Why Do We Have Leap Years? The actual time between two yearly solar events works out to be 365.2422 days — resulting in almost one full extra day every four years. The federal government's chief of time and frequency explains the finer points of telling time.
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Why Do We Have Leap Years?

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Why Do We Have Leap Years?

Why Do We Have Leap Years?

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JOE PALCA, host:

From NPR News, this is TALK OF THE NATION: SCIENCE FRIDAY. I'm Joe Palca.

By now, everyone knows that today, Friday, February 29th is leap day, that bonus day that comes with every leap year. It turns out that leap year is a pretty good marketing tool. We're told to we use this extra day to volunteer for a worthy cause. If you're a single woman to propose to your sweetie, although that may be a little old fashioned now. There's a leap year cocktail, even McDonald's is giving away free leap year food, I guess.

So you know it's leap year and that today is the extra day, but do you know the science behind the day. We decided to check in with the folks at NIST, the National Institute of Standards and Technology, they are the ones charged with keeping the most accurate time in the world, and we figured they'd have the answers to all our leap year questions.

Joining me now from NIST is Tom O'Brian. He is the chief of the time and frequency division at the National Institute of Standards and Technology in Boulder, Colorado.

Hello and welcome to the program, Dr. O'Brian.

Dr. TOM O'BRIAN (Chief, Time and Frequency Division, National Institute of Standards and Technology): Thanks, Joe. Happy leap day.

PALCA: Thanks very much. You have tell me what the special celebrations you're doing out there are.

But first, I want to remind people that we're anxious to get your leap questions, so call us at 800-989-8255, that's 1-800-989-TALK. And if you want more information about what we're talking about this hour, go to the Web site www.sciencefriday.com where you can go find links to our topics.

So, are there any special celebrations out in Boulder?

Dr. O'BRIAN: Well, Joe, I'm sorry to disappoint you but, no, we're not having any special celebrations, we're just keeping our atomic clocks running as usual. We have the world's most accurate atomic clock it's good to about one second in 80 million years, and we have some research atomic clocks for future generations that are good to about one second in a billion years. So we're celebrating by just keeping those going.

PALCA: I see. That's good. You know, it kind of raises the question, though, I mean - all right it's a good thing to be on time and we were making jokes about, you know, starting the show on the right hour and all that stuff. But really, what does it mean to be that accurate, who needs that kind of accuracy?

Dr. O'BRIAN: It might at first seem like that's kind of a geeky thing to do and why are we spending our time trying to measure things so accurately. But actually, Joe, very accurate time and synchronization are an integral part of our modern technology and society. Every time you make a cell phone call, at least on the type of network that's called CDMA, you're relying on synchronization that network to better than a millionth of a second per day.

When you flip on the electric power switch, you're relying on the grid spanning the whole continent, more than 10,000 power generating stations to be synchronized to better than a millionth of a second a day for efficient and safe transmission of the power. And when you use the Global Positioning System, GPS, you're relying on timing much better than a billionth of a second to be able to give you your location anywhere on the surface of the Earth. So, there are many other examples, but I hope I've made it…


Dr. O'BRIAN: …a little bit clear that…

PALCA: I got it.

Dr. O'BRIAN: …we're all using a very accurate time everyday.

PALCA: But is there a limit, I mean, is there something that we're not able to do, because we don't have accurate enough time pieces?

Dr. O'BRIAN: Part of the reason we're trying to make even better atomic clocks is to be able to improve things like navigation, telecommunication, positioning, also that we'll be able to do some fundamental studies such as the ultimate state of the universe. And actually when you get extremely accurate clocks, the local environmental conditions affect the ticking rates and the clocks become the most sensitive measures of magnetic field and gravitational field, et cetera.

So, for example, we're exploring even right now using atomic clocks as magnetic field measuring devices to measure the magnetic fields generated by biological activity from the heart, from the mind, et cetera.

PALCA: Well that, I know that's quite interesting. Well, let's take a step back now because we've been talking about the finest divisions of time; let's talk about the more broad divisions that allow us to have this leap day. What - I mean just to put it simply, why do we need them?

