Wait just a (leap) second

By Phil Plait | January 23, 2012 7:00 am

Clock at midnightThis summer will be a little bit longer than usual. A tiny little bit: one second, to be precise. The world’s official time keepers are adding a single second to the clocks at the end of June. This "leap second" is needed to keep various time scales in synch. It’s a bit of a pain and won’t really affect people much, but if it weren’t done things would get messy eventually.

This gets a bit detailed — which is where the fun is! — but in short it goes like this. We have two systems to measure time: our everyday one which is based on the rotation of the Earth, and a fancy-schmancy scientific and precise one based on vibrations of atoms. The two systems aren’t quite in synch, though, since the Earth counts a day as a tiny bit longer than the atomic clocks say it is. So every now and again, to get them back together, we add a leap second on to the atomic clocks. That holds them back for one second, and then things are lined up once again.

There. Nice and simple. But that’s spackling over all the really cool details! If you want a little more info, you can read the US Naval Observatory’s press release on this (PDF).

If you want the gory details, then sit back, and let me borrow a second of your time.


Time after time

There are lots of ways of keeping time. The basic unit day is based on the physical rotation of the Earth, and year is how long it takes to go around the Sun. But we need finer units than those! So we decided long ago to divide the day into 24 hours, and those into 60 minutes each, and those into 60 seconds each. In that case, there are 86,400 seconds in a day. OK, easy enough.

For most of us, that is enough. But scientists are picky (or "anal" if you want to be technical) and like to be more precise than that. And the thing is, the Earth is a bit of a sloppy time keeper. Tidal effects from the Sun and Moon, for example, slow it a bit. Other effects come in as well, changing the rate of the Earth’s rotation.

To account for this, in 1956 the International Committee for Weights and Measures made a decision: we’ll base the length of the second on the year, not the day. In fact, we’ll take the year as it was in the year 1900 (a nice round number, so why not) and say that the length of the second is exactly 1/31,556,925.9747 of the year as measured at the beginning of January 1900*.

OK, fine. Now scientists have their anal precise definition, normal people have calendars, and we’re all happy, right?

Right?


Sunrise, sunset

Yeah. Not so much. Defining the second as a fraction of a year is fine and all, but it kind of leaves the unit of "day" out in the cold. To define that, we use what’s called a mean solar day, which is essentially the time it takes for the center of the Sun’s disk to pass a point in the sky twice. So basically look at the Sun, mark its position and note the time, and wait for it to pass that point in the sky again. That’s a solar day.

But this doesn’t depend on the second or the year! And remember, the length of the day is slowly increasing. You can ask yourself – and you should – hey, when were there exactly 86,400 seconds (as we now define them based on the year 1900) in one solar day? Because if the day is getting longer, there are more seconds in a day now than there were a hundred years ago.

The answer, it turns out is in the year 1820. More or less, but close enough. Back then, a solar day had 86,400 of these new-fangled seconds in it. But that’s not true any more. The Earth has been slowing, the day getting longer, and now, almost two centuries later, there are about 86,400.002 seconds in a day.

Yup. The day has 2 extra milliseconds in it. That may not sound like much, but it adds up. Over the course of a single year we have an extra 365 of those 2 millisecond slices of time. That adds up to about 0.73 seconds every year. After a year, the calendar is off by about 3/4 of a second because the Earth’s rotation is slow. In two years that’s 1.46 seconds, and after ten years it’s over 7 seconds!

We can’t have that, obviously. But what can you do?


The Atomic Age

In 1972 a new timekeeping system was adopted: Coordinated Universal Time, or UTC, based on an atomic clock. The idea is that atoms are pretty good timekeepers, and in particular a cesium atom makes an excellent clock (I describe why here). In a sense using an atomic clock makes it unnecessary to use the Earth as a clock… but we humans have this pesky desire to use clocks in our everyday lives, and to base them on things like sunrise and sunset — in other words the Earth’s rotation (the aforementioned system, which is called Universal Time 1 or UT1).

And we know the Earth’s rotation is a bit slower than it was, and every year we have these dangling 0.73 seconds. That means that the Earth time is lagging behind the atomic time by that much every year.

