Why Do We Have Leap Years?

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Sundial in the evening sun

The actual time between two yearly solar events isn't 365 days exactly. It's actually 365.2422 days — so every four years there's approximately one extra day left over. iStockPhoto.com hide caption

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Pope Gregory i

Pope Gregory XIII, circa 1583, introduced the reformed Gregorian calendar. Hulton Archive/Getty Images hide caption

itoggle caption Hulton Archive/Getty Images
Pope Gregory

Pope Gregory XIII, circa 1583, introduced the reformed Gregorian calendar.

Hulton Archive/Getty Images

This year is a leap year, with an extra day added at the end of the month of February to help the modern Gregorian calendar sync up with the astronomical calendar. The adjustment is needed because the actual time between two yearly solar events isn't a clean 365 days. It works out to be 365.2422 days — resulting in almost one extra day every four years.

Of course, since it's not quite one full day, the rules for leap years are a little more complicated than "every fourth year." If a year is divisible by four, it's a leap year — unless it's also divisible by 100, in which case it's not a leap year. But if the year is divisible by 400, then it is a leap year. (Meaning that while 1800 and 1900 were not leap years, the year 2000 was.) Confused yet?

So on Friday, Feb. 29 — a date that occurs only once every 28 years — Thomas O'Brian, the federal government's chief of time and frequency, talks with guest host Joe Palca to explain leap years, leap days — even leap seconds — and the finer points of telling time.

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