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The Future of Leap Seconds
Posted by
michael
on Fri Apr 18, 2003 05:50 PM
from the stuff-you-thought-you'd-never-think-about dept.
from the stuff-you-thought-you'd-never-think-about dept.
@10u8 writes "Since 1972 precision clocks around the world have ticked using atomic seconds, but earth rotation is slowing down. Leap seconds have been inserted in order to keep noon happening at noon, but they upset some timekeepers. Recent discussions have considered
discontinuing leap seconds in UTC, and a colloquium in Torino next month will present results. It is a matter of international significance."
Related Stories
[+]
Vote To Eliminate Leap Seconds 531 comments
Mortimer.CA writes "As discussed on Slashdot previously, there is a proposal to remove leap seconds from UTC (nee 'Greenwich' time). It will be put to a vote to ITU member states during 2008, and if 70% agree, the leap second will be eliminated by 2013. There is some debate as to whether this change is a good or bad idea. The proposal calls for a 'leap-hour' in about 600 years, which nobody seems to believe is a good idea. One philosophical point opponents make is that the 'official' time on Earth should match the time of the sun and heavens."
[+]
US DoD Poll On Leap Seconds 314 comments
@10u8 writes "For time scales to leap, or not to leap, has been the question here before. The ITU-R will be considering leap seconds again in a few weeks. This week the USNO posted a survey about leap seconds by the US DoD. The issue has civil implications as well as technical ones, and there is a demonstrated way to respect the history, remove leaps from navigation and POSIX time, yet keep the sun overhead at noon."
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Why? (Score:5, Interesting)
Accuracy isn't everything... (Score:5, Insightful)
Oh the horror...
Accuracy isn't everything...
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Oh, but it is... (Score:5, Funny)
It's about time someone did something to correct these errors.
(it's funny, go ahead and laugh, willya?)
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Re:Why? (Score:5, Informative)
Jason
ProfQuotes [profquotes.com]
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Re:Why? (Score:5, Insightful)
The concept of leap years is because the ratio of the length of a year divided by the length of an earth day is not an iteger. No calendar can get around that fact. You either add intercalation days whenever the remainders of your divisions exceed 1, or you keep track of huge numbers and cycles that greatly complicate your timekeeping.
The Julian roman calendar did suck because they didn't get the ratios quite right and it drifted. (The Gregorian calendar fixed this for all practical purposes.) However, prior to Julius Caesar, it sucked even more because there was no mathematical formula. Instead, priests were supposed to observe the sun each year and decide when leap days were needed.
The priests were also involved in politics, so they chose to shorten political terms more often than not by omitting leap days. IIRC, by the time the Julian calendar was instituted, the Romans were off by several months due to these partisan shenanigans.
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Re:Why? (Score:5, Funny)
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Re:Why? (Score:4, Interesting)
There is time as used by humans vs other measures. Its purpose was to define the time that the trains ran on and is very convenient and has only passing relationshipt to the position of the sun -- those at the edges of timezones can be off by an hour or two or more.
Leap seconds are a fine correction for gear that, unlike humans, cares about nanosecond accuracy.
The serious problem with leapseconds is that they make minutes context sensitive and essentially all computer software presumes seconds are not context sensitive.
The simple fix is to keep leap seconds as a correction factor but not confuse it with the time that humans and their computers use for normal use.
The leap second is the kind of bug that appears when you have experts who know too much and are totally clueless about any usage other than what the care about.
It was simply a stupid mistake to foist it on humans and there. They should apologize and simply keep their mitts of social mechanisms like the clock.
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Re:Why? (Score:5, Informative)
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Re:Why? (Score:5, Interesting)
Obviously, there have been all sorts of tweaks and modifications to this formula in the past 200+ years or so, but the basics are the same: You need to know what time it is to know where you are. Your precious little GPS receivers wouldn't work if they could get as accurate a time measurement as possible from the US Naval Observatory.
