In this episode, we'll explore the fascinating history of the metric system.
In the 18th century, two French astronomers set out on a gruelling journey to define a measurement so "perfect" that it is still in use today.
[00:00:05] Hello, hello hello, and welcome to English Learning for Curious Minds, by Leonardo English, the show where you can listen to fascinating stories and learn weird and wonderful things about the world at the same time as improving your English.
[00:00:21] I'm Alastair Budge, and today, we are going to be talking about the creation of the metric system.
[00:00:28] All around the world, with a few notable exceptions, the vast majority of the world’s population uses the same system to measure stuff. Metres, grammes, litres.
[00:00:40] It's something we take for granted, but it is, of course, a man-made creation.
[00:00:46] So, in this episode, we are going to dive into the wonderful, unlikely, revolutionary, but not absolutely perfect story of the metric system.
[00:00:57] Right, let’s not waste a minute and get right into it.
[00:01:03] The other day, I was talking with a Swedish friend.
[00:01:06] We were talking about different cities in Sweden, and he mentioned that a town was only 20 miles away.
[00:01:16] My first reaction was, “Wow, my Swedish geography is really bad, I thought the town we were talking about was a lot further away than 20 miles”.
[00:01:27] And then I thought, “Hang on, why is this Swede talking about miles? I thought that was something only the Brits and Americans used, a hangover from many centuries ago?”
[00:01:40] Seeing the confusion on my face, he corrected himself, “Swedish miles, I mean. A Swedish mile is 10 kilometres”, he added.
[00:01:50] In case you weren’t aware, as I wasn’t, there is such a thing as a Swedish mile, or a Scandinavian mile, as it’s also known.
[00:02:00] It is exactly 10 kilometres, and it is still a commonly used measurement in Sweden and Norway.
[00:02:09] Thankfully, it is relatively easy to understand, and you can quickly translate this to kilometres by adding a zero to the end. Simple, once you know what you’re doing.
[00:02:21] But imagine a world in which every country, every city, and in some cases, every town or even village had a different unit of measurement.
[00:02:33] What I might call a “mile” is a slightly different length to what you might call “a mile”, or you might not call it a mile at all.
[00:02:42] It would be chaos, but for much of human history, in many countries, this was the case, not just for distances but for practically everything.
[00:02:53] In pre-revolutionary France, for example, if you travelled just a short distance, you might find a new foot, a different pound, and a host of other local measurements with baffling names and ever-so-slightly varying lengths or weights.
[00:03:12] It was confusing, and made trade more complicated.
[00:03:16] Buyers and sellers never knew if they were getting a fair deal.
[00:03:21] It was such a problem that in some countries, Sweden, for example, the crime of falsifying weights or measurements was even made a capital offence, it was punishable by death.
[00:03:35] Across Europe, royal edicts had tried to impose common standards for measurements, but they rarely stuck.
[00:03:43] There were just too many competing local traditions, each with its proud defenders.
[00:03:50] And the measurements were often based on inexact objects in the real world.
[00:03:58] A common example is the cubit, the unit of measurement used in Ancient Egypt that was defined as the length between someone’s elbow and the tip of their middle finger.
[00:04:10] Everyone has an elbow and a middle finger, but this is not an exact, universal measurement. My “cubit” is different from your “cubit”, so it is an inexact measurement.
[00:04:24] In the UK, we still use “feet” to measure certain distances, like height or the dimensions of rooms, and horses are still measured in “hands”. It isn’t too hard to figure out where those came from, but they are imperfect because they vary from person to person.
[00:04:45] So, for a unit of measurement to be perfect, for it to be indisputable, it needs to come from somewhere or something that was universally observable and applicable. Something that never changed.
[00:05:01] Now, let us move into the late 18th century.
[00:05:05] In France, revolutionary fever was in the air.
[00:05:09] The old institutions were under attack, and the leaders of the new France believed society had to be rebuilt from the ground up.
[00:05:19] Laws, religion, social structures—everything needed reform.
[00:05:25] And right at the heart of this spirit of change was an ambitious plan: to unify the chaotic patchwork of French measurements by creating a system that would be universal, scientific, and, crucially, based on nature itself.
[00:05:44] No more random definitions like the length of a king’s foot or the weight of a stone from a particular riverbank. It would come from somewhere that could be verified by anyone, or rather any scientist, anywhere, at any point in time.
[00:06:02] The answer was right under their feet; everything would come directly from the size of the Earth.
[00:06:11] They would call it the “metric” system, and the unit of measurement would be the “metre”. The name was chosen from the Greek word μετρέω, meaning to measure or count.
[00:06:25] The work began in 1792, when the French National Convention charged two astronomers, Pierre Méchain and Jean-Baptiste Delambre, with a daunting mission: to measure the distance between Dunkirk in northern France and Barcelona in Spain.
[00:06:45] Why these two places, you might be thinking?
[00:06:49] Well, it was because these two locations were on the same longitude. Importantly, they were on the Paris Meridian, the French equivalent to the now more popular Greenwich Meridian, which is a straight line going directly from north to south.
[00:07:07] If you were to draw a line from the North Pole to Paris, then it would have passed through Dunkirk. And if you kept going all the way to the South Pole, you would pass directly through Barcelona, or to be precise, Castell de Montjuïc in Barcelona.
[00:07:27] This was the distance that the two men were tasked with measuring.
[00:07:32] But how would this help them come up with the distance for a metre, you might well ask?
[00:07:37] The idea was to use this measurement—from Dunkirk to Barcelona—to estimate the total distance from the North Pole to the Equator. Astronomers already knew what fraction of that total distance the Dunkirk-Barcelona segment represented.
[00:07:55] And by extrapolating this proportion, they could calculate the full pole-to-equator distance.
[00:08:04] They would then divide this distance into 10 million units, and each of those units would be called a metre.
[00:08:13] This would mean that the circumference of the Earth would be exactly 40 million metres.
[00:08:20] Remember that, as we’ll address this in more detail a little bit later on.
