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==See also==
==See also==
*[[Clock drift]], phenomenon where a clock gains/loses time compared to another clock
*[[Clock drift]], phenomena where a clock gains/loses time compared to another clock
*[[Leap year]], a year containing one extra day
*[[Leap year]], a year containing one extra day
*[[Unix time]], a common representation of time for computer systems which ignores leap seconds
*[[Unix time]], a common representation of time for computer systems which ignores leap seconds

Revision as of 03:12, 2 March 2012

Announced leap seconds to date
Year Jun 30 Dec 31
1972 +1 +1
1973 0 +1
1974 0 +1
1975 0 +1
1976 0 +1
1977 0 +1
1978 0 +1
1979 0 +1
1980 0 0
1981 +1 0
1982 +1 0
1983 +1 0
1984 0 0
1985 +1 0
1986 0 0
1987 0 +1
1988 0 0
1989 0 +1
1990 0 +1
1991 0 0
1992 +1 0
1993 +1 0
1994 +1 0
1995 0 +1
1996 0 0
1997 +1 0
1998 0 +1
1999 0 0
2000 0 0
2001 0 0
2002 0 0
2003 0 0
2004 0 0
2005 0 +1
2006 0 0
2007 0 0
2008 0 +1
2009 0 0
2010 0 0
2011 0 0
2012 +1  
Year Jun 30 Dec 31
Total 10 15
25
Current TAI − UTC
34

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 ΔでるたT). 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.[2]

Reason for leap seconds

Leap seconds are necessary partly because the length of the mean solar day is very slowly increasing, and partly because the atomic, fixed-length SI second, when adopted, was already a little shorter than the current value of the second of mean solar time.[3] Time is now measured using stable atomic clocks (TAI or International Atomic Time), whereas the rotation of Earth is much more variable.

Deviation of day length from SI based day, 1962–2010

Originally, the second was defined as 1⁄86400 of a mean solar day (see solar time) as determined by the rotation of the Earth around its axis and around the Sun. By the middle of the 20th century, it was apparent that the rotation of the Earth did not provide a sufficiently uniform time standard, and in 1956 the second was redefined in terms of the annual orbital revolution of the Earth around the Sun. In 1967 the second was redefined, once again, in terms of a physical property: the oscillations of an atom of caesium-133, which were measurable by an atomic clock.[4] But the solar day becomes 1.7 ms longer every century due mainly to tidal friction (2.3 ms/cy, reduced by 0.6 ms/cy due to glacial rebound).[5]

The SI second counted by atomic time standards has been defined on the basis of a history going back to the former standard time scale of ephemeris time (ET). It can now be seen to be close to the average second of 1⁄86400 of a mean solar day between 1750 and 1892. The current SI second was defined in 1967, as 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. (the time it takes for a caesium 133 atom to emit 9,192,631,770 oscillations of a radio wave.) This number first arose from calibration of the caesium standard by the second of ephemeris time: in 1958, the second of ephemeris time was determined as the duration of 9,192,631,770 ± 20 cycles of the chosen caesium transition,[6] (while at about the same time, and with the same caesium standard, the then-current mean length of the second of mean solar time (UT2) had been measured at 9,192,631,830 cycles).[7] Later verification showed that the SI second referred to atomic time was in agreement, within 1 part in 1010, with the second of ephemeris time as determined from lunar observations.[8] Time as measured by Earth's rotation has accumulated a delay with respect to atomic time standards. From 1961 to 1971, the rate of (some) atomic clocks was (for purposes of Coordinated Universal Time) constantly slowed to remain synchronised with Earth's rotation. (Before 1961, broadcast time was synchronized to astronomically determined Greenwich Mean Time.) Since 1972, broadcast seconds have been exactly equal to the standard SI second chosen in 1967.

Coordinated Universal Time is counted by atomic clocks, but is kept approximately synchronised with UT1 (mean solar time) by introducing a leap second when necessary. This happens when the difference (UT1 − UTC) approaches 0.9 seconds, and is typically scheduled either at the end of June 30 or December 31 (though leap seconds can be applied at the end of any month). On January 1, 1972, the initial offset of UTC from TAI was chosen to be 10 seconds, which approximated the total difference which had accumulated since 1958, when TAI was defined equal to UT2. The table shows the number of leap seconds added since then. The total difference between TAI and UTC is 10 seconds more than the total number of leap seconds.

Graph showing the difference between UT1 and UTC. Vertical segments correspond to leap seconds.

