What is a Leap year?

Wednesday is Feb. 29, or leap day, a date that happens only once every four years (mostly).

What is a leap year?

A leap year is a year in the Gregorian calendar, the one we use, with one extra day added to the end of February, making the month 29 days long. The day was established to keep the seasons and the calendar in sync.

A year is measured by the amount of time it takes the sun to return to the spot directly above the Earth's equator. Although it actually takes 365.2422 days, we round it to 365. But this results in an error of 0.2422 days, or almost six hours, a year. Without leap day, this calendar would be more than 24 days ahead of the seasons after 100 years.

Years that end in "00" are leap years only if they are divisible by 400. So 1900 was not a leap year, but 2000 was.

Why February?

Julius Caesar is said to be the "father of leap year." Ancient Egyptians created the basis for the modern-day calendar. But by Caesar's time, it had slipped out of sync with Earth's seasons. The 355-day Roman calendar called for an extra 22- or 23-day month every two years to keep the seasons on track. He decided to create a 365-day calendar, but the actual calculations were made by Caesar's astronomer, Sosigenes. He added one extra day to every fourth February. Why February? It was the last month of the year in Roman times.

What's a leapling?

Someone born on Leap Day might be called a "leapling." They usually celebrate their birthdays on Feb. 28 or March 1 when it's not a leap year.

Celebrity leaplings include actor Dennis Farina, who turns 68 on Wednesday; motivational speaker Tony Robbins, 52; actor Antonio Sabato Jr., 40; and rapper Ja Rule, 36.

Is it a curse?

Greek superstition holds that bad luck will come to couples who marry during a leap year. Supposedly, one in five engaged couples in Greece will avoid having their weddings during a leap year.

By the numbers

■ Odds of a leap birthday are 1 in 1,461.

■ According to the 2000 U.S. Census, there are about 200,000 Americans born on Feb. 29 and 4.1 million people worldwide.