What in the World Is Leap Year?

Today, February 29, 2012, is leap dap, a day that appears on our calendars approximately once every 4 years on leap year. Which then got us thinking, what in the world is leap year?

Today, February 29, is a day that appears on our calendars only once every 4 years (aka leap year). But what the heck is leap year anyway? Is it a year where we magically jump forward in time? And what about people born on leap days? Since their birthdays only occur once every 4 years, does it mean they’re 4 times younger than the rest of us?

While we pride ourselves on having answers to the most complex insurance questions, when it comes to calculating time and space, we’re just as bemused as the next guy. So we did a little digging and here’s what we found.

How leap year works

Most of us are used to thinking of a year as 365 days. But in fact, a year on earth — the time it takes the earth to orbit around the sun — is actually 365 days, 5 hours, 48 minutes, and 46 seconds. Simply put, every year, we lose about 6 hours (just think of what you could do with that time!).

Over days, decades, and centuries, this misalignment could throw our lives off kilter. For instance, if we didn’t account for that quarter of a day every year, after just 4 years, our measurement of time would be off by a day. After 100 years, we’d be about 25 days behind, Christmas would occur in mid-January, our calendar of the seasons would be off, and we wouldn’t know the right time to plant certain crops and harvest others.

How often does leap year happen?

To give the world back its lost time and keep our calendar as accurate as possible, an extra day (technically known as an intercalary day) is added to the calendar every 4 years. So because 2012 is a leap year, it has 366 days instead of the usual 365.

The history of leap year

Julius Caesar, the guy who came, saw, and conquered, introduced leap day along with the Julian calendar in 45 BCE. Caesar’s decision to add an extra day to the 365-day calendar system was based in astronomy and not (as could be argued) the result of a megalomaniac desire to control both empire and time.

The old Roman calendar consisted of 10 months and 304 days. In a society that relied on the solstices and equinoxes to mark important religious holidays and manage the planting and harvest seasons, this lag in time eventually resulted in chaos.

So Caesar, acting on the advice of Greek astronomer Sosigenes, reformed the calendar to 365 days with an extra day added to February (which in Caesar’s time was the last month of the year) every fourth year.

But the system still wasn’t perfect. In fact, adding a leap day every 4 years made each calendar year too long by 11 minutes and 14 seconds. And as we know, this little drift forward in time can cause big disorder in the long run.

Then in 1582, Pope Gregory XIII introduced the Gregorian calendar — the calendar we still use today — to rectify this miscalculation. After doing some fancy Copernican math, Pope Gregory and his astronomers added a new rule to Caesar’s system to account for the drift in time: leap years should not occur in years ending in “00,” unless the year is divisible by 400. So though 2000 was a leap year, 1900 was not.

Leap year 2012

If all those numbers have you frazzled, don’t worry. Just know that thanks to the Gregorian reform, our calendar year and the solar year are only half a minute off and the movement of our calendar aligns (more or less) with the earth’s rotation around the sun.

So thanks, leap day, for giving us back our lost time and keeping us all on track. And for those people born on February 29, happy birthday, leaplings!

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