Leap Seconds, Smear Seconds, and the Slowing of the Earth

Season 1 • Episode 9

The earth’s spinning is slowing down. Any clocks pegged to the earth’s rotation are therefore drifting out of alignment with our far more precise atomic clocks—only by a thousandth of a second every 50 years, but that’s still a problem for the computers that run the internet, cellphones, and financial systems. 

In 1972, scientists began re-aligning atomic clocks with earth-rotation time by inserting a leap second every December 31, or as needed. It seemed like a good idea at the time—until computers started crashing at Google, Reddit, and major airlines. Google engineers proposed, instead, a leap smear: fractionally lengthening every second on December 31, so that that day contains the same total number of seconds. But really: If computer time drifts so infinitesimally from earth-rotation time, does anybody really care what time it is? 

Guests: Theo Gray, scientist and author. Geoff Chester, public affairs officer for the for the Naval Observatory. Peter Hochschild, principal engineer, Google.

Episode transcript

Leap Seconds Script

The earth’s spinning is slowing down. Any clocks pegged to the earth’s rotation are therefore drifting out of alignment with our far more precise atomic clocks—only by a thousandth of a second every 50 years, but that’s still a problem for the computers that run the internet, cell phones, and financial systems. 

In 1972, scientists began re-aligning atomic clocks with earth-rotation time by inserting a leap second every December 31, or as needed. It seemed like a good idea at the time—until computers started crashing at Google, Reddit, and major airlines. Google engineers proposed, instead, a leap smear: fractionally lengthening every second on December 31, so that that day contains the same total number of seconds. 

But really: If computer time drifts so infinitesimally from earth-rotation time, does anybody really care what time it is? Guests: Theo Gray, scientist and author. Geoff Chester, public affairs officer for the Naval Observatory. Peter Hochschild, principal engineer, Google. 


The earth’s rotation is slowing down. Very gradually, but enough that it’s drifting out of sync with our atomic clocks—the ones that run our internet, cell phones, and financial systems.

Theo Since the late 50s, they’ve drifted apart by 37 seconds cumulatively. 

David: So how do we— 

Theo: Well, so leap, leap seconds is what people do. 

Yes, leap seconds. We add one second to each year, as needed. Which is great—unless it crashes your software.

Peter And there were funny reports   of some crashes in Google’s servers . And that really caught our attention.

Today on “Unsung Science:” The Earth’s time…computer time…and the battle to manage the difference.  


Season 1, 9: Leap Seconds, Smear Seconds, and the Slowing of the Earth. The perfect topic for the end of the year.

In a way, this episode has been in the works since I was in fourth grade. 

[school bell + kids ambi + music]

I will NEVER forget this. A guest came in to talk to our class: Some astronomy professor, hired to try to infuse a little interest in science to elementary schoolers like us.

He was one of these audience-participation dudes. At one point, he was like, “Who knows how many days in a year?”

And I knew that. I was a little smarty pants. I yelled out “365 days!” Because everyone knows that. 

But the guy goes, “Nope! Guess again!” 

What!? That’s not wrong! I was really steamed. That’s the kind of kid I was.

So the other kids started shouting their answers. They thought maybe I’d gotten the numbers mixed up. “356!”

And the guys’ like, “Nope!”

“365! 366! 364! A hundred! 52!”

And the guy was like, “Nope! No, no, no. Try again!”

Honestly, he let it drag on for way too long.

He finally told us his answer: 365 and a quarter days. Which is why we need a leap year every four years. Those four quarter days add up to what we know as February 29 every four years. 

It’s kind of awkward that the earth’s trip around the sun is not evenly divisible by whole days. So we add a leap day to keep our clocks in sync with the earth’s motion.

That explanation was good enough for fourth graders. But it wasn’t quite the whole story.

Theo The man who has one clock always knows what time it is. If you have two clocks, you’re never quite sure.   Like, whose clock are you going to believe? And suppose that you do have two clocks, or three clocks, and they’re never going to exactly agree.  

This is Theo Gray. I love Theo. We’ve appeared together on a few “Nova” specials on PBS, and I collect his big glossy hardbound photo books of elements, and molecules, and machines. By profession, he’s a—what is he? 

Theo I don’t really have a career, as such, I don’t think.   I mean, I did work as a software engineer for 23 years, cofounding and developing Mathematica, Wolfram Research, Wolfram Language, etc., but I haven’t done that for a while and I think probably you call me an author at this point, because I just make a living writing books, and people pay me to do that. And that’s fun. 

