Leap Year 2012: The Science And History Behind Feb. 29 (VIDEO)

02/29/2012 11:40 am ET

Once every four years in February, we're rewarded with a little gift: an extra day at the end of the month before we have to pay the rent. And for about 4 million people around the world born on Feb. 29, the date is a doubly special occasion as it's been four years since they've been able to celebrate the anniversary of the actual day they were born.

But why exactly does February get an extra day every four years? Luckily, a video posted to YouTube yesterday answers some common questions about the day and why "Leap Years" are necessary. But first, let's take a look at the historical context of the 366-day-long year.

Why do we need leap years?

In short, the purpose of adding an extra day to February every four years is to sync up the 365-day Gregorian calendar with the solar calendar, which is 365 days, 5 hours, 48 minutes, and 46 seconds long -- the (near) exact amount of time it takes the earth to revolve around the sun.

Sure, ushering in the new year 5 hours before the earth has fully revolved around the sun doesn't seem like a huge deal, but over time it starts to add up. So much, in fact, that every four years our calendar is about one day behind where it should be to reflect the earth's actual position in its journey around the sun.

So that's why we need an extra day in February. Otherwise, after enough years had gone by, we'd be celebrating Christmas in July.

Why is the extra day added in February?

Leap Year was "invented" back in the first century BC, when Julius Caesar and his team of astronomers noticed that their 355-day Roman calendar had somehow slipped out of sync with the seasons. Caesar and his astronomer Sosigenes did some calculations and came up with a 365-day calendar that would sometimes add an extra day to the last month of the year, which was February. That became known as the Julian calendar.

Where did our current calendar come from?

The Julian calendar worked for a while, but something still wasn't adding up, and by the 16th century the calendar and the seasons were off by about 11 days. So in March of 1582, Pope Gregory XII moved up the calendar by 11 days and teamed up with astronomers to formulate a new calendar. Surely, it must have been strange for them to go to bed on March 11 and wake up on March 22.

The Pope's astronomers discovered that in fact, the Julian calendar was about 11 minutes too long. So they instituted a slight change that would prevent the calendar from getting ahead of itself over time. According to the new rule, a century year could only be a leap year if it was divisible by 400. Thus, while 1900 was not a leap year, the year 2000 was. This small change formed the basis for the Gregorian calendar used today.

Okay, so is the Gregorian calendar good to go?

Not exactly. Though the Gregorian calendar put the calendar year extremely close to the solar year, it's still a little bit off. But it will take about 3,000 years for this slight difference to add up to an extra day in error.

So astronomers have a few years to figure out how to deal with that one.

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