"UTC" redirects here. For the time zone between UTC−1 and UTC+1, see UTC+00:00. For other uses, see UTC (disambiguation).
Current time zones
Coordinated Universal Time (UTC) is the primary time standard globally used to regulate clocks and time. It establishes a reference for the current time, forming the basis for civil time and time zones. UTC facilitates international communication, navigation, scientific research, and commerce.
UTC has been widely embraced by most countries and is the effective successor to Greenwich Mean Time (GMT) in everyday usage and common applications.[a] In specialized domains such as scientific research, navigation, and timekeeping, other standards such as UT1 and International Atomic Time (TAI) are also used alongside UTC.
The coordination of time and frequency transmissions around the world began on 1 January 1960. UTC was first officially adopted as a standard in 1963 and "UTC" became the official abbreviation of Coordinated Universal Time in 1967.[2] The current version of UTC is defined by the International Telecommunication Union.
Since adoption, UTC has been adjusted several times, notably adding leap seconds in 1972. Recent years have seen significant developments in the realm of UTC, particularly in discussions about eliminating leap seconds from the timekeeping system because leap seconds occasionally disrupt timekeeping systems worldwide. The General Conference on Weights and Measures adopted a resolution to alter UTC with a new system that would eliminate leap seconds by 2035.[3]
Etymology
The official abbreviation for Coordinated Universal Time is UTC. This abbreviation comes as a result of the International Telecommunication Union and the International Astronomical Union wanting to use the same abbreviation in all languages.[4] The compromise that emerged was UTC,[5] which conforms to the pattern for the abbreviations of the variants of Universal Time (UT0, UT1, UT2, UT1R, etc.).[6]
McCarthy described the origin of the abbreviation:
In 1967 the CCIR adopted the names Coordinated Universal Time and Temps Universel Coordonné for the English and French names with the acronym UTC to be used in both languages. The name "Coordinated Universal Time (UTC)" was approved by a resolution of IAU Commissions 4 and 31 at the 13th General Assembly in 1967 (Trans. IAU, 1968).[2]
The westernmost time zone uses UTC−12, being twelve hours behind UTC; the easternmost time zone uses UTC+14, being fourteen hours ahead of UTC. In 1995, the island nation of Kiribati moved those of its atolls in the Line Islands from UTC−10 to UTC+14 so that Kiribati would all be on the same day.
UTC is used in many Internet and World Wide Web standards. The Network Time Protocol (NTP), designed to synchronise the clocks of computers over the Internet, transmits time information from the UTC system.[7] If only milliseconds precision is needed, clients can obtain the current UTC from a number of official internet UTC servers. For sub-microsecond precision, clients can obtain the time from satellite signals.
Amateur radio operators often schedule their radio contacts in UTC, because transmissions on some frequencies can be picked up in many time zones.[9]
Mechanism
UTC divides time into days, hours, minutes, and seconds. Days are conventionally identified using the Gregorian calendar, but Julian day numbers can also be used. Each day contains 24 hours and each hour contains 60minutes. The number of seconds in a minute is usually 60, but with an occasional leap second, it may be 61 or 59 instead.[10] Thus, in the UTC time scale, the second and all smaller time units (millisecond, microsecond, etc.) are of constant duration, but the minute and all larger time units (hour, day, week, etc.) are of variable duration. Decisions to introduce a leap second are announced at least six months in advance in "BulletinC" produced by the International Earth Rotation and Reference Systems Service.[11][12] The leap seconds cannot be predicted far in advance due to the unpredictable rate of the rotation of Earth.[13]
Nearly all UTC days contain exactly 86,400 SI seconds with exactly 60seconds in each minute. UTC is within about one second of mean solar time (such as UT1) at 0° longitude,[14] (at the IERS Reference Meridian). The mean solar day is slightly longer than 86,400 SI seconds so occasionally the last minute of a