The fix for that is to add leap seconds when needed. The International Earth Rotation and Reference Systems Service (IERS) are the folks who keep track of these things, and they decided that when the lag between UT1 and UTC gets to be more than 0.9 seconds, they add a leap second to the end of a convenient month, and the two get back to being much closer in synch.

That’s why we were adding one at the end of June.

And we’re done. Right?

Right?


Stop the Earth, I wanna get off!

Nope. I want to clear up a misconception (used by creationists sometimes) first… and then I have to add one more thing.

Some people think that because we have to add leap seconds every year or so, the Earth must be slowing by a tremendous rate. But that’s not the case: the reason we add leap seconds is because the two time systems have clocks that tick at different rates, essentially. It’s not much, but over time it adds up. I’ve written about this before:

Imagine you have two clocks. One thinks there are 86,400 seconds in a day, the other thinks that there are 86,401, so the second clock runs a tad bit slower than the first. Every day, it’s one second behind, clicking over to midnight one second after the first clock does. Mind you, it keeps accurate time according to its own gears: every day has 86,401 seconds, so it’s not slowing down.

However, to keep it synchronized with the other clock, we’d either have to subtract a second from the second clock (yikes, terminology is a bit confusing there!) or add one to the first clock every day. So we’d need a leap second every day, but not because the clock is slowing. It’s only because it runs at a different (but constant) rate.

So it’s not that the Earth is slowing down so much. If the Earth were to maintain its current spin rate from now on, we’d still have to add leap seconds every now and again, because it’s running slow. In that case, we could simply insert leap seconds at regular intervals and everyone’s happy.

But there’s still one more thing. Of course there is.


Staggering seconds

The final issue is that the Earth isn’t slowing at a constant rate. There are a lot of factors that change the Earth’s spin — weather, damming rivers, earthquakes, and so on — that add up over time. So it’s not just that the Earth is slow compared to an atomic clock, but also that the amount it’s slow changes! We can’t just add a leap second every 15 months or whatever; we have to watch the Earth’s spin carefully and add them in as needed. Since 1972 when all this started, there have been 24 leap seconds added to atomic clocks. Sometimes the interval between them has been less than a year, and sometimes it’s been quite a bit more. The last one was added on December 31, 2008.


Cutting the cord

After all this, you might be wondering why we bother. Why not just decouple the two clocks, and let them go their separate ways?

A lot of scientists do in fact feel this way. But it turns out to be really, really complicated to do that. A lot of computer systems (including satellite navigation systems) have software written a while ago, and changing that would be difficult and have unforeseen consequences. Fiddling with that may be dangerous.

At a recent meeting of the American Astronautical Society, there was a session on this topic. A paper was written (PDF) giving an overview of it. It gets a little technical, but it might make for interesting reading for any chronological geeks out there.

I think I lean toward the present system of keeping the systems in tune by using leap seconds. It’s a small price to pay for having our own internal clocks based on the spinning planet.

… but I wonder. There will come a day when we leave this planet for others. The Moon, Mars, and beyond. When we do, we’ll have to worry about timekeeping there as well. We do have one for Mars, but it will have the same inherent problems we do now.

It’s actually kind of neat: there may — there will — be a time when we do need a truly Universal time system. What will we do then?

Images courtesy of zoutedrop’s Flickr stream; NASA; Ozymandias.


* There are different ways of measuring the year, too so they went with a tropical year, which was a pretty decent choice.


Related posts:

- Take a flying leap second
- Followup: leap seconds
- Another orbit? Why, you don’t look a rotation older than 4.56 billion years!
- Why we have leap days (one of my favorite posts of all time)

CATEGORIZED UNDER: Cool stuff, Science, Time Sink, Top Post

Comments (58)

Links to this Post

  1. This summer will be longer, by one second | January 24, 2012
  2. No, Wait « CollTales | January 26, 2012
  3. TMQ watch: January 31, 2012. « Whipped Cream Difficulties | January 31, 2012
  4. Quantum Leap « CollTales | February 29, 2012
  1. Carey

    I am definitely a chronological geek. These posts, and the ones about sidereal vs. tropical vs. anomalistic years get my geeky juices flowing!

  2. Cindy

    But Phil, you haven’t even discussed Julian Dates! ;-)

  3. GeH

    > there will — be a time when we do need a truly Universal time system. What will we do then?