[00:08:26] Anyway, this might all sound elegant in theory, but this was in 1792.
[00:08:34] How were they actually going to measure the distance between these two points?
[00:08:39] They couldn’t just walk from Dunkirk to Barcelona with a long piece of rope.
[00:08:44] Today, we know—thanks to modern satellite technology—that this distance is 1,075 kilometres. But Méchain and Delambre had to calculate it from scratch, and from that, determine the precise length of a metre.
[00:09:03] As you can probably appreciate, Méchain and Delambre were tasked with one of the most challenging surveying operations in history.
[00:09:12] To calculate the distance, they used the method of triangulation, which involves taking angular readings from carefully chosen high points—church spires, mountaintops, and miscellaneous tall buildings—and then they used geometry to calculate exact distances over large expanses of land.
[00:09:37] Essentially, they stood in one high place, observed two points of interest in the distance, and measured the angles between them.
[00:09:47] But because this was imperfect, they would measure the angles from multiple different places, and use all of those calculations to reduce the margin for error.
[00:09:59] And they would do this over and over and over, moving all the way from Dunkirk to Barcelona.
[00:10:08] Or technically, Delambre did the northern part, from Dunkirk to a town called Rodez in central France, and Méchain was responsible for the southern part, from Rodez down to Barcelona.
[00:10:22] Even though they divided the work, it was incredibly laborious, requiring delicate instruments and precise observations, often repeated many times to eliminate errors.
[00:10:36] And these instruments were cumbersome: they were heavy brass and steel devices that had to be carried up hills, across rivers, and through towns in the midst of political unrest.
[00:10:49] Remember, this was all taking place in the middle of the French Revolution.
[00:10:55] France declared war on Austria in 1792, the year Méchain and Delambre had set out.
[00:11:02] The following year, King Louis XVI was tried and guillotined; the year after that, Robespierre, one of the revolutionaries, was overthrown and executed.
[00:11:13] It was the most turbulent political climate in French history, so one can just imagine Méchain and Delambre trying to explain to suspicious locals why they were standing on their church roof at three o’clock in the morning, fiddling with a strange-looking metallic contraption.
[00:11:32] And it wasn’t just the human component that they were up against.
[00:11:36] The two scientists also had to deal with the fact that the Earth is not a perfect sphere.
[00:11:43] It’s technically something called an oblate spheroid, meaning it bulges slightly around the Equator like it has had a little too much for lunch.
[00:11:53] This makes measuring an arc of the Earth’s meridian trickier than you might think.
[00:11:59] It was a gargantuan effort, and every small error in the angles, every imprecise reading, would multiply over hundreds of kilometres, so they had to be meticulous.
[00:12:13] They repeated each measurement multiple times under different conditions—morning, afternoon, nighttime, clear skies, cloudy skies—always cross-checking for tiny differences.
[00:12:26] They kept notebooks full of corrections for temperature changes that could affect the metal in their instruments.
[00:12:34] They also had to correct for atmospheric refraction, the way light bends slightly as it travels through layers of air with different densities.
[00:12:45] Any slip-up, any mistake, could lead to a miscalculation of the final metre.
[00:12:52] But perhaps the scientific part, the observations and the calculations, were not the greatest challenge the pair faced.
[00:13:01] One of the most serious obstacles faced was actually on the Spanish side of the border, where Méchain had to deal with local suspicion during a time of war.
[00:13:12] He had the requisite papers with details of his mission, but he was frequently mistaken for a spy, someone measuring fortifications or planning a military route.
[00:13:25] He was arrested, hassled, and delayed for months.
[00:13:29] Meanwhile, Delambre, on his northern journey, was often met with confusion or outright hostility from villagers.
[00:13:39] Despite all of this, the pair inched closer to their goal with extraordinary determination.
[00:13:47] By 1798, after almost seven years of gruelling, nerve-wracking work, the two astronomers finally had enough data to calculate the length of the meridian arc and, with it, define the metre.
[00:14:03] The calculations were complete, and the true distance of a metre could finally be unveiled.
[00:14:10] For the first time in history, there would be a universal measurement based on the circumference of the Earth.
[00:14:18] In 1799, a platinum bar was cast to represent this new standard.
[00:14:26] That bar, known as the “mètre des Archives,” became the reference point for all future measurements, and this is where it gets even more interesting.
[00:14:37] Once the metre was set, scientists could define a smaller unit, the decimetre—one-tenth of a metre.
[00:14:46] A cube that measured one decimetre on each side was declared to be one litre in volume.
[00:14:54] Ta-da, a new universal measurement of volume.
[00:14:58] Then, the weight of a litre of water at a specified temperature [close to 4°C] was considered to be exactly one kilogramme.
[00:15:09] And ta-da, a new measurement of weight.
[00:15:13] Suddenly, length, volume, and mass were all interlinked, rooted in a single measurement: the metre.
[00:15:23] And because everything was in multiples of ten, conversions were vastly simpler than in the old imperial or local systems.
[00:15:33] No more 12 inches in a foot, 3 feet in a yard, 1,760 yards in a mile; now it was tens, hundreds, and thousands—a clean, decimal arrangement that anyone but the most ardent supporter of pounds, feet and gallons can recognise as a superior system.
[00:15:56] Of course, theory and practice don’t always align immediately, and plenty of people in France, especially in rural areas, resisted the new system.
[00:16:08] Napoleon himself, seeing how unpopular it could be, allowed many of the old measures to stay in use.
[00:16:15] But the metric system was already out in the world, and it was only a matter of time before its clarity and simplicity caught the attention of scientists in other countries.
[00:16:29] Throughout the 19th century, the idea of a universal, decimal-based system of measurement spread across Europe and beyond.
[00:16:38] Newly independent countries in Latin America adopted it as a mark of modernity and a break from colonial influences.
[00:16:47] By the late 1800s, the metric system had become a near-global concern, and in 1875, seventeen countries signed the Metre Convention, setting up the International Bureau of Weights and Measures near Paris.