In his book “How things Work,” he’s got this huge section on the history of clocks. 

Theo I mean, obviously the simplest, the crudest, the most universal clock, is a sundial, or otherwise known as a stick in the ground.   It’s a nice clock, but it has some disadvantages. It –it only works when the sun is shining. So, you know, clouds, rain, whatever, nighttime. This is the problem.   I think what’s really the most remarkable about sundials is that up until 1955, they were the most accurate clocks in the world.  And, you know, you might say, what are you talking about? That’s ridiculous.  

But it’s true. Until the 1800’s, the famous clock in Greenwich, England, the official world standard of time, was based on the sun’s position in the sky. At some point they switched to observation of stars’ position in the sky, because they’re sharper and therefore easier to see.  

Theo And yeah, but it was all based on the rotation of the Earth. In other words, a glorified sundial. 

There was actually a clock, which is still there to this day, outside the main gate at the Greenwich Observatory.  And then there’s a giant red ball on a tower and the top of the observatory, which drops at noon precisely. And you can see that ball from central London. So all the bankers could, you know, see that clock, that that ball drop. And that was how the time was communicated. 

Until 1955. Then the cesium atomic clock was invented. 

Ah, yes—the atomic clock. 

Theo Get yourself some cesium atoms, put them in a controlled environment, low temperature, beam in a certain microwave frequency   and then count the cycles. You count the correct number of cycles of that, that cesium atomic resonance frequency. That’s a second. 

Anybody can do it anywhere in the world. And you don’t have to have—you don’t have to go to Paris.   You just read the definition, build the machine and you’ve got the thing. And systematically—  time was replaced. The Earth was replaced with cesium atomic clocks. 

Now, atomic clocks are going to be the star of this show, so I should probably take a moment to tell you where yours are. The ones that tell your phone, your computer, and your internet what time it is.

Well, they’re in Maryland, at the US Naval Observatory. 


If you’ve ever heard of the Naval Observatory, you probably think of it as the home of the U.S. Vice President—and yes, that’s where he or she always lives. But if you’ve ever benefited from the internet, cell phones, airplanes, GPS, the financial system, or the military, you might care much more about the Naval Observatory’s second function: As the home of the United States master clock. 

We go now to the library of the US Naval Observatory—an astonishing rotunda lined with 90,000 books about navigation, astronomy, and time.

CHESTER: If it looks like I’m in a closet, it’s because I am in a closet. This is room W, which is where we keep the journal archives for our library. But it’s quiet.  

POGUE If you had six inches of books to put under it on that table, it would be then closer to your mouth and sound a lot better. 

Geoff So let’s see, OK: “Monthly Notices of the Royal Astronomical Society.” That ought to do it. 

Pogue That’ll make it sound really good. 

Thus began my Zoom interview with Geoffrey Chester, the public affairs officer for the Naval Observatory. The official keepers of time and location. 

Geoff We are a Navy command. All told, we have about 150 on staff and so it’s mostly civilian.   When we get a new superintendent,  the first thing I tell them is you may not necessarily want to go to sea with this crew, but I can guarantee you that we will never get lost and we always know what time it is. 

David Why is it the Navy’s job to set the standards for time and space? 

Geoff All the countries that had big navies had observatories so they can calibrate chronometers and create almanacs, so that they knew where the heck they were going. 

And so we are actually kind of the new kids on the block, because we started in 1830, when most of these other institutions started back in the 1700s. But we are the only one that is still associated with the Navy of its host country. 

See, dear listener? Four minutes in, and you’ve already learned something fascinating. I know you didn’t know that.

Geoff Here at the Naval Observatory, we are responsible for—for determining and disseminating a long term, precise time scale. So we operate about one hundred clocks here.   What we do essentially is, we take a weighted mean of our 100 or so atomic clocks.   We have computers that do this about every two minutes.  

We have the ability to keep a time scale here that on some level can be measured down to the femtosecond level. A femtosecond is ten to the minus 15, or 1 1000th of a trillionth of a second.   We actually built those clocks that keep it that precise. We actually built those in-house, because we could not find a commercial supplier that could do it for us at a price we can afford. 

David I’ve had the privilege of visiting the Naval Observatory. I’ve been in the room that has all those clocks. Can you describe it for the folks at home? 

Geoff Most of the clocks are kept in a building that was specially built.   The temperature does not vary by more than one tenth of a degree centigrade throughout the year, and the humidity stays within three percent of a nominal mean.   So you go into that building in the dead of winter or in the heat of a Washington summer, and the temperature and humidity are the same. It’s a great place to hang out.  