Dr. O'BRIAN: Basically, we have a leap day to try to keep our calendar synchronized with the seasons. It takes, we say, casually, 365 days equals one year. But in fact it takes a little bit more than that for the Earth to make one complete orbit around the sun. After 365 days, we're a little bit short of getting all the way around, it really takes about 365 and a quarter days. So, if we didn't have a leap year every four years what you would find is that the calendar would be moving ahead compared to our seasons.

A specific example, the start of winter is usually December 21st, something we call the winter solstice. If we didn't have leap year after four years winter would start on December 22nd, maybe that's not such a big deal but after 70 years of human lifetime, winter would be starting about a little more than two weeks later, about January 7th or so. And after 700 years, winter would be starting in late June. So, if it's important to you that winter start in December and summer start in June then you want to keep the calendar synchronized with the Earth's orbit around the sun.

PALCA: I see, I see. And I did a story some years ago talking about leap seconds.

Dr. O'BRIAN: Yup.

PALCA: Now, what are those?

Mr. O'BRIAN: Leap seconds come about because the rotation of the Earth - the time that it takes for the sun to come to its highest position in the sky and then, again, it's not exactly 24 hours, it's not exactly 86,400 seconds. In fact, the Earth's rotation rate is constantly slowing down and speeding up by very small amounts, by about a millionth of a second per day. Again, that might not sound like a lot, but over time that has a big impact.

So what we do is we try to keep atomic clock time synchronized to Earth rotation time to within one second, and that requires us occasionally to add a leap second to atomic clock time to make sure that it does stay synchronized to Earth rotation time or astronomical time.

PALCA: But I've read, also, that there was some discussion of abandoning leap seconds or - at least for the foreseeable future.

Mr. O'BRIAN: There is indeed. For the vast majority of human history, timekeeping has been based on the Earth's rotation basically, or other astronomical phenomenon - how long it takes the Earth to make one rotation or the moon to appear to move across the sky or the sun to appear to move across the sky, et cetera. And that was the best clock that human beings had for the vast majority of history.

But about - almost 60 years ago when atomic clocks were invented - and I'll do a little publicity here, they were invented at NIST - atomic clocks were invented, it was determined that the Earth's rotation really was not quite as stable or as constant as we thought. That it's actually changing. There's a long-term slowing down and that comes primarily from the gravitational tug of the moon slowing down the Earth. And built on to that long-term slowing down -there are seasonal variations because of where snow and ice are deposited and where they melt. There are seasonal variations because of changes in the circulation of the ocean and the atmosphere. And there are variations that are not predictable because of changes in the circulation of the molten rock, the magma inside of the Earth.

And so all these things add up to - making it actually very difficult to predict exactly how long a day is going to be. And that's why the leap seconds were added. Now, some people don't like the idea of saying, gee, suddenly, our time scale can make a big jump. For example, if you're running a positioning system based on atomic clocks like GPS, there is no way that you could tolerate a jump of a second, because light travels…

PALCA: Quite a bit. Yeah.

Mr. O'BRIAN: Light travels about one foot in a billionth of a second. So if you had a jump of a second, you have an offset of 186,000 miles, approximately. You can't tolerate that. So as we have moved from time based strictly on astronomical phenomena to time based more on atomic-clock-type phenomena, there has been a corresponding move to try to make our timekeeping based on atomic clock time.

So some people are saying, look, would it make a difference to our technology or to our society if we did let the length of the day - excuse me - the time that noon occurs drift by a few seconds, would that make a difference that we could perceive or is it more important to keep our time scale more steady and get rid of leap seconds.

So that's an ongoing area of debate and it's still open to the discussion. In fact, a representative from NIST will be going to an international meeting to talk about that in a couple of weeks.

PALCA: Excellent. All right. Let's hear what our callers have to say, and let's start with Rock(ph) in Payson, Arizona. Rock, welcome to the program.

ROCK (Caller): Good afternoon, gentlemen.

PALCA: Good afternoon.

