The Rest Is Science - What Day Is It, Really?

Episode Date: December 30, 2025

What day is it, really? And who decided? What happens to time when we leave the Earth? And when might future humans be counting down to the dawn of a New Year in the middle of the day? From missing days and meddling popes to atomic clocks and vanishing centuries, Professor Hannah Fry and Michael Stevens dive headfirst into one of the slipperiest questions in science and society: what IS the time and date? On the way they trace a tiny error by Julius Caesar, a correction by Pope Gregory XIII, and how time itself has bent to human will. In the process, faiths have quarrelled, centuries have blurred or been deleted altogether and yet, we crave precision in a universe that drifts, producing clocks so exact they must pause to let the Earth turn. ------------------- For more information about Cancer Research UK, their research, breakthroughs and how you can support them, visit ⁠⁠⁠https://cancerresearchuk.org/restisscience⁠⁠⁠ Cancer Research UK is a registered charity in England and Wales (1089464), Scotland (SC041666), the Isle of Man (1103) and Jersey (247). A company limited by guarantee. Registered company in England and Wales (4325234) and the Isle of Man (5713F). Registered address: 2 Redman Place, London, E20 1JQ. ------------------- Find The Rest Is Science all over the internet by ⁠clicking ⁠here⁠.⁠ ------------------- Video Producer: Adam Thornton Video & Social: Bex Tyrrell Assistant Producer: Imee Marriott Producer: Becki Hills Senior Producer: Lauren Armstrong-Carter Head Of Digital: Samuel Oakley Exec Producer: Neil Fearn Learn more about your ad choices. Visit podcastchoices.com/adchoices

This episode is brought to you by Cancer Research, UK. Dinosaurs walked the earth 180 million years ago. But as you know, cancer was part of their story too. Scientists have found tumors in ancient fossils. Well, that is part of the reason why cancer is a big, big part of our story, right? It's the other side of evolution. It's the most complex disease that we face. There are more than 200 types of cancer in total, each with distinct characteristics, challenges and mysteries.

And that complexity demands scale. Cancer Research UK is the world's largest charitable funder of cancer research, with more than 4,000 scientists, doctors and nurses working across more than 20 countries in the search for answers. And then sharing their discoveries beyond borders. And the impact of this collaboration is clear, because over the last 50 years, the charity's pioneering work has helped to double cancer survival in the UK.

That is more people who are living longer, better lives. can show us the past, but research is shaping the future. And for more information about Cancer Research UK, their research, breakthroughs, and how you can support them, visit cancerresearchuk.org forward slash rest is science. Welcome to the rest is science. I'm Professor Hannah Fry. And I am Michael Stevens, creator of Vsauce on YouTube. Hey, happy new year, Michael. Okay. Are you teasing me?

Why? Is it really a new year? really 2026? Like, are we sure people have been counting without missing, you know, a day or a year here or there? We may have been lied to about what time it is. And that is what we're going to be talking about on this episode. We are going to look at days that never happened, centuries that disappears, and whether or not we have been keeping count. All right. So let's begin by talking about what day everyone says it is. Like, we pretty much all agree. It's the 2020s, according to the Gregorian calendar. And that was decided for us in 1582 by Pope Gregory the 13th.

Unlucky 13, if you ask me. It's quite a lot of Gregory to have happened since the year zero. Or I guess the year one, because the number zero really wasn't in regular usage until Shakespeare was walking on the earth. That's true, but not even then, because in the year one, Jesus hadn't made Peter the first Pope yet. So, like, the Gregs are busy. The Greg's busy. Okay, but why, so why did Pope Gregory decide that this was the calendar that everybody should have? Okay, let's do the long and short of it, and please pop in and help me, because I'm not completely up to snuff on my Greggs.

But before the Gregorian calendar, a lot of Europe was using the Julian calendar. And this calendar had started in the year 45 BC, okay? And it was pretty good. I'm sure you've got a lot of criticisms. And it shows, okay? The Julian calendar assumed that there were 365 and a quarter days in the year. You're absolutely right. I've got criticisms because that is out.

That is out for how long it takes for the earth to actually rotate around the sun by 11 minutes and 14 seconds every single year. Burned. Oh, Julius. You idiot. You idiot. 11 minutes off. And yeah, what was really relevant to them at the time Wasn't exactly the astronomical year,

but the tropical year, the time between the equinoxes. That was something that you could measure very easily from Earth. When is the day and the night the same length? Okay, and that would happen twice in a year. And they tried to make the organization system work, but because in reality a tropical year is 365.2422 days, it wasn't quite right. And the Julian calendar was off by a little bit of time.

