Why Is There A Leap Day? Explaining With Science And History

June 7, 2019 | Jamie Hayes

Why Is There A Leap Day? Explaining With Science And History

Our system for keeping track of the date and time is far from perfect. For instance, almost every child, at some point, has learned that the year has 365 days—except for leap years. Those have a 29th day added to February, bringing us to a grand total 366 days, once every four years.

Like so many of our world’s weird quirks, many children will ask “But…why?” when confronted with this information. And, like so many of our world’s weird quirks, the simplest answer to the question “Why is there a Leap Day” is “Just…because.”

There’s an answer, but to really understand it you need knowledge of history, mathematics, and astronomy.  So, if your kid simply will not take “Because” for an answer, first of all, congratulate them on their curious mind, then sit them down, put on a pot of coffee, and tell them this story.

Why Do We Have Leap Years?Getty Images

If Only it Were So Simple

It all comes down to the fact that this dumb rock we’re all spinning on just refuses to make things easy for any would-be calendar architects. In a perfect world, the amount of time it takes for the Earth to spin around its axis (a day) would be nicely related to how long it takes to spin around the sun (a year). But it's not.

To be fair, it’s close. People in Ancient Sumeria divided the year up into 12 months of 30 days each, which would bring the year to a nice round 360 days. In fact, it’s believed that it was this (optimistic) division of the year that led us to use 360 degrees to denote one full rotation.

Unfortunately, that nice, convenient 360-day year didn’t quite add up, and people were fairly quick to notice. After all, if your year is five days too short, it’ll be off by an entire month after just six years, and soon enough, you’re celebrating the Fourth of July in the dead of winter.

Why We Have Leap DaysUnsplash

Getting Warmer

The Ancient Egyptians calculated that a year was more like 365 days. But, since they liked the 12 X 30-day month calendar, they came up with a simple solution‚ and it was pretty amazing. They kept the same calendar, but they added five days of partying to the end of every year. This brought them to the 365 day year and left them with a dating-system that worked a whole lot better than anything that came before it.

GettyImages-483100235-808x1024.jpgGetty Images

But while it worked better, that doesn’t mean it worked.

Astronomers around the world were starting to realize that one solar year, ie. the time it takes the Earth to rotate around the sun, is much closer to 365.25 days. That may not seem like much, but that extra quarter of a day can really throw things off (eventually we'll be back to the winter in July dilemma). We still had a ways to go before we had a calendar that didn’t wander from the seasons.


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He Came, He Saw, He Made a Better Calendar

The Egyptian calendar was about the best people had for centuries, and many cultures copied it. But its imprecise nature became more and more obvious with each passing year, and so in the first century BC, none other than Julius Caesar took it upon himself to set it straight.

Caesar brought in some of the best mathematicians that the Classical world had to offer to develop a calendar that didn’t eventually leave us celebrating the first day of spring in October. When they had come up with the Julian Calendar, he made it the official dating system of the Roman Empire.

All he had to do was make that year last a whopping 445 days in order to reset the clock, so to speak. The extra-long 46 BC then became known as the Year of Confusion, but it made the winter months happen in winter again, the spring months in spring, etc. Caesar then instituted the leap year, where a 366th day would be added once every four years, solving the seasonal drift forever!

Not quite…


Not Enough Leap Year

Remember how I said that the Earth refuses to make things easy for us? Well, certainly a 360-day year would be too easy, but it turns out that a 365.25 day year is still too simplified. Don’t blame Classical scholars for not nailing it exactly, but the Earth actually takes 365.24217 days to orbit the sun (any children still paying attention at this point deserve some serious kudos, by the way).

Now, we’re talking about tiny, tiny fractions of a day. That couldn’t possibly throw off the Julian calendar that much, right? Well, unfortunately, time just stubbornly marches on and on, years pass, and after just a few measly centuries, Caesar’s genius calendar was starting to look pretty darn flawed.

Under Caesar’s system, a calendar year was about 11 minutes shorter than a solar year. That meant that every 128 years, the calendar would be one full day off. It was a gradual change, but by the 16th century, the Julian calendar was 10 days askew. 



Christmas in January

10 days might not seem like a huge deal, especially considering it took over 1,500 years to get to that point, but the Catholic Church at the time was pretty displeased with the discrepancy. The flawed calendar made it obvious that important Christian holidays like Easter and Christmas were not occurring on the days they were meant to.

Why We Have Leap DaysGetty Images

Enter: Pope Gregory XIII. Unwilling to allow the Church’s holy days to be improperly observed, he commissioned yet another new calendar—and this time, we got it right (mostly).

In 1582, the Gregorian calendar was unveiled. It worked pretty similarly to its Julian predecessor, but with one key difference: On years divisible by 100, we would skip the leap year—unless that year was also divisible by 400 (this is the point where I expect even the most academic and curious children to have passed out from boredom).

To put it simply, we don’t do leap years on most of the 100s, (so 1700, 1800, 1900) but we do every four centuries (1600, 2000, 2400). This calendar, which accounts for a year that is 365.245 days long, is what we continue to use to this day.

Leap Days Forever

Now, if your child is, by some miracle, still listening, and managed to notice that I said that a year is 365.24217 days long, not the 365.245 assumed by the Gregorian calendar, they might now be asking you “Won’t this calendar eventually wander just like all the others?” I highly, highly doubt they are, but hey, it’s a good question.

The answer is yes. That’s right, our hallowed Gregorian calendar, that we’ve used for centuries, will eventually drift off from the solar year—but don’t worry about it too much. It’ll be another 3,300 years before it’s even off by a day.

And that’s not even considering the leap seconds that we occasionally add to make up for anomalies in the Earth’s rotational axis. As I said, this dumb rock we’re all spinning on just refuses to make things easy us.

So that’s why we have leap years. Now, that pot of coffee is surely empty and your child has long since fallen asleep, but now you know the answer. And, if you’re lucky, your kid might end up taking “Just…because” as an acceptable answer next time they ask you a tough question, though if they ask "Why is the sky blue?" or "Why do cats purr?", we've got you covered there.

Why We Have A Leap Day


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