Leap second: Difference between revisions

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# [[130 JulyJune]] [[1972]]

# [[131 JanuaryDecember]] [[19731972]]

# [[131 JanuaryDecember]] [[19741973]]

# [[131 JanuaryDecember]] [[19751974]]

# [[131 JanuaryDecember]] [[19761975]]

# [[131 JanuaryDecember]] [[19771976]]

# [[131 JanuaryDecember]] [[19781977]]

# [[131 JanuaryDecember]] [[19791978]]

# [[131 JanuaryDecember]] [[19801979]]

# [[130 JulyJune]] [[1981]]

# [[130 JulyJune]] [[1982]]

# [[130 JulyJune]] [[1983]]

# [[130 JulyJune]] [[1985]]

# [[131 JanuaryDecember]] [[19881987]]

# [[131 JanuaryDecember]] [[19901989]]

# [[131 JanuaryDecember]] [[19911990]]

# [[130 JulyJune]] [[1992]]

# [[130 JulyJune]] [[1993]]

# [[130 JulyJune]] [[1994]]

# [[131 JanuaryDecember]] [[19961995]]

# [[130 JulyJune]] [[1997]]

# [[131 JanuaryDecember]] [[19991998]]

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Civil 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, measured by the [[UT1]] timescale, is irregular; the solar day is gradually but unevenly becoming longer, mainly due to the [[tidal acceleration]] of the [[Moon]]. In order to keep solar time close to civil time, UTC is corrected by a leap of 1 second.

The instruction to insert a leap second will be given whenever the difference between UTC and UT1 is expected to exceed ±0.9 s. After UTC 23:59:59, an additional second at 23:59:60 is counted, before the clock jumps to 00:00:00 of the next day. Negative leap seconds are also possible if the Earth's rotation becomes slightly faster, but this has never happened. In that case, 23:59:58 would be followed by 00:00:00.

Leap seconds can occur only at the end of a month, and have only ever occurred at the end of a [[30 June 30]]^th or [[31 December 31]]^st. Unlike [[leap day]]s, they occur simultaneously worldwide; for example, a leap second on 31 December will be observed as 6:59:60 pm [[Time_zone#UTC-5_(EST_-_Eastern_Standard_Time)|U.S. Eastern Standard Time]].

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 [[1972]] 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.