NASA: National Aeronautics and Space Administration

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  1. Question

    What does the earth's orbit have to do with leap year?

    The issue is not the orbit exactly, but the length of time it takes the Earth to circle the sun. This time, the length of the year, is not an even number of days. There is no reason in nature to expect the length of the year to be divisible evenly by the length of the day, but this disparity has created a challenge for calendars throughout history. The ancient Romans figured out that there are very nearly 365.25 days in a year, so they invented the system of leap years, with an added day every fourth year so that the average would be 365.25. During the renaissance this was made more precise by omitting one leap year every 400 years, giving us the "Gregorian" calendar that is now used throughout the world.

    David Morrison
    NAI Senior Scientist

    October 14, 2008