I think it added in too many leap days as well. And so by the year 1582, the Equinox was not happening on the day that it should. Should. Who says should? Well, the astronomers care quite a lot because the equinox or equinox, depending on which side of the Atlantic you happen to be from, it's the moment when the sun crosses the equator, right? So it's that moment where the tilt of the earth and so on, it's that moment.

And I mean, maybe if you're not an astronomy, you wouldn't care that much. But the Catholic Church themselves, they also really cared about this because Easter is set as the first Sunday after the first full moon after the spring equinox, which they said was the 21st of May. But there kind of came a point. I mean, hundreds of years before Pope Gregory came along. where, you know, this day was slipping further and further backwards in time. Because, okay, it doesn't sound like very much if you're only 11 minutes, 14 seconds or whatever it was, out every single year. But if you are, you know, if you're doing that every single year, it means that by two years in, you're 22 minutes behind. You know, three years in, you're 33 minutes behind.

It starts to compound these errors. So even by the 1200s, the astronomers. the astronomers were like, guys, you know, this Equinox is like, it's getting further and further away. And then there was one guy called Roger Bacon who wrote this big document, sending it to the Pope, begging them to change the calendar and was like, eventually Easter is going to be a summer holiday. You need to sort of do something about this. Yeah, and that might not have been a problem. You know, I think people like Bacon were upset with Easter shifting throughout the year.

but also in the year 325 AD, the Council of Nicaea got together to try to figure out some of this stuff. I mean, they got together to figure out what Christianity was. And determining when Easter should be was one of the problems. And there was a lot of fights about whether they should continue to follow the Jewish lunar calendar or not. And they never decided when Easter should be. I mean, they didn't even decide it should be on a Sunday back then. All they did is that they said, it is hereby decrees. that the Vernal Equinox, the spring equinox, must happen on the 21st of March.

So they started dictated to the stars and the moon, basically. They dictated that and they also set this really important rule for all Christians, which was that Easter had to be celebrated simultaneously worldwide by all Christians. So they needed to standardize how they named and numbered the days, because they all had to choose the same date. And by the time of Pope Gregory in 1582, Gregory the 13th, want to respect the Greggs, by his time, the Vernal Equinox was happening not on the 21st of March as decreed by the Council of Nicaa, but instead on March 10th. So something had to be done. What did he do then?

Well, he decided, screw it. We're going to just jump 10 days forward and we're going to rejigger the algorithm of the calendar, name it after myself. It's going to be better. We're going to do leap days every four. four years unless the year is divisible by four and a hundred. I mean, it's kind of a whole complicated system. We still use this today because it's okay. It's better than the Julian calendar.

And this is how they get around those 11 minutes, right? It's that we have leap years every four years, but with a twist. So if a year is divisible by 100, it's not a leap year, unless it's also divisible by 400. So 1900 wasn't a leap year, but 2000 was. And that's essentially how they do the mass, how they manage to keep the calendar in single. with the Earth orbit. That's right. And so, on the 4th of October, 1582,

the next day should have been the 5th of October. Pope Greg was like, nope, it's the 15th of October. Scyke. And... Unlucky if your birthday's on the 10th. Isn't that unlucky? So I always love to, if I don't know a historical answer, I always say, oh, I think that happened on October 7th, 1582.

And everyone's like, really? And they look it up and they go, there never was a 7th, of October 1582. He's a wizard. And I'm like, guilty. He also didn't change the actual days, though, right? Didn't he, he said, okay, the date is changing. But even though it was a, I don't know, a Tuesday,

tomorrow is still going to be a Wednesday. Yeah, it was a Thursday. October 4th, 1582 was a Thursday. And the next day was Friday, October 15th. And so if you're ever, like, feeling bad on a Monday, just think that if it hadn't been for the Pope, it would be Friday. This is why I'm late all the time, right?

Because it's like, it's actually Tuesday right now. Exactly. Whatever day of the week you're listening to this. We are still living in a world reverberating with the consequences of this 13th grade. The thing is this old Greg, old Greg 13th. I mean, this is the Catholic Church, right? This is a point where they are extremely powerful, but not everybody in Europe in particular is underneath the Catholic Church. And the Protestants in particular were very suspicious of this.

