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Monday, February 29, 2016

The Surprising History Behind Leap Year

Written by Brian Handwerk

It's that time again: Today, 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.

The evidence lies in a long history of wildly shifting dates and accompanying civil, agricultural, and religious chaos.

That's why 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.

“We've made a calendar that comes close,” Lowe says, “but to make it work you have to do these leap day tricks that have some quirky rules.”

Ancient Timekeeping
Efforts to make nature's schedule fit our own have been imperfect from the start. Some ancient calendars, dating to the Sumerians 5,000 years ago, simply divided the year into 12 months of 30 days each. Their 360-day year was nearly a week shorter than our annual journey around the sun. (Also see "Where Our Fear of Friday the 13th Came From.")

The practice of adding extra days to the year is at least as old as these 360-day systems.

“When the Egyptians adopted this calendar they were aware that there was a problem, but they didn't add any more days to the calendar,” says Lowe. “They just added an extra five days of festivals, of partying, at the end of the year.”

Earlier Egyptians (prior to about 3100 B.C.) and other societies from China to Rome once used lunar calendars to track time. (See National Geographic's moon facts.)

But lunar months average 29.5 days and years only about 354. So societies that kept lunar time quickly drifted well out of sync with the seasons due to the 11-day lag.

The Romans regularly tried to tweak this calendar by irregularly adding days or months, but those patchy efforts only highlighted the need for reform.

"Year of Confusion"
By the time Julius Caesar enjoyed his famed affair with Cleopatra, Rome's calendar had diverged from the seasons by some three months. But Egypt was observing a 365-day year, and as early as the third-century B.C. had even established the utility of a leap-year system to correct the calendar every four years.

Julius adopted the system by decreeing a single, 445-day-long Year of Confusion (46 B.C.) to correct the long years of drift in one go. He then mandated a 365.25 day-year that simply added a leap day every fourth year.

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