[00:17:04] Even the United States, which still clings to miles and pounds in daily life, was a founding signatory.
[00:17:12] And over the next century, definitions became even more precise.
[00:17:18] Originally, the metre was defined by a platinum bar—literally a metal stick kept under lock and key in Paris.
[00:17:28] If anything happened to that bar—if it got a tiny scratch, or if the metal warped over time—our definition of the metre would change.
[00:17:38] Scientists realised that basing the metre on a single physical object wasn’t ideal, so in 1983, they redefined the metre using the speed of light in a vacuum, which is one of the most reliable properties in the universe.
[00:17:55] We say that the metre is the distance light travels in 1/299,792,458 of a second.
[00:18:08] Because the speed of light doesn’t change, we get an incredibly precise, unchanging definition of what a metre actually is.
[00:18:18] And a similar logic applies to the kilogramme. For a long time, the official kilogramme was a platinum-iridium cylinder, also kept in Paris.
[00:18:30] Just like the metre bar, any tiny imperfection or contamination could change its mass, even if only by an infinitesimal amount.
[00:18:41] So, in 2019, scientists decided to define the kilogramme in terms of Planck's constant, a fundamental number in quantum physics.
[00:18:53] If we measure Planck's constant accurately and link it to the idea of mass, we get a definition of the kilogramme that doesn’t rely on a single metal cylinder sitting in a vault.
[00:19:06] It’s tied, instead, to a property of the universe that remains the same no matter where you go.
[00:19:14] This can all be traced back to Méchain and Delambre’s incredible precision.
[00:19:20] But, and it is a big but, they did make a tiny mistake.
[00:19:26] As you’ll remember, they had intended the distance from the North Pole to the equator to be exactly ten million metres, making the Earth’s polar circumference exactly 40 million metres.
[00:19:41] That figure was close, but not exact.
[00:19:45] Thanks to a series of tiny errors—some the result of the Earth’s irregular shape, others from errors in the triangulation work—the final calculation was off by a few hundred metres.
[00:20:00] And if you trace the sources of this discrepancy, most fingers point to Pierre Méchain.
[00:20:08] He was an obsessively meticulous man who discovered inconsistencies in his results but never fully corrected them, partly out of fear of undermining the entire endeavour.
[00:20:21] As more precise measurements emerged in the decades and centuries that followed, scientists realised that the meridian distance should have been slightly larger, meaning our metre–the metre that you and I know–turned out to be just a fraction smaller than intended.
[00:20:41] But by then, it was too late.
[00:20:45] The metre had already become the reference for countless measurements, scientific instruments, legal contracts, textbooks, and everyday transactions.
[00:20:56] There were discussions about fixing it—simply readjusting the value so that the polar circumference lined up perfectly with 40 million metres—but doing so would have rendered every existing metre stick obsolete.
[00:21:12] Redefining the metre in that way would have caused enormous disruption.
[00:21:17] Instead, the world decided to keep the metre as it was.
[00:21:21] After all, it was only a fraction of a millimetre off per metre, which was more than accurate enough for 18th-century practical purposes.
[00:21:32] Later refinements to the definition of the metre, like tying it to the speed of light, have made it infinitely more precise, but the original length is still the same.
[00:21:44] In other words, Méchain’s tiny error became enshrined in history.
[00:21:49] The Earth isn’t exactly 40 million metres in circumference, but the metric system has worked so well that it doesn’t really matter.
[00:21:59] It was a small price to pay for the simplicity and universality that the metric system brought to the world.
[00:22:07] OK then, that is it for today's episode on the metric system, and the story of how and why a metre is a metre.
[00:22:16] I hope it's been an interesting one and that you've learned something new.
[00:22:20] As always, I would love to know what you thought of this episode. Did you know this story beforehand, and what other scientific discovery stories like this should we make?
[00:22:30] I would love to know, so let’s get this discussion started.
[00:22:33] You can head right into our community forum, which is at community.leonardoenglish.com and get chatting away to other curious minds.
[00:22:42] You've been listening to English Learning for Curious Minds by Leonardo English.
[00:22:47] I'm Alastair Budge, you stay safe, and I'll catch you in the next episode.
[00:00:05] Hello, hello hello, and welcome to English Learning for Curious Minds, by Leonardo English, the show where you can listen to fascinating stories and learn weird and wonderful things about the world at the same time as improving your English.
[00:00:21] I'm Alastair Budge, and today, we are going to be talking about the creation of the metric system.
[00:00:28] All around the world, with a few notable exceptions, the vast majority of the world’s population uses the same system to measure stuff. Metres, grammes, litres.
[00:00:40] It's something we take for granted, but it is, of course, a man-made creation.
[00:00:46] So, in this episode, we are going to dive into the wonderful, unlikely, revolutionary, but not absolutely perfect story of the metric system.
[00:00:57] Right, let’s not waste a minute and get right into it.
[00:01:03] The other day, I was talking with a Swedish friend.
[00:01:06] We were talking about different cities in Sweden, and he mentioned that a town was only 20 miles away.
[00:01:16] My first reaction was, “Wow, my Swedish geography is really bad, I thought the town we were talking about was a lot further away than 20 miles”.
[00:01:27] And then I thought, “Hang on, why is this Swede talking about miles? I thought that was something only the Brits and Americans used, a hangover from many centuries ago?”
[00:01:40] Seeing the confusion on my face, he corrected himself, “Swedish miles, I mean. A Swedish mile is 10 kilometres”, he added.
[00:01:50] In case you weren’t aware, as I wasn’t, there is such a thing as a Swedish mile, or a Scandinavian mile, as it’s also known.
[00:02:00] It is exactly 10 kilometres, and it is still a commonly used measurement in Sweden and Norway.
[00:02:09] Thankfully, it is relatively easy to understand, and you can quickly translate this to kilometres by adding a zero to the end. Simple, once you know what you’re doing.
[00:02:21] But imagine a world in which every country, every city, and in some cases, every town or even village had a different unit of measurement.
