Leap second

Leap second

Overview


A leap second is a positive or negative one-second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....

 adjustment to the Coordinated Universal Time
Coordinated Universal Time
Coordinated Universal Time is the primary time standard by which the world regulates clocks and time. It is one of several closely related successors to Greenwich Mean Time. Computer servers, online services and other entities that rely on having a universally accepted time use UTC for that purpose...

 (UTC) time scale
Time scale
A time scale specifies divisions of time.*Time standard, a specification of either the rate at which time passes, or points in time, or both*Duration , a quantity of time...

 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
Civil time
In modern usage, civil time refers to statutory time scales designated by civilian authorities, or to local time indicated by clocks. Modern civil time is generally standard time in a time zone at a fixed offset from Coordinated Universal Time or from Greenwich Mean Time , possibly adjusted by...

, is maintained using extremely precise atomic clock
Atomic clock
An atomic clock is a clock that uses an electronic transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element...

s. To keep the UTC time scale close to mean solar time, UTC is occasionally corrected by an intercalary adjustment, or "leap", of one second.
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Encyclopedia
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
Year Jun 30 Dec 31
Total 9 15
24
TAI − UTC
34


A leap second is a positive or negative one-second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....

 adjustment to the Coordinated Universal Time
Coordinated Universal Time
Coordinated Universal Time is the primary time standard by which the world regulates clocks and time. It is one of several closely related successors to Greenwich Mean Time. Computer servers, online services and other entities that rely on having a universally accepted time use UTC for that purpose...

 (UTC) time scale
Time scale
A time scale specifies divisions of time.*Time standard, a specification of either the rate at which time passes, or points in time, or both*Duration , a quantity of time...

 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
Civil time
In modern usage, civil time refers to statutory time scales designated by civilian authorities, or to local time indicated by clocks. Modern civil time is generally standard time in a time zone at a fixed offset from Coordinated Universal Time or from Greenwich Mean Time , possibly adjusted by...

, is maintained using extremely precise atomic clock
Atomic clock
An atomic clock is a clock that uses an electronic transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element...

s. 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
International Earth Rotation and Reference Systems Service
The International Earth Rotation and Reference Systems Service , formerly the International Earth Rotation Service, is the body responsible for maintaining global time and reference frame standards, notably through its Earth Orientation Parameter and International Celestial Reference System ...

 (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 within a year the length of day (LOD) would have to decrease by 1/365 second, or about three milliseconds. While LOD does fluctuate, it is currently much as it was in 1820, and historic eclipse observations show that LOD tends to increase by roughly two milliseconds per century. A decrease of 4 milliseconds did occur around 1920, but that required more than a decade of continuous acceleration.

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
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....

, when adopted, was already a little shorter than the current value of the second of mean solar time.
Time is now measured using stable atomic clocks (TAI or International Atomic Time
International Atomic Time
International Atomic Time is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid...

), whereas the rotation of Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 is much more variable.
Originally, the second was defined as 1/86400 of a mean solar day (see solar time
Solar time
Solar time is a reckoning of the passage of time based on the Sun's position in the sky. The fundamental unit of solar time is the day. Two types of solar time are apparent solar time and mean solar time .-Introduction:...

) as determined by the rotation
Rotation
A rotation is a circular movement of an object around a center of rotation. A three-dimensional object rotates always around an imaginary line called a rotation axis. If the axis is within the body, and passes through its center of mass the body is said to rotate upon itself, or spin. A 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
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

 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
Caesium
Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C , which makes it one of only five elemental metals that are liquid at room temperature...

-133, which were measurable by an atomic clock. 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).

The SI
International System of Units
The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...

 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
Ephemeris time
The term ephemeris time can in principle refer to time in connection with any astronomical ephemeris. In practice it has been used more specifically to refer to:...

 (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. 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 cycles of the chosen caesium transition,
(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). 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.
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
Greenwich Mean Time
Greenwich Mean Time is a term originally referring to mean solar time at the Royal Observatory in Greenwich, London. It is arguably the same as Coordinated Universal Time and when this is viewed as a time zone the name Greenwich Mean Time is especially used by bodies connected with the United...

.) 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
Universal Time
Universal Time is a time scale based on the rotation of the Earth. It is a modern continuation of Greenwich Mean Time , i.e., the mean solar time on the Prime Meridian at Greenwich, and GMT is sometimes used loosely as a synonym for UTC...

 (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.


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
Rotation period
The rotation period of an astronomical object is the time it takes to complete one revolution around its axis of rotation relative to the background stars...

 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
Unix time
Unix time, or POSIX time, is a system for describing instants in time, defined as the number of seconds elapsed since midnight Coordinated Universal Time of Thursday, January 1, 1970 , not counting leap seconds, which are declared by the International Earth Rotation and Reference Systems Service...

 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
Tidal acceleration
Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite , and the primary planet that it orbits . The "acceleration" is usually negative, as it causes a gradual slowing and recession of a satellite in a prograde orbit away from the primary, and a corresponding...

 slows down Earth's rotation, causing the number of SI seconds in a mean solar day
Day
A day is a unit of time, commonly defined as an interval equal to 24 hours. It also can mean that portion of the full day during which a location is illuminated by the light of the sun...

 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
Moment of inertia
In classical mechanics, moment of inertia, also called mass moment of inertia, rotational inertia, polar moment of inertia of mass, or the angular mass, is a measure of an object's resistance to changes to its rotation. It is the inertia of a rotating body with respect to its rotation...