(Some historians have suggested that the US won the war in the Pacific because US ships had more accurate clocks.)
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Re:Why? (Score:5, Informative)
Not true. GPS receivers get all the information they need directly from the GPS satellites - which track their own "GPS Time" that dispenses with the leap-seconds.
You're right that having an accurate astronomically-relevant time is important for navigation - if you are determining your position with a sextant. It's the decreasing relevance of sextants to the world of navigation, and the increasing need to keep electronic equipment of all sorts in lock-step, that is driving this movement away from the leap-seconds.
See a summary of the issues from one of the US Naval Observatory scientists in charge of this stuff: PDF [navy.mil], Postscript [navy.mil].
-renard
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Re:Why? (Score:5, Informative)
Initially the precise measurement of time was the province of astronomers and ship navigators. Time was fundamentally the measurement of the orientation of the Earth. Time was a function of location. Noon was when the sun was at zenith. If you could know the difference in time measurements at two locations, you could determine the difference in longitudes of the two locations. In order to determine the differences in time systems, mankind developed precise mechanical time measuring systems. The new time measuring systems allowed man to measure the durations between events very precisely.
Eventually man developed atomic clocks that could use the decay of atoms to provide an incredibly stable time reference. However, some time ago, we reached a point where the mechanical time measuring systems became more stable than the Earth's rotation. So the atomic clocks which were counting down seconds very accurately were now getting out of synch with the Earth's rotation which was slowing down (and not smoothly slowing down, either).
Since no one who was concerned with the durations between events wanted seconds that varied in length, which is what would happen if you fixed the varying length of the day at 86400 seconds, the concept of using seconds of fixed duration (based on an atomic standard) was developed. The ever accumulating count of these seconds is TAI (Time Atomique Internationale aka International Atomic Time). The time which represents the orientation of the Earth is Universal Time (UT). (This is a simplification, there are a number of subtle variations on UT that I'm not going to go into, but which aren't important for the purposes of this discussion.)
If left alone, the difference between UT and TAI would grow. So, many years ago the concept of UTC (Universal Time Coordinated) was invented. This time standard uses the standard TAI second, but at irregular intervals, an additional second may be added (on either June 30th or Dec. 31st) to always keep UTC and UT to within half a second of each other.
The bottom line is that for people who have to deal with durations, especially long durations, having those irregular additional seconds is a bookkeeping pain and for those who need to be very concerned about the orientation of the Earth, a half second isn't nearly accurate enough. The latter group are undoubtedly using much higher resolution correction data that is produced by the IERS (International Earth Rotation Service). For most civilians, the fact that noon is shifting off by a second every couple of years just doesn't matter. (Especially since the railroads introduced the concept of time zones a little over a hundred years ago, which means that the sun is rarely at zenith when the clock says its noon.)
A lot of people in the field have questioned for some time whether in the era of modern computers where using the higher resolution IERS corrections is trivial, the leap second has any use. Now it may finally be going away.
Now if you want to get really esoteric, here is something to ponder: For astronomy and celestial mechanics, time is defined as the independent variable in the equations of motion of the universe. For physicists and those who use atomic time standards, time is defined as the independent variable in the decay of atomic particles. Noone, to my knowledge, has ever been able to detect a difference in these two independent variables, but it is not a given that they are the same.
For those who'ld like to know more, the University of Texas teaches a graduate level course in the Aerospace Eng. Dept. on the "Determination of Time".
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IERS (International Earth Rotation Service) (Score:4, Funny)
Is France a member?
Do they take requests? ("I'd like an extra long sunset this Friday night... I have a date!")
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Re:IERS (International Earth Rotation Service) (Score:4, Funny)
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Re:Why? (Score:4, Informative)
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Re:Why? (Score:5, Informative)
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I hope... (Score:5, Funny)
S2B bugs, here we come! (Score:3, Funny)
Excuse me while I stock up on food, medicine and ammo.
