Leap second

A leap second is a positive or negative one-second adjustment to the Coordinated Universal Time (UTC) time scale that keeps it close to mean solar time. UTC, which is used as the basis for official time-of-day radio broadcasts for civil time, is maintained using extremely precise atomic clocks. To keep the UTC time scale close to mean solar time, UTC is occasionally corrected by an intercalary adjustment, or "leap", of one second. Over long time periods, leap seconds must be added at an ever increasing rate (see ΔT). The timing of leap seconds is now determined by the International Earth Rotation and Reference Systems Service (IERS). Leap seconds were determined by the Bureau International de l'Heure (BIH) prior to January 1, 1988, when the IERS assumed that responsibility.
When a positive leap second is added at 23:59:60 UTC, it delays the start of the following UTC day (at 00:00:00 UTC) by one second, effectively delaying the UTC clock. Negative leap seconds have never been needed. For one to be needed, the length of day (LOD) would have to be below the 1750–1892 average LOD for sufficiently long to accumulate one second of time. Except for fluctuations of up to 4 milliseconds per day, LOD has remained much as it has been since 1700.[1] However, historic eclipse observations show that LOD has increased by about 1.7 milliseconds per century since 700 BC.


The International Earth Rotation and Reference Systems Service (IERS) announces the insertion of a leap second whenever the difference between UTC and UT1 approaches 0.6 s, to keep the difference between UTC and UT1 from exceeding 0.9 s. IERS publishes announcements every six months, whether leap seconds are to occur or not, in its "Bulletin C". Such announcements are typically published well in advance of each possible leap second date — usually in early January for June 30 and in early July for December 31. Because the Earth's rotation rate is unpredictable in the long term, it is not possible to predict the need for them more than six months in advance.
The most recent leap second was added at the end of December 31, 2008. The next leap second will be added at the end of June 30, 2012.
After 23:59:59 UTC, 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 which case, 23:59:58 would be followed directly by 00:00:00—but they have not yet been used. 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, the leap second on December 31, 2005 occurred at 23:59:60 UTC. This was 18:59:60 (6:59:60 p.m.) U.S. Eastern Standard Time and 08:59:60 (8:59:60 a.m.) on January 1, 2006 Japan Standard Time.


Historically, leap seconds have been inserted about every 18 months. From June 1972 through June 2012, the BIH/IERS gave instructions to insert a leap second on 25 occasions, after an initial 10 second offset from TAI on January 1, 1972. The seven-year interval between January 1, 1999 and December 31, 2005 was the longest period without a leap second since the system was introduced.
Some time signal broadcasts give voice announcements of an impending leap second.


On July 5, 2005, the Head of the Earth Orientation Center of the IERS sent a notice to IERS Bulletins C and D subscribers, soliciting comments on a U.S. proposal before the ITU-R Study Group 7's WP7-A to eliminate leap seconds from the UTC broadcast standard before 2008. (The ITU-R is responsible for the definition of UTC.) The Wall Street Journal noted that the proposal was considered by a U.S. official to be a "private matter internal to the ITU", as of July 2005.[13] It was expected to be considered in November 2005, but the discussion has since been postponed.[14] Under the proposal, leap seconds would be technically replaced by leap hours as an attempt to satisfy the legal requirements of several ITU-R member nations that civil time be astronomically tied to the Sun.
Several arguments for the abolition have been presented. Some of these have only become relevant with the recent proliferation of computers using UTC as their internal time representation. For example, currently it is not possible to correctly compute the elapsed interval between two instants of UTC without consulting manually updated and maintained tables of when leap seconds have occurred. Moreover, it is not possible even in theory to compute such time intervals for instants more than about six months in the future. The uncertainty of leap seconds introduces to those applications needing accurate notions of elapsed time intervals either fundamentally new (and often untenable) operational burdens for computer systems (the need to do online lookups) or insurmountable theoretical concerns (the inability in a UTC-based computer to accurately schedule any event more than six months in the future to within a few seconds).
A number of objections to the proposal have been raised. Dr. P. Kenneth Seidelmann, editor of the Explanatory Supplement to the Astronomical Almanac, wrote a letter lamenting the lack of consistent public information about the proposal and adequate justification. Steve Allen of the University of California, Santa Cruz cited the large impact on astronomers in a Science News article. He has an extensive online site devoted to the issues and the history of leap seconds, including a set of references about the proposal and arguments against it.
Chunhao Han of the Beijing Global Information Center of Application and Exploration said China has not decided what its vote will be in January, but most Chinese scholars consider it important to maintain a link between civil and astronomical time due to Chinese tradition.