They thought that actually this was a trick that the Catholic Church were pulling to make it. So the Protestants celebrated Easter on the wrong day and therefore would damn their souls to hell. It was just a mean trick. And the astronomer, Jonas Kepler, he said that Protestants would rather disagree with the sun than agree with the Pope. I think it's probably fair to say that Catholics and Protestants,

not the best of friends, Michael, but maybe we'll leave that one to the rest of history, shall you? As far as long as said, there were some other people who were really worried that Catholics were trying to steal time so that they could live longer and different countries adopted it at different times. So Poland, they were like,

okay, we're going to do whatever Pope Greg says, we're happy with this. They adopted it almost immediately. But their neighbour, Prussia at the time, they took another 30 years to decide that they were going to adopt it, which meant that if you lived on the border, you could send a letter from Poland

on October the 15th, and it could arrive in Prussia on October the 6th. Yeah. I mean, because all of this confusion is funny, but it makes sense. I mean, imagine if tomorrow Donald Trump said, that's it, I'm changing the calendar. Year zero is now my birth date. It's actually the year 79. Like, I can't wait for the year 80. Happy New Year.

Not everyone would go along. There would be all kinds of resistance. And, yeah, it took the Gregorian calendar centuries. finally get adopted. I mean, technically, Saudi Arabia didn't even accept it until 2016. Well, I think the Ethiopian Orthodox Church still hasn't. Their calendar uses one that's about seven to eight years behind the Gregorian calendar. Yeah, and good for them. They think it's 2018 at the moment, which means someone should really warn them about what's coming in 2020. Yeah, right? So, I mean, that actually, that joke actually gets at two very different

kinds of time. There's physical time, the tapestry on which events occur in, and then there's also organized, human standardized time that is a series of languages and words that describe and name them. And so, you know,

I've always joked that if, like, Jesus had been born 25 years later, in sync would have been the hottest band of the 70s. And that also toys with the fact that there's time, which is the order, the chronology of events, and then there's what number we give them. And yeah, we're still all kind of like using multiple calendars. I don't want to date this episode really specifically, but it is what, the 10th of December,

but if you're still using the Julian calendar, it's only November 26th. I do often wonder also whether our perspective on the year would be very different if we had decided, instead of tying it to the birth of Jesus, but had tied it to the beginning of human civilization or the agricultural revolution or whatever it might be. Like I think that if we were living in the year, a hundred and, you know, 25,000, 782, you know, looking back to, say, the Second World War or even, you know, things that happened in the Middle Ages, it would feel like they would feel like much more present than they do at the moment. Well, I like the Holocene calendar proposal, which is that we just, just add 10,000 years. So right now it's actually the year 12,025.

And that one, instead of starting at the birth of Jesus for the positive integer years, we start at the end of the last glacial recession, which is really when humanity started to kind of be the dominant force of life-changing Earth. I think it makes you feel more connected to your ancestors in a lot of way, actually. Just that tiny, subtle change of adding a one to the beginning of the year. Yeah.

And the change of not putting BC on years so that it seems like an alien world. If instead Aristotle was doing stuff in the year 8,000, and we were in 10,000, it would still feel like we were more contemporaneous, right? Yeah, yeah. Which in a lot of ways we are. We are. Yeah. I mean, not to get too far off field, but like we're still living in the Stone Age. I mean, our world is still made out of things that we find in rocks, concrete, steel.

Like, we have not actually moved on. Oh, silicon? That's still a stone. It's sand, you guys. It's sand. We are still stone age people. We were never modern. Like, let's get over it.

I mean, lucky for us that it turns out you can do quite so much with sand. We probably wouldn't go as far if you needed gold in order to build computers. Oh, man. Imagine that. Only squillionaires would be allowed to have them. It's actually almost weirdly lucky that... It is weirdly lucky, isn't it? It's so easy to make computers out of the dirt all around us.

Maybe even suspiciously lucky. Suspiciously. Suspiciously lucky. We'll do that one for another episode, don't we? Here's something else that's suspicious. We need to talk about Pope Gregory the 13th again. He added 10 days to the calendar. He jumped 10 days forward because that's how far the Julian calendar had drifted off of

the actual motions of the earth and the sun. But if you do the math from 45 BC until his time, 1582, it all should have been off by not 10 days but 13. So what happened? Well, in the 90s,

this guy named Herbert Illig, great name, by the way, started to put together a hypothesis that he called the Phantom Time Hypothesis. And the idea is that the Pope was off because the popes had lied to us about what year it was, that there were three missing centuries, which accounts for those three days that they didn't have to add on. Isn't thou shan not lie, one of the Ten Commandments?

