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The Future of Leap Seconds
@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."
This site may be more helpful, especially in clearing up some of the problems with leap seconds (and their ultimate creation of an offset from both TAI and GPS time)
If I understand what I read correctly, essentially the problem they're trying to solve is this: the Earth's rotation is slowing, but they can't predict exactly how much it's going to slow at any given time. It is a real, physical thing, and while they can model its orbit with extreme and unchanging accuracy (things are widely separated enough that the mathematical abstractions work fine), modeling its rotation isn't really possible. There's all sorts of liquid sloshing around everywhere, both liquid water on the surface and molten rock in the center. All they can do is measure it, and every once in awhile, determine that sunrise is happening just a little late.
:-)
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.
Because time is crucial to all sorts of physical and scientific endeavors, such as planetary motion, navigation, GPS, etc. We need an accurate standard, or stuff quits working.
A really interesting guy on this topic is Tom Van Baak, the fellow that runs leapsecond.com. As a measure of the level of obsession a person can obtain, this guy has multiple cesium frequency standards, but he had to go out and buy a crazy russian hydrogen maser so he could get better than a microsecond a year accuracy. He's also got some interesting information about the leapsecond debate on his website.
Me, I'm a simple guy, I just need to keep NTP locked to a couple of microseconds to sleep well.
jeff
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, Postscript.
-renard
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".
This is not a legal opinion, no representation is expressed or implied.