[00:02:33] What I might call a “mile” is a slightly different length to what you might call “a mile”, or you might not call it a mile at all.
[00:02:42] It would be chaos, but for much of human history, in many countries, this was the case, not just for distances but for practically everything.
[00:02:53] In pre-revolutionary France, for example, if you travelled just a short distance, you might find a new foot, a different pound, and a host of other local measurements with baffling names and ever-so-slightly varying lengths or weights.
[00:03:12] It was confusing, and made trade more complicated.
[00:03:16] Buyers and sellers never knew if they were getting a fair deal.
[00:03:21] It was such a problem that in some countries, Sweden, for example, the crime of falsifying weights or measurements was even made a capital offence, it was punishable by death.
[00:03:35] Across Europe, royal edicts had tried to impose common standards for measurements, but they rarely stuck.
[00:03:43] There were just too many competing local traditions, each with its proud defenders.
[00:03:50] And the measurements were often based on inexact objects in the real world.
[00:03:58] A common example is the cubit, the unit of measurement used in Ancient Egypt that was defined as the length between someone’s elbow and the tip of their middle finger.
[00:04:10] Everyone has an elbow and a middle finger, but this is not an exact, universal measurement. My “cubit” is different from your “cubit”, so it is an inexact measurement.
[00:04:24] In the UK, we still use “feet” to measure certain distances, like height or the dimensions of rooms, and horses are still measured in “hands”. It isn’t too hard to figure out where those came from, but they are imperfect because they vary from person to person.
[00:04:45] So, for a unit of measurement to be perfect, for it to be indisputable, it needs to come from somewhere or something that was universally observable and applicable. Something that never changed.
[00:05:01] Now, let us move into the late 18th century.
[00:05:05] In France, revolutionary fever was in the air.
[00:05:09] The old institutions were under attack, and the leaders of the new France believed society had to be rebuilt from the ground up.
[00:05:19] Laws, religion, social structures—everything needed reform.
[00:05:25] And right at the heart of this spirit of change was an ambitious plan: to unify the chaotic patchwork of French measurements by creating a system that would be universal, scientific, and, crucially, based on nature itself.
[00:05:44] No more random definitions like the length of a king’s foot or the weight of a stone from a particular riverbank. It would come from somewhere that could be verified by anyone, or rather any scientist, anywhere, at any point in time.
[00:06:02] The answer was right under their feet; everything would come directly from the size of the Earth.
[00:06:11] They would call it the “metric” system, and the unit of measurement would be the “metre”. The name was chosen from the Greek word μετρέω, meaning to measure or count.
[00:06:25] The work began in 1792, when the French National Convention charged two astronomers, Pierre Méchain and Jean-Baptiste Delambre, with a daunting mission: to measure the distance between Dunkirk in northern France and Barcelona in Spain.
[00:06:45] Why these two places, you might be thinking?
[00:06:49] Well, it was because these two locations were on the same longitude. Importantly, they were on the Paris Meridian, the French equivalent to the now more popular Greenwich Meridian, which is a straight line going directly from north to south.
[00:07:07] If you were to draw a line from the North Pole to Paris, then it would have passed through Dunkirk. And if you kept going all the way to the South Pole, you would pass directly through Barcelona, or to be precise, Castell de Montjuïc in Barcelona.
[00:07:27] This was the distance that the two men were tasked with measuring.
[00:07:32] But how would this help them come up with the distance for a metre, you might well ask?
[00:07:37] The idea was to use this measurement—from Dunkirk to Barcelona—to estimate the total distance from the North Pole to the Equator. Astronomers already knew what fraction of that total distance the Dunkirk-Barcelona segment represented.
[00:07:55] And by extrapolating this proportion, they could calculate the full pole-to-equator distance.
[00:08:04] They would then divide this distance into 10 million units, and each of those units would be called a metre.
[00:08:13] This would mean that the circumference of the Earth would be exactly 40 million metres.
[00:08:20] Remember that, as we’ll address this in more detail a little bit later on.
[00:08:26] Anyway, this might all sound elegant in theory, but this was in 1792.
[00:08:34] How were they actually going to measure the distance between these two points?
[00:08:39] They couldn’t just walk from Dunkirk to Barcelona with a long piece of rope.
[00:08:44] Today, we know—thanks to modern satellite technology—that this distance is 1,075 kilometres. But Méchain and Delambre had to calculate it from scratch, and from that, determine the precise length of a metre.
[00:09:03] As you can probably appreciate, Méchain and Delambre were tasked with one of the most challenging surveying operations in history.
[00:09:12] To calculate the distance, they used the method of triangulation, which involves taking angular readings from carefully chosen high points—church spires, mountaintops, and miscellaneous tall buildings—and then they used geometry to calculate exact distances over large expanses of land.
[00:09:37] Essentially, they stood in one high place, observed two points of interest in the distance, and measured the angles between them.
[00:09:47] But because this was imperfect, they would measure the angles from multiple different places, and use all of those calculations to reduce the margin for error.
[00:09:59] And they would do this over and over and over, moving all the way from Dunkirk to Barcelona.
[00:10:08] Or technically, Delambre did the northern part, from Dunkirk to a town called Rodez in central France, and Méchain was responsible for the southern part, from Rodez down to Barcelona.
[00:10:22] Even though they divided the work, it was incredibly laborious, requiring delicate instruments and precise observations, often repeated many times to eliminate errors.
[00:10:36] And these instruments were cumbersome: they were heavy brass and steel devices that had to be carried up hills, across rivers, and through towns in the midst of political unrest.
[00:10:49] Remember, this was all taking place in the middle of the French Revolution.
[00:10:55] France declared war on Austria in 1792, the year Méchain and Delambre had set out.
[00:11:02] The following year, King Louis XVI was tried and guillotined; the year after that, Robespierre, one of the revolutionaries, was overthrown and executed.
[00:11:13] It was the most turbulent political climate in French history, so one can just imagine Méchain and Delambre trying to explain to suspicious locals why they were standing on their church roof at three o’clock in the morning, fiddling with a strange-looking metallic contraption.