    I think that standard is called Star Date.

  4. Naked Bunny with a Whip

    Leap seconds are just a money-making scam by Big Cesium.

  5. Robert Gibson

    And when we do travel to other planets, and maybe even other stars, won’t relativistic effects throw a huge wrench into our timekeeping?

  6. Sometimes I am amused by how fanatical we get over a relative construct like time. :) I’m sure it presents great opportunities for educational posts like this one though, so I am glad for it.

  7. Tara Li

    STARDATE! 23! 47! Point! 9! WHIle in orBIT around Fakus Latinus Aleph Null, WE!!!! disCover we forgot to wind the clock. SPOCK! says its. 11:47am WHICH means I can’t HAve that DRINK with Bones yet, BUT!! SCOTTY! says anytime IS a GOOD TIME FOR whisky. UHUra says it’s 2,487,490 cesium OscILLATIONs past tacky.

  8. A truly universal time is a doomed endeavour, especially if we start travelling at relativistic speeds. I know, this is not happening anytime soon. But the point is, relativity has rendered obsolete the concept of simultaneity. Unless we define (arbitrarily select) “one true inertial reference frame”, there can be no truly universal time.

  9. Gary

    Yeah, time for stardates.

  10. Zach

    I don’t really see the downside of decoupling the clocks. Realistically, clocks don’t match observed time now. When the clock shows noon in the Eastern time zone, the sun is in a different spot in the sky for people in Boston and those in Philly. Not much, of course, which is why it doesn’t matter, but a bit.

    After a century of leaving the day and second decoupled, observable time would only be off by a little over a minute – far less than the acceptable ~30 minutes of difference we consider negligible for people living on the edges of time zones.

    In fact, it would take something like 1,200 years before clocks were off by 15 minutes, the point when observable noon was closer to the edge of a time zone than the center. And that’s so far into the future as to be pointless to plan for.

  11. John Baronian

    very interesting

  12. Time always seems so simple until you look at it closely.

  13. Chris A.

    “…scientists are picky (or “anal” if you want to be technical)…”

    I _do_ want to be technical, so I’ll point out that what you really mean is “anal-retentive,” to distinguish it from “anal-expulsive,” in the Freudian vernacular. (Roughly, anal-retentive = Felix Unger; anal-expulsive = Oscar Madison). :)

  14. Zach

    I just realized my math was wrong in my last post. I didn’t take into account the additional slowing of the Earth. I’m on my phone, so I can’t really do the math right, but my back of the envelope guesstimate is 500 years or so before observable noon is off by more than 15 minutes. Which is still far enough into the future that it’s not worth worrying about.

  15. CB

    @ Tara Li:
    That was great. :)

  16. Pete Jackson

    Excellent article, BA. And timely, too.

    The problem that timekeepers have with leap seconds is that one doesn’t know when they are going to be inserted until only about a year or so before they happen.

    A compromise solution for leap seconds would be to make them regularly spaced, based on a mean solar day being equal to very close to 86400.002 atomic seconds. Having a leap second once a year would correspond to a mean solar day equal to 86400.00274 atomic seconds. I propose having a leap second on December 31 every year, except leap years. This would correspond to a mean solar day of 86400.002075 atomic seconds. This can be easily programmed into computers and satellites, and would keep us aligned with the Sun for a very long time to come.

  17. Chris S

    I claim Geek of Chronology by having a first edition of Calendrical Calculations. (The Millennium edition was for people who didn’t think ahead.)(Last week we passed through 26.0 into 25.999… while counting down to the Unix rollover. This is your last chance to *test* those systems that calculate ahead 25 years to ensure they will still run fine after January 19th (time varies by zone) of next year.)

    You don’t even need relativistic issues to make multi-planet time complicated. The issue of simultaneity causes no end of hard-thinking even today. When a lander sets down on Mars, is it happening “now”, or is “now” when we get the light-speed signal? Even thinking in multiple time zones that are in different north/south hemispheres when you are near daylight savings changes can make a brain hurt.

    Trying to pick “one inertial reference frame” will cause us grief, because the surface of the Earth is non-inertial reference frame. In such a scenario, you would need to adapt everyday event timings into “Agreed Upon Reference Time”.

