Leap second: Difference between revisions

Revision as of 08:50, 11 December 2004

Revision as of 03:35, 22 December 2004

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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 rotation of the earth is now already a bit slower than it should be in order to have a day of exactly 24 hours. If the rotation would now remain constant, leap seconds would be necessary in regular intervallsintervals. Because the earth is continually slowing down, the interval between two leap seconds is becoming smaller over longer timescales. 50000 years in the future one can expect to have more than one leap second every day if the time system is not going to be changed.

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.