Leap second: Difference between revisions

Revision as of 21:24, 16 September 2002

Revision as of 00:29, 21 October 2002

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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 result 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 [[Luna|Moon]]. Thus, in order to not go out of step with day and night, 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 GMT becomes 0.9 s. The insertion only ever occurs at the end of 30 June or 31 December. (According to the official rules, the [[International Earth Rotation 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 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. (The IERS can also omit a second at the above mentioned points instead, if the difference between UTC and GMT is sufficentlysufficiently negative -- however, this has never happened.)

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.