Why do we have leap years?

09 June 2002

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Question

Why do we have leap years?

Answer

the reason is that a year - the time it takes the earth to complete its journey around the sun - doesn't actually have a full number of days in it - in other words there is a small amount of extra time left over at the end of the year. So we add an extra day every 4 years to keep things synchronised. But, this still leaves a small error which calendar-makers correct for by skipping a leap-day 3 times every 400 years ! To work out which ones to leave out if the first 2 digits of the year are not divisible by 4 then they are skipped - so in the last 400 years, 1700, 1800, 1900 were NOT leap years (because 4 doesn't go into 17, 18 or 19), but 2000 was.

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