But in that room, we have racks of equipment.  These are beige boxes that look like they kind of look like a stereo amplifier. Except they have a little display that ticks off the time.  

OK. So remember how everything changed with the invention of the cesium atomic clock in 1955? Not long thereafter, the world officially adopted the atomic clock as its master timekeeping machine. The rotation of the earth was retired. 

GEOFF: In 1967, the definition of the second was changed. It was no longer one 86,400th part of a mean solar day. It was from thence forward until the present, the interval of 9,192,631,770 hyperfine transmissions of a neutral cesium 133 atom in its ground state. When I started working here, I thought a second was one Mississippi, so… 

That’s my favorite joke of the podcast year. 


But now we go from comedy to tragedy, or at least something that’s existentially depressing.

See, in one regard, the atomic clock’s astonishing precision is actually a problem, because it exposed the flaws of our old, earth-based system. What it revealed is—ready for existential depression?—that the earth’s spin is slowing down. Here’s Theo Gray.

Theo: It’s so bad that, like, we’re now I think it’s 37 seconds off from the, the late 1950s, when we first started to be able to measure more accurately.

David Oh, I used to worry about the sun exploding—now I have to worry about the world stopping spinning? 

Theo You know, that is actually a very good question, that somebody should do the math—like which is going to happen first? Like I say, we always worry about the sun’s going to burn us all up in five billion years. But if the earth stops turning before then, we’d be in a lot more trouble, or different trouble anyway.  

David I guess at some point I’m relieved that this is the one way that we are not responsible for destroying the way things are supposed to be. 

Theo Yeah, well, I mean — global warming, because it would make the earth on average warmer, is going to drive more moisture into the atmosphere and is therefore going to slow the earth down a little bit more. 

David Oh, great. 

Theo So just, you know, if you want to go that way and you want to feel guilty about it, I’ll give you a way to feel bad about it. 

David Oh, man. Well, how much has it slowed down already, measurably? 

Theo I think the figure is one second per 500 years.  The days used to be significantly shorter. 

David The days have not always been 24 hours long?!   

Theo Yeah, no, I mean, I think in the dinosaur era, they were, what, maybe 20 hours?  

You think there’s not enough time in a day in your life? How’d you like to have been one of the workers who built the Egyptian pyramids? 

Theo I counted, they had nine seconds less per day to work on the pyramids. 

I need to insert an audio footnote here so I don’t get scientist hate mail. To be absolutely clear, the earth’s spin isn’t just slowing down.

Theo The Earth’s rotation is not steady. Like, it wobbles a little bit from day to day, and it systematically speeds up and slows down throughout the seasons.  You know, when the earth is warmer, more of the water kind of migrates up to higher up in the atmosphere. And when it’s colder, the water, rain falls down, and it’s lower. So, you know, the classic example of the figure skater who, you know, spins faster when they pull their arms in, when the earth pulls its water in closer, it spins faster. And when the water goes out, higher up into the atmosphere, the earth goes slower. 

But the overall result of all of this wobbling—the long-term trend—is a slowing.

Theo It’s slowing down systematically because of things like tidal friction. So, you know,  the water on the earth, the ocean being pulled by the moon, it sloshes around—that’s, you know, that’s friction. 

David This is all well-known to physicists and scientists, the fact that the Earth not only does not rotate steadily, but is slowing down? 

Theo Oh, absolutely. Yeah. You might think, well, who cares—  one second a year, really. But if you’re trying to run, for example, a GPS satellite network or even a financial system, this is a big deal.  So there’s one rule of thumb, a very convenient fact, that the speed of light is one nanosecond per foot. 

So, you know, your car would be halfway to the moon if GPS were off by a second. 

David Hate when that happens. 

Theo Let’s say you’ve got a billion dollar interbank loan at some interest rate, right? And, you know, you’re charging interest by the microsecond or something on this billion-dollar loan. You know, it actually makes a difference.  

David So here’s –here’s what’s deep down bugging me. We’ve got these cesium atomic clocks that are not susceptible to slowing down and the variability of the earth’s rotation. And then we’ve got the earth, which is. Aren’t they eventually going to drift apart?

Theo Well, they do, yes.  since the late 50s, they’ve drifted apart by 37 seconds cumulatively. 

David So how do we— 

Theo Well, so leap, leap seconds is what people do. 