Yeah, right. Or maybe not right, because we need to investigate this phantom time hypothesis a bit more. I'll tell you, I'll tell you some more of the details. So, according to Illig's theory, the years 614 to 911 never happened. Okay, they were completely fabricated because at the time, Pope Sylvester II, not only wanted that putty cat, but also wanted to rain during the year 1000. So they said, let's just fudge the numbers. It's not, we're not in the 600s. Oh my gosh, you guys, it's almost the year 1,000.

Ha ha! And they fabricated all of the history that today we are taught happened in those three centuries. That means Charlemagne never existed. That means that the Carolinian period was a complete fabrication. They just made it all up to trick everyone into thinking that it was the year 1000. Hold on those. So it's the idea that the Catholic Church went and created all of these original sources. I mean, Charlemagne's essentially fan fiction.

Is that what we're saying? Constructed, put in place. Wow. Okay. That's, I mean, that's quite a bold statement. Are they doing that on the basis of just thinking that Pope Sylvester was really into branding of like wanting to hit the year 1000? Yes, yes. And I think also just a suspicion of authorities that like if they could do something dastardly like this, they would.

And Illig's only evidence really was the aforementioned 10 days jumping forward when it should have been 13. And also the kind of vague like hand wavy. I mean, can you even name something that happened in the year 700? There aren't a lot of historical records from that time period. And he was like, well, that's because it's all men made up. So the idea then is that the dark ages are kind of like dark matter. It's just sort of a bit of a kind of shady, unknown region. You just give it and even move on.

That's a really good analogy because it's like, I mean, it happened in that the events they say happened matter. It has an effect on us, but you can't see it. You can't interact with it. because it never physically happened. Now, unfortunately, because it would be very funny if it was the case, it'd be like the greatest lie ever told. Illig's theory never got very popular for a few reasons. First of all, the Gregorian calendar's 10-day jump forward instead of 13 is easily explained

by the fact that the Julian calendar had not remained untouched since 45 BC. The Council of Nicaa adjusted it a little bit, and that completely answers that problem. Well, there's other ways too. I mean, like science steps in to really help us out on this one. Because there is, there's an area of research, which is called it Dendocrinology. This is like super fascinating. So the idea is that you chop down a tree, it has tree rings. You get one tree ring a year. And the reason for that is because of the seasons, right? So the way that as a tree grows, it grows faster and slower, depending on the conditions that are surrounding it. So if it is warmer, you'll get, you get more growth or so. sort of get a faster growth. You'll get a wider ring. If it's cold, you'll get it kind of narrowing, which is what happens over winter. But it also changes depending on whether there's a lot of rainfall one year,

also how much sunlight there might be. And what you can do is you can take two trees that have existed in a similar place, and when you cut them down, there is a way to cross-compare those tree rings and see the same pattern of growth appear in both of them. So maybe one tree is born 50 years after the other. but you can work out sort of daisy chain them as it were

work out where the overlap is and where one tree out survived another so what scientists have done is they've managed to do this really successfully with trees all across Europe also in America as well German pine is amazing at this you get really really old German pines

but also in the the bog lands of Ireland which is where my family are from the bog people that's who we are I didn't know that we are absolutely the right in the centre of Ireland and the heart where the peat bogs are, you get this sort of like semi-phossilised Irish oaks in there, which can go thousands of years old,

and using this exact same technique with modern oaks that are around now and then all the way back, you can daisy-chain this so that we have basically an unending record of the sort of climate of a particular region that goes back over 10,000 years. Now that on its own doesn't tell you anything about, what year the church we're calling it. Yeah. But what you can do is you can go into a building that is, say, built in the year 800 AD, right? Or 1,000 AD, whatever it might be.

And if there is oak timbers in there, they can be analyzed and slot it into this timeline that we have about what was happening in the climate of the world. And every time you do that, they slot in exactly without. those 300 years that are missing. And there's kind of more evidence for this because in 536 AD there was this gigantic volcanic eruption in Iceland. It was so bad

that across Scandinavia crops failed, people abandoned villages you find all of these sort of hidden gold in parts of Scandinavia which around that time they think that people were sort of making offerings to bring the sun back because the volcanic ash was so dramatic.

