Why Do We Have Leap Year?

Chaos, basically, without Feb. 29. Thank goodness for Julius Caesar, Pope Gregory XIII, and leap years to keep our calendar from misbehaving.

"If we didn't have leap years, our calendar would be totally scrambled," says Geoff Chester of the U.S. Naval Observatory in Washington, D.C., the nation's timekeepers. "Otherwise, the year would be totally out of sync with the seasons."

Why? Basically the Earth doesn't circle the sun in exactly 365 days. Rather, it takes 365.2425 days, instead of a nice round number, says physicist Dennis Duke of Florida State University in Tallahassee. Those extra digits add up to almost a full day, but not quite, every four years.

Since the Roman ruler Caesar first added a leap day to the calendar every four years around 46 B.C., "with some help from astronomers," Duke says, roughly 500 leap days have come and gone.

That still wasn't perfect, because the Julian calendar was adding too many days.

"By the 1500s, Easter was slipping back into winter, about 10 days off," says John Lowe of the federal National Institute of Standards and Technology's office in Boulder, Colo. So, the modern calendar used today dates to 1582, when Pope Gregory XIII instituted the Gregorian calendar, with its first leap year of 1584.

Under its rules, every year that is evenly divided by 4 is a leap year, except for turn-of-the-century ones ending with an "00" such as 1900 or 2100, that aren't divisible by 400.

"Without this extra adjustment, the seasons would gradually slip out of adjustment with the calendar," Duke says. "But at a pretty slow rate — about eight days every 1,000 years."

Today, about 5 million people worldwide share a Feb. 29 birth date, making them "leapsters" or "29ers," according to the Honor Society of Leap Year Day Babies.

"It's nice to be really different," says Bailey Cox of Merritt Island, Fla., who will be 2 or 8 today, depending on how you see things.

Either way, she'll be bringing cupcakes today to her classmates at Lewis Carroll Elementary School in her hometown.

Contributing: Britt Kennerly, Florida Today