Why Do Days Start At 12 O'Clock?
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On this podcast, we take questions from curious kids just like you, and we find answers. I'm Jane Lindholm, your host. We have gotten questions from kids in all 50 states now, thanks to Elise and Cooper in South Dakota who sent us a question just a few days ago. And we've heard from many of you in other countries as well. We're up to about 48 different countries that you have sent us questions from. That makes us really happy.

You know, it felt like it took a long time to get questions from all 50 states in the United States. But depending on how you look at it, it was also a really short amount of time, less than two years to hear from so many different places. That got us thinking, how does time work anyway? Why do some things feel like they take forever, like if something is boring or unpleasant, but it can feel like no time at all when you're having fun?

And how does time work, anyway? This week, we're focusing on time. How do people decide to have one minute or one hour be one minute, 60 minutes, or 60 seconds? How is time created? Why do the days start at 12 o'clock in the morning? Why do clocks have to go clockwise?

Before we get started, I want to reveal something to you. We've really struggled with this episode. It's actually very hard to explain time in a way that is easy to understand. Time is complicated. Time is essential to the study of physics, which is a branch of science that looks at the way things work, how everything works, really, but things like matter and energy, the movement of the earth, the reason heat travels better through copper than wood.

Physics deals with heat, light, sound, energy, magnetism, mechanics. And time is a big part of the study of physics. And physics can be complicated to explain just talking about it. Plus, some things that can be explained by science still feel kind of mysterious to us. Like how a day can take forever if it's the day before something really exciting is going to happen to you. Or why time feels faster to adults than it does to children.

So we're going to do the best we can. Now, here's one thing that's pretty universal for most of us anyway. Our daily lives rely on routine. And routines rely on a standard idea of a clock and a day and an hour and a minute. But people had to agree on the idea of what a minute is, what an hour is, how we measure time and how we record it.

So we're going to get into this and we're going to get help in this episode from someone who has a lot of expertise. Hi, this is Andrew Novick. I work at the National Institute of Standards and Technology in the Time and Frequency Division. That was a big title. But what's interesting about Andrew's job is that he basically works on time for the federal government. Governments need reliable measurements of time. So some people have a career thinking about time on behalf of their governments.

Here's our first question for Andrew. My name is Allison. I live in Englewood, California, and my question is, who invented time? And who chose what time of day would be what hour? And why is time segmented into 12-hour periods?

That's a lot of questions, Allison. So let's have Andrew take them one by one. First, why is time segmented into 12-hour periods? The ancient Egyptians had a base-12 number system, and we use a base-10 number system. The base-12 number system is called the duodecimal system, and it might have been because of the 12 lunar cycles during the year.

Or it may be because they were using their fingers for counting, and they used their thumb counting each joint of their other four fingers. And they were the inventors of the sundial in about 1500 BC. So that's why we have 12 hours during the day based on the sundial. Sundials were used to tell time before mechanical clocks were invented.

I don't know if you've ever seen a sundial, but you could kind of think of it like a plate with a stick sticking out of it. The stick gets pointed to the north, and then when the sun shines down on the stick, it casts a shadow on the face of the sundial. And with that shadow moving through the path of the sun, it can tell you how time is passing throughout the day.

Andrew also mentioned base 12 and base 10. That's kind of confusing, but basically most of us think of things in units of 10. If you use the metric system, things very naturally fall into units of 10. But older systems of measurement were based around the number 12. And you know how we often use our 10 fingers for counting? Well, another way of doing it would be to use the fingers on, let's say, your right hand.

So bend your fingers on your right hand. Are you doing it? Do you see how there are three segments on each finger? When you have it bent, you can see three different sections. If you add the three segments from each of your four fingers together, that makes 12. 4 times 3 is 12. And you could touch each segment with your thumb as you count them to keep track.

It's just another way of creating a system of counting or keeping track of things. And one of the main things about how we think about time is as a way to keep track of things. The second part of Allison's question was who invented time? Now, no one actually invented time itself. Time exists whether we keep track of it or not, whether there are people on this earth or not. We can't stop or start the passage of time or slow it down.

But how we think about time is something that humans have invented systems around. There's been lots of different ways that different cultures have to perceive of time or to record time or to measure time, from calendars all the way to atomic clocks. So there's no really one entity or person who invented time.