Re:S2B bugs, here we come! (Score:4, Funny)
2. I'm actually rather smart, but sometimes instead of a word's last letter I type the next word's last letter. You may classify it as mild dyslexia; it's mostly not a problem since most people can understand what I meant from context.
3. The person sitting next to me right now has been using excessive amounts of glue for the last few minutes; I'm not sure what its purpose was, but some of you CSI fans can no doubt explain how certain types of industrial-strength glue can cause synapses to misfire.
4. Again, I apologize for any inconveniece. As a token of good will, please accept this coupon, good for ignoring one (1) typo or grammar error in any future slashdot posting. Expires 6/18/2003.
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WWV (Score:5, Informative)
Is it time to change time? (Score:3, Interesting)
Although if there are going to be changes to the time standards I can always add converters [webcalc.net] to my calculator site for people to use.
Re:Is it time to change time? (Score:5, Funny)
What's this "today's precision" bullshit? We've been having this problem for thousands of years.
For centuries, peoples dickered back and forth about how many 28-day lunar cycles ("months") were in a year. Some said 12, some said 13. The concept of fractions apparently escaped these simple folk.
Then the Romans came along. Not to be daunted by silly things like "reality," they declared that the lunar cycle was really 30 days, and that there were exactly 10 of those months in a year. While they were at it, they delcared pi to be exactly 3.125. To make things work out a little better, they threw in some extra days between a few months whenver they felt the need. The Latin names for these periods translates roughly to "Not really a month."
That got real old real fast so they threw in two more "real" months, and juggled around the lengths of the months just because they could (They're Romans, they ruled the world, yadda yadda yadda).
That one actually lasted a few centuries, at which point they started to admit that things were still screwy (in the sweltering heat of January), which was also the same time when some guy named after a salad was thinking he was Alexander the Great. The Senate told saladm boy to go fix things (apparently smacking cheese-eating surrender monkeys around qualifies one to make a calendar). Lo and behold he did, declaring that a year was 365.25 days long (ie. every fourth year had a full extra day). Ever humble, he took the time to re-name a month after himself. Feeling insufficient in some department, he also shifted around some days so that his own personal month was longer than most.
For a group of people that had no concept of zero, this system worked pretty well for a millenium or so, when it became obvious that things were getting screwy again. This time, the folks in Rome asked some guy with a funny white hat named Greg to help sort things out. Apparently not as experienced in invading France as salad boy was, he decided he had to plagiarize somebody else's work. He found some insane scribblings from some Poloc kook who figured out that a year was "really" 365.2425. Because the Poloc was loony (he said the earth went around the sun, of all things!), Greg felt comfortable with taking that number without admitting where he found it.
The system work, but the folks Greg worked with had a bit of a bad reputation at the time. To the west, there were a bunch of political crackpots who called themselves "Complainers" or something like that, and to the east there were a bunch of people in even funnier hats than Greg's who all spoke Greek. So it took a little bit of time for "Greg's Nifty Calendar" (why name a month after yourself when you can rename the whole damned calendar?) to catch on, which is why nobody can figure out when Washington's birthday is and why the October Revolution took place in September.
And even now there are all sorts of people from Newton to Einstein that say that even that calendar is screwed up! Greg's plagiarized number was off by 0.0003 days! That's almost 26 seconds! Heck, after a few thousand years that's a hole big enough to drive a Mack truck through, and then where will we be?
"With our global economy (weather we like it or not) times need to be synced across the world."
Between the Greek-speaking folks still thinking Greg was the anti-Christ and two groups of people blowing themselves up in the Middle East still trying to make the whole lunar cycle thing work out, we can't even agree on what day it is!
Leap second, no leap second, it doesn't really matter to me. I'll always have NTP.