    Strange as it may seem, I think we already have the solution — use cesium as a “physical constant” for time duration (unless it can be replaced with something more accurate), and adapt everything else to that. There will never be a good or single nice reference for absolute time.

  18. Jim Baerg

    Re: Interplanetary Time Keeping.

    See kiloseconds, Megaseconds Gigaseconds etc. as used by some cultures in Joan Vinge’s _Heaven Belt_ stories, & by the Qeng Ho in Vernor Vinge’s _A Deepness in the Sky_.

  19. Chris S

    Easy first order reference: Pi seconds is a nanocentury

  20. JMW

    @7 Tara Li
    <raised eyebrow>Humour. It is not logical.</raised eyebrow>

  21. It’s actually kind of neat: there may — there will — be a time when we do need a truly Universal time system. What will we do then?

    Stardate 6054.9: Captain’s log, supplemental. A search of old Earth archives pulled up the above quote. We can now answer with certainty that the solution was developed by S(*()^^%$**#+:YgghH/.,,.//

    NO CARRIER

  22. Well that’s just great! Go on, you revel in your longer summer, don’t give a thought to those of us in this hemisphere who’ll have to endure a longer winter!

  23. What I don’t really understand is this.

    The reason we want to add leap seconds is because we want day and night to stay much as they are, right? And the drift is always in one direction — we only ever add leap seconds, not take them away.

    We’ve added 24 in 40 years, but apparently we can’t just add leap seconds at regular intervals because we don’t drift at a constant rate of 0.6 seconds per year.

    So what, though? If we had saved up until 2070 or whenever and applied a whole leap minute at once, nobody would be complaining that day and night had got all messed up in the meantime.

    And by 2070, maybe our understanding of planetary orbits will be better enough that by then we can predict how many leap seconds we would have to apply for the next thousand years. Only instead of doing so, we could just adjust the definition of the length of a second so that instead of being the duration of 9,192,631,770 periods of whatever, it’s 9,192,631,945 periods or whatever.

    And I’m willing to bet that we could make a go of that now, that we could probably make a more accurate estimate than 0.6. So why don’t we? Some atomic clocks will need to be reprogrammed, but that would have to happen considerably less often than every single clock in the world needs to be adjusted under the current system.

  24. Tara Li

    I am – and always have been – confused by this idea that something doesn’t happen until you see it, when applied to relativity. That’s a *QUANTUM* thing, and in quite a different respect of meaning. Planet X and Planet Y don’t disagree on when Planet E was blown up (it was the little black guy, in the green helmet, with the Illudiam Q-36 Explosive Space Modula-torrrrrrr), they disagree on when they *saw* it. When they correct for distance, and relative speeds between them, I expect they agree quite well on just *WHEN* it happened.

  25. HP

    So, when is some enterprising programming geek going to come up with a web-based clock we can use to watch it tick over 11:59:60 on Leap Second Day, so we can organize drinking parties around it?

    “. . . six, seven, eight, SIP! Woo-hoo!”

  26. JB of Brisbane

    Actually, isn’t “leap second” a misnomer? It should be a “leap minute”, in the same way a leap year has 366 days, a leap minute has 61 seconds.

  27. Mike

    +10 for extremely thorough post.

  28. Andrew

    I swear, Fly Like an Eagle came on the radio just as I started reading this post…

    “Time keeps on slippin’, slippin’, slippin’, into the future.”

  29. Messier Tidy Upper

    @25. JB of Brisbane :

    Actually, isn’t “leap second” a misnomer? It should be a “leap minute”, in the same way a leap year has 366 days, a leap minute has 61 seconds.

    Yes but there is a leap day – Feb. 29th – which has just the usual 24 hours so having leap second is consistent with that.

    PS. Happy Chinese New Year everyone for yesterday! :-)

    PPS. Guess that’s at least one area they’ll always be behind us Westerners. ;-)

    *****

    “Something to think about when trying to fathom these numbers : 1 million seconds is about 12 days. One billion seconds is about 31 years.”
    - Stephen Jones, letter in the Weekend Australian newspaper 2011, April 9th-10th.