ROCK: I have a question. In my studies, I've read quite often that the Pre-Columbian Mayans and the Aztecs did not have to have a leap year because their calendars are so accurate. Can you explain this to me?

PALCA: Interesting question. Thanks, Rock.

Mr. O'BRIAN: Sure. I'll first have to give a caveat with that; I'm certainly not an expert in the history of all kinds of calendars. But, indeed, the Mayans and many other Pre-Columbian people did have extremely accurate calendars. For many people, not just those in Mezzo-America, but those across the ancient world, observing the motions, the apparent motions, of the stars and the planets, was a crucial part of their understanding of their place in the universe and of their religious activities.

So some of them made extremely accurate observations and kept track of those over a number of centuries. Of course, that requires a civilization that did span a number of centuries, and they would make adjustments to that. And indeed, the Mayans did have a very accurate calendar.

PALCA: Interesting. All right. Let's take another call now and go to Mark(ph) in Portland, Oregon. Mark, welcome to the program.

MARK (Caller): Hi. Thanks for taking my call, Joe.

PALCA: Sure.

MARK: You see I'm just sitting in my vehicle; I'm on the cell phone on the streets of Portland. So I apologize for that, there may be a rough sound. Maybe I'll step back while I'm talking.

(Soundbite of laughter)

MARK: As an astronomy teacher of sorts in Portland for years and across the nation and overseas, I found that it's interesting that many people are interested in astronomy, yet they don't really realize what the subject is. They may think it's something that requires a lot of physics and math, but of course it starts with simple observation. And as I listen to your guest's start at the beginning there, I was waiting for him to get the point about discussing our observations, our celestial mechanics and motions of the stars and, of course, our own planet - the Earth - was unrealized probably until about the time of Newton and the great astronomers as to what was moving and changing and turning.

Until people realized one day, that this all appears to be a big clock, of course, that we've started changing our calendars and adjusting even with religious holidays added in to make up the differences in the calendar coming up short in time. But I think it's interesting that - and the discussion right before this, I was listening to a guest talk about the importance of education today, and that we need to do more. And I think that something as simple as going out, looking at stars, and learning the night sky, learning the celestial mechanics, the motions of the night sky is something that may be lacking today in our education systems. And that…

PALCA: Interesting. You know, I think - no, you make a good point and - Mark. It seems to me that a lot of the study of time, of course, was - and the study of astronomy was linked up with the Navy. It's interesting if you happen to hear the start of this program, it was - it's the Naval Observatory's talking clock that we use here for accurate timekeeping. So, I guess, observing the sky and time have always been partners in a sense, isn't that right, Thomas O'Brian?

Mr. O'BRIAN: Absolutely. And I agree with your caller that, unfortunately, we do have a lack of people being aware of the apparent motions of the stars and the planets and the sun and the moon across the sky largely, because I think people spend less time outdoors. They're perhaps spending more time indoors on computer activities and watching TV, et cetera. And also, as more and more people live in cities, it just becomes harder to observe the motion of the celestial objects at night because of the great light pollution, it becomes very difficult to see more than a few stars and therefore it's harder to learn. And I agree with your caller, absolutely, that trying to decipher and decode these very complex, apparent motions of stars and of the sun and of the planets led to wonderful advances in mathematics and mechanics and science in general.

PALCA: We're talking with Thomas O'Brian the chief of time and frequency at NIST about leap year and leap days and leap seconds. I'm Joe Palca, and this is TALK OF THE NATION from NPR News.

And let's take another call now and go to Mike(ph). Mike in Rockford, Michigan, welcome to SCIENCE FRIDAY.

MIKE (Caller): Oh, I have a question for you. Speaking of frequencies, I'd read this a while back that there is a family - I believe in one of the Scandinavian countries, possibly Norway - that he has three children, all three of them born four years apart on a leap year day.

(Soundbite of laughter)

PALCA: Ah, that's a - well, that means - I don't know. Have you - has NIST got an official position on this, Tom O'Brian?

