Leap Second To Be Added Dec 31, 2008 255
ammorris writes "Don't be the laughingstock of your friends when you shout 'Happy New Years' a second too early ... The International Earth Rotation and Reference Systems Service has announced that a leap second will be added on December 31, 2008 at 23h 59m 60s, meaning that this year will be exactly one second longer. The last leap second occurred Dec 31, 2005; they are added due to fluctuations in the rotational speed of the earth. You can read all about leap seconds on Wikipedia."
legally speaking, it's the first leap for the US (Score:5, Informative)
Re:Gee, thanks for the notice (Score:3, Informative)
Re:Gee, thanks for the notice (Score:5, Informative)
The bulletin is dated 4 July 2008, it's just the Slashdot article that's late. Or even, just on time as a reminder.
Longer, or shorter? (Score:4, Informative)
"Don't be the laughingstock of your friends when you shout 'Happy New Years' a second too early ... this year will be exactly one second longer."
So... wouldnt we be shouting it one second later than everyone else?
Re:legally speaking, it's the first leap for the U (Score:5, Informative)
Yes, yes, that's Nix vs Hedden and it was ruling by the U.S. Supreme Court in 1893. The court ruled that in the common parlance of the time a tomato was seen to be a vegetable and not a "fruit of the vine", working from the assumption that most people at it for a main course instead of a dessert. I think that if you were going to pick up on the ridiculous nature of the case you'd focus on the reason behind the court case — that taxes needed to be paid on imported vegetables and yet not on imported fruit.
That's UTC (Score:5, Informative)
Re:Gee, thanks for the notice (Score:5, Informative)
Re:Gee, thanks for the notice (Score:5, Informative)
Re:Added When (Score:5, Informative)
The length of the second doesn't change. An extra second is added. I work with precision timing systems where this is an issue.
The sequence is:
23:59:59 UTC
23:59:60 UTC
00:00:00 UTC
00:00:01 UTC
That means that the valid range for seconds is 0..60 and it is possible to have 61 seconds in a minute. You need to know this if you are using a programming language with range checks.
GPS uses its own time scale that isn't affected by leap seconds.
Re:Fluctuations? (Score:5, Informative)
I'm sorry? Fluctuations in the rotation of the earth? You mean the earth is accelerating and breaking?
Yes, that's exactly what we mean (well, "braking" rather than "breaking"). The earth does not have a constant angular velocity. To conserve angular momentum, as the mass distribution of the earth changes (e.g. due to glacial rebound), the spinning of the earth speeds up and slows down. It also slows down a little due to tidal braking. So a "day", as measured by the rotation of the earth relative to the fixed stars, is not exactly 86400 seconds. It's generally a little more, around 86400.001 seconds at present, and it varies from day to day and from year to year. Now that civil time (UTC) is kept with atomic clocks, this is a genuine problem. Leap seconds are introduced to keep UTC close to UT1 (astronomical time).
It has nothing to do with the fact that a rotation around the sun is not exactly 365.25 rotations around our own axis? hmm...
That's right. Leap seconds have nothing whatsoever to do with that. They don't affect the calendar. That's what leap days are for. Leap days keep the calendar in sync with the seasons (by setting the average calendar year length to 365.2425 days, very close to the vernal equinox year which is currently 365.242374 days).
Re:Added When (Score:2, Informative)
Re:Fluctuations? (Score:2, Informative)
It has nothing to do with the fact that a rotation around the sun is not exactly 365.25 rotations around our own axis?
No, it does not. Leap days are about keeping the calendar in sync with the seasons. Leap seconds are about keeping the clock in sync with the rotation of the earth. These are two different components of motion, and they are handled with different measures.
Re:Gee, thanks for the notice (Score:3, Informative)
Leap second at UTC, not Local midnight! (Score:5, Informative)
The UTC second 60 gets added at midnight only at those locations where UTC == local time, i.e. places like England.
For us in the rest of Europe, the leap second will be added an hour after local midnight, i.e. at 01:00:60 CET.
Terje
Re:legally speaking, it's the first leap for the U (Score:3, Informative)
Re:Second! (Score:2, Informative)
Re:Gee, thanks for the notice (Score:3, Informative)
Re:Fluctuations? (Score:5, Informative)
Re:Leap second at UTC, not Local midnight! (Score:4, Informative)
00:59:60 CET, you mean.