  30. Infinite123Lifer

    So wait just one second . . . I mean, so add just one second :)

  31. tmac57

    Imagine you have two clocks. One thinks there are 86,400 seconds in a day, the other thinks that there are 86,401, so the second clock runs a tad bit slower than the first.

    Hmmm…Wouldn’t the 2nd clock be running faster? It manages to rack up one more second in the same amount of time.

  32. gdave

    @tmac57:

    The 2nd clock is running slower. Remember, a “second” has been arbitrarily defined and both clocks agree on how long it takes for a second to elapse. If you reset both to 0, they would agree on the total number of elapsed seconds since the reset. But the first would begin Day 2 after reset at 86401 (Day 1 had 86400 seconds), the second wouldn’t start Day 2 until 86402, and so on, with Clock 2 “losing” a second every day, and gradually falling behind Clock 1 in terms of elapsed days.

  33. Daniel Schealler

    Once we need a universal time system, won’t we necessarily be distributed so broadly throughout the universe that relativistic effects would kick in and make maintaining everything by reference to some kind of Universal Galilean Clock even more of a pain than things are right now?

  34. Infinite123Lifer

    “I swear, Fly Like an Eagle came on the radio just as I started reading this post…”

    What perfect timing! Although apparently there is no perfect timing.

  35. JB of Brisbane

    @Messier Tidier Upper #25 – Well maybe. I’ve always heard of the 29th of February referred to as “Leap Year Day”, and never as “Leap Day”. Of course, it may well be called that where you are.

  36. Gibbon1

    My take on this is really they need to break down and have two parts, Universal Time, which is constant and monotonic, and an offset that changes. Once you do that you can change the offset as much as you like without bothering people that need to do calculations based on time stamps.

  37. Nigel Depledge

    At the risk of having been pre-empted . . .

    The BA said:

    . . . for any chronological geeks out there

    Actually, I think you meant horological.

    Horology is the science of timekeeping. A “chronological geek” would, I guess, be fanatical about putting events into sequential order.

  38. Carey

    @Hamish #23: The rate at which leap seconds are added isn’t predictable enough. An earthquake can cause a change in speed of earth’s rotation (I believe the Japan earthquake last year shortened the solar day by a couple microseconds). And since we don’t know how to predict earthquakes, we can’t predict the changes in the Earth’s rotation speed.

    And this precision down to a second must be maintained because of the wide use of GPS satellites, which rely on a precise timestamp to be able to give you your location.

  39. tmac57

    gdave- The reason I view the 2nd clock as running faster based on Phil’s scenario is this analogy:
    If you have two wall clocks synchronized to start at 12 midnight,and you check them exactly 24 hrs later,and one clock reads 12:00 and the 2nd clock reads 12:00:01,which clock would you say is running faster? The 2nd one. You would need to subtract a second from the 2nd clock to match the 1st ,right? Now here is what Phil said from the article quoted:

    “However, to keep it synchronized with the other clock, we’d either have to subtract a second from the second clock (yikes, terminology is a bit confusing there!) or add one to the first clock every day. So we’d need a leap second every day, but not because the clock is slowing. It’s only because it runs at a different (but constant) rate.”

  40. Kenneth Polit

    A truly universal time? How about going to Stardates?

  41. Anchor

    *hiccup*

    Pardon.

    @#42Kenneth Polit: WHAT “Stardates”? You’re after some universal reference, and there isn’t any. One is obliged to pick a particular reference to go by. Trouble is, any you choose will inevitably show some variation over sufficiently long periods of time. You must realize, of course, that the fictional “Stardate” of Star Trek has no basis that makes any sense: the Milky Way galaxy which we inhabit is over 100,000 light-years across, and any attempt to correlate the timing of its estimated 200-400 billion stars is a fool’s errand. (Think about it). The Earth is as good as any reference for the purposes of us Earthlings, even WITH the occasional hiccup. And we can spot whenever it needs corrections like this one since we stare at vibrating atoms, which supply the fine-scale tuning. Good luck to anybody who thinks they can stably correlate atomic clocks that are positioned throughout the galaxy, what with the stupendous opportunity of intervening gravitational fields in the form of stars, giant molecular clouds and variations in the dark matter distribution rendering time-keeping for any particular spot in the galaxy like ours a headache better attended to by future galaxy grav-field dynamicists.