But you see this scar effect inside of the rings of trees all across the region. But you also, of course, right, if the sun gets blacked out by a volcanic ash, there's also this written record of it. You know, monks are writing about this. People are sort of, you know, writing about the villages being abandoned. And it all ties up exactly. I mean, if these 300 years really were missing,

then the tree records and the written records would be out by 300 years. But we know that they're not. I didn't know about that. That's beautiful. I love that the Earth can like hold us accountable. I had heard about the problems of solar eclipses, that there have been all these reports of solar eclipses at certain times and they all line up exactly when they should with this specifically this 297 year gap that the Phantom Time hypothesis says didn't happen. Unfortunately, it looks like it did. Because you could wind back the clock with the simulations that we have of the solar system now and work. out precisely the day that solo eclipse has happened in the past. And I love to imagine, I mean, this is one of my hobbies, basically, is to imagine when you had an event like that, before we

really understood what was going on in the sky, how unimaginably terrifying it must have been, to be like, okay, everybody, today the sun disappeared for like a few minutes and then it came back, what on earth does it mean? I know. And you better believe that people are writing about that. In between all the plagues and the murder and stuff, there was lots of people writing about those solar eclipses. It must have been so terrifying. Because when

a solar eclipse happens now, it terrifies me. It's just such a big event that I cannot stop. Like, I'm very much a person who's grown up in a society where I can press pause and I can decide when I do what.

And yet, the eclipse is going to happen whether I want it to or not, whether I'm ready or not. And I've, I've been visiting like every eclipse that I can. And I get so anxious just before it happens. It's a very scary thing that these two things that are bigger than I can even imagine are about to do something. And it's going to happen when it happens. But imagine not even knowing that it was going to happen or how long it would go on or what it meant. Yeah. And that then being used as proof that the Catholic Church were telling the truth all along. Imagine that on top. Imagine that cherry on top. Maybe the sun and the moon and the trees are actually in cahoots with the Catholic Church.

I like how you're thinking. The tree of knowledge, more like the tree of ignorance and lies. Okay, I'll tell you what. We'll agree for now on what day it is. That's fine. But as for what time it is, I mean, that's where stuff gets messy. Join us off to the break. This episode is brought to you by Cancer Research UK.

We often think of beating cancer as treatment, but imagine stopping it before it begins. After years of work, cancer research UK scientists are launching a clinical trial of lung vax, the first vaccine designed to prevent lung cancer. It builds on TracerX, the world's largest cancer evolution study, which tracked lung cancer cells over many years to uncover the disease's earliest warning signs. Lung Vax is designed to train the immune system to spot these. signs early on, destroying 40 cells before cancer develops. So it's not treatment, but preventative, with the potential to stop lung cancer before it starts. The first stage of the trial starts this

year focusing on people at higher risk. It shows what long-term research makes possible. For more information about Cancer Research UK, their research breakthroughs and how you can support them, visit cancer researchuk.org forward slash the rest is science. Welcome back. We are today asking, when is today? Is it today? When is it really? What time even is it, Michael? That's where we're at today. Think about New Year's, right? There's sort of nothing cosmological, nothing agricultural, nothing seasonal about the reason why it's got to be the first of January. It's just sort of, it's kind of a collective lie that we've all brought in on. But why?

I actually have no idea why January 1st was chosen to be the beginning of the year. It comes back to the Roman. So the original Roman calendar, they actually only had 10 months. They started in March. Decimal month system. I like that. Right. But this is still there, right?

So October, the 8th month, November, the 9th month, December, the 10th month. And then they just never bothered with January February because they were like, it's dark. There's no crops. Don't worry about it. It's just, you know, get through it. Get through it. That makes sense. They also, this is the reason why you've got 31 and 30 days because odd numbers were lucky.

They preferred odd numbers. Think of sort of Roman numerals. You kind of wanted a little stick in the end. So they were sort of like, oh, let's just sprinkle some odd numbers through these 10 months. Then in about 713 BC, they were like, okay, maybe we'll count these extra ones. This extra bit of time, we'll add in January and February. And then in 153 BCE, I agree with you about the calendar, not liking it. Let's call it in the year 9,000.

Thank you, Holocene calendar. I love it. 9,847, that's the sort of it would have been. Wow, you should be a math professor at Cambridge. You know what? I might just consider it. Thank you. Anyway, so this is the Roman Empire, the days that we're in, it's all kicking off in Spain.

Spain are revolting, there's all of the stuff going on. And the Romans need to appoint two new consuls. These are the people who order the army what to do. But they've got this rule that they're only allowed to do that at the beginning of the year. Now, up until this point, the first day of the year was the first of March, the beginning of spring, the beginning of their calendar. Okay. But now they're stuck, right? Because it's all going off in Spain.

