Leap second: Difference between revisions

Revision as of 16:52, 23 November 2001

Revision as of 18:00, 23 November 2001

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A '''leap second''' is inserted into civil clock time occasionally in order to keep clock time close to the mean solar time (see [[GMT]]).

The reason for these occasional insertions is that 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 is irregular and is not fit for accurate time keeping. A clock day has exactly 86400 [[SI]] [[second|seconds]], whereas the mean solar [[day]] (a productresult of the rotation period of the Earth, and the revolution period of the Earth around the Sun) slightly increases in length mainly due to the [[tidal acceleration]] of the [[Moon]]. Thus, in order to not go out of step with day and night, UTC is corrected by a leap of 1 second whenever the [[International Earth Rotation Service]] ('''IERS''') declares it necessary. On average, a leap second is inserted every 18 months or so. (Between January [[1970]] and November [[2001]], the IERS gave instructions to insert a leap second on 22 occasions).

The instruction to insert a leap second will be given whenever the difference between UTC and GMT becomes 0.9 s. The insertion only ever occurs at the end of 30 June or 31 December. It is implemented as follows: after clock time 23:59:59, an additional second at 23:59:60 is counted, before the clock jumps to 0:00:00 of the next day.

As noted above, itIt is the responsibility of the [[International Earth Rotation Service]] ('''IERS''') to announce leap seconds:; seebetween January [http://www.iers.org/iers/publications/bulletins/bull_c/[1970]] theand IERSNovember Bulletin[[2001].], the PreviousIERS eventsgave areinstructions listedto atinsert [http://www.iers.org/iers/earth/rotation/utc/table2.htmla theleap IERSsecond archive]on 22 occasions.

'''See also:''' [[leap year]]

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'''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 Archive, to view old announcements: http://www.iers.org/iers/earth/rotation/utc/table2.html