  42. Anchor

    Even with Phil’s fine exposition, it seems some commenters here are under the impression that time itself has acquired an extra second. That’s not so. Time hasn’t changed. The rotation rate of the Earth can change. Like any mechanical oscllator (which our rotating Earth may be regarded) it can slow down or speed up. Mostly, however, the Earth’s rotation is subject to various forms of friction, which slows it down. But as Phil points out, that isn’t always the reason for correction. Its not even the primary reason. Extremely accurate clocks can get out of synch just because they may happen to be located at different altitudes. Clocks at the same altitude but in different locations can get out of synch just because they’re under (very subtly) different gravitational acceleration fields. (Special Relativity kicks in, and the effect is detectable by the clocks onboard the GPS satellites, for example, which not only deal with the extra altitude, but with their orbital velocity). Then there are the inevitable mechanical issues. But, yeah, it’s mostly true that the addition of ‘leap seconds’ is mostly in response to the slowing of Earth’s rotation rate over time due to various frictions. (The most famous and direct source is that induced by solar and lunar tides). But then events such as earthquakes and even ocean currents can episodically SPEED UP the Earth’s rotation rate. One encounters a curve over time that jumps back and forth over an overall trend of slowing rotation. (It’s vastly amusing to encounter climate change deniers who look at this very same data and cannot see the essential resemblance to the curve which demonstrates global warming, which shows precisely the same kind of long-term trend punctuated by short-term variation!). Whenever it is required, the clock folks whose business it is to notice these time asynchronicities decide to add a second whenever it becomes necessary…as now, when it gets off by as much as a second. They’re not looking at any single “universal’ reference clock of any kind. They’re looking at DOZENS of clocks that all tell a slightly different tale, and they figure the mean time between all of them to give them their reference.

    I think I’ll spend my “extra second” blinking.

  43. Nigel Depledge

    tmac57 (40) said:

    gdave- The reason I view the 2nd clock as running faster based on Phil’s scenario is this analogy:
    If you have two wall clocks synchronized to start at 12 midnight,and you check them exactly 24 hrs later,and one clock reads 12:00 and the 2nd clock reads 12:00:01,which clock would you say is running faster? The 2nd one. You would need to subtract a second from the 2nd clock to match the 1st ,right?

    I’m not sure I accept this.

    What if your clocks instead say 12:00:00 and 11:59:60? After all, they are both defining a day, and one of them has an extra second to that day compared with the other. The only way to firt that extra second in is if one of your “minutes” in the day is longer than the others. Now, move on one minute, and the clocks will say 12:01:00 and 12:00:59. Which one is fast? And yet both clicked over from one day to the next at 12:00:00.

    You seem to be of the opinion that the clock that defines its day as having more seconds has shorter seconds, but it is the other way around – it has a slightly longer day. The duration of each second is the same for both clocks.

  44. tmac57

    Nigel-For the purpose of my example,I am assuming that both clocks are starting from the exact same time,and the second one is finishing up having clocked up 1 second more than the first,within the exact same span of time.That last bit is the key.the actual objective duration measured by both clocks was the same. Now,is there anyone out there who sets their watch exactly by some know stable standard,who later checks it against that standard,and finds that it is any amount of time forward from what it should be, (1 second,1 minute,1 hour) that wouldn’t consider that watch to be running ‘fast’?
    Maybe we are not considering the same scenario,but those were my assumptions,as that was what I believed that Phil was describing.My head hurts. ;)

  45. Fred Bjorklund

    Time is a fundamentally misunderstood construct of the human mind, both in the scientific and general population, as this article demonstrates. Time is simply consequence of change. If there is no change, there is no time. This extends from the sub quantum realm and
    beyond, to macro universe and beyond. There is no time travel to the. past. There are no wormholes that will take us into the past, because the past has already occurred and is not infinite. Einsteins’ work suggests that we can travel into the future, the mechanism of which is traveling through space at very high velocities. This is not time travel in the typical sense that most of us perceive it to be. His work suggests that “time” slows down as velocity

    increases. Time actually slows down because the molecular components of the system slow with increased velocity.