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That's great, but Indiana won't participate (Score:5, Funny)
Re:That's great, but Indiana won't participate (Score:5, Funny)
Given Indiana's history with keeping time, probably half the counties will participate and half won't. And some will just switch to the Mayan calander and be done with it.
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Proud to live in a weirdo state! (Score:4, Informative)
A few years ago somebody was running radio commercials here to support a bill that would have put us on DST. The tug-your-heartstrings appeal was that "there are two states that don't use DST: Arizona because of the heat, and Indiana because of special interest groups" (always the Bad Guys here in Indiana.) who want our kids to wait for the school bus in the dark.
Funny because DST makes it darker at any given hour of the morning, and everybody knows that the "special interest groups" who came up with DST in the first place are commerical interests who wanted to extend their business hours. Either the writers of that commercial were really, really stupid, or it was a brilliant piece of social engineering. Either way, I don't think Indiana will be adopting DST any time in the near future.
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Obligitary Hitch Hiker quote (Score:4, Funny)
Time measuerments that make sence... (Score:5, Funny)
Me: Wanna go have sex?
Hot Girl: OK! When?
Me: I'm on lunch break in 3 Maxtors and a Tape.
Hot Girl: I'll pay for the Hotel room.
Re:Time measuerments that make sence... (Score:5, Funny)
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For Those Interested About Leap Seconds In General (Score:5, Informative)
The easiest solution to all this is (Score:5, Funny)
I propose we keep the earth spinning at a constant rate by detonating thousands of nukes at certain places once every four years. This will produce a Catherine Wheel effect and the earth will speed back to its original spin rate.
I am going to patent this idea but I fear itll be 500 years before I get it processed.
Re:The easiest solution to all this is (Score:4, Funny)
You know your 25 MHz computer from 10 years ago? Guess what, now that days are longer, it's 25.001 MHz!!!
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Leap second: history and possible future (Score:3, Informative)
This is the link to a summary of the issues involved, written at a slightly less technical level.
(don't have to pay, don't have to register, etc.)
http://www.cl.cam.ac.uk/~mgk25/c-time/metrologia-
an attempt at a summary.... (Score:5, Informative)
There are two major timekeeping systems: TAI, which is "absolute time" and is never adjusted, and UTC, which is "civilian time". Because UTC is used by normal people, they try to keep it synced to the Earth's rotation, which in theory at least makes it more useful for us mere mortals. (knowing that the sun will rise at exactly X time on X date at sea level, for instance.). So, gradually, UTC diverges from TAI, because one rotation of the Earth is just a little longer than 24 hours, and over time this divergence adds up to be greater than a second. When it's getting close, they add a leap second. These additions are not at regular intervals, because they can't predict exactly when any given second should be added.
There are occasional problems when they add the leap seconds (programs that don't expect 61 seconds in a minute, for example), or programs that don't realize that there are X number of seconds (15 or so?) that simply didn't exist since 1970. (sometimes this stuff matters).
Thus, they're debating about doing away with leap seconds altogether. One possible substitute is a 'leap hour' every thousand years.
It seems like a rather anal-retentive thing to argue about, but these people are paid to be precise to a degree we can't even imagine.
A worthy slashdot story. This is serious geekery.
Re:an attempt at a summary.... (Score:5, Funny)
Why not?
Asshats from the Industrial Revolution days make us do a frickin' "leap hour" twice a year anyways, one of which violates causality. Fuckin' Daylight Savings Time.
What drooling asshat decided that it'd be a good idea if, every year, there was one day when everyone's heart/respiration rates slowed down to one beat/breath per hour, and about six months later, these same people should be able to start a 20 minute download that finishes 40 minutes before it started?
Fine if you've got a black hole nearby for the former, and fine if you can travel faster than light for the latter.
The day we have those technologies, fine. Until then, no, no, no, no, no, these are bad, bad, bad, bad, bad ideas.
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precision timekeeping is real interesting stuff (Score:5, Insightful)
Read the article!