The leap second adjustment (which equates to approximately 0.6 seconds per year) is necessary because of the difference between the length of the SI day (based on the mean solar day between 1750 and 1892) and the length of the current mean solar day (which is about 0.002 seconds longer). The difference between these two will increase with time, but only by 0.0017 seconds per century. In other words, the adjustment is required because we have decoupled the definition of the second from the current rotational period of the Earth. The actual rotational period varies due to unpredictable factors such as the motion of mass within Earth, and has to be observed rather than computed.

For example, suppose an atomic clock is used to count seconds from the Unix epoch of 00:00:00 on January 1, 1970. UTC and mean solar time (UT1) were almost identical at that time. After Earth makes one full rotation with respect to the mean Sun, the counter will register 86400.002 seconds (once again, the precise value will vary). Based on the counter, and assuming that a day is 24×60×60 = 86400 seconds long, the date will be calculated as 00:00:00.002 January 2, 1970. After 500 rotations, it will be 00:00:00 May 16, 1971 in solar time (UT1), but the counter will register 43,200,001 atomic seconds. Since 86400 × 500 is 43,200,000 seconds, the date will be calculated as 00:00:01 on May 16, 1971, as measured by atomic time. If a leap second had been added on December 31, 1970, then the date would be computed as 00:00:00 on May 16, 1971. The system involving leap seconds was set up to allow TAI and UT1 to have an offset of 10 seconds on January 1, 1972.

Tidal braking slows down Earth's rotation, causing the number of SI seconds in a mean solar day to increase by approximately 2 milliseconds every century (meaning a projected increase from the current 86400.002 to 86400.004 by the early part of the 22nd century). Additionally, events or processes that cause a significant change to the mass distribution of the earth, thereby changing its moment of inertia, can affect the rate of rotation due to conservation of angular momentum. Most notable in recent times is the 2004 Indian Ocean earthquake which, according to theoretical models, is thought to have decreased the solar day by 2.68 microseconds.[9] For unknown reasons, the earth's rotation speed increased in 1999, so the mean solar day has become 1 ms shorter and fewer leap seconds have been needed after year 2000.[10]

Announcement of leap seconds

Screencapture of the clock at time.gov showing the leap second added December 31, 2008, as seen in the Central Standard Time zone.

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.[11][12]

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.

Proposal to abolish leap seconds

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[15] 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.[16] He has an extensive online site[17] devoted to the issues and the history of leap seconds, including a set of references about the proposal and arguments against it.[18]

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.[19]

Arguments against the proposal include the unknown expense of such a major change and the fact that universal time will no longer correspond to mean solar time. It is also answered that two timescales that do not follow leap seconds are already available, International Atomic Time (TAI) and Global Positioning System (GPS) time. Computers, for example, could use these and convert to UTC or local civil time as necessary for output. Inexpensive GPS timing receivers are readily available and the satellite broadcasts include the necessary information to convert GPS time to UTC. It is also easy to convert GPS time to TAI as TAI is always exactly 19 seconds ahead of GPS time. Examples of systems based on GPS time include the CDMA digital cellular systems IS-95 and CDMA2000.

At the 47th meeting of Civil Global Positioning System Service Interface Committee in Fort Worth, Texas in September 2007, it was announced that a mailed vote would go out on stopping leap seconds. The plan for the vote is:[20]

  • April 2008: ITU Working Party 7A will submit to ITU Study Group 7 project recommendation on stopping leap seconds
  • During 2008, Study Group 7 will conduct a vote through mail among member states
  • October 2011: The ITU-R released its status paper, Status of Coordinated Universal Time (UTC) study in ITU-R, in preparation for the January 2012 meeting in Geneva; the paper reported that, to date, in response to the UN agency's 2010 & 2011 web based surveys requesting input on the topic, it had received 16 responses from the 192 Member States with "13 being in favor of change, 3 being contrary."[21]
  • January 2012: The ITU decided to postpone a decision on leap seconds to the World Radio Conference in 2015. France, Italy, Japan, Mexico and the US were reported to be in favor while Canada, China, Germany and the UK were reportedly against.[22] Others including Nigeria, Russia and Turkey called for more study. The BBC states the ITU decided further study of broader social implications was needed.[23]
  • 2015: World Radio Conference set to decide issue.[needs update]

See also

  • Clock drift, phenomena where a clock gains/loses time compared to another clock
  • Leap year, a year containing one extra day
  • Unix time, a common representation of time for computer systems which ignores leap seconds
  • Delta-T (ΔでるたT), the time difference obtained by subtracting Universal Time from Terrestrial Time
  • Shortwave radio stations that continuously broadcast UTC