And they've got to wait a couple of months before they are, before it's the beginning of the year when they're allowed to appoint new consoles. So rather, rather than changing the rule, they decided, let's just change when the year begins. So basically, the reason why it's the 1st of January, it's admin. It's the father of this modern holiday. So that's how it happened. Yeah, that's how it happened.

Wow. And just like the Gregorian calendar, I mean, no everyone was happy. This is, you know, again, it was all over the place. You had, in England, it was the 25th of March. And in France, it was Easter was the beginning of the year. In Scotland, it was, you know, January and Russia, it was September. It wasn't until sort of the 1750s that everyone sort of started to slot into place

with this idea that the Gregorian calendar and the 1st of January was the correct date. Yeah, it takes a long time, especially back then. I think as the world becomes more connected, you have to confront. the importance of standardization. No one really cared until people were moving around enough that they needed to sort of cross-reference it. I mean, it's the same as time. You know, for most of human history,

I mean, it didn't matter what time it was. You get up, when you get up, you go to sleep, when you go to sleep. Yeah, if you didn't have to coordinate what the year was or what the time was with people in a different country, then you didn't need to think too much about it. As long as it worked locally, it was fine. I think that that idea about coordinating with another location, we didn't actually have to worry about what time it was at all

until we had ships and trains. When it comes to ships in particular, if you, there's a strange truth, which is that if you want to know where you are, you need to know when it is. So north and south is easy. You just look at the angle that the sun makes in the sky at midday. So, of course, if you're, if you're, if you're, if you're, you know, really at the North Pole, the sun will be very low in the sky. If you're at the equator, the sun will be very high in the sky.

So you can work at how far north you are. Right. But how far east and west you are is much, much, much, much harder. It relies on knowing what time it is relative to a fixed position. So, for instance, if you know when it's midday in London, you can calculate when it's midday where you are, if it's two hours difference, you can work out how far around the earth you are. And this was such a gigantic problem for so much of history of how do you hold London time when you leave the city effectively.

People had all of these different mechanisms that they used to try and like hold on to working out what time it was. They had they had candles that was you could melt them down. They had sort of water clocks. They had, you know, even when mechanical clocks were invented, you know, a person. pendulums, they tried to put those on ships and realize that actually they, you know, pendulums don't really work on a rocking ship. Yeah.

And then there was this really big maritime disaster in 1707 this is. I mean, which we're already quite far into, you know, the idea of sort of naval warships. There's this Royal Navy fleet that's heading home from Spain, from Gibraltar, in fact, part of the war of succession in Spain. And they had done their calculations based on a clock that was supposedly set to London time, but that wasn't very good at keeping the correct time, could be off by minutes a day or even hours in a day. And they did their calculations and they thought that they were really safely in the English Channel.

They thought they were almost home, basically. It was very bad weather as well. But they were actually dozens of miles further west than they thought they were. and so they went aground on a reef and the whole ship sank in a couple of minutes and nearly 2,000 sailors ended up dying and this was the big motivator there was this great big prize

being offered sort of like you know a 1700s X factor that's kind of what we're talking about here of anybody who could make a timepiece that could keep time at sea and eventually a watch is created for exactly that purpose Okay, so wait, wait, these sailors died because they were in the wrong place because they didn't know what time it was.

Exactly, exactly. Thousands of people in this massive disaster, yeah. I honestly don't know this very well. So if you know, tell me, to determine how far east and west you are on Earth's surface, what would they do? They needed to know the time. And then they could say, well, the sun should be here. this time. But if it's not, it's because I've gone east. And the difference allowed them to determine how far away they were from, say, London. Precisely. So you keep a clock ticking on

London time. On your ship. On your ship. Let's imagine it's a watch, right? You keep your watch on London time. And when you're on your ship, you also record when it's midday. And at midday locally, you look and see what time it is in London. And midday is like a solar observation. Absolutely. When the sun is at its highest point and starts going back down, you've hit midday. You know it's midday. So then you look and see what time is in London. And if you are an hour ahead of London, then you know that you must be further to the east of London. Right, some number of degrees east of London. That was hard to do, though, because you needed a really good clock.

