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Leap Day

February 29, 2016

Today marks an oddity in the calendar that comes around once every 1,461 days: Leap Day, a day that’s thrown in to keep the calendar in sync with the seasons.

The concept of a leap day is as old as the calendar itself. The ancient Roman calendar, which was based on the motions of the Moon, threw in leap days, weeks, and months to make things balance. After centuries of such additions, though, the calendar was a mess. So Julius Caesar ordered the creation of a new one in the year 46 B.C.

The resulting Julian calendar consisted of a 365-day year, with a leap day added to every fourth year. That created an average of 365-and-a-quarter days per year. The true year, however, is about 11 minutes shorter than that, so the Julian calendar gradually drifted out of alignment with the seasons.

To correct that problem, in 1582 Pope Gregory XIII instituted a small update. Under the Gregorian calendar, three leap days were dropped for every 400 years. With this change, the difference between the calendar year and the true year adds up to just one day in 3300 years.

Incidentally, the name “leap year” comes from the fact that the extra day causes succeeding days to leap over a day of the week. For example, Independence Day fell on a Friday in 2014 and a Saturday in 2015. This year, however, the date leaps over Sunday and falls on Monday — a leap that helps keep the calendar aligned with Earth’s motion around the Sun.


Script by Damond Benningfield

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