  46. Nigel Depledge

    tmac57 (46) said:

    Nigel-For the purpose of my example,I am assuming that both clocks are starting from the exact same time,and the second one is finishing up having clocked up 1 second more than the first,within the exact same span of time.That last bit is the key.the actual objective duration measured by both clocks was the same. Now,is there anyone out there who sets their watch exactly by some know stable standard,who later checks it against that standard,and finds that it is any amount of time forward from what it should be, (1 second,1 minute,1 hour) that wouldn’t consider that watch to be running ‘fast’?

    It would, but . . .

    Maybe we are not considering the same scenario,but those were my assumptions,as that was what I believed that Phil was describing.

    I got the opposite impression, that what the clocks disagreed on was not the length of a second, but the length of a day. AFAICT, we have a single definition of “second” but 2 or 3 definitions of “day”. The mean solar day and the sidereal day have different numbers of seconds in them. And, IIUC, the defined mean solar say no longer exactly matches the Earth’s rotation (the actual solar day), hence the need for leap seconds.

  47. tmac57

    Nigel-After rereading (again) what Phil wrote,I think your view is correct. I got derailed on to a different frame of reference (not the 1st time I assure you :) ) Thanks for setting me straight.And you too gdave.

  48. And if you listen to the BBC that night, you’ll hear 7 beeps instead of the usual 6.
    http://en.wikipedia.org/wiki/Greenwich_Time_Signal

  49. Anchor: @#42Kenneth Polit: WHAT “Stardates”? You’re after some universal reference, and there isn’t any. One is obliged to pick a particular reference to go by. Trouble is, any you choose will inevitably show some variation over sufficiently long periods of time. You must realize, of course, that the fictional “Stardate” of Star Trek has no basis that makes any sense: the Milky Way galaxy which we inhabit is over 100,000 light-years across, and any attempt to correlate the timing of its estimated 200-400 billion stars is a fool’s errand. (Think about it). The Earth is as good as any reference for the purposes of us Earthlings, even WITH the occasional hiccup. And we can spot whenever it needs corrections like this one since we stare at vibrating atoms, which supply the fine-scale tuning. Good luck to anybody who thinks they can stably correlate atomic clocks that are positioned throughout the galaxy, what with the stupendous opportunity of intervening gravitational fields in the form of stars, giant molecular clouds and variations in the dark matter distribution rendering time-keeping for any particular spot in the galaxy like ours a headache better attended to by future galaxy grav-field dynamicists.

    This. And galaxy, heck, this is already an issue with space probes, and it’ll be an even bigger issue once we start establishing crewed missions throughout the solar system. Even if you’ve got your atomic clocks at Cloud Station Valhalla, Venus and Labrum Marineris, Mars perfectly synchronized, they’re going to appear to drift due to their orbital velocity and the Sun’s gravity well.

  50. DaKine

    I grew up on Kauai in Hawaii where some of the people are who actually have to PUT the leap second in to the clock. Here’s a link to them at station WWVH (atomic clock photos included) operated by the National Institute of Standards and Technology:

    http://tf.nist.gov/stations/wwvh.htm

    Love your piece, Phil

  51. Nigel Depledge

    @ tmac57 (49) -

    No worries, glad to help.

  52. MacNicol

    …”The final issue is that the Earth isn’t slowing at a constant rate. There are a lot of factors that change the Earth’s spin — weather, damming rivers, earthquakes, and so on — that add up over time.”

    What is the mechanism by which the above natural occurences change the Earth’s spin rate?

  53. PISCES DREAM

    TIME IS A SOCIAL CONSTRUCT…. WHAT TIME WOULD IT BE IF YOU DIDNT KNOW WHAT TIME IT IS??? HOW OLD OR YOUNG WOULD YOU BE? THE CONCEPT OF TIME IS ANOTHER ATTEMPT OF MAN TO CONTROL HIS SURROUNDINGS….ANCIENT PEOPLE LOOKED AT THE SUN RISING AND SETTING….THEY LIVED MORE PRODUCTIVE, LESS HARRIED, LESS STRESSFUL AND HAPPIER/SPIRITUALLY BASED LIVES…TIME IS SET FOR THE MARKETS…WE LIVE OUR LIVES IN NANOSECONDS TO BUY AND DIE!!!

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