February 29, 2000

Today is Leap Day. It is also Venerable Bede Day, which may require some explanation.

We all know that there are 365.24219 days in a year, or in other words nearly a fourth of day more than 365 days, even. So, every fourth year we add a day, leap day on February 29th, to make up for the “missing” time and keep our calendar in alignment with the earth’s orbit around the sun.

However, in 730 AD a monk, Venerable Bede, accurately determined that a quarter-day is too much, 11 minutes and 14.78 seconds too much, to add every year. While that may not seem like much, over time it does add up. Over the run of 400 years, it makes 3 days, 2 hours, 53 minutes and 20 seconds too much.

Inevitably, the Julian calendar – which came up with the leap day idea – grew farther and farther off until by the late middle ages it had accumulated eleven days of error. Then in 1522 Pope Gregory XIII authorized a new calendar, known since as the Gregorian calendar. This calendar takes out 3 days every 400 years by adjusting the rule of leap year. Exactly how three days are removed is to declare that leap year falls on every year divisible by 4, except those years which are divisible by 100 but not 400. That is, in any century, ’04, ’08, ’12, …, ’88, ’92, and ’96 are all leap years, but ’00 is not. However, ’00 *is* leap year in the years 1600, 2000, 2400, ….

Since the Gregorian calendar went into effect in the 16th Century, 1600 was the first time this rule was invoked (note that 1600 would have been a leap year anyway under the Julian calendar). Subsequently, the years 1700, 1800 and 1900 were not leap years. This year, 2000, is the only the second occurrence of leap year on a “400” year in the history of the Gregorian calendar.

What about that remaining error? Those 2 hours, 53 minutes and 20 seconds, or 10400 seconds, accrue over a period of 400 years. This means the Gregorian calendar does not match the earth’s orbit around the sun by a rate of 26 seconds ever year, or less than 1 second every two weeks. In fact, only very recently could mankind ever measure time so accurately; previously no time keeping machines were accurately enough over long periods of time to be able to measure this small amount of error. Now, with atomic clocks such accuracy is possible, and when detailed time measurements reveal the world reference time standard is out of alignment with the earth’s orbit, “leap seconds” are added as needed. Since this really matters to only the scientific community, it occurs without fanfare. Leap seconds are added now and then and most of the people on earth are unaware of it.

We all see many leap years, and most of us will see leap-exception years (on multiples of 100’s). Some may even see two years in which leap year is put in abeyance, but there are none here today who will be around to see the next Venerable Bede Day, which occurs February 29th, 2400.