That’s right. There are leap seconds. You’ve lived through at least several of them. There is, of course, a global committee in charge of leap seconds—and it has a fantastic name: the International Earth Rotation Service. (OK, in 2003, they bulked up the name. Now it’s the International Earth Rotation and Reference Systems Service, but that’s not nearly as excellent.)

Once every few years, as needed, IERS scientists schedule one extra second, tacked on to June 30 or December 31, to bring the atomic clocks back into alignment with the earth’s spinnature. In recent years, we’ve enjoyed the luxury of that extra one second in 2005, 2008, 2012, 2015, and 2016. 

David So as it is right now, if I were watching my phone on, on the day when a leap second is scheduled, would I see one of the hours go 57, 58, 59, 60, then the next minute begins? 

Theo Yes, you see a hard jump. It’s the same second repeats twice. 

David Right. 

Theo And, you know, I mean, most people don’t notice that, right? Like you’d have to watch pretty close. It’d be exciting. It’s like watching your odometer, you know, turn over. So it’s not like this is a big issue for most people most of the time.  


But it is a big issue for scientists and other technical people. Like, if you’re a surveyor or an astronomer, your work is still connected very much to the earth and its movements, so you probably rely on the original earth-based time scale. But if you’re an internet or banking or space company, you need absolute precision—you use the atomic clock. 

So I asked Geoff Chester how we manage two different timekeeping systems. 

David So it sounds like if the earth spinning is variable and slowing overall, it sounds like these atomic clocks are more precise than the planet spinning. Eventually, aren’t they going to drift out of sync with each other? 

Geoff So this was something that was recognized early on after they defined the second in terms of the atomic frequency standard. And so they hammered out this idea, which they essentially codified in 1972, that there would be two concurrent timescales. 

So there is what is called International Atomic Time, or the Temps Atomique Internazionale, because the International Bureau of Weights and Measures is headquartered in France. So TAI is the time that’s kept by atomic clocks. 

And then there is what is called Coordinated Universal Time. 

Coordinated Universal Time is based on the earth’s rotation. That’s the time we adjust with leap seconds. And they call it UTC, because, once again, that’s the acronym for its French name. 

Geoff: (continues) So these two concurrent time scales, one essentially based on atomic time and the other based on Earth rotation time, have a cumulative error of roughly one and a half milliseconds, that compounds on a day-to-day basis. So after about 500 days, you have a difference of one second between the two time scales. 

And…when that happens,   —there is a provision in the 1972 definition that essentially allows us to stop atomic time for one second to let the earth catch up. And that’s what’s known as a leap second. 

Aha! Very cool! So let me see if we’ve got this straight. Because the earth is slowing down, there aren’t exactly 24 hours in a day anymore. But that means that we wind up with two time scales: the one based on atomic clocks, and the one based on the earth’s rotation. And we introduce a leap second as necessary to keep them synced up.

[Begin fake conclusion music]

YAY! We did it! We saw a problem, and we fixed it with science! Let’s celebrate the ingenuity of the leap second. Everyone come to my house for a party next time we have a leap second! It’ll be a really short party, but, you know. Nerdy and fun.

And that concludes this episode of— 

Geoff should interrupt me (and the music) – [needle slowdown fx]

Geoff: So today, leap seconds are a real problem. 

Wait, what? That’s not the end of the story?

Geoff: The thing is, we can’t predict with any real precision more than about six months in advance whether or not a leap second is necessary at a particular time. So when it is determined that a leap second will need to be inserted,   we can give the world about six months’ notice saying, “hey, you know, here’s a leap second coming up, get your networks ready,” whatever. But every year, typically about 10 percent of the world’s networks fail, and sometimes they can be very spectacular. I think it was 2012. I believe it was Qantas Airlines that botched the leap second in their enterprise, and they lost a day of revenue because their system was kaput. 

It was 2012, and it wasn’t just Qantas. The 2012 leap second also took down Reddit, Gawker, LinkedIn, FourSquare, Mozilla, and Yelp that year. But every time there’s a leap second, somebody suffers. In 2008, it was Oracle’s computers and Sun’s computers. In 2015, it was Twitter and Android. In 2016, it was Cloudflare, the website security company. 

Well dang! If leap seconds aren’t the ultimate solution, how else are we supposed to fix the discrepancy between earth time and atomic time?

Peter We thought about various possible approaches to the problem, and the leap smear seemed to be by far the most practical and reasonable. 

Yes, this Google engineer just said “leap smear.” After the break: The future of time discrepancies, as rethought by Google.


Book Ad 


Welcome back. Before the break, you’d absorbed some heady science: Like the fact that the earth’s gradual rotational slowing has thrown off our timekeeping, and now atomic clocks and earth-based clocks gradually drift out of sync.

