What is a leap year?

0:47 Welcome to Maths is Not Boring by kidopoly.com. I'm Mira. And I'm Finn. Before we start today, a huge hello to six-year-old Shivanch in Australia. Hey Chevanch! We hear you are a Leap Year baby and you wanted to know all about leap years. So Shivanch, today's story is especially for you. And for everyone else.

1:11 If you haven't already, tap follow wherever you're listening so you never miss an episode. Okay, so leap years. That means February gets 29 days instead of 28, right? That is what it looks like on paper. But to see what a leap year really is, you have to imagine something completely impossible happening.

1:33 Impossible like what? Imagine you live on a farm. You watch the calendar closely, waiting for July, because that is when summer comes and the crops need sunshine. Makes sense. You have to know when to plant things. But when July arrives, the trees are bare, the rivers freeze over, snow starts falling from the sky.

1:55 The calendar says it is summer, but the world is freezing. Wait, what? Snow in July? Did the weather just break? The weather is fine! It is the calendar that is lying. Over hundreds of years, the months have silently slipped out of place. How can months just slip out of place? A calendar is just a list of days on a piece of paper.

2:19 To understand why, we have to look up from the paper and look out into space. Finn, what is a year, really? Oh, I know this. A year is how long it takes the Earth to travel all the way around the Sun. One big circle in space. Exactly. And how many days does that take? 365 days. That is what people thought for a very long time.

2:46 But about 2,000 years ago, around the year 46 BCE, a leader in Rome named Julius Caesar noticed a huge problem. Was he getting snow in the summer? Yes. The Roman festivals for the harvest were happening before the crops were even ready to be picked. The seasons were drifting away from the calendar. That would be so confusing.

3:11 Imagine waking up and not knowing what season you were actually in. Caesar brought in the brightest stargazers to figure it out, and they discovered that 365 days is wrong. Wrong? But we still use that number today. It is wrong because the Earth does not finish its lap around the sun at midnight on New Year's Eve.

3:34 It takes 365 and a quarter days. Wait, a quarter of a day? How can you have a quarter of a day? The sun either comes up or it doesn't. Picture this. It is midnight. The fireworks are going off for the new year. People are cheering. Okay, I am picturing it. Confetti everywhere. But up in space, the Earth has not actually reached the finish line of its orbit.

4:02 It still has six hours of flying left to do. So the calendar starts the new year before the earth is even finished with the old one. Yes, and six hours is one quarter of 24 hours. So every year the calendar runs a quarter of a day ahead of the Earth. Okay, but a quarter of a day is just a morning. That doesn't seem like enough to cause snow in the summer.

4:28 Not in one year, but what happens after four years? Four quarters makes one whole day. The calendar is now a full 24 hours ahead of reality. Oh, so if you let that keep going for hundreds of years, the calendar would be weeks ahead. The calendar thinks it is summer, but the earth is still in spring. You just figured out exactly what Julius Caesar figured out.

4:53 To fix it, he created the Julian calendar. He added one extra day to February every four years to hold the calendar back and let the earth catch up. The leap day. So that completely fixed it. Case closed. That is what people thought for over a thousand years. But here is the part that will really bend your brain.

5:15 Caesar was wrong. What? But four quarters make a whole. How could he be wrong? Because nature does not like neat little fractions. A year is not exactly 365 and a quarter days. Well, how long is it then? It is 365.2422 days. It is a tiny bit shorter than a quarter of a day. So Caesar's leap year added a whole day every four years.

5:45 But the Earth didn't actually need a whole day to catch up. Exactly. He added about 11 minutes too much time every year. Which means slowly, silently, the calendar started drifting again in the other direction. Over a thousand years. 11 minutes a year is a really big problem. By the year 1582, the calendar had drifted by 10 whole days.

6:13 So Pope Gregory XIII had to step in with an even cleverer rule. What was his rule? Gregory said we keep the leap years. But to fix those extra 11 minutes, we have to skip three leap days every 400 years. Wait, skip them? So sometimes a year that is supposed to be a leap year just gets cancelled? Yes, that is called the Gregorian calendar.

6:38 And every phone calendar in your house follows that exact rule today. But the change caused chaos at the time. Why? Did people not like the new maths? It wasn't just the maths. Different parts of the world switched at different times. Britain waited until the year 1752 to switch. But by 1752, the calendar would have drifted even more than 10 days, right?

7:03 11 days. To fix it, Britain had to instantly delete 11 days from reality. People went to sleep on September 2nd, 1752. And when they woke up the next morning, the calendar said it was September 14th. They just woke up and it was almost two weeks later. Imagine if your birthday was September 5th. You just wouldn't get a birthday that year.

7:28 Exactly. It sounds like magic, but they were just snapping the paper calendar back into line with the giant rock we live on. Alright, Finn, quiz time. Let's see what you remember. Okay, I am ready. Lay it on me. First question. What do we call the special year that has one extra day added to it? Next one.

7:52 How long does the Earth actually take to orbit the Sun? Down to the fraction? Question 3. Who was the Roman leader who first added a leap day about 2,000 years ago? Here is number four. What was the name of the Pope who made the clever 400-year skip rule? Last question. In 1752, what date did people in Britain wake up on after going to sleep on September 2nd?

8:33 Great job, let's see how you did. The answer to the first question is a leap year. For question 2, the earth takes 365.2422 days. The third answer is Julius Caesar. For number four, the Pope was Pope Gregory the Thirteenth. And the final answer is September 14th. It is so wild to think about deleting 11 whole days just to make sure a piece of paper matches a giant space rock.

9:10 I know! It just shows how hard humans have worked to map out something as huge as the Earth's orbit. Hey, if you love having your mind bent by stories of skipping 11 days, drop down and leave us a five-star review. Just scroll down and tap the stars. And make sure you tap follow so new episodes turn up on their own.

9:32 Oh, and we have sister shows too. History, science, and philosophy is not boring. Go give those a listen and a follow as well. And if you want a shout out on the show, like Chivanch, or just want to say hi, email us at hello at kidopoly.com. Thanks for exploring with us today. See you next time on Maths Is Not Boring.

9:54 Bye!