How did they invent the metre?

0:47 Welcome to Maths Is Not Boring by kidopoly.com. I'm Mira. And I'm Finn. Before we start, a big hello to Ofi, listening all the way in Pennsylvania.

0:58 And a very happy ninth birthday to Peyton in Irvine, California, who listens with his brother Trevan.

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1:11 So, Mira, you told me today's idea starts with a tree house?

1:15 Yes. Imagine you and I are building a tree house then. I ask you to cut a plank of wood exactly three feet long.

1:23 Easy. I grab my ruler, measure three feet, and cut it.

1:28 Right, but what if your ruler was completely different from my ruler? What if your three feet was way shorter than my three feet?

1:37 The treehouse would be completely lopsided. None of the wooden pieces would fit together. We'd have gaps everywhere. Exactly.

1:45 And believe it or not, that is exactly what the country of France was like in the 1780s. A foot or a pound changed from town to town. Wait, really? Every town had a different sized ruler. Yes. Imagine you live in an old French village. You buy some rope, and the shopkeeper measures out ten lengths of his arm.

2:07 But his arm is short. Then you travel to the next town, and the carpenter there uses a totally different piece of wood as his standard measurement.

2:17 That sounds like chaos. How did anyone build anything or trade fairly?

2:22 It was chaotic, and sometimes it was a way for people to cheat each other. The king had his own foot measurement, called the Pied du Roi, but people out in the countryside didn't want to use it. So why not just invent a new ruler and tell everyone to use it? That is exactly what they decided to do. But they didn't want a length that belonged to the king, or a length that belonged to Paris.

2:47 They wanted a length that belonged to absolutely everyone.

2:50 A length that belongs to everyone. What does that even mean? What do we all share?

2:56 What is the one giant thing we all live on? The Earth! Yes. The scientists decided to create a brand new length based on the Earth itself. They called it the meter, from a Greek word meaning measure. So how big was it going to be? They decided a meter would be exactly one ten millionth of the distance from the north pole, straight down to the equator.

3:21 Whoa, one ten millionth! So if you stacked ten million of these metres end to end, you'd go from the north pole all the way along the surface to the equator. You got it!

3:32 It was a beautiful mathematical idea.

3:35 Okay, that is a beautiful idea. But how do you actually figure out how long that is? They didn't have a giant tape measure in the 1700s.

3:44 They had to actually measure the earth. In 1792, two brilliant astronomers took on the job. Their names were Jean-Baptiste Delombe and Pierre Mechan.

3:56 Jean-Baptiste Delombe and Pierre Mechan. What was their plan?

4:01 They set out to measure a huge slice of the earth, from Dunkirk in the north of France, all the way down to Barcelona in Spain. Delambe measured the northern part down to a town called Chodes, and Meschat measured the southern part from Barcelona up to Hodez.

4:20 That's hundreds and hundreds of miles. Were they just walking the whole way, counting their steps?

4:26 No, they use maths. They used a method involving giant invisible triangles. If you know the exact length of one small side of a triangle, and you can measure the angles, you can figure out all the other sides without walking them.

4:43 Oh, that's smart. So they just sat in a field doing maths?

4:47 No, they had to climb! They spent their days climbing up huge church towers, dragging heavy brass telescopes up steep hills and mountain peaks, just to look out at the next hilltop and measure the exact angles. That sounds exhausting. It was an incredible adventure. And remember the year they started, 1792.

5:10 The French Revolution was happening. There were riots and castles being taken over.

5:15 Wait, they were measuring triangles in the middle of a revolution.

5:19 Yes, people were very suspicious of these two men with weird metal instruments climbing their local towers. Sometimes they were chased out of towns or even thrown in jail because people thought they were spies. It took them seven whole years working through a revolution and a war. Seven years just to make a ruler?

5:42 Yes, after dragging their equipment through freezing rain, hiding from mobs and climbing countless towers, they finally brought all their measurements together. And did they get the magic number? The 110 millionth? They did. In 1799, they used their math to create a solid bar out of a precious metal called platinum.

6:04 It was exactly one metre long, based on their math, and it was locked safely in the National Archives in Paris. It was called the Maître des Archives. They did it, the ruler for the whole world. They did, mostly. But Finn, here is the jolt. Here is the part that's going to bend your brain a little.

6:24 Oh no, what happened?

6:26 Their answer was actually a tiny bit off. The platinum bar was about a fifth of a millimetre shorter than the real distance.

6:33 Wait, seven years climbing mountains, dodging a revolution, and they got it wrong? Just by the tiniest fraction of a millimetre.

6:42 You see, they already knew the spinning Earth was a tiny bit squashed at the poles. It bulges at the middle. But when they did their math, they slightly miscalculated exactly how squashed it was.

6:54 Oh, like if you pressed down on a squishy ball, the sides poof out.

6:59 Exactly. So the distance from the pole to the equator was slightly different than they thought. Plus, Michal had made a tiny mistake reading his instruments down in Barcelona.

7:10 Oh man, that must have been so frustrating.

7:13 It really was, but honestly, measuring hundreds of miles with heavy brass telescopes during a revolution was still a truly incredible achievement.

7:22 But wait, did that mistake ruin the meter? If the bar was slightly off, did they have to throw it away?

7:30 No, and that is the most beautiful part of this idea. Even though it wasn't perfectly one ten millionth of the Earth anymore, it didn't matter. The bar of platinum in Paris became the true meter.

7:44 Because everyone just agreed to share it.

7:47 Exactly! The idea of a length that everybody shares is what won. That tiny error didn't stop the world from connecting. Every ruler in your pencil case, every tape measure, every running track at the Olympics is all copied from that one beautiful idea.

8:06 That is completely wild. Every time I measure a piece of wood for a treehouse, I'm holding a length that those guys climbed mountains for.

8:15 You are. But, Finn, I promised you a sideways connection. Today, the solid platinum bar is resting quietly in Paris. We don't use it to define the meter anymore.

8:28 We don't? Then what do we use?

8:31 We use something even more perfect. Something that belongs to the whole universe. The speed of light. Light? Like from a flashlight. Yes. Light travels at the exact same speed in empty space, or a vacuum. So today, one metre is officially defined as the exact distance that light travels in a vacuum in a tiny fraction of a second.

8:58 That is amazing. We went from a squishy earth to a metal bar to a beam of light. Alright, Finn, time for the quiz! Okay, I'm ready. Let's do this. First question.

9:10 What giant thing did the scientists decide to use to create the new ruler? Next one. What is the name of this universal length they created? Question three. Who were the two astronomers that set off to measure the Earth in 1792? Here's number four. How many years did their crazy journey take? Last question.

9:47 Today we don't use a metal bar for the meter. What do we use to define it now? Great job! Let's see how you did. The first answer is they use the earth itself! The biggest ruler ever. For question two, the universal length is called the meter.

10:08 Got that one.

10:09 Number three, the two astronomers were Jean-Baptiste Delombe and Pierre Mechard.

10:15 The ultimate climbing team.

10:17 Question four. Their journey took seven long years. Through a whole revolution. And the final answer is, today the metre is defined by the speed of light.

10:28 Still my favourite fact. It's just so cool to think about. Every single time I use a tape measure now, I'm going to imagine those two astronomers dragging telescopes up a church tower.

10:39 Me too, Finn. It's like you're holding a tiny piece of history. A tiny piece of the earth right in your hands. If you loved exploring the meter with us today, please scroll down right now and leave us a five-star review. Yeah, just tap the stars. It really helps out. And be sure to tap follow so you never miss an episode.

11:01 And check out our sister shows.com. See you next time on Maths Is Not Boring. Bye!