And we learned that the international masters of time thought they’d solved the problem by adding one leap second as needed to keep the two time systems in sync.

And then we learned that the leap-second solution is actually a problem. Once again, here’s Geoff Chester, of the U.S. Naval Observatory.

GEOFF: As large scale computer networks began to spring up, ultimately leading to the Internet and things like that, leap seconds became kind of a colossal pain. Because if you are a system manager and you do not incorporate the leap second across your entire enterprise, at the same instant in time, your network will fail. And if your network fails, you don’t make any money and if your company doesn’t make any money, the odds are you’re going to get fired.

Now I’d like to introduce you to a man who will not be getting fired any time soon. He’s too important.

Peter I’ve spent a number of years working on making Google’s computers synchronized.  It’s not a great cocktail party conversation, but it’s surprisingly pretty popular. 

Pogue Oh, you wait ‘till this episode drops. They’ll be stopping you on the streets. 

Peter Okay. 

Peter Hochschild is a principal engineer at Google. He is basically Google’s director of time. His job entails keeping all of Google’s servers perfectly synchronized—and Google has a lot of servers. Google search, and Gmail, and YouTube, and Android, and Chrome, Google Docs, Google Maps, and on and on.

Peter If everything in the world was done by one computer, that wouldn’t be so difficult. That computer would know the order that things happened in. But that isn’t how the world works. Everything is — all jobs, more or less, are divided among multiple computers, and now they, they have to agree with each other about the order of events, or else chaos will ensue. 

Pogue We wouldn’t want that. 

Peter You wouldn’t want that. 

Peter first heard about leap seconds one day in 2008. 

Peter Somebody wandered into our office and said, “Hey, you people know about leap seconds, don’t you?” And I said, “No, I’ve never heard of a leap second.” I had never heard of a leap second. 

Right away, he thought that they sounded like trouble.

The way computers keep time and handle leap seconds, it turns out,is the computer clock jumps back one second at the leap second. 

That made us very uneasy, because — I don’t know, the one thing you know about time is that it goes forwards. And here’s a case where time seems to go backwards. 

Yeah—trying to teach your computer network to run in reverse for one second sounds like a recipe for crashes. 

Now, Google was a relatively small company in 2008—small enough that Peter and his team could easily get access to its computer logs. At the time, the most recent leap second had arrived in 2005. Out of curiosity, he checked the records. 

Peter And there were funny reports written from the very last day of 2005 of some crashes in Google’s servers, some fairly widespread crashes that were not fully understood. And that really caught our attention, we thought, “Oh, that’s interesting.” The computer programs — first of all, they are assuming that time just goes forwards, because everybody assumes that. And they also made some assumptions about the rate at which time advances, and those are perfectly good assumptions at every instant, except around a leap second. 

And that set us to work, because we realized for two reasons we had to deal with leap seconds. One was to make the internet more reliable, because things would break if we handled leap seconds the old way they were handled. And because we wanted in the future much better synchronization across the computers, and the sudden step back in time would wreck that. 

But if the leap second wasn’t the perfect solution to the difference between the earth and the atomic clocks, what was? Google’s time team came up with a different idea, which those clever dogs decided to call—the leap smear. 

Peter What we would have the computers do is very slowly smear out that one second — leap second — rather than do it all of a sudden. And by doing it slowly, they would all be in very close alignment. And there would be no sudden discrepancy that would break the coordination of the multiple computers.  The basic idea is don’t do it suddenly, spread it out over time. 

Pogue And as a handy bonus, time doesn’t go backwards. 

Peter Correct.  And that’s a huge, that’s a huge thing, because   that’s, first of all, hard to think about. And secondly, it’s really hard to test and it only happens every few years. And it’s, it’s not so much fun. 

Pogue So you still do the leap smear on the day when a leap second is prescribed? 

Peter Correct.   The smear is centered on the actual leap second, and it extends to the previous and following noon. 

Pogue Is, is there more than one Leap Smear Proposal, or is Google the sole holder of this idea? 

Peter We published it pretty widely, because we thought, “Look, this is a great way to make the whole internet more reliable.” It’s not a competitive thing. It’s just good for everybody. And so   several companies use the same smear, and I believe there are organizations that use somewhat different smears.  

Actually, even Google did several. The first one that we did in — at the end of 2008, when we were scared, because we’d never done it before, we actually made a more complicated one that sort of started —started slow, sped up even more, and then slowed back down again. And then we measured very carefully what happened during that first leap smear, and we realized, “OK, this worked great, we can simplify it and we’ll just do a straight-line correction.”

