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A leap second is an intercalary, one-second adjustment that keeps broadcast standards for time of day close to mean solar time. Leap seconds are necessary to keep time standards synchronized with civil calendars, the basis of which is astronomical.
Broadcast standards for civil time are based on "Coordinated Universal Time" (UTC), which is maintained using extremely precise atomic clocks. In contrast, the rotation of the Earth, indicated by the UT1 timescale, is slightly irregular; over eons the solar day gradually becomes longer, mainly due to tidal acceleration from the Moon. In order to keep the UTC broadcast standard close to mean solar time, UTC is occasionally corrected by an intercalary adjustment, or "leap", of one (1) second. The atomic second is a bit faster than it should be in order to have an average day of exactly 86400 atomic seconds. Because Earth is also very gradually slowing down, the rate of leap-second insertion will eventually increase over longer timescales. Roughly 50000 years in the future, one can expect to have a day of 86401 seconds if the definition of the SI second is not eventually changed.
The announcement to insert a leap second is given whenever the difference between UTC and UT1 approaches one-half second, to keep the difference between UTC and UT1 from exceeding ±0.9 s. After UTC 23:59:59, a positive leap second at 23:59:60 would be counted, before the clock indicates 00:00:00 of the next day. Negative leap seconds are also possible should the Earth's rotation become slightly faster; in that case, 23:59:58 would be followed by 00:00:00.
Leap seconds occur only at the end of a UTC month, and have only ever been inserted at the end of June 30 or December 31. Unlike leap days, they occur simultaneously worldwide; for example, a leap second on 31 December will be observed as 6:59:60 pm U.S. Eastern Standard Time.
Historically, leap seconds have been inserted about every 18 months. However, the Earth's rotation rate is unpredictable in the long term, so it is not possible to predict the need for them more than a year in advance. 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. On January 14, 2005, the IERS announced that there will NOT be a leap second at the end of June 2005.
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, typically published every six months.
Note that leap seconds have nothing to do with leap years.
- IERS Bulletin C, where leap seconds are announced
- IERS information about Bulletin C and when leap seconds may occur
- IERS Archive, to view old announcements
- USNO article on leap seconds
- UTC might be redefined without Leap Seconds
- Efforts to abolish leap seconds
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