for National Geographic News
Friday, February 29, marks leap day, a day that's added to the month of February almost every four years—that is, every leap year—to keep the modern calendar in line with the celestial cycles that frame it.
But where did leap year come from? How does it work? And have other cultures, with their own systems for tracking time, needed to use it, too?
The short answer is: Yes. Think of leap year as the little trick the world uses to make up for lost time.
The Quarter-Day Conundrum
We observe the modern leap year because Earth orbits the sun every 365.242 days—not an easy number for a calendar to accommodate.
(See an interactive map of the solar system.)
As a result, many cultures since ancient times have taken on the practice of adding extra days, or even months, to round out the calendar year.
Early calendars were often based on lunar months, which average 29.5 days. But a year of such months totals only about 354 days.
This discrepancy resulted in annual events—like festivals, agricultural milestones, religious observations, and other important dates—drifting out of alignment with their intended seasons as the years passed.
"Civilizations like Rome would add months to try to correct the drift of the lunar calendar," said David Ewing Duncan, author of the book, Calendar: Humanity's Epic Struggle to Determine a True and Accurate Year.
But Duncan describes the Roman solution as "sloppy."
"It played havoc with everything from religious holidays to market times," he said.
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