[00:11:32] And it wasn’t just the human component that they were up against.
[00:11:36] The two scientists also had to deal with the fact that the Earth is not a perfect sphere.
[00:11:43] It’s technically something called an oblate spheroid, meaning it bulges slightly around the Equator like it has had a little too much for lunch.
[00:11:53] This makes measuring an arc of the Earth’s meridian trickier than you might think.
[00:11:59] It was a gargantuan effort, and every small error in the angles, every imprecise reading, would multiply over hundreds of kilometres, so they had to be meticulous.
[00:12:13] They repeated each measurement multiple times under different conditions—morning, afternoon, nighttime, clear skies, cloudy skies—always cross-checking for tiny differences.
[00:12:26] They kept notebooks full of corrections for temperature changes that could affect the metal in their instruments.
[00:12:34] They also had to correct for atmospheric refraction, the way light bends slightly as it travels through layers of air with different densities.
[00:12:45] Any slip-up, any mistake, could lead to a miscalculation of the final metre.
[00:12:52] But perhaps the scientific part, the observations and the calculations, were not the greatest challenge the pair faced.
[00:13:01] One of the most serious obstacles faced was actually on the Spanish side of the border, where Méchain had to deal with local suspicion during a time of war.
[00:13:12] He had the requisite papers with details of his mission, but he was frequently mistaken for a spy, someone measuring fortifications or planning a military route.
[00:13:25] He was arrested, hassled, and delayed for months.
[00:13:29] Meanwhile, Delambre, on his northern journey, was often met with confusion or outright hostility from villagers.
[00:13:39] Despite all of this, the pair inched closer to their goal with extraordinary determination.
[00:13:47] By 1798, after almost seven years of gruelling, nerve-wracking work, the two astronomers finally had enough data to calculate the length of the meridian arc and, with it, define the metre.
[00:14:03] The calculations were complete, and the true distance of a metre could finally be unveiled.
[00:14:10] For the first time in history, there would be a universal measurement based on the circumference of the Earth.
[00:14:18] In 1799, a platinum bar was cast to represent this new standard.
[00:14:26] That bar, known as the “mètre des Archives,” became the reference point for all future measurements, and this is where it gets even more interesting.
[00:14:37] Once the metre was set, scientists could define a smaller unit, the decimetre—one-tenth of a metre.
[00:14:46] A cube that measured one decimetre on each side was declared to be one litre in volume.
[00:14:54] Ta-da, a new universal measurement of volume.
[00:14:58] Then, the weight of a litre of water at a specified temperature [close to 4°C] was considered to be exactly one kilogramme.
[00:15:09] And ta-da, a new measurement of weight.
[00:15:13] Suddenly, length, volume, and mass were all interlinked, rooted in a single measurement: the metre.
[00:15:23] And because everything was in multiples of ten, conversions were vastly simpler than in the old imperial or local systems.
[00:15:33] No more 12 inches in a foot, 3 feet in a yard, 1,760 yards in a mile; now it was tens, hundreds, and thousands—a clean, decimal arrangement that anyone but the most ardent supporter of pounds, feet and gallons can recognise as a superior system.
[00:15:56] Of course, theory and practice don’t always align immediately, and plenty of people in France, especially in rural areas, resisted the new system.
[00:16:08] Napoleon himself, seeing how unpopular it could be, allowed many of the old measures to stay in use.
[00:16:15] But the metric system was already out in the world, and it was only a matter of time before its clarity and simplicity caught the attention of scientists in other countries.
[00:16:29] Throughout the 19th century, the idea of a universal, decimal-based system of measurement spread across Europe and beyond.
[00:16:38] Newly independent countries in Latin America adopted it as a mark of modernity and a break from colonial influences.
[00:16:47] By the late 1800s, the metric system had become a near-global concern, and in 1875, seventeen countries signed the Metre Convention, setting up the International Bureau of Weights and Measures near Paris.
[00:17:04] Even the United States, which still clings to miles and pounds in daily life, was a founding signatory.
[00:17:12] And over the next century, definitions became even more precise.
[00:17:18] Originally, the metre was defined by a platinum bar—literally a metal stick kept under lock and key in Paris.
[00:17:28] If anything happened to that bar—if it got a tiny scratch, or if the metal warped over time—our definition of the metre would change.
[00:17:38] Scientists realised that basing the metre on a single physical object wasn’t ideal, so in 1983, they redefined the metre using the speed of light in a vacuum, which is one of the most reliable properties in the universe.
[00:17:55] We say that the metre is the distance light travels in 1/299,792,458 of a second.
[00:18:08] Because the speed of light doesn’t change, we get an incredibly precise, unchanging definition of what a metre actually is.
[00:18:18] And a similar logic applies to the kilogramme. For a long time, the official kilogramme was a platinum-iridium cylinder, also kept in Paris.
[00:18:30] Just like the metre bar, any tiny imperfection or contamination could change its mass, even if only by an infinitesimal amount.
[00:18:41] So, in 2019, scientists decided to define the kilogramme in terms of Planck's constant, a fundamental number in quantum physics.
[00:18:53] If we measure Planck's constant accurately and link it to the idea of mass, we get a definition of the kilogramme that doesn’t rely on a single metal cylinder sitting in a vault.
[00:19:06] It’s tied, instead, to a property of the universe that remains the same no matter where you go.
[00:19:14] This can all be traced back to Méchain and Delambre’s incredible precision.
[00:19:20] But, and it is a big but, they did make a tiny mistake.
[00:19:26] As you’ll remember, they had intended the distance from the North Pole to the equator to be exactly ten million metres, making the Earth’s polar circumference exactly 40 million metres.
[00:19:41] That figure was close, but not exact.
[00:19:45] Thanks to a series of tiny errors—some the result of the Earth’s irregular shape, others from errors in the triangulation work—the final calculation was off by a few hundred metres.
[00:20:00] And if you trace the sources of this discrepancy, most fingers point to Pierre Méchain.
