Blogtrek

Leap Year Day

Today was Leap Year Day, 2004 February 29. It was a Sunday, so I went to church for the fifth time in the month. Five Sundays in February is rare; the last time it occurred was in 1976. The day was warm, but it seemed like any other day to me. But it was out of the calendar. Saying "A year from now" does not make sense, for instance, since there will be no 2005 February 29. February 29 has special properties; for example, women can ask men for dances, dates, and marriage today, but many of them do anyway in today's egalitarian age, so that does not make any difference. The only difference now is that women may ask women for dances, dates and marriage, and men may ask men.

So how often does this day occur? In the original Julian calendar, any year that was divisible by 4 was a leap year. That means the years 0, 324, 1776, and 1900 were leap years, and 1801, 1946, and 2003 were not. This assumes the year is 365.25 days long. It isn't. The year is no nice multiple of the day in length, and the number of days in a year continually changes due to variations in the motions of the Earth, Sun, Moon, and other planets. It comes close to being 365.242199 days long. This is a tiny fraction, but over centuries, this added up to 10 days. So Pope Gregory XIII ordered his astronomical operations research analysts to come up with a way of correcting the situation. They decided that from now on, century years such as 1900 and 2000 were leap years only if they were divisible not only by 4, but by 400. Since the Pope thought that March 21 as spring was ordained presumably by God, he would not change that, so he ordered 10 days chopped from the calendar. 1582 Oct 4 was followed by 1582 Oct 15, causing massive accounting headaches and a few rebellions ("give us back our 10 days!"). He could have left the beginning of spring at March 11. But for once the Church and the Pope came up with a good decision. However, since the Pope was associated with it, Protestant nations such as England would have nothing to do with it and Britain did not convert until 1752, chopping 11 days out of September, including September 11.

The resulting Gregorian calendar has done well for us. The most unusual Leap Year Day in 2000 came by and went without any Y2K or any other trouble. This year's Leap Year Day is only an ordinary Leap Year Day, but still it is unusual enough to notice. There are two proposals for ensuring accuracy in the millennia ahead. Herschel would make 0, 4000, 8000, years, divisible by 4000, common years, as by then the Gregorian calendar would be off by a day. That has the disadvantage of being unstable at year 0. First it is a leap year, now it is a common year. Another change, the New Orthodox calendar, would from now on make a century year a leap year if and only if it leaves a remainder of 200 or 600 upon division by 900. This is more accurate than the Gregorian calendar and even the Herschelian calendar. 2400 is a leap year in both calendars, so the first difference is 2800 is a leap year in the Herschelian calendar, but 2900 is the leap year in the New Orthodox calendar. But that is for people living 750 years or so from now to figure out. Right now our calendar satisfies our needs, even if February gets short-changed, leap year or no.