Allison also asked how we chose what time of day would be what hour, and we're going to get into that a little deeper in a couple of minutes. But here's another time question that gets us thinking about our system of measuring.

Hi, my name is Sarah, and I am nine years old, and I live in Aberdeen, Maryland. And my question is, how do people decide to have one hour be 60 minutes or one minute 60 seconds?

I'm going to skip over some of the complicated history of the different types of number systems. So if that stuff really gets you excited, I hope you'll ask an adult in your life to help you do some research on base-12 numbering systems and base-60 numbering systems. But let's think instead about how time is connected to a day and the rotation of the Earth.

People were able to figure out how to calculate, using things like sundials, how long it took for the Earth to make a complete rotation. Now, for a long time, people thought the Sun revolved around the Earth because that's what it looks like to us standing here on the ground. So they were thinking about how long they thought it took the Sun to move around the Earth.

We now know that it's the Earth that moves around the Sun, and as the Earth is moving around the Sun, the Earth is also spinning. And it takes 24 hours, approximately, for the Earth to make a complete spin around. And with that numbering system that uses 12 as a base, it's about 12 hours of daylight and 12 hours of night per day.

But if you wanted to tell your friend to meet you at the playground, you need to be more specific than just a day. I mean, you don't want to be waiting around for a whole day if you said, hey, meet me today while the sun is up. So with a base 12 numbering system that breaks down 12 months, you can further break down those months into days and then those days into hours and hours into minutes and minutes into seconds and so on.

60 is actually a number that breaks down really easily when you want to split it into different parts. So people chose 60 minutes in an hour and 60 seconds in a minute, in large part because 60 is easily divisible, easily divided by 12. So let's turn to a question from Charlotte. I live in Longwood, Florida, and I'm 60 years old. I would like to know,

Why do the days start at 12 o'clock in the morning? That goes back to sundials, where the shadow on the sundial comes from where the sun is in the sky. And so when the sun is highest overhead and the shadow goes straight up to the top of the sundial, that's noon. And as the sun goes overhead, the shadow moves until sunset, when then the shadow disappears.

And then we have 12 more hours where the sundial doesn't work, and that's at night. And then at sunrise, the sundial shadow shows back up, and we keep going. So we had to account for that time that there's no shadow. And so because the middle of the day with the sun as highest overhead is noon, then the opposite of that would be what we call midnight. And so that would be the 12 starting over again, and that's why the day starts at 12. Hello?

Hello, I'm Harry, and I'm four years old, and I want to know about how clocks work. Okay, so Harry wants to know about how clocks work. In order to have a clock, you have to have something periodic, and you have to have a way to count it.

For instance, if you have a pendulum clock and the arm of the pendulum swings back and forth and you count those swings, then you can measure time by seconds or minutes or hours. The counter in the pendulum clock is actually the gears in the clock. So every time the arm swings, a gear moves. And that gear moves 60 times and that gear gears a minute hand. And then that gear moves an hour hand.

You may have seen a clock like this, maybe hanging on the wall, maybe a big grandfather clock, and it has this swinging pendulum that goes back and forth. That swinging arm, that pendulum, moves a gear. Each time the pendulum swings, it moves the gear.

But many clocks that you see don't use a pendulum. They use a different mechanism, a quartz crystal. If you have a watch with a quartz crystal, that crystal actually vibrates at 32,768 hertz, or cycles per second. That quartz crystal vibrates because voltage is applied to it from a power source,

usually a battery. That frequency is 2 to the 7th power, so 2 times 2 times 2 times 2 times 2 times 2 times 2. The reason why we use that number is because with digital electronics, it's actually pretty easy to divide by 2. So if you have a sine wave from an oscillator and you just count every other cycle, then in essence you're dividing by 2. So with digital electronics, we do that 7 times, and that 32,768 cycles per second is divided down to 1 cycle per second, or 1 second.

Then you can easily derive minutes and hours from the seconds. The more vibration, the better the accuracy. That vibrating quartz pulses. It's more or less the tick that the pendulum of a grandfather clock would make. And that moves the gears that move the hands of the clock or the watch to let you know the time. What amazes me is that I look at a clock on the wall and it all seems so simple.

