Friday, February 27, 2004

Extra day fine tunes our modern calendar



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Here's why you should learn to love Leap Day:

The Earth doesn't circle the sun in exactly 365 days. The cycle of the seasons, or "tropical year" as it's called, is more like 365.2422 days. Close enough, you say? No. Over a century, the calendar and seasons would move apart by about 24 days, if no correction were made. Which means spring in the northern hemisphere wouldn't begin until April 13.

Julius Caesar tried to correct the problem with his Julian calendar, introduced in 46 B.C. A leap day was added every four years, making the average length of a calendar year 365.25 days. That was an improvement, but over the course of 16 centuries the beginning of spring shifted by 12 days.

So in 1582 Pope Gregory XIII created what's now known as the Gregorian calendar. It isn't quite perfect, either. But it comes very close to synchronizing the calendar with the tropical year.

The Gregorian calendar introduces 97 leap days every 400 years. Years evenly divisible by 4 are leap years, except for centurial years not evenly divisible by 400. (So 1900 and 2100 aren't leap years, but 2000 and 2400 are.)

As a result, the average Gregorian calendar year is 365.2425 days long. Not perfect, but darn close to agreeing with the tropical year.

Using this system, it will be about 3,300 years before we're a single day out of synch with the seasons.

John Johnston

Source: U.S. Naval Observatory




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