Pogue Okay, great. So at this point, how long is a second on leap smear day? 

Peter You’ll love the way we describe it. A second is 11 parts per million slower during the leap smear. 

Pogue Is it the same as saying 11 millionths of a second ? 

Peter Yes. Yes.  Sorry, I was just being a nerd. I apologize. 

Pogue So on Leap Second Day, the leap smear adds one, 60 x 60 x 24th of a day to every second of the day?

Peter Yes. Perfect. 

Pogue Okay, thank you. I’m not going to embarrass myself.

Believe it or not, leap seconds and smear seconds aren’t the only solutions people are kicking around. Maybe we don’t need the TAI and UTC clocks to be exactly in sync, down to the second. Some scientists maintain that adding a leap second every few years is overkill; maybe we can wait a couple hundred years to accumulate those seconds into a single leap minute. Here’s Peter from Google again:

Peter There have been interesting proposals, and they’re clever, in a way, of saying, “Well, instead of correcting by one second every so often, why don’t we make at least a pro forma leap minute that would happen once every few centuries? And that would at least kick the problem way down in the future.” And there might be an argument, a sensible argument for that — as science develops, there may be further corrections to the way humans keep time. And so it’s perhaps not a crazy proposal. 

Now, none of these time-adjustment solutions is universally beloved. Every one of them involves compromise and causes problems for somebody.

So maybe this is the time to mention what might be the most radical solution of all. This might freak you out, so I hope you’re sitting down. Preferably reclining. Ready? 

Maybe it doesn’t matter if the earth clock and the atomic clocks drift apart. 

I mean, what, really, is the issue? What if they do get of sync? What’s the worst that could happen?


Sure, we’re used to certain times of day matching up with certain numbers on our watches. If you’re used to seeing the sun directly overhead at 12 noon, maybe you’re disturbed by the notion of a future where the sun is actually low to the horizon at 12 noon.

But first of all, that noticeable a difference will take thousands of years to come about. And second, keep in mind that we deliberately set our clocks off that familiar pattern by a whole hour every year! It’s called Daylight Savings Time. If we value the “12 noon sun overhead” thing so much, why on earth do we go out of our way to mess it up by a whole hour? 

This radical notion has occurred to the international time-keeping bodies, too. Here’s Google’s Peter Hochschild.

Peter Every decade or so, there’s a fairly serious discussion in one of the international standards bodies. Should we keep leap seconds or should we stop doing them?  

And sure enough: the ITU, the United Nation’s International Telecommunication Union, periodically polls its members on whether or not to abolish the leap second. They go around this huge auditorium and let each country’s representative make its case.

UN audio

US: The use of leap seconds introduces discontinuities into what would be otherwise continuous time stream. 

UK: In the view of the UK, leap seconds have already been inserted without causing difficulty.

Canada: Canada does not see any compelling reason to change its definition.

Chair: I believe that the appropriate course of action is that we return this draft revision for further work.

US: Thank you, Mr. Chairman.

They went through this debate in 2005…in 2008…in 2012…and in 2015. And every single time, the members of this august body made the same decision, which is —not to decide. To kick the decision down the road. The vote is scheduled to come up again in 2023, at which point it will probably be postponed again.

In the meantime, our species will continue to use leap seconds, leap smears, and maybe other approaches to keeping the earth’s clock and our computer clocks in sync. It’s a challenge that will only get harder as our atomic clocks get better. Here’s Geoff Chester from the Naval Observatory.

Geoff Our most recent clocks are what we call rubidium fountain clocks.   These things are really cool. These are clocks which are designed   to incorporate five Nobel prizes in physics.

 We can provide a time scale that’s precise down to about one hundred picoseconds. That’s one hundred-trillionth of a second. But I can guarantee you that   somebody is going to find a need for a more precise time scale.   And that’s why we are now building these prototype optical frequency standards. 

[music starts fading in]

In a cesium atomic clock, we use microwaves to bombard cesium atoms. But in the optical clock he’s talking about, we’ll shoot light at atoms—laser beams—for an astonishing leap in precision. 

Geoff:  Optical frequencies are five orders of magnitude higher than microwave frequencies.  

(continued) So I would say that in 10 years, we are going to have optical frequency standards, we are going to redefine the second.   And that means I will have to memorize a number with five more digits in it. So I think I’m going to retire before that happens because, my brain just won’t absorb that anymore.