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[[eo:Supersekundo]] [[de:Schaltsekunde]]
A '''leap second''' is inserteda intoone-second adjustment to civil clock time occasionally in order to keep clock timeit close to the [[mean solar time (see [[GMT]]).
The reason for these occasional insertions is that civilCivil clock time is based on [[UTC|"Coordinated Universal Time" (UTC)]], which is maintained by (extremely precise) [[atomic clock]]s. In contrast, the rotation of the Earth, ismeasured irregularby and is not fit for accurate time keeping. A clock day has exactly 86400the [[SIUT1]] [[second|seconds]]timescale, whereasis irregular; the mean solar [[day]] (ais resultgradually ofbut theunevenly rotationbecoming period of the Earthlonger, and the revolution period of the Earth around the Sun) slightly increases in length mainly due to the [[tidal acceleration]] of the [[Luna|Moon]]. Thus, inIn order to notkeep gosolar outtime ofclose stepto withcivil day and nighttime, UTC is corrected by a leap of 1 second every 18 months or so.
The instruction to insert a leap second will be given whenever the difference between UTC and GMTUT1 becomesis expected to exceed 0.9 s. TheAfter insertion only ever occurs at the end of 30 June or 31 December. (According to the official rules, the [[International Earth Rotation and Reference Systems Service]] ('''IERS''') can also insert leap seconds at the end of the first or third quarter, although the IERS has never done so in practice.) It is implemented as follows: after clock timeUTC 23:59:59, an additional second at 23:59:60 is counted, before the clock jumps to 000:00:00 of the next day. (The IERSNegative canleap alsoseconds omitare aalso secondpossible atif the aboveEarth's mentionedrotation becomes pointsslightly insteadfaster, ifbut thethis differencehas betweennever UTChappened. and GMTIn isthat sufficientlycase, negative23:59:58 --would however,be thisfollowed hasby never happened00:00:00.)
Leap seconds can occur only at the end of a month, and have only ever occured at the end of a [[30 June]] or [[31 December]]. Unlike [[leap day]]s, they occur simultaneously worldwide; for example, a leap second on 31 December will be observed as 7:59:60 pm [[CET#UTC-5_(EST_-_Eastern_Standard_Time)|US Eastern Standard Time]].
It is the responsibility of the IERS to announce leap seconds; between January [[1970]] and November [[2001]], the IERS gave instructions to insert a leap second on 22 occasions.
Historically, leap seconds have been inserted about every 18 months. However, as the slowing of the Earth is irregular, it is not possible to predict more than a relatively short time in advance whether a leap second will have become necessary. Between January [[1970]] and November [[2001]], the IERS gave instructions to insert a leap second on 22 occasions. The most recent leap second was 1998-12-31 23:59:60 UTC; the interval since then has been the longest period without a leap second.
It is the responsibility of the [[International Earth Rotation and Reference Systems Service]] to measure the Earth's rotation and determine whether a leap second is necessary. Their determination is announced in Bulletin C, published every six months.
See also: [[leap year]]
External references:
* IERS site: http://www.iers.org
* IERS Bulletin, where leap seconds are announced: http://www.iers.org/iers/publications/bulletins/bull_c/
* IERS information about Bulletin C and when leap seconds may occur: http://hpiers.obspm.fr/eoppc/bul/bulc/BULLETINC.GUIDE
* IERS Archive, to view old announcements: http://www.iers.org/iers/earth/rotation/utc/table2.html
* [http://tycho.usno.navy.mil/leapsec.html USNO article on leap seconds]
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