[00:20:08] He was an obsessively meticulous man who discovered inconsistencies in his results but never fully corrected them, partly out of fear of undermining the entire endeavour.
[00:20:21] As more precise measurements emerged in the decades and centuries that followed, scientists realised that the meridian distance should have been slightly larger, meaning our metre–the metre that you and I know–turned out to be just a fraction smaller than intended.
[00:20:41] But by then, it was too late.
[00:20:45] The metre had already become the reference for countless measurements, scientific instruments, legal contracts, textbooks, and everyday transactions.
[00:20:56] There were discussions about fixing it—simply readjusting the value so that the polar circumference lined up perfectly with 40 million metres—but doing so would have rendered every existing metre stick obsolete.
[00:21:12] Redefining the metre in that way would have caused enormous disruption.
[00:21:17] Instead, the world decided to keep the metre as it was.
[00:21:21] After all, it was only a fraction of a millimetre off per metre, which was more than accurate enough for 18th-century practical purposes.
[00:21:32] Later refinements to the definition of the metre, like tying it to the speed of light, have made it infinitely more precise, but the original length is still the same.
[00:21:44] In other words, Méchain’s tiny error became enshrined in history.
[00:21:49] The Earth isn’t exactly 40 million metres in circumference, but the metric system has worked so well that it doesn’t really matter.
[00:21:59] It was a small price to pay for the simplicity and universality that the metric system brought to the world.
[00:22:07] OK then, that is it for today's episode on the metric system, and the story of how and why a metre is a metre.
[00:22:16] I hope it's been an interesting one and that you've learned something new.
[00:22:20] As always, I would love to know what you thought of this episode. Did you know this story beforehand, and what other scientific discovery stories like this should we make?
[00:22:30] I would love to know, so let’s get this discussion started.
[00:22:33] You can head right into our community forum, which is at community.leonardoenglish.com and get chatting away to other curious minds.
[00:22:42] You've been listening to English Learning for Curious Minds by Leonardo English.
[00:22:47] I'm Alastair Budge, you stay safe, and I'll catch you in the next episode.
[00:00:05] Hello, hello hello, and welcome to English Learning for Curious Minds, by Leonardo English, the show where you can listen to fascinating stories and learn weird and wonderful things about the world at the same time as improving your English.
[00:00:21] I'm Alastair Budge, and today, we are going to be talking about the creation of the metric system.
[00:00:28] All around the world, with a few notable exceptions, the vast majority of the world’s population uses the same system to measure stuff. Metres, grammes, litres.
[00:00:40] It's something we take for granted, but it is, of course, a man-made creation.
[00:00:46] So, in this episode, we are going to dive into the wonderful, unlikely, revolutionary, but not absolutely perfect story of the metric system.
[00:00:57] Right, let’s not waste a minute and get right into it.
[00:01:03] The other day, I was talking with a Swedish friend.
[00:01:06] We were talking about different cities in Sweden, and he mentioned that a town was only 20 miles away.
[00:01:16] My first reaction was, “Wow, my Swedish geography is really bad, I thought the town we were talking about was a lot further away than 20 miles”.
[00:01:27] And then I thought, “Hang on, why is this Swede talking about miles? I thought that was something only the Brits and Americans used, a hangover from many centuries ago?”
[00:01:40] Seeing the confusion on my face, he corrected himself, “Swedish miles, I mean. A Swedish mile is 10 kilometres”, he added.
[00:01:50] In case you weren’t aware, as I wasn’t, there is such a thing as a Swedish mile, or a Scandinavian mile, as it’s also known.
[00:02:00] It is exactly 10 kilometres, and it is still a commonly used measurement in Sweden and Norway.
[00:02:09] Thankfully, it is relatively easy to understand, and you can quickly translate this to kilometres by adding a zero to the end. Simple, once you know what you’re doing.
[00:02:21] But imagine a world in which every country, every city, and in some cases, every town or even village had a different unit of measurement.
[00:02:33] What I might call a “mile” is a slightly different length to what you might call “a mile”, or you might not call it a mile at all.
[00:02:42] It would be chaos, but for much of human history, in many countries, this was the case, not just for distances but for practically everything.
[00:02:53] In pre-revolutionary France, for example, if you travelled just a short distance, you might find a new foot, a different pound, and a host of other local measurements with baffling names and ever-so-slightly varying lengths or weights.
[00:03:12] It was confusing, and made trade more complicated.
[00:03:16] Buyers and sellers never knew if they were getting a fair deal.
[00:03:21] It was such a problem that in some countries, Sweden, for example, the crime of falsifying weights or measurements was even made a capital offence, it was punishable by death.
[00:03:35] Across Europe, royal edicts had tried to impose common standards for measurements, but they rarely stuck.
[00:03:43] There were just too many competing local traditions, each with its proud defenders.
[00:03:50] And the measurements were often based on inexact objects in the real world.
[00:03:58] A common example is the cubit, the unit of measurement used in Ancient Egypt that was defined as the length between someone’s elbow and the tip of their middle finger.
[00:04:10] Everyone has an elbow and a middle finger, but this is not an exact, universal measurement. My “cubit” is different from your “cubit”, so it is an inexact measurement.
[00:04:24] In the UK, we still use “feet” to measure certain distances, like height or the dimensions of rooms, and horses are still measured in “hands”. It isn’t too hard to figure out where those came from, but they are imperfect because they vary from person to person.
[00:04:45] So, for a unit of measurement to be perfect, for it to be indisputable, it needs to come from somewhere or something that was universally observable and applicable. Something that never changed.
[00:05:01] Now, let us move into the late 18th century.
[00:05:05] In France, revolutionary fever was in the air.
[00:05:09] The old institutions were under attack, and the leaders of the new France believed society had to be rebuilt from the ground up.
[00:05:19] Laws, religion, social structures—everything needed reform.
[00:05:25] And right at the heart of this spirit of change was an ambitious plan: to unify the chaotic patchwork of French measurements by creating a system that would be universal, scientific, and, crucially, based on nature itself.