And the clock that you had, any of the clocks that you had, would run fast and slow or be affected by the motion of the waves. And so you couldn't be sure that it really was midday in London. Your clock might say that it's midday in London, but actually it's, you know, half 11 or 12.03 or whatever it might be. Yeah. Which to, you know, it doesn't maybe. seem like it matters, but when those three minutes gets translated to miles on the ocean, it can make this

really gigantic difference. To even track the time, to agree what time it is in London, to measure it precisely is all about humans moving around. Yeah. By the time trains come along, steam locomotives

come along, now you can move really quickly across land. And people start to realize that there's like this jet lag thing, that you'll get somewhere fast enough that you're hungry and yet it's not actually the middle of the day when you normally eat like shoot and you talk to people there and they're like yeah no it's not noon and you're like what the heck well okay we've all got to agree on what time it is like if it's noon in new york but now i'm in say st louis we needed to standardize time across cities across entire

countries. We need to invent time zones. And so we still live in a world defined by the consequences of the train. We do, but we no longer live in a world where the time is dictated by the rotation of the earth. That's right. Yes. Because it's atomic clocks now that are the ultimate timekeepers that ultimately tell us what time it is. It is. And it is such a significant moment in human history, when we went from allowing the sun and the earth to tell us what time it was, to us saying, no, shut up, we know what time it is. And we stopped looking at the position of the sun and sunrise and sunset. And we started to say, let's measure time more accurately than you guys are. You know the idea for atomic clocks? They date back to Maxwell in the 1870s. No kidding. Him and William Thompson, and they realize that in theory, Hold on, Thompson. This is the electron guy. Yeah, yeah, yeah.

Wow. I mean, they'd only just discovered the atom at this point. Yeah. It's phenomenal. But they realized that because this particle was like, it was stable, you know, unchanging, identical. Each atom of a particular atomic weight was identical to each other. They realized that in theory it could make this perfect clock because every atom is, in a way, this tiny, perfect little pendulum. So lots of atomic clocks at the moment they're made of cesium.

And the thing is, if you give an atom of cesium exactly the right amount of energy, then its electrons will jump between energy levels at this frequency that is absolutely specific. So for cesium 133, this tick rate is 9 billion, 192, 631, 770 oscillations per second. I mean, it's a big number. Now, the way that you do this is you need to give it exactly the right amount. of energy in order for that to happen. And so what you do is you take a microwave, not like one you'll find in your kitchen. It's a sort of much more scientific, perfectly tuned thing.

And then you have this like cloud of cesium sort of moving around. And you tune this microwave to blast that cloud of cesium atoms with energy. And if you hit it at exactly the same frequency that the cesium atoms like, that 9 billion, whatever it was, that's the point where the atoms will start flipping their electrons. So what you can do is you can adjust this microwave until the frequency of the microwave changes the energy state of these cesium atoms, and you know you've exactly hit that precise, precise, precise frequency.

And that then becomes the tick, as it were, of this clock, when 9 billion, 192 million, 6008-1, 770,000 of those oscillations have occurred, and then you know that's one second, exactly. Wow. Now, these things are so phenomenally accurate that they would lose less than a second over 15 billion years. I mean, it's like, it's mind-boggling the level of precision that you are capable of getting via this method. That's incredible. I don't know if you know the answer to this, but why do you still lose a second every 15 billion years? Because the cesium atom is vibrating in a way that's independent of what temperature it is. or its orientation in space.

But like, what's causing it to lose time? Well, check you out, the perfectionist, Michael Stevens. One billion. Yeah, answer. Answer for yourself. Okay. Not having it with one second of 15 billion years. I mean, the reality is if you're doing this experiment in the real world,

you are going to have these variables that you can't control. You have quantum uncertainty. You have maybe background magnetic fields that will interact with the experiment. you have these tiny, tiny, tiny, environmental variations, essentially, that can accumulate over time. But really, really, really, really not very much. Yeah, not much at all. You know these atomic clocks, though, so you can't have just one, because even though it's precise, there's still measurement error, there's still, you know, things that could go wrong.

So the way that they have this, the way that we actually agree on what time is now, is you have these atomic clocks, there's a number of them that's sort of all over the world. and once a month they run these experiments where they do something like they'll ping a laser from one clock to another clock via a satellite. Both of those centres will write down precisely the atomic time on their clocks at which that laser arrived. And because we know the speed of light, you can work out exactly how those clocks are in sync with one another.

But the thing that I like so much about this is that once a month, all of that data gets collected by a group in Paris who, do sort of weighted average and work out exactly what the time is. But they communicate via email. So the way that we know what time is is that we do it by email. Oh, can you imagine, like everything gets messed up because that email went into the spam box of some guy in Denver and now suddenly America is like off and France is living in the future? I mean, we may all still be off though. I mean, like we haven't solved it. This idea of what time is, we haven't solved it because atomic clocks, incredibly precise metronomes, our planet, a wobbly rock, rotating around other wobbly rocks in the vastness of our solar system.

