2020 is a leap year: What is this? And why do we skip leap day every 100 years, and other questions answered

Every four years, February gets an extra day. And it isn’t just because we feel bad for the little guy since it usually only has 28, either.

If you’ve ever wondered, “What’s the deal with leap years?”, don’t worry. We’ve got some answers.

Why do we have Leap Years?

Well, we know that a year is the length of time that it takes for the Earth to revolve once around the sun. In that span, the Earth fully rotates on its axis 365 times. Each rotation is 24 hours, i.e. one day. That’s all pretty straightforward, right? But here’s where it gets tricky. The Earth actually rotates slightly more than 365 times in the span of a year. Four years worth of extra rotation adds up to one extra full rotation, and thus one extra day.

So the Earth rotates exactly 365.25 times per year?

Nope! That would make things too simple. The Earth actually rotates something like 365.24 times in the span of one year. A full extra day every four years is actually a little bit too much.

Is that enough of a difference to affect the calendar?

In the short term, no. But over a few decades, it becomes noticeable - that’s a one day difference every one hundred years. In the time since we’ve adopted the current Gregorian calendar in 1582, we’d have an extra four days to add somewhere. After a few more centuries, our calendar would be out of alignment with the current seasons, and we’d be celebrating Thanksgiving in the summer and none of the pumpkins would be ripe and that would be chaos!

How does that get balanced out?

Thankfully, the Gregorian calendar does have a built-in fix for this problem. Every 100 years, we don’t have a Leap Year. In the years 1800 and 1900, for instance, there was not a Feb. 29.

Does that fix the problem?

Again, not exactly. Though removing those Leap Years once a century helps, it still leaves you with one day every 400 years. And to address that hiccup, we do have a Leap Year if the century is divisible by 400. That explains why, if you remember, we did have a Leap Year in 2000. But when 2100 finally rolls in, we will not.

So the problem is finally and completely solved there, right?

Of course not! This only means our calendar is off by one day every 8,000 years or so. But I think that’s awfully darn close. Plus, the Earth’s rotation is actually slowing by a minuscule amount, so in a few thousand years the whole thing will be off by another couple of seconds anyway.