Yesterday was leap day, so I thought it would be fun to take a closer look at why the leap year is the way it is.

And since so much thought is being given to the calendar, I'm having a sale on Day One, my updated and simplified approach to the classic day of the week for any date feat. Through Sunday, I'm selling it for 30% off - $6.99!

Now, let's get back to the leap year itself. Why do we even have it?

It would be nice if leap year didn't have to exist in the first place. The basic problem is that a day is measured by how long it takes the Earth to rotate once about its own axis, while a year is measured by how long it takes for the Earth to go around the Sun. These two measurements have no meaningful relationship to each other whatsoever, except for our desire to match them up.

C. G. P. Grey has a great analogy in this video, in which he asks you to imagine a ballerina doing repeated pirouettes on the back of a flatbed truck that's going around a closed track. Working out the relationship of the Earth's day to its year, then, is akin to trying to work out how long it will take the truck to complete 1 run around the track by counting the number of pirouettes the ballerina is doing.

The original Roman calendar had a length of 355 days, and occasionally had longer years of either 377 or 378 days to compensate for the errors. The problem with this approach was that the need for these years had to be calculated, and they weren't consistently applied throughout all parts of the Roman empire. If you wandered too far from home at this time, you couldn't be sure what day it was!

It was our old friend Eratosthenes who was behind the first push for a 365-day year, with a 366-day year every 4 years. However, the change wouldn't be made for almost another 200 years by Julius Caesar. The Egyptians were already using a 365-day calendar at that time, and the Romans could appreciate the wisdom of having a calendar that didn't require regular re-calculation, thus making standardization much easier.

So why did we need another change? Numberphile's video explains the astronomical problems simply in this video:



If I asked you what an astronomer is, you would reply probably reply that it is someone who studies the objects and behaviors of bodies in outer space. However, the original definition of an astronomer was someone who studied the movement of the Sun and the planets, in order to work towards a more accurate calendar.

After the fall of the Roman empire, it was the Catholic Church who had the major influence over society, including the calendar. One of their biggest celebrations was Easter, but this caused a few problems.

The first problem, of course, is that the Julian calendar was already drifting off by 1 day every 400 years. The next problem was the definition of Easter itself: It was to take place on the first Sunday after the first full moon after the vernal equinox (beginning of spring). See the problem? Now the orbiting of the moon must be thrown into the mix, as well! Even worse, that was only one possible definition of Easter, and there were many others.

The original solution was to try and approximate the correct date for Easter with a 19-year cycle. There was a way to make sure this stayed in sync with the Sun and the moon, but it could only be checked every 312.5 years!

By the 1500s, the years, cycles, and calculations for holidays were so out of sync, reforming the calendar became a major concern. The challenges to the church authority by Martin Luther provided the church with the perfect opportunity to put such large-scale reforms in place.

There was one more subtle problem that no one expected. Remember when I mentioned that the original definition of an astronomer as someone who was working towards a more accurate calendar? More accurately, that definition was the study of the movements of the Sun and the planets around the Earth, in order to work towards a more accurate calendar.

This assumption of the planets and the Sun going around the Earth was so basic, it wasn't something you'd ever really question. Unfortunately, since the calendar reform required taking a closer look at this motion, the discovery of the orbit caused a few problems.

As James Burke explains in the “Infinitely Reasonable” episode of The Day The Universe Changed, this caused it's own problems:



As you might expect, the calendar reform in 1582 didn't take hold everywhere all at once. Britain and its American colonies didn't adopt the calendar until 1752. Several countries, including Greece and Russia, didn't adopt this calendar until the 1920s!

That's only the nutshell version of the weird and twisted tale of how our current leap year system came to be.

The New York Times offers some fun Leap Year lessons you might enjoy. Some are geared towards a classroom, but others you can try out on your own, if you wish.