Friday, February 29, 2008

Happy Leap Day

I couldn't let the first Leap Day of my blogging career pass unremarked. A year is approximately 365¼ days long, so about every 4th year, an extra day is added to the calendar. But that number is actually closer to 365.242374 days than 365.25. For that reason, century years are not leap years unless they are divisible by 400; thus, 2000 was a leap year, but 1900 was not and 2100 will not be. That modification — the difference between the Gregorian and Julian calendars — results in an average year of 365.2425 days. At that rate, the calendar will be off by a full day in a bit less than 8,000 years, except of course that the astronomical year is gradually getting a little bit longer, just as the day is gradually getting longer.

The day is slowly lengthening, caused primarily by friction as the moon's gravity pulls on the earth's oceans, causing tides but also slightly slowing the earth's rotation. For that reason, it is occasionally necessary to add a "leap second." On those occasions, determined by detailed and precise measurements of the earth's rotation, the normal sequence is modified slightly. Ordinarily, 23:59:59 is followed one second later by 00:00:00 of the following day; when a leap second is added, 23:59:59 is followed by 23:59:60 and then 00:00:00. The last leap second was added at the end of 2005; it will probably be at least a couple of years until the next one, so you won't be able to use leap second as an excuse for being late to work.

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