Why do we have Leap Years?

Ever wonder why we get that extra day every four years?

The math can be dizzying (see the video above) but those 24 hours are very important in keeping our calendar year synchronized with the trip around the sun.

Our modern calendar (the Gregorian calendar) contains 365 days. However, one orbit actually takes a slightly longer amount of time - about 365.2422 days. "Only a quarter of a day?" you may say, but over long periods of time it can add up. Without leap years, our calendar and the seasons would fall out of sync.

But it's not as simple as just adding the extra 24 hours and calling it a day (no pun intended!) There are still some rules in place to further keep things from getting out of whack. With the leap day, we average out to get an extra 11.2 minutes each year. To counteract that, we skip leap years if they fall on the start of a century - unless that year is divisible by 4.

With all rules in place, it would take us over 3,300 years to get a day off, so at some point way down the line this will need to be further addressed.

Calendar length has been a point of contention for thousands of years. While his math was a little off, Julius Caesar was the one who actually introduced the concept of the leap year in the hopes of avoiding the calendar confusion that had existed up until that point.

Before that, many ancient calendars (Hebrew, Chinese and Buddhist to name a few) used to have an entire leap month!  As opposed to the modern calendar that indicates our position relative to the sun, these calendars also took into account the position of the Moon.

It probably goes without saying that being born on a leap day is pretty rare. There are only about 5 million people living that were born on February 29 - roughly 0.07 percent of the world's population! The odds of this are about 1-in-1,461.

You may have also heard about a leap second. Earth's rotation can occasionally be sped up or slowed down  - ever so slightly - due to geological events like earthquakes and volcanoes. Much like the way a leap year keeps accounts for a fractionally longer orbit, the leap second is periodically added on either June 30 or December 31 to compensate for when the rotation slows down. By doing this, we keep everything in sync with our atomic clocks.

Test your knowledge on leap years below!

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