# Neil deGrasse Tyson Explains Why Leap Day is More Than a 4-Year Event

Today is February 29, a magical day that only really exists in our imagination…I think.

Okay, we all know how this works. The earth’s rotation is not really 365 days, it’s slightly longer, so every four years we add an extra day to our shortest month (cause god forbid we freaking add it to June or July), and presto! The calendar is all set.

Except it’s not that simple, and to truly make things right, we don’t need four years, we need four hundred. In this video from National Geographic, the world’s coolest astrophysicist, Neil deGrasse Tyson, explains how and why we have Leap Days, and why sometimes we don’t.

It does take roughly 365 and 1/4th days for our planet to go around the sun, hence the extra day every four years. That correction predates our current Gregorian calendar; it dates back to the Julian calendar. However, a Leap Day actually overcorrects, because that roughly 1/4th of an extra day every year is “a little bit less.”

“So you wait four years, you put in a whole day, then you put in too much,” explains Tyson, “So, you have to wait to put in a whole day too much, then you take out a Leap Day that would have otherwise appeared.”

That’s right, eventually we skip a whole Leap Day, once every 100 years (a “century year”), because we’ve overcorrected by an entire day. This is why 1700, 1800, and 1900 didn’t have Leap Days.

Got it? Good, because there’s more.

When we take out that “century” Leap Day every year 100 years, we overcorrect, again, but in the other direction. So every four hundred years, that whole skip-the-century-year rule is thrown out, and the Leap Day is put back in.

“This sounds complicated, but it has profound implications.”

He’s absolutely right. That means every 100 years, unless it’s divisible by 400, we won’t have an excuse for our favorite February 29 tradition.

Just kidding. We don’t need an excuse to watch those two kids get together.

Anyway, it’s actually kind of cool to know that today isn’t just a part of some small equation every four years, but a small part of a much bigger one that takes centuries to work itself out.

What other calendar quirks interest you? Leap into our comments section below to let us know.

Featured Image: National Geographic
Poster: Universal Pictures