How Often Does a Leap Year Happen? Not Every Four Years, That’s for Sure

Thinking leap year happens every four years is a common mistake.

Generally, that’s true. However, there’s a little more science behind leap year that you may not know about. We’re here to clarify some things for you.

So how often does a leap year happen?

Unless you were alive in 1900, you probably haven’t noticed that leap years don’t occur on century years such as 1700, 1800, 1900 and so on. But here’s the twist, leap days do occur on century years that are divisible by 400. It’s a lot of math, but all you really need to know is that 1600 and 2000 were leap years but 2100 will not be.

So every so often, leap years happen every eight years.

But, the amount of times a leap year occurs starts with the history of the leap year.

How did the leap year begin?

For the past 400 years, we’ve followed the Gregorian calendar, which syncs up with how long it takes for Earth to orbit the sun once. Since a calendar year is 365 days, then it must take 365 days for Earth to orbit the sun once right? Well, not really. It takes approximately 365.2422 days, which translates to 365 days, five hours, 48 minutes and 45 seconds.

To make up for the extra .25 hours of our day that we lose everyday, we added an extra day every four years.

On the other hand, the Roman Calendar had 355 days with an extra 22-day month every two years. Sounds ridiculous right? Well, Julius Caesar seemed to think so too. So, he made his astronomer come up with something better. So, Sosigenes of Alexandria came up with the idea of changing the calendar to 365 days a year with an extra day added every four years.

Caesar implemented this suggestion in 46 B.C.

Then, 500 years later, Pope Gregory XIII came along and his astronomers decided to tweak the calendar to make it more accurate so they agreed to lose three days every 400 years.

The Gregorian calendar was introduced in 1582.

So now we know why leap years exist, but did you know a leap second also exists?

What’s a leap second?

According to NASA, the last day of June or December is a second longer than every other day due to Earth’s rotation. The average length of a day syncs up with how long it takes for Earth to rotate.

By our time standard, each day is 86,400 seconds. But, it takes Earth 86,400.002 seconds to rotate. This is due to the fact that Earth’s rotation is slowing down because of a “braking force caused by the gravitational tug of war between Earth, the moon and the sun.”

We would have to go into a full-on astronomy lesson to explain that to you, but, ultimately, Earth’s rotation is gradually slowing down.

So, that .002 a day seems irrelevant right? Since its repeated every day of every year, those .002 seconds add up to a full second. And since we’re all about making up lost time, an extra second is added to the end of June or the end of December every year. So instead of the clock moving from 23:59:59 to 00:00:00, it goes from 23:59:59 to 23:59:60 to 00:00:00.

I know, wild stuff.