Mr. O'BRIAN: Unfortunately, we haven't studied this issue. But it looks like an interesting one to go into. I'd like to know what was going on nine months before February 29th on each of those leap years.

PALCA: I see. I see. Do you know anybody born on February 29th?

Mr. O'BRIAN: I don't. Well, let me put it this way. I don't - I'm not aware of any…

PALCA: You don't know that you do. Yeah.

Mr. O'BRIAN: …February 29th, but of course if we had a random sample of the population, we'd expect about 1 out of 366 would have that birthday.

PALCA: See, there you go, there you go. Always the scientist being careful. Let's get another call now from Ashley(ph) in San Antonio, Texas. Ashley, welcome to the program.

ASHLEY (Caller): Hi there. Thanks for taking my call. It's actually related to the last one. It sounds a little silly, but I was wondering if somebody is born on a leap year, February 29th, how did they do it for the following year? You know, do they celebrate their birthday on the 28th or do they celebrate it on first?

(Soundbite of laughter)

PALCA: That's great.

Mr. O'BRIAN: Are you born on a leap day?

ASHLEY: I'm not. I was born on the 25th. And I was thinking, gosh, my parents are pretty (unintelligible). But…

Mr. O'BRIAN: Right, but how wonderful is that? Because you know you could be a 40-year-old individual, you could just explain to people that you're 10 and ageless.

(Soundbite of laughter)

AHLEY: I was just wondering, I've actually been thinking about this, and so when you guys were talking about this, like, I wonder if there's an actual answer for that?

PALCA: Another one I suspect that NIST hasn't been doing too much research on. But Ashley, thanks for the call.

Mr. O'BRIAN: Yeah. There is no official government standard on how to celebrate a birthday from a leap year. I assume some people do it on February 28th and some on March 1st.

PALCA: Good point, one more - let's see, I think we could fit in one more call. Brooke(ph) in Princeton, New Jersey. Welcome to the program.

BROOKE (Caller): Hi, thank you. My question is about the measurement of time and how it has changed in its calculations, whether or not the same basis that was originally used to calculate the change in time is still used today?

PALCA: Interesting question. So are the - is it still a matter of observation for adding these leap seconds or is there some other - or leap years or leap days or is there some other factor involved?

Mr. O'BRIAN: Well, it is now a combination of observation and atomic clocks to determine the size of the seconds that's strictly based on atomic clocks. It used to be, of course, based on the rotation of the Earth. And when the switch was made from astronomical-based time to atomic-clock-based time, scientists took the best care they could to try to make sure that the atomic-clock-size second, if you will, was commensurate with the seconds from Earth's rotation time.

But our time scale now is a hybrid. It is based on atomic clock time telling you the size of the seconds, but occasionally adding those leap seconds, or sometimes it could be a suppression of a leap second, to make sure that our atomic-clock-based time stays synchronized to Earth's rotation time within about one second. And the same observational techniques are used as have been used for a quite a long time to determine that astronomical time.

PALCA: Okay. Tom O'Brian, while I've got you, I;ve got to share with you one idea that's been bugging me for a long time, and I want you to consider it carefully. I think we should think about going to metric time and that would be a 10-hour day with 100-minute hours and 100-second minutes.

Mr. O'BRIAN: Well, that's an interesting proposal, and you might realize you're not the first one to propose that.

PALCA: Oh, darn. I thought I was…

Mr. O'BRIAN: That was after the French Revolution, when they were introducing the general metric system, they did indeed propose a metric time, and your proposal would have a total of about 100 - would have a total of exactly 100,000 seconds in a day. That's pretty close to the current 86,400 seconds we have, so it would be not too different, you know.

PALCA: All right. I got to tell you, radio directors across the world will revolt, because they finally figured out how to back time in 60 seconds. But I'm afraid we're out of time for this hour. So thank you very much Tom O'Brian.

Mr. O'BRIAN: Thank you, Joe. Good to talk to you.

PALCA: Tom O'Brian is the chief of time and frequency division at the National Institute of Standards and Technology in Boulder, Colorado.

For NPR News in Washington, I'm Joe Palca.

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