And there are other timezones besides Greenwich Mean Time [wikipedia.org] and Central European Time [wikipedia.org] in Europe too: Eastern European Time [wikipedia.org], Moscou Time [wikipedia.org] and even a smattering of Samara Time [wikipedia.org] and Yekaterinburg Time [wikipedia.org].
Re:Longer, or shorter? (Score:1, Informative)
Not only that, but the summary assumes you live in UTC. From Wikipedia:
"Unlike leap days, [leap seconds] occur simultaneously worldwide; for example, the leap second on December 31, 2005 occurred at 23:59:60 UTC. This was 6:59:60 p.m. U.S. Eastern Standard Time and 0:59:60 a.m. on January 1, 2006 Central European Time."
Re:legally speaking, it's the first leap for the U (Score:3, Informative)
Leap days and leap seconds serve different purposes.
Leap days are because our definition of a day (and thus a year) are not exact. A year is actually ~365.25 days, so we add an extra day every 4 years to compensate.
Leap seconds are needed as there's another small random variance in the length of a day (The mean solar day lengthens by about 1.7ms per century, due to slowing of the earth's rotation), so we occasionally need to add a second to keep us in sync with astronomical time.
Re:Gee, thanks for the notice (Score:5, Informative)
"Slew the time"? What system does that? According to the following page, the NTP server announces the leap second in advanced, and "well-behaved" kernels count the extra second like they are supposed to; i.e. there is no slewing:
http://www.cis.udel.edu/~mills/leap.html [udel.edu]
Re:Fluctuations? (Score:4, Informative)
There is a drift, and there are fluctuations.
Regarding the drift: The day length is getting gradually longer by about 1.7 milliseconds every century (+2.3ms due to tidal braking, -0.6ms due to glacial rebound). In about 1820 the day was 86400 seconds; now it is longer than that. In a thousand years, the day will be about 86400.017 seconds, and we will need a leap second every couple of months.
[Note: I am simplifying a little here for the sake of clarity by ignoring the difference between a solar day and the stellar and sidereal days, which are about 4 minutes shorter].
Regarding the fluctuations: There are fluctuations of the earth's angular velocity on many timescales. It fluctuates with weather, with the seasons, and with major events on the surface (e.g. a dam creating a new reservoir) and in the earth's crust (e.g. an earthquake or major volcanic eruption) and deeper interior (e.g. we don't really know). All these events are minor rearrangements of the mass of the earth, which change its moment of inertia. Conservation of angular momentum dictates that the angular velocity must change, and it does. Of course the earth isn't a rigid body and that complicates all this. Learn about Geodesy if you want to know more.
In the 1990s the day length was approximately 86400.003 seconds, so we needed a leap second every year. For poorly-understood reasons (possibly some sort of deep mantle activity), the earth's rotation speeded up around the year 2000, and for a while the day length was about 86400.0004 seconds. Now it is slower again, about 86400.001 seconds. These changes all come under the "fluctuations" heading.
There is an organisation called the IERS [iers.org] - International Earth Rotation and reference Systems Service - which collects measurements of all this stuff to very high accuracy and produces all sorts of reports, bulletins, data sets, etc etc.
Leap leap... (Score:5, Informative)
No it isn't. It's a 86401 seconds longer. Than last year. Or 86400 longer than the previous leap-second-year 2005. Oh, yeah, it's exactly 1 second longer than 2004 and 1996.
I confess enjoying myself as a time nazi. Should not forget to count february 29th...
Re:Leap leap... (Score:5, Informative)
I'd mod you up if I could. Instead, I'll add these bits of trivia:
The last time we had a leap second and a leap year was in 1992. The last time we had it on December 31 was 1976.
The only time we had two leap seconds (June 30 23:59:60 and December 31 23:59:60) was on leap year 1972, the first year leap seconds were applied, and making 1972 the longest year.
Re:Gee, thanks for the notice (Score:5, Informative)
The NTP daemon is normally used to interface with GPS clocks and to distribute time around a LAN. It never allows time to just jump. It always slews the clock.
This, of course, is wrong.
First, by default it steps the time on startup, with a default limit of 1000 seconds offset, but you can disable this limit.
Second, after startup, by default it slews the time unless the offset is greater than 128ms, in which case it steps the time. The 128ms value is configurable via the "tinker" command, but it is not recommended that it be changed.
Re:Gee, thanks for the notice (Score:2, Informative)
Re:why rely on hh/mm/ss instead of millis elapsed. (Score:2, Informative)
Um, no, that is not true. Unix time is kept in non-leap seconds elapsed since a defined point in time. Look it up [wikipedia.org].