Calendrical Complications and Moveable Feasts

February 2000 should have included a leap day, because any year divisible by four has an extra day. On the other hand, century years Ð ending in double zeros Ð ordinarily do not have a February 29; 1700, 1800, and 1900 didnÕt have this additional day. But once every four centuries, the double-oh year is a leap year, and 2000 is one of them so there was a February 29 this year?

How did this get so complicated? Basically people like whole numbers, but nature does not oblige. Measured against the stars, an Earth year lasts 365.2422 days, or 365 days, 5 hours, 48 minutes, and 46 seconds. If the year was 365.25 days, or about 11 minutes longer, things would be simple: Adding one leap day every fourth year eliminates the fraction. But the Earth circles the sun slightly faster than that, so we occasionally have to skip the once-every-four-year leap day. Passing three of them every 400 years may be confusing, but it gets the job done. Pope Gregory XIII introduced this extra adjustment in 1582, and it keeps our calendar accurate to one day every 3,300 years. Years divisible by 4,000 are not leap years, which fixes that little glitch.

If it werenÕt for this elaborate system of leap years, each January 1 would begin at a slightly different point in EarthÕs circuit around the sun. That might not seem important, but eventually the seasons would start at odd times. Christmas would come in the summer if this went on long enough. This explains why Christmas, the Winter Solstice and New YearÕs Day should be celebrated on the same day Ð they all celebrate the same festival Ð the end of autumn and beginning of winter, the time when the sunÕs light is at its minimum.

Our previous calendar, in which all century years were leap years, was good enough for Julius Caesar but its annual 11-minute error did add up. By the 1500s, equinoxes and solstices took place a full 10 days too early.

In 1582, Pope Gregory boldly returned the seasons to their intended place by simply eliminating 10 days from the calendar. That year, October 4 was followed by October 15. England did not adopt the Gregorian calendar until 1752, by which time the calendar was 11 days behind. In that year September 3rd became September 14th, to the amazement of people who did not understand. The populace was riled, believing that they had "lost" eleven days, and took to the streets shouting, "Give us back our eleven days!" He also decreed that all century years divisible by 400 would be leap years.

The Gregorian calendar also allows the equinoxes to drift behind as each century progresses, only to push them ahead a bit too much by the omitted leap year at most centuriesÕ end. People over 30 may recall a time when the first day of spring fell on March 21. Long since then the date slid to March 20. The insertion of an extra day this February keeps the start of spring on the twentieth and even allows it to slip back to March 19 later this century.

A T Mann