It's important for systems programmers, and lots of folks here are at least systems programming fanboys.
It's important for navigation. Yeah, that includes your GPS toys.
It's important for a number of scientific disciplines, including a number of subdisciplines of radio astronomy.
It's also really interesting that the change in the Earth's rotation can't yet be predicted with enough accuracy to set a schedule in advance for adding leap seconds, but must be measured. This is relatively prosaic stuff that's nonetheless at the limits of our current understanding. Doesn't anyone get excited or curious about science anymore?
What are leap seconds? (Score:5, Interesting)
One year = the time it takes for the Earth to revolve around the Sun.
One day = the time it takes for the Earth to rotate on its axis.
The problem is, there are really about 365-1/4 days in a year - it doesn't work out evenly to 365 days. So, every four years we add an extra day (Feb. 29), and then it all averages out. Otherwise, if we only had 365 days in a year, over many years seasons would start getting earlier and earlier on the calendar.
One day = the time it takes for the Earth to rotate on its axis
One second = the time it takes for Cesium 133 to oscilate about 9.19 billion times (because it's something constant we can measure)
The problem, again, is that there aren't exactly 86400* seconds in a day. So, we add leap seconds periodically to account for it. As I understand it, this isn't necessarily done at fixed intervals, but rather whenever it's decided that it needs to be done. The Network Time Protocol used to synchronize clocks over the Internet supports leap seconds; they can be announced over NTP in advance, so everybody adds them at the correct moment.
Why is it important? It's not important to most people, but computers like things to be precise and accurate for various reasons, and that means we have to agree on exactly what time it is.
* BIND now lets you write "1d" in a zone file, but how many of you still have this number memorized?
Time zones (Score:5, Insightful)
The Right Way to solve this problem is for computers to work with TAI internally, and treat the difference introduced by leap seconds as part of the time zone, for human consumption only. Instead of defining PST to be UTC - 08:00, define PST = TAI - 08:00:22.
Computers can keep their straightforward time system, humans can keep our astronomically synchronized system. No need to lose either of those qualities.
I know who's fault this is! (Score:3, Funny)
When he messed around with the Earth's rotation to save Lois Lane, he got lazy and messed it up by a tiny bit. Now look whats happened, we're off by a couple seconds now.
This is what happens when you get an alien to do a human's job.
Why Stop at Leap Seconds? (Score:4, Funny)
It seems to me that we should get rid of the concept of seconds altogether. The second was devised in the Sumerian culture, along with such bizarre ideas as a circle having 360 degrees.
The French of course stole the concept of decimalization from Thomas Jefferson and applied it to a variety of measurements, but failed to carry it to a good conclusion by decimalizing time (it seems everything French starts off well but is never really completed).
It seems to me that real progress should be made by dividing the day up into decimal units of time, and the circle into decimal units of arc, thus eliminating the second as a unit of measure.
Enough Earth-centrism! (Score:4, Insightful)
Big Deal (Score:5, Funny)
running though is not so popular among this crowd...
The science behind the length of a day (Score:4, Informative)
It looks like the day is getting an average of 2ms longer per century, but it fluctuates 4-5ms away from that on a decade timescale plus some shorter-term noise.
leapsecond.com (Score:5, Informative)
Me, I'm a simple guy, I just need to keep NTP locked to a couple of microseconds [mock.com] to sleep well.
jeff
America Is To Blame (Score:5, Funny)
America is to blame! We are only 5% of the Earth's population, but we use 80% of the angular momentum. Scientists have warned us for years about global slowing, but big business Republicans, and Democrats with large angular momentum consuming projects in their districts refuse to address the issue. The only viable solution is to make papier mache puppets and parade them down Pennsylvania Avenue.