Until you hear Andrew explain how a clock stays accurate and tells the time and how it works, then it doesn't seem quite so simple, does it?

You may have noticed that the little arms on a clock or a watch, the kind of clock or watch that has a circle and 12 numbers, and a long hand and a short hand that move around the circle to tell you what time it is, not a digital watch that tells you the numbers, but I'm talking about the hands on an old-fashioned or analog kind of clock, they move in one direction. It's helpfully known as clockwise direction.

Hello, my name is Bentley, and I'm 8 years old. I am from Greeley, Colorado, and my question is, why do clocks have to go clockwise? My name is Kirill. I'm 5 years old. I'm from California, and my question is, why do clocks from the bottom go left and from up go right?

This goes back to the invention of the sundial in ancient Egypt, which is in the northern hemisphere. In the northern hemisphere, the sun rises in the east and sets in the west. So when the sun goes from the east to the west, the shadow goes from the left to the right. This would be opposite in the southern hemisphere. Most of the population historically is in the northern hemisphere. And so the idea of the shadow going from the left to the top and then to the right derived the definition of clockwise.

We now use clocks instead of sundials, but it's because of the way the sun moves in the northern hemisphere that clocks all around the world go clockwise. Hello, my name is Asha. I'm seven years old. I live in Toronto, Canada. And my question is, what is the real time? Thank you. I love your show.

I would say what the real time is, is what we call Coordinated Universal Time, or UTC abbreviated. And that's the official time. But it's actually an average of a lot of clocks all over the world that contribute to the average. So we at the National Institute of Standards and Technology create what we call UTC NIST. Where you live in Canada, the National Research Council, or NRC,

creates UTC-NRC. So there's countries all over the world that create their own time, and then we average all those together at the Bureau of International Measurements outside of Paris, France, and they come up with UTC for the world, the official time. You can actually find the UTC online with help from your adults.

We've been talking a lot about the science of time, how it's broken down into measurable quantities, how clocks work. That's sometimes called linear time. But let's go even deeper. Hello, my name is Ivan. I'm 11 years old. I'm from Austin, Texas, and I would like to know if time has a beginning or an end. Hi, my name is Emilio. I'm seven years old and

And I live in Regina, Canada. And my question is, how is time created? That's a really tricky question because is time created or does it just exist? In other words, did we create time because we've named it time and we have these ways that we define a minute and an hour and a second? Is time a human creation because that's what we've called it?

Or does time just exist in the universe, whether we know how to calculate it or not, whether we call it time or something different? Sometimes astrophysicists, people who study the stars and the galaxies in our universe, they try to get an understanding of when time started and what existed before it. Because how can something come from nothing? How could there have been something before time that wasn't just more time?

Is nothing something? If nothing exists after our universe, long into the future, will time stop? Is something else happening after that? Physicists, astronomers, religious philosophers, they've all grappled with different ways to think about this.

There are theories, but of course no one existed before time, and we don't know for certain. So Emilio and Ivan, I hope you will keep searching for answers to those questions, questions that we humans find so very difficult to understand.

But back to the basics. Fundamentally, we're still not really sure how time works here in this episode, are we? Hello, my name is Lily and I am eight years old and I live in Kathmandu, Nepal. And my question is, how does time work and why does it happen?

It's tricky to describe how time works. It's kind of a philosophical question at that point, where in physics, it's really how we define entropy. It's the one directional variable that we use to describe other things in physics as they're changing. So they describe time as the flow of time or the arrow of time, as proposed by the British astronomer Arthur Settington in 1927.

He talks about the arrow of time and how it's a one-directional way that we can measure process or existence. And because it's only one-directional, it's a different type of variable in physics than up or down or hot or cold. But as anything in physics changes, we measure that change over time. So it is a kind of hard variable to understand.

We've talked a little bit about how time keeps moving forward. You get older, you never get younger. The years keep going forward, not backward. But lots of us have wondered if there's any possible way we could move backward through time. My name is Judah. I live in Honolulu. I am seven years old, and my question is, is time travel possible?

Hi, my name is Buddy and I'm seven years old and I'm from Glendale, California. And my question is, why can't you travel in time? So thanks, Buddy. You answered Judah's question that you can't travel in time. It's not possible because time is connected to the speed of light, which is a constant. So you can't really travel faster or slower unless you want to talk about Einstein's theory of relativity.