, 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. 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.

Announcement of leap seconds


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 [ftp://hpiers.obspm.fr/iers/bul/bulc/bulletinc.dat 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.

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
Japan Standard Time
Japan Standard Time or JST is the standard timezone of Japan, and is 9 hours ahead of UTC. For example, when it is midnight in UTC, it is 09:00 in Japan Standard Time. There is no daylight saving time, though its introduction has been debated several times. Japan Standard Time is the same as...

.

Historically, leap seconds have been inserted about every 18 months. From June 1972 through December 2008, the BIH/IERS gave instructions to insert a leap second on 24 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
Time signal
A time signal is a visible, audible, mechanical, or electronic signal used as a reference to determine the time of day.-Audible and visible time signals:...

 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
ITU-R
The ITU Radiocommunication Sector is one of the three sectors of the International Telecommunication Union and is responsible for radio communication....

 is responsible for the definition of UTC.) The Wall Street Journal
The Wall Street Journal
The Wall Street Journal is an American English-language international daily newspaper. It is published in New York City by Dow Jones & Company, a division of News Corporation, along with the Asian and European editions of the Journal....

noted that the proposal was considered by a U.S. official to be a "private matter internal to the ITU", .
It was expected to be considered in November 2005, but the discussion has since been postponed. 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
University of California, Santa Cruz
The University of California, Santa Cruz, also known as UC Santa Cruz or UCSC, is a public, collegiate university; one of ten campuses in the University of California...

 cited the large impact on astronomers in a Science News
Science News
Science News is an American bi-weekly magazine devoted to short articles about new scientific and technical developments, typically gleaned from recent scientific and technical journals. Science News has been published since 1922 by Society for Science & the Public, a non-profit organization...

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.

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
Global Positioning System
The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...

 (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
IS-95
Interim Standard 95 is the first CDMA-based digital cellular standard by Qualcomm. The brand name for IS-95 is cdmaOne. IS-95 is also known as TIA-EIA-95....

 and CDMA2000
CDMA2000
CDMA2000 is a family of 3G mobile technology standards, which use CDMA channel access, to send voice, data, and signaling data between mobile phones and cell sites. The set of standards includes: CDMA2000 1X, CDMA2000 EV-DO Rev. 0, CDMA2000 EV-DO Rev. A, and CDMA2000 EV-DO Rev. B...

.

At the 47th meeting of Civil Global Positioning System Service Interface Committee in Fort Worth, Texas
Texas
Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...

 in September, 2007, it was announced that a mailed vote would go out on stopping leap seconds. The plan for the vote is:
  • April 2008: ITU
    Itu
    Itu is an old and historic municipality in the state of São Paulo in Brazil. The population in 2009 was 157,384 and the area is 641.68 km². The elevation is 583 m. This place name comes from the Tupi language, meaning big waterfall. Itu is linked with the highway numbered the SP-75 and are flowed...

     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
    ITU-R
    The ITU Radiocommunication Sector is one of the three sectors of the International Telecommunication Union and is responsible for radio communication....

     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."
  • 2012: If 70% of member states agree, the Radiocommunication Assembly will approve the recommendation A meeting of scientists and government representatives in November 2011, near Milton Keynes, UK, did not reach a consensus. Merali, writing for Nature News, reports the outcome of the vote difficult to predict.
  • 2017: If 70% of member states have voted to abolish the leap second, application of leap seconds will stop and UTC will become a continuous time scale, otherwise they will stay the same.

See also

  • Clock drift
    Clock drift
    Clock drift refers to several related phenomena where a clock does not run at the exact right speed compared to another clock. That is, after some time the clock "drifts apart" from the other clock. This phenomenon is also used for instance in computers to build random number generators...

  • Leap year
    Leap year
    A leap year is a year containing one extra day in order to keep the calendar year synchronized with the astronomical or seasonal year...

  • Unix time
    Unix time
    Unix time, or POSIX time, is a system for describing instants in time, defined as the number of seconds elapsed since midnight Coordinated Universal Time of Thursday, January 1, 1970 , not counting leap seconds, which are declared by the International Earth Rotation and Reference Systems Service...

    , a common representation of time for computer systems which ignores leap seconds
  • Delta-T
  • Some stations that continuously broadcast UTC
    • CHU (radio station)
      CHU (radio station)
      CHU is the call sign of a shortwave time signal radio station operated by the Institute for National Measurement Standards of the National Research Council of Canada.- History :...

    • WWV (radio station)

Further reading

  • Finkleman, David, et al. The Future of Time: UTC and the Leap Second, American Scientist
    American Scientist
    American Scientist is the bimonthly science and technology magazine published since 1913 by Sigma Xi. Each issue includes four to five feature articles written by scientists and engineers. These authors review research in all fields of science...

    , July-August 2011, 99(4):312-319. DOI: 10.1511/2011.91.1
  • Kamp, Poul-Henning The One-Second War, Communications of the ACM
    Communications of the ACM
    Communications of the ACM is the flagship monthly journal of the Association for Computing Machinery . First published in 1957, CACM is sent to all ACM members, currently numbering about 80,000. The articles are intended for readers with backgrounds in all areas of computer science and information...

    , May 2011, 54(5):44-48. DOI:10.1145/1941487.1941505

External links



UTC redefinition