And you have all of these other factors that are going on too. You've got, you know, magma sloshing around in the centre. You've got winds, you've got kind of currents, you've got tides, you've got earthquakes. I mean, it's just, it's jiggling about all over the place. And if you have it, compared to this extremely precise scientific. measurement, the two are going to deviate from one another, which essentially is why you need leap seconds to just regularly nudge it back into place. Right. So we're looking at these atomic clocks being like, you're doing a great job, like keep it up. But we're going to need you

to pretend it's been an extra second. We're going to do a leap second so that the earth can keep up. Exactly. Sacrificing a couple of pieces in chess so your kid can win. That's what the leap second is. It's like us being kind to Earth. Yeah. Well, if we figure out a way to all agree on exactly what time it is and we measure the passage of time, the physical flow of time with precision that's just amazing, we still are going to wind up with new problems as we explore the solar system. Okay. That's when nature rears its head and says, uh-uh, you don't want what you've created. You've built a monster because we go to Mars.

Guess what? A day on Mars, the time it takes between, you know, midday and midday again, is not 24 hours. It's like 24 hours and then another 36 minutes. So every few months, they're a day off. And communicating with Earth when you live on Mars is going to become a much bigger nightmare than time zones are today, where at least we're all still like spinning around at the same rate and going around the sun at the same rate. But Mars goes around at a different speed, goes around the sun at a different speed, and it goes around its own axis at a different speed. And so I feel like one of the biggest, most immediate crises we're going to have as a interplanetary species

is that we will be segregated by what time we think it is and how we live. It's interesting, again, that it's the fact of us traveling that is going to change the way that we view day and time. That's right. And no amount of physics and atomic clocks will fix the fact that, the natural day on Mars and the natural day on Earth are what they are. I mean, I guess the simplest solution isn't the solution, which is that we do ignore the fact that the planets are moving and just go to atomic time. But there's consequences to that too, right?

Because the Earth's rotation is different to that on Mars, but it's also different now to what it was in the past. That's right. It's a big sloppy mess. It's a big sloppy mess. So, I mean, the earth is slowing down, right? We know that it's slowing down on its axis. You've got this tidal pull that's effectively acting like a break. You can see this in ancient coral.

If fossilized ancient coral, you can count up, it's a bit like tree rings, right? You can count up how many days they were in a year, these big changes that happened seasonally, but also through night and day. And you can see that 400 million years ago, there were 420 days in the year, right? The earth was spinning a lot faster. And so if you go forwards in time and you stick to atomic clocks, we're slipping by 1.7 milliseconds a century, which doesn't sound like much, right? But the thing is, that's 1.7 milliseconds that the earth is too slow every year. So, you know, today it's, you know, in 100 years, it's 1.7 milliseconds too long the day.

the day after it will also be an additional 1.7 milliseconds too long. After 10 days, you're not 1.7 milliseconds behind, you're 17 milliseconds behind. And after 100 years, you've lost 30 seconds, right? Which means that actually if you want to flip midday to midnight, if you want to be celebrating New Year around lunchtime, you actually only have to wait until the year 5,700 AD, or 15,700. in policy

calendar. So I guess all of this makes it really clear that time is both very precise as a concept but also

when it comes to living in time it's very it's like smoke escaping through our fingers and I think the ancients knew that because let me give you

a little fact I looked up just before the podcast. Time the word time T-I-M-E probably comes from this

proto-Indo-European root word for divide. Time is how we divide up the sequence of events in our lives. But now let's talk about time, the herb that we eat. Do you know where that word comes from? No. T-H-Y-M-E. The herb time, or herb, as you might say,

comes possibly. We're not entirely sure, but it might actually come from the root word for smoke. That's why we have Greek words like timon, which is incense. Okay. Or timio, which means to fumigate. So maybe time has been time all along. Just this smoky essence that you can't quite grab onto, but you keep trying to hold on to.

What an excellent point for us to end the episode. So there you go, everybody. Your New Year's plans, your New Year's resolutions, stop trying to hold onto the smoke, everybody. Just let it go. That's right. Just go with the Gregories. All we have to hold on to is each other. And of course, Pope Greg.

Well, if you've got questions for field notes, you can send those into The Rest of Science at Gollhanger.com. And check in next Thursday. And sign up for our free newsletter at the rest is.com forward slash science.