[00:05:44] No more random definitions like the length of a king’s foot or the weight of a stone from a particular riverbank. It would come from somewhere that could be verified by anyone, or rather any scientist, anywhere, at any point in time.
[00:06:02] The answer was right under their feet; everything would come directly from the size of the Earth.
[00:06:11] They would call it the “metric” system, and the unit of measurement would be the “metre”. The name was chosen from the Greek word μετρέω, meaning to measure or count.
[00:06:25] The work began in 1792, when the French National Convention charged two astronomers, Pierre Méchain and Jean-Baptiste Delambre, with a daunting mission: to measure the distance between Dunkirk in northern France and Barcelona in Spain.
[00:06:45] Why these two places, you might be thinking?
[00:06:49] Well, it was because these two locations were on the same longitude. Importantly, they were on the Paris Meridian, the French equivalent to the now more popular Greenwich Meridian, which is a straight line going directly from north to south.
[00:07:07] If you were to draw a line from the North Pole to Paris, then it would have passed through Dunkirk. And if you kept going all the way to the South Pole, you would pass directly through Barcelona, or to be precise, Castell de Montjuïc in Barcelona.
[00:07:27] This was the distance that the two men were tasked with measuring.
[00:07:32] But how would this help them come up with the distance for a metre, you might well ask?
[00:07:37] The idea was to use this measurement—from Dunkirk to Barcelona—to estimate the total distance from the North Pole to the Equator. Astronomers already knew what fraction of that total distance the Dunkirk-Barcelona segment represented.
[00:07:55] And by extrapolating this proportion, they could calculate the full pole-to-equator distance.
[00:08:04] They would then divide this distance into 10 million units, and each of those units would be called a metre.
[00:08:13] This would mean that the circumference of the Earth would be exactly 40 million metres.
[00:08:20] Remember that, as we’ll address this in more detail a little bit later on.
[00:08:26] Anyway, this might all sound elegant in theory, but this was in 1792.
[00:08:34] How were they actually going to measure the distance between these two points?
[00:08:39] They couldn’t just walk from Dunkirk to Barcelona with a long piece of rope.
[00:08:44] Today, we know—thanks to modern satellite technology—that this distance is 1,075 kilometres. But Méchain and Delambre had to calculate it from scratch, and from that, determine the precise length of a metre.
[00:09:03] As you can probably appreciate, Méchain and Delambre were tasked with one of the most challenging surveying operations in history.
[00:09:12] To calculate the distance, they used the method of triangulation, which involves taking angular readings from carefully chosen high points—church spires, mountaintops, and miscellaneous tall buildings—and then they used geometry to calculate exact distances over large expanses of land.
[00:09:37] Essentially, they stood in one high place, observed two points of interest in the distance, and measured the angles between them.
[00:09:47] But because this was imperfect, they would measure the angles from multiple different places, and use all of those calculations to reduce the margin for error.
[00:09:59] And they would do this over and over and over, moving all the way from Dunkirk to Barcelona.
[00:10:08] Or technically, Delambre did the northern part, from Dunkirk to a town called Rodez in central France, and Méchain was responsible for the southern part, from Rodez down to Barcelona.
[00:10:22] Even though they divided the work, it was incredibly laborious, requiring delicate instruments and precise observations, often repeated many times to eliminate errors.
[00:10:36] And these instruments were cumbersome: they were heavy brass and steel devices that had to be carried up hills, across rivers, and through towns in the midst of political unrest.
[00:10:49] Remember, this was all taking place in the middle of the French Revolution.
[00:10:55] France declared war on Austria in 1792, the year Méchain and Delambre had set out.
[00:11:02] The following year, King Louis XVI was tried and guillotined; the year after that, Robespierre, one of the revolutionaries, was overthrown and executed.
[00:11:13] It was the most turbulent political climate in French history, so one can just imagine Méchain and Delambre trying to explain to suspicious locals why they were standing on their church roof at three o’clock in the morning, fiddling with a strange-looking metallic contraption.
[00:11:32] And it wasn’t just the human component that they were up against.
[00:11:36] The two scientists also had to deal with the fact that the Earth is not a perfect sphere.
[00:11:43] It’s technically something called an oblate spheroid, meaning it bulges slightly around the Equator like it has had a little too much for lunch.
[00:11:53] This makes measuring an arc of the Earth’s meridian trickier than you might think.
[00:11:59] It was a gargantuan effort, and every small error in the angles, every imprecise reading, would multiply over hundreds of kilometres, so they had to be meticulous.
[00:12:13] They repeated each measurement multiple times under different conditions—morning, afternoon, nighttime, clear skies, cloudy skies—always cross-checking for tiny differences.
[00:12:26] They kept notebooks full of corrections for temperature changes that could affect the metal in their instruments.
[00:12:34] They also had to correct for atmospheric refraction, the way light bends slightly as it travels through layers of air with different densities.
[00:12:45] Any slip-up, any mistake, could lead to a miscalculation of the final metre.
[00:12:52] But perhaps the scientific part, the observations and the calculations, were not the greatest challenge the pair faced.
[00:13:01] One of the most serious obstacles faced was actually on the Spanish side of the border, where Méchain had to deal with local suspicion during a time of war.
[00:13:12] He had the requisite papers with details of his mission, but he was frequently mistaken for a spy, someone measuring fortifications or planning a military route.
[00:13:25] He was arrested, hassled, and delayed for months.
[00:13:29] Meanwhile, Delambre, on his northern journey, was often met with confusion or outright hostility from villagers.
[00:13:39] Despite all of this, the pair inched closer to their goal with extraordinary determination.
[00:13:47] By 1798, after almost seven years of gruelling, nerve-wracking work, the two astronomers finally had enough data to calculate the length of the meridian arc and, with it, define the metre.
[00:14:03] The calculations were complete, and the true distance of a metre could finally be unveiled.
[00:14:10] For the first time in history, there would be a universal measurement based on the circumference of the Earth.