But we'll save that for another day. Even though it's been the story in many books and movies over the years, time travel is sadly not possible. At least not yet. No one has figured out a way to trick the laws of physics and travel back in time. But if you could go back in time, what period would you like to travel to?

Would you want to go way back and see the dinosaurs? Or maybe not that far back, but to some time when humans came along. Or maybe you'd just like to see what the adults in your life were like when they were kids. Or go back to when you were younger and do something differently a second time around.

All right. In practically no time at all in the scheme of things, we're going to talk about some of the devices we use to keep track of time, like calendars. And also... Why does time seem faster when you're having fun? This is But Why, a podcast for curious kids. I'm Jane Lindholm, and today we're answering questions about time.

Understanding time is important to the study of physics, but it's also key to how we keep our society in order. So many things run on set times, from our bus and train systems to our school days and sports games to television and radio schedules to celebrations and holidays.

Time is involved in practically everything we do, so we better all know how to tell what time it is and what day it is and plan ahead for what's going to happen in the future. We've been answering your questions about time with Andrew Novick, a scientist from NIST. That's the National Institute for Standards and Technology. So how do we keep track of the passage of time? Well, with something like a calendar.

That helps us tick off the days of the week and years. But we can't understand how calendars work unless we understand how days and years work. My name is Tyson, and I'm from Los Angeles, California, and I'm four years old. And my question is, why don't the days stop?

Why don't the days stop? The days have to do with the rotation of the Earth on its axis. So the Earth does not stop. The rotation of the Earth keeps going over and over. And the rotation of the Earth in one full rotation is one day. And then it just keeps going. My name is Georgia and I'm seven years old and I live in Fort Wayne, Indiana. And I'm a

I want to know why a year is 365 days. So the definition of a year really has to do with how long it takes the Earth to precess around the sun. In ancient times, they had to use astronomy to learn about the motion of the Earth. And early calendars by the Babylonians, the Greeks, and the Romans had 300 days per year. So their measurements were pretty far off. By 45 BC, in Julius Caesar's time,

the Julian calendar had 355 days per year, which was pretty close. They were using seven-day weeks, but it wasn't until the time of Pope Gregory in 1582, the Gregorian calendar, that they had adjusted the length of the months and the number of days in the year to be 365 and one-fourth.

And so that's not too convenient, but they realized it wasn't an even number of days it took the Earth to go around the sun. Because it was 365 and 1 4th, every four years, they had to add an extra day. So that's how we get leap days. That was actually one of the questions we got for this episode.

My name is Grayson and I live in Kelowna. My age is eight and I want to know why is there an extra day in February every four years? The Earth does not go around the Sun in an even number of days and actually it

It's not even exactly 365 days and a fourth. That actually overestimates the amount of time it takes. So every 100 years, what would have been a leap year, we leave out. We take off one leap day every 100 years. That gets us very, very close. But that goes a little bit too far the other way. So every 400 years, we do have...

a leap year, and that's kind of a special leap year on a century year. So the year 2000 was a special case, but 1900 and 2100 and 2200 are not leap years, even though they would fall in the typical four-year leap year cycle. I didn't know that. That's very cool.

I'm Ethan. I live in Houston, Texas. I'm 10 years old and I would like to know why different cultures have different calendars. The same question could be asked of language. In other words, a certain culture uses what's convenient or what works for them. Different cultures have developed different languages and different calendars and so it wasn't until people started traveling and comparing each other's calendars and languages before they could translate between each other's definitions.

Most of the world uses the Gregorian calendar. That's the one that says we're in the year 2019. But if we know the world has been around a lot longer than 2019 years, why are we only counting from back that far? Here's Liam. I'm eight years old and I live in Arlington, Virginia. My question is,

If there were more centuries than actually 21 centuries, why are we only counting centuries from when Christ died? So Liam's referring to the time of B.C. and A.D. I know that sounds confusing, but it's actually an after-the-fact representation of the calendars. In other words, the dates back then had usually more to do with the number of years of the founding of a city or the number of years of the rule of a king. They weren't thinking of it as a continuing calendar.

