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Q. Is 2000 a leap year or not? — Lucretia McC.
A. A calendar year is 365 days long, unless the year is exactly divisible by 4, in which case an extra day is added to February to make the year 366 days long.

If the year is the last year of a century -- such as 1700, 1800, 1900, 2000 -- then it is only a leap year if it is exactly divisible by 400. For that reason, 1900 wasn't a leap year but 2000 will be.

The reason for the rules is to bring the average length of a calendar year in line with the length of Earth's orbit around the Sun, so that the seasons always occur during the same months each year.

Learn more about calendars and leap years: Royal Greenwich Observatory  »»

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