History Behind Leap Year

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History Behind Leap Year

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Most of the modern world has adopted the Gregorian calendar and its leap year system to allow days and months to stay in step with the seasons.
Photograph by Abraham Nowitz, National Geographic
By Brian Handwerk

PUBLISHED February 26, 2016

It's that time again: This Monday, February 29, is a leap day, the calendar oddity that occurs (almost) every four years.

For centuries, trying to sync calendars with the length of the natural year caused confusion—until the concept of leap year provided a way to make up for lost time. “It all comes down to the fact that the number of Earth's revolutions about its own axis, or days, is not equal to or connected in any way to how long it takes for the Earth to get around the sun,” says John Lowe, leader of the National Institute of Standards and Technology (NIST)'s Time & Frequency Division. The solar, or tropical, year is approximately 365.2422 days long. No calendar comprised of whole days can match that number, and simply ignoring the seemingly small fraction creates a much bigger problem than one might suspect.

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