It was about in a time that would be represented as 532 A.D. when they backdated the calendar and they figured out the number of years to when A.D. started and then went backwards from there for B.C. as they represented dates that happened. But Liam's question mentioned Christ. The calendar we use today was created by people who were Christians, who believe in a man named Jesus Christ.

Because Christ is so important in the Christian religion, the time that was before Christ and the time that came after Christ were sort of natural separations. So that's where B.C. and A.D. come from. B.C. means before Christ, and A.D. comes from a Latin phrase, and it signifies all the years since Jesus Christ was born.

Now, certainly not everyone in the world is a Christian. Far from it. More people in the world are not Christian than are Christian. But this is still the calendar most of the world uses that calculates dates from the start of the era of Jesus Christ. A lot of that has to do with global military and political and religious power, which is a topic for a different episode.

And today, we also use this calendar in many parts of the world, regardless of whether we celebrate any religion at all, because of convenience. It's easier to coordinate things when we all use the same calendar. But you should know that not everyone uses the Gregorian calendar. A few countries use different calendars or use the Gregorian calendar and another calendar. And some groups of people within countries also use other ways to record the passage of time.

We've talked a lot about how time works, but what about how time feels? Hi, my name is Jacob, and I'm five years old, and I live in Tampa, Florida. And my question is, why does time go so fast when you're playing? Hi, my name is Quentin. I come from San Jose, California, and I am six years old.

And my question is, why does time seem faster when you're having fun? It has to do with the perception of time and your neurotransmitters, how your brain reacts to what you're doing. There's been a lot of studies on this, and it has to do with distraction versus attention. They say a watched pot never boils, which just means it seems like it takes a long time because you're focusing your attention on one thing.

So time seems to be going slower. If you're distracted and you're out playing and having fun, time seems to go much faster. So all of these things wrap up into why time flies when you're having fun. Because you're not focusing on time when you're doing something fun or something that takes a lot of concentration, time seems to go by quicker.

Here's a question from Jake, who's six and lives in Chester Springs, Pennsylvania. Why does time go slow when you're a kid and faster when you're an adult? This is another question about the perception of time. Time doesn't actually go slower when you're a kid. It's just that it seems like it goes slower. My understanding is that it has to do with the percentage that you've been alive. In other words, a week of time when you're 80 years old...

It's a very tiny percentage of how long you've been alive because you've lived so many weeks in your life. When you're a kid, a week is actually a lot longer because you've only been alive for six years in your case. And so your perception of how time passes really is a relationship to how old you are. This is one of the most interesting things about time.

We've spent a lot of our episode talking about how we calculate time and how all over the Earth time is calculated the same way so we can all operate on the same schedule and so the world can work in an organized fashion.

But even though a second is a fixed amount of time, and a minute is 60 seconds here, and 60 seconds in Mumbai, and 60 seconds in Adelaide, and even though most of us agree that a year is 365 days, that doesn't mean we all feel time the same way. An hour is an hour, but it might have felt like a very long time for you and a very short time for your cousin.

This very episode might have felt very long or very short, depending on how interesting you've been finding it. Andrew talked about how an older person has a lot more life experience and might perceive time differently than a very young person. But some scientists think it also has to do with how our brain processes light and images.

If you process more images more quickly, as you do when you're younger and when you're more alert, time seems to pass more slowly. But when you get older and you process fewer images per second than the young person next to you, that actually makes time feel faster.

Scientists also think that some animals, especially things like tiny little flies with very fast metabolism and eyes that process light and movement more quickly than humans do, kind of see in slow motion. And because of that, they have a very different perception or feeling of time. I love thinking about how different we all are and how your sense of time and my sense of time might be very different.

It makes me wish you and I could actually swap bodies and minds for a couple of hours, just to see how the other one thinks and feels. But alas, like time travel, swapping brains and bodies is so far not humanly possible. So before our brains explode with all this thinking about the nature of time, let's end this episode.

I want to give an extra special thank you to Andrew Novick from NIST. Now, if you have a question about anything, have an adult record it. They can email the file to questions at butwhykids.org.

We will do our best to get an answer for you. But Why is produced by Melody Beaudet and me, Jane Lindholm, at Vermont Public Radio. Our theme music is by Luke Reynolds. Special thanks today to John Van Heusen. We'll be back in two weeks with an all-new episode. Until then, stay curious. From PR.