Talk:Leap second
Is there any practical considerations with regards to leap seconds? How does it affect anything? I suppose, for example, that the Unix epoch which counts seconds starting from January 1, 1970 would be about 22 (?) seconds out of sync with UTC, right? I think that discussions on such practical aspects needs to placed in the article. --seav 06:05, Jan 3, 2004 (UTC)
- I believe it has something to do with GPS systems, airline computers and the like.... but i'm just drawing that from my head, from an article I read about the latest spin correction... I don't have enough info to add to the entry. Lyellin 06:12, Jan 3, 2004 (UTC)
- Here someone asks a few relevant questions
- * http://www.mail-archive.com/leapsecs@rom.usno.navy.mil/msg00051.html
- and here is some related discussion, I think
- * http://www.metrology.asn.au/leapseconds.htm
- Essentially, there is a tension between our convention that a day is a fixed length, and our recording devices, because it isn't quite as we wish; I expect the tension will increase as the accuracy of our recording and measuring devices increase. However, I'm far from any kind of expert on the matter! :) Kyk 06:32, 3 Jan 2004 (UTC)
- The following page has a very useful explanation of leap seconds:
The only reason for leap seconds, in fact calendars vs timekeeping is to keep the Earth's surface position, and season timings in sync with the traditional points in the sky which mark the events. The reason for the gregorian calendar adjustment was to insert days into the mix to move easter back to occuring in it's traditional relation with the sky positions. (I don't recall these exactly and the details are not relevant to my point). The leap seconds are gradually realiging the earth's position back to day alignment as the leap year days adjust the calendar.
The earths nutation has caused the slipage and it was traditionally ignored, but there is still a significant amount to be accounted for, if I recall, it is a matter of inserting the adjustments to minimize other factors.
- "Roughly 50000 years in the future, one can expect to have a day of 86401 seconds if the definition of the SI second is not eventually changed."
I'm not sure, but I think in the year 4000 the Gregorian Calendar won't work as it does now a days... so this phrase would be irrelevant... could someone check it out? I'm busy... --Henriquevicente 01:12, Apr 26, 2005 (UTC)Henrique Vicente
- You may be falling into a (popular!) intellectual trap here, namely confusing the length of the day with the length of the year. The two are independent oscillations. Changing the leap-year rule near 4000 (no serious astronomer is confident to predict the exact length of the year that far into the future) would not affect the length of the day after that.
Frequency of changes
The article states that "Historically, leap seconds have been inserted about every 18 months". I did a quick check of the table and it looks like the interval has been 18 months only 5 times, while it has been 12 months 14 times. Might it be better to start that paragraph at "The Earth's rotation rate is unpredictable..."?
Looking at the frequency of updates and the fact that we have gone 7 years without one, the higher frequency up to 1998 would appear to be some sort of gradual 'catch up'. Would that be a correct interpretation?
82.43.52.87 16:16, 30 July 2005 (UTC)
- The frequency of about every 18 months is correct for all leap seconds between 1 January 1972 and 31 December 2005, including the initial 6 month interval and the final 84 month interval, because the mathematical average during that period was 17.7 months. A graph showing the relationship of UTC to UT1 can be found at [1], showing that there was no attempt to 'catch up'. Rather, Earth's rate of rotation actually sped up slightly during 1997. — Joe Kress 21:15, 30 July 2005 (UTC)
I quite liked this: http://www.post-gazette.com/pg/05210/545823.stm although I have no idea if it's suitable as an external link on the article.
- I also like it and I'll add it. — Joe Kress 03:54, 31 July 2005 (UTC)
Simple naming question
Shouldn't this be called a leap day? Like... a leap year has an extra day, a leap day has an extra second? :-P --67.172.99.160 23:51, 5 August 2005 (UTC)
- Interesting question. Unfortunately, "leap day" is already the name for the extra day in a leap year. There was no doubt some discussion in the years leading up to their adoption in 1972 concerning what to call them. We only know the result of the discussion—the ITU decided that they will be called leap seconds. — Joe Kress 03:31, 6 August 2005 (UTC)
Removed incorrect information
The reason we need leap seconds is that the rotation of the Earth is slowing down. The solar day does gradually become longer by about 1.7 ms every century, mainly due to tidal acceleration from the Moon. The SI second that is counted by atomic time standards has been defined in such a way that its length did match the nominal second of 1/86400 of a mean solar day some time during the 19th century. Since that time the length of the solar day has been slowly increasing. Therefore the time as measured by the rotation of the Earth has been accumulating a delay with respect to atomic time standards. Whenever the accumulated delay approaches one second, a leap second is added to UTC.
This misconception weas contradicted in the paragraph that followed it. Ben Arnold 07:05, 8 August 2005 (UTC)
- The paragraph you removed is completely correct. So is the paragraph which follows it in its own way. What seems to be missing is an obvious linkage between the apparently slow rate at which Earth is slowing down (milliseconds per day per century) and the apparently much more rapid rate at which leap seconds are added (many seconds per century). That is immediately achieved by restating the rate at which Earth's rotation is slowing down, 1.7 milliseconds/day/century (actually 1.7 ms/(d·cy)), in square centuries. To do this we multiply by the number of days in a century, which is 36525 days per Julian century (to two significant digits, it does not matter what kind of century is used, but astronomical equations are always stated in terms of Julian centuries, never in terms of Gregorian centuries or mean tropical centuries). That yields a rate of 62 seconds/century², which is obviously the proper magnitude to yield many leap seconds per century. But the actual mathematical equations should be given, which requires integration. I am composing a rewrite. — Joe Kress 17:40, 8 August 2005 (UTC)
Actually, it is not correct that the reason we need leap seconds is because the Earth is slowing down. We need leap seconds because the SI (atomic) second is not exactly 1/86400 of a mean solar day. Even if the Earth's speed could be locked today, and never changed again, leap seconds would still be needed at about the same rate we're used to. It's the difference in the two units, not the rate of change that matters.
Besides, it's not really that we "need" them, so much as that we have decided to use them. When UTC was established in 1972, harmonizing the definition of the civil second and the atomic second, we chose to adjust for the inevitable differences between the rock-solid UTC and the highly variable UT1 (earth rotation time) in one-second jumps.
Prior to that, the adjustments were still being done, but it was done as a fractional disseminated-frequency offset from the atomic time definition. That is, an adjustment trim was applied to the disseminated GMT time and frequency standards in order to keep them in fairly close agreement to actual earth-rotation time.
Then, by '72, it was decided to scrap GMT, and quit trying to keep adjusting it for minor earth-rotation variations. UTC was established, the disseminated frequency offset was set to zero, and, since then, civil and atomic clocks have ticked in sync. Standards organizations stopped trying to adjust the time scales, and let them drift apart, but published the predicted and measured differences between UTC and UT1, for those interests that required earth-rotation time to a precision of less than one second. Only when the differences threatened to accumulate to more than 700 ms would an ajustment to UTC be scheduled. These are the familiar leap seconds, scheduled (or waived) six months in advance for 23:59:60 UTC on the last day of June or December. --Jeepien 05:03:27, 2005-08-09 (UTC)