Sunday, February 24, 2008

Leap Year: The Best We can Do in Time Keeping

Tomorrow is a day that comes around only once every four years. It is leap day, February 29. Leap day, occurring in leap years, is a semi-permanent fix to a problem that has plagued official time keepers from the beginning of time.

There is really nothing natural about time keeping. And throughout history scholars have had a tough time figuring out exactly what time it was.

The word “month” is derived from “moon.” It was originally used to measure lunar cycles. One lunar cycle is about 28 days, which includes four phases of about seven days each.

Unfortunately, nature did not cooperate in synchronizing the lunar cycles with the seasons. Although lunar cycles were important to many cultures for religious reasons, the solar cycle and the seasons were important to most agricultural societies. People needed to know when spring arrived.

In addition, the early, overly-powerful, Roman Catholic Church had to have a fixed date for spring, because that is how they computed their holiest day of the year – Easter. Easter always occurs on the first Sunday following the first full moon that occurs on or after the vernal equinox, or first day of spring. So for Christians, the lunar cycle and solar cycle had to be intermingled, and it all had to fit with the changing of the seasons.

It was quite a task to come up with a suitable calendar. Caesar’s calendar, known as the Julian calendar, was pretty good, but it was out of synch with the sun by a factor of about seven days every 1000 years.

In 1582, Pope Gregory XIII decreed that ten days would be stripped from the current year, in order to bring the calendar in phase with the seasons. From then on, leap years, in which an extra day is added to February, would occur in all years divisible by 4 except in those century years that were not also divisible by 400. So the year 2000 was a leap year, but 1900 was not.

Pope Gregory’s calendar, called the Gregorian calendar, was much more accurate than Caesar’s, but it is still not perfect. It is off by a day or so every few thousand years. But we can live with that for now. It is the calendar we use today, because it is the best at keeping things straight, although it doesn’t even bother trying to fit the week in neatly.

There have been failed attempts at revising the calendar again. It would be nice, for example, to have a calendar in which every month began on a Sunday and ended on a Saturday, with the same number of days in each. All holidays would always fall on the same day of the week and date of the year. But creating such a calendar would mean several “leap days” would be left over at the end of every year. They would not be given dates or names, but would simply be extra days, probably designated as holidays.

As for now, leap year is as good as we can get at reconciling the differences between the sun cycles and moon cycles. Leap years are also good at keeping track of presidential election years and the Summer Olympics, both of which are events that happen only during a leap year.

And as far as calendar reform is concerned, don’t look for any such reform to happen soon. It took over 500 years for Pope Gregory’s reforms to be adopted by the whole world.

No comments: