First things first: In case you forgot, let this article serve as a helpful reminder that 2020 is a leap year – meaning we get an extra day at the end of the month (February 29th)!

Now, if you’re like most of us, you’ve probably never really given the leap year a second thought.

And, sure, it’s probably not the first thing you think of when you think about “innovation” – but trust us, this one’s actually quite an important one… and more complicated than you think!

In this article, you’ll learn why we have leap years (and what would happen without them). We’ll do our best to keep it simple, but fair warning: there are a lot of numbers ahead.

Why do we have leap years?

The simple answer is that one “real” year is not 365 days long.

In fact, one full year is actually 365.24219 days long – but we can round that number up to 365.25 (a much easier number to work with).

Why is this?

Well, as you probably know, a year is approximately the amount of time it takes Earth to complete one rotation around the sun. The precise time period of such a “real year” is 365.24219 days – which is called a “solar year”.

(This also explains seasons, by the way: Earth is slightly tilted, so at any given moment, about half the world is pointed towards the sun, while the other half is pointed away from the sun.)

Now, surely you can imagine the problem here. Our planet takes 365.25 days to revolve around the sun, but we can’t really add a “quarter day” to the calendar – or else night and day would swap before long!

Here’s a fun fact before we continue: Did you know that a day is not exactly 24 hours, either? That’s right – the length of a day actually changes throughout the year! However, the average length is 24 hours, so it’s much easier to keep math simple by using this number.

Okay, so a day isn’t 24 hours and a year isn’t 365 days. What now?!

Alright… so now we know a year isn’t exactly a year, and a day isn’t exactly a day. But what can we do to solve this?

Well, many suggestions have been proposed. We could have a “short day” added each year, that only lasts about 6 hours… but that would mess up night and day for the rest of the year!

We could also ignore it completely… but within four years, the calendar would be off by a day. After 120 years, we’d be a full month off – and before long, we’d be celebrating Christmas in the summer! (Although to be fair, that would mean our friends in the Southern Hemisphere would finally have a chance at a white Christmas…)

The answer is, of course, to add an extra day every four years – which we now call the “leap year”. But it didn’t come so easily.

Who invented the leap year?

The first leap year was in 45 B.C. – and it’s all thanks to Julius Caesar.

At the time, this seemed to solve the problem of our seasons slowly shifting. However, remember how the year isn’t exactly 365.25 days?

This still creates an extra eleven minutes per year… or one full day every 128 years.

By 1582, when Pope Gregory was in power, the extra time had added up quite a bit. But thankfully for our collective sanity, Pope Gregory had an innovative, relatively simple solution.

First, he removed ten days from the following year (to get it back in sync). Then, he proposed a new rule: centennial years (such as 1900, 2000, etc.) cannot be leap years… unless they are evenly divisible by 400. This calculation was based on a year length of 365.2425, which is only a tiny bit (0.00031) off from the actual solar year.

That means we’ll still be off by a day eventually… but it will take about 3,000 years for that to happen. Pope Gregory’s solution was so brilliant, they even renamed the calendar after him. (It makes us wonder: Does the person who fixes it in 3,000 years also get to rename it after him or herself?)

Alright! So now we have a near-perfect system. In fact, you might think that there’s no other way to do it – since this is so complicated. At least, that’s what we thought… but alas, we were wrong!

Does every calendar use leap years?

Simply put: Nope!

There are many different calendars, and while the Gregorian calendar (named after the Pope who fixed it) has come up with this innovative solution, others use their own.

For example, the Chinese calendar uses “leap months” – about every three years (7 times in 19 years), an extra “leap month” is observed.

In Iran, they use the Solar Hijri calendar. Instead of using math, they follow the equinoxes (the moment when the equator passes through the center of the sun) – making it one of the most accurate calendars in the world. They usually have a leap year every four years, like the Gregorian calendar… but every fourth “cycle”, they wait an extra year to add the extra day.

See – we told you it wasn’t so simple!

There’s actually a lot more happening when it comes to the calendar. It’s truly one of the greatest innovations of all time! However, our head hurts from all these numbers. So we’ll give you a break and end today’s article here.

If you’d like to know more about this (or any other innovative topic), just let us know in the comments below. We really enjoy hearing from you!