[00:14:18] In 1799, a platinum bar was cast to represent this new standard.
[00:14:26] That bar, known as the “mètre des Archives,” became the reference point for all future measurements, and this is where it gets even more interesting.
[00:14:37] Once the metre was set, scientists could define a smaller unit, the decimetre—one-tenth of a metre.
[00:14:46] A cube that measured one decimetre on each side was declared to be one litre in volume.
[00:14:54] Ta-da, a new universal measurement of volume.
[00:14:58] Then, the weight of a litre of water at a specified temperature [close to 4°C] was considered to be exactly one kilogramme.
[00:15:09] And ta-da, a new measurement of weight.
[00:15:13] Suddenly, length, volume, and mass were all interlinked, rooted in a single measurement: the metre.
[00:15:23] And because everything was in multiples of ten, conversions were vastly simpler than in the old imperial or local systems.
[00:15:33] No more 12 inches in a foot, 3 feet in a yard, 1,760 yards in a mile; now it was tens, hundreds, and thousands—a clean, decimal arrangement that anyone but the most ardent supporter of pounds, feet and gallons can recognise as a superior system.
[00:15:56] Of course, theory and practice don’t always align immediately, and plenty of people in France, especially in rural areas, resisted the new system.
[00:16:08] Napoleon himself, seeing how unpopular it could be, allowed many of the old measures to stay in use.
[00:16:15] But the metric system was already out in the world, and it was only a matter of time before its clarity and simplicity caught the attention of scientists in other countries.
[00:16:29] Throughout the 19th century, the idea of a universal, decimal-based system of measurement spread across Europe and beyond.
[00:16:38] Newly independent countries in Latin America adopted it as a mark of modernity and a break from colonial influences.
[00:16:47] By the late 1800s, the metric system had become a near-global concern, and in 1875, seventeen countries signed the Metre Convention, setting up the International Bureau of Weights and Measures near Paris.
[00:17:04] Even the United States, which still clings to miles and pounds in daily life, was a founding signatory.
[00:17:12] And over the next century, definitions became even more precise.
[00:17:18] Originally, the metre was defined by a platinum bar—literally a metal stick kept under lock and key in Paris.
[00:17:28] If anything happened to that bar—if it got a tiny scratch, or if the metal warped over time—our definition of the metre would change.
[00:17:38] Scientists realised that basing the metre on a single physical object wasn’t ideal, so in 1983, they redefined the metre using the speed of light in a vacuum, which is one of the most reliable properties in the universe.
[00:17:55] We say that the metre is the distance light travels in 1/299,792,458 of a second.
[00:18:08] Because the speed of light doesn’t change, we get an incredibly precise, unchanging definition of what a metre actually is.
[00:18:18] And a similar logic applies to the kilogramme. For a long time, the official kilogramme was a platinum-iridium cylinder, also kept in Paris.
[00:18:30] Just like the metre bar, any tiny imperfection or contamination could change its mass, even if only by an infinitesimal amount.
[00:18:41] So, in 2019, scientists decided to define the kilogramme in terms of Planck's constant, a fundamental number in quantum physics.
[00:18:53] If we measure Planck's constant accurately and link it to the idea of mass, we get a definition of the kilogramme that doesn’t rely on a single metal cylinder sitting in a vault.
[00:19:06] It’s tied, instead, to a property of the universe that remains the same no matter where you go.
[00:19:14] This can all be traced back to Méchain and Delambre’s incredible precision.
[00:19:20] But, and it is a big but, they did make a tiny mistake.
[00:19:26] As you’ll remember, they had intended the distance from the North Pole to the equator to be exactly ten million metres, making the Earth’s polar circumference exactly 40 million metres.
[00:19:41] That figure was close, but not exact.
[00:19:45] Thanks to a series of tiny errors—some the result of the Earth’s irregular shape, others from errors in the triangulation work—the final calculation was off by a few hundred metres.
[00:20:00] And if you trace the sources of this discrepancy, most fingers point to Pierre Méchain.
[00:20:08] He was an obsessively meticulous man who discovered inconsistencies in his results but never fully corrected them, partly out of fear of undermining the entire endeavour.
[00:20:21] As more precise measurements emerged in the decades and centuries that followed, scientists realised that the meridian distance should have been slightly larger, meaning our metre–the metre that you and I know–turned out to be just a fraction smaller than intended.
[00:20:41] But by then, it was too late.
[00:20:45] The metre had already become the reference for countless measurements, scientific instruments, legal contracts, textbooks, and everyday transactions.
[00:20:56] There were discussions about fixing it—simply readjusting the value so that the polar circumference lined up perfectly with 40 million metres—but doing so would have rendered every existing metre stick obsolete.
[00:21:12] Redefining the metre in that way would have caused enormous disruption.
[00:21:17] Instead, the world decided to keep the metre as it was.
[00:21:21] After all, it was only a fraction of a millimetre off per metre, which was more than accurate enough for 18th-century practical purposes.
[00:21:32] Later refinements to the definition of the metre, like tying it to the speed of light, have made it infinitely more precise, but the original length is still the same.
[00:21:44] In other words, Méchain’s tiny error became enshrined in history.
[00:21:49] The Earth isn’t exactly 40 million metres in circumference, but the metric system has worked so well that it doesn’t really matter.
[00:21:59] It was a small price to pay for the simplicity and universality that the metric system brought to the world.
[00:22:07] OK then, that is it for today's episode on the metric system, and the story of how and why a metre is a metre.
[00:22:16] I hope it's been an interesting one and that you've learned something new.
[00:22:20] As always, I would love to know what you thought of this episode. Did you know this story beforehand, and what other scientific discovery stories like this should we make?
[00:22:30] I would love to know, so let’s get this discussion started.
[00:22:33] You can head right into our community forum, which is at community.leonardoenglish.com and get chatting away to other curious minds.
[00:22:42] You've been listening to English Learning for Curious Minds by Leonardo English.
[00:22:47] I'm Alastair Budge, you stay safe, and I'll catch you in the next episode.
