encountered in my researches. The article is long, includes lots of internal and external links (plus a list of “See Also’s”), and offers some interesting graphics and useful charts.