Examine the Purpose of Timekeeping (Score:4, Insightful)
Removing the leap second makes most history recorded with reference to time of day pretty useless. Noon is defined by most people as the time that the sun is in the middle of the sky. Let's keep it that way. If method of keeping time based on exact seconds from one point in time to another (which is actually pretty useless for most things that happen within timeframes longer than a couple of minutes) then let a separate system be designed for it. Start reading off an atomic clock and never account for leap seconds, but don't screw up the rest of the world to please a few.
Re:Examine the Purpose of Timekeeping (Score:4, Informative)
Too late. In most places, local solar noon hasn't been used as a time standard for more than a century [greenwichmeantime.com]. Depending upon where you live within your time zone, the local solar noon can be different from standard time by a half hour or worse--and I'm not going to mention the impact of Daylight Saving Time.
Correcting--or not correcting--the time through use of leap seconds makes a difference of less than half a minute per century. The leap second correction is too coarse for almost any scientific work, and much too fine for the average person on the street.
Why not have a leap minute, where necessary, once every two or three centuries? It will still be dark at midnight, and we reduce the hassle of dealing with time discontinuities by a couple orders of magnitude.
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Old saying (Score:4, Insightful)
A man with a watch always knows what time it is. A man with two watches is never quite sure...
several timescales (Score:5, Informative)
The leap second reconizes the fact that the "second" is defined in terms of particle physics (a quantity of state changes) which is very stable (it's always going to take the same amount of time for the same quantity of state changes), where as the idea of time really comes from the cosmos. When the sun is directly overhead it's 12:00.
Where the earths orbit around the sun is very stable, 265.24 days, the rotation of the earth is very unstable. In fact, there's a provision (though never used) to remove a second from the day! The speed of the rotation is constantly changing. Over the long term it's pretty stable with a stable decay, but in the short it could be necessary to add a second rather quickly to keep the civil time within
The long term average is that we need to add a second to the day about every 18 months, but we haven't needed a leap second since the end of 1998 (over four years!) so in the short term the stability of the earths rotation is low compared to the order of magnitude we measure.
In order to handle this a desicion is made every six months as to a new leap second at the end of June or December (or to remove a second). This is a problem because some systems can't handle the addition of a second on six months notice such as the submarine!
One proposed solution is to allow UT1 (cosmic time) and UTC (civil time) to be out of sync by as many as 10 seconds. This would allow for ample time for warnings to be produced and everyone to know exactly what is going to happen and how to handle it. I don't know if the protocol would add 10 seconds at once, or warn everyone a few years in advance that a second is going to be added at several different points in time.
One interesting side note. Most computer systems don't handle leap seconds. Time keeping software slows the computers clock down (since it's important not to have events which have happened (past) in the future (future). This means that if your measuring anything else based on time that measurement is going to be wrong. The theory being that the accuracy in what time it *is* is more important than what time it *was*. The reason I bring this up is that time is something that can be measured with amazing percision, where as other things can't be measured as well. If you can convert one measurement to time you can measure it more percisely. For example, how fast does the ISS move? If you know it's altitude by measuring how long it takes to bounce a light off of it, and you know how long it takes to get from A to B (or from A to A again), you know how far it moved and how long it took to move and voila, speed, all by measuring time. If a leap second got thrown in while you weren't paying attetion during your measurement, your speed will be wrong.
Re:Why does this mean anything? (Score:5, Funny)
Yeah, let's bicker over whether we should be bickering over leap seconds. Much more productive.
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Re:How cool a job is "Timekeeper"? (Score:5, Funny)
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Re:i got affected by leap second (Score:4, Interesting)
one way to look at this experiment is like this. you have a very faint object that you are photographing. you also want 360 degree view and are using 10 cameras at different angles. Due to shaky-ness, you can't use long exposures. So you use multiple photos which you later combine in your computer. Assume that the object was moving randomly but you know the exact motion. Now if you forget to remember what time, each frame was taken, there is no way to do motion compensation and hence no way to superimpose the frames. now if your computer was too slow to superimpose the images, it may not be worth doing trial and error.
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