References

Notes

  1. ^ Excess of the duration of the day to 86400s ... since 1623, accessed January 5, 2012.
  2. ^ Steve Allen (June 8, 2011). "Extrapolations of the difference ( TI - UT1 )". ucolick.org. Retrieved December 9, 2011.
  3. ^ :(1) In "The Physical Basis of the Leap Second", by D D McCarthy, C Hackman and R A Nelson, in Astronomical Journal, vol. 136 (2008), pages 1906–1908, it is stated (page 1908), that "the SI second is equivalent to an older measure of the second of UT1, which was too small to start with and further, as the duration of the UT1 second increases, the discrepancy widens."
    (2) In the late 1950s, the caesium standard was used to measure both the current mean length of the second of mean solar time (UT2)(measured over a period of about a year) (result: 9192631830 cycles, probable error not stated), and also the second of ephemeris time (ET) (measured over a period of about three years) (result:9192631770 ± 20 cycles): see "Time Scales", by L. Essen, in Metrologia, vol. (1968), pp. 161–165, on p. 162. As is well known, the 9192631770 figure was chosen for the SI second. L Essen in the same 1968 article (p.162) stated that this "seemed reasonable in view of the variations in UT2".
  4. ^ "Leap Seconds". Time Service Department, United States Naval Observatory. Retrieved December 27, 2008.
  5. ^ F.R. Stephenson, L.V. Morrison. "Long-term fluctuations in the Earth's rotation: 700 BC to AD 1990". Philosophical Transactions of the Royal Society of London, Series A 351 (1995) 165–202.
  6. ^ W Markowitz, R G Hall, L Essen, J V L Parry (1958), 'Frequency of caesium in terms of ephemeris time', Phys Rev Letters v1 (1958), 105–107.
  7. ^ L Essen, "Time Scales", Metrologia, vol.4 (1968), pp.161–165, at p.162.
  8. ^ Wm Markowitz (1988) 'Comparisons of ET(Solar), ET(Lunar), UT and TDT', in (eds.) A K Babcock & G A Wilkins, 'The Earth's Rotation and Reference Frames for Geodesy and Geophysics', IAU Symposia #128 (1988), at pp 413–418.
  9. ^ Cook-Anderson, Gretchen; Beasley, Dolores. "NASA Details Earthquake Effects on the Earth." National Aeronautics and Space Administration (press release). January 10, 2005.
  10. ^ "Extra Second Will be Added to 2005". LiveScience. Retrieved November 26, 2009. "And then, in 1999 for reasons still unknown, the rotation of the Earth speeded up a bit, so we haven't had to add a second since then," O'Brian told LiveScience in a telephone interview.
  11. ^ Gambis, Daniel (July 4, 2008). "Bulletin C 36". Paris: IERS EOP PC, Observatoire de Paris. Retrieved April 18, 2010.
  12. ^ Andrea Thompson (December 8, 2008). "2008 Will Be Just a Second Longer". Live Science. Retrieved December 29, 2008.
  13. ^ Why the U.S. Wants To End the Link Between Time and Sun by The Wall Street Journal
  14. ^ Leap second talks are postponed by BBC News
  15. ^ UTC redefinition or change by Kenneth Seidelmann
  16. ^ Cowen 2006
  17. ^ UTC might be redefined without Leap Seconds by Steve Allen
  18. ^ Proposed US Contribution to ITU-R WP 7A
  19. ^ Merali 2011.
  20. ^ "47th CGSIC Meeting - Timing Subcommittee" (PDF). September 25, 2007. p. 9. Retrieved November 18, 2007.
  21. ^ "WP7D - Status of Coordinated Universal Time (UTC) study in ITU-R" (Word 2007). International Telecommunication Union – Radiocommunication Sector (ITU-R) Release: Pg.2 (Pgs.2). 2011-10-04. Retrieved October 24, 2011. To date, the BR received replies from 16 different Member States for the latest survey (out of a total of 192 Member States, 55 of which participate in the formation of UTC) - 13 being in favor of the change, 3 being contrary. {{cite journal}}: |pages= has extra text (help)
  22. ^ "Wait a second: leap-second verdict goes into extra time". Ottawa Citizen. January 19, 2012.
  23. ^ "Leap second decision is postponed". BBC News. January 19, 2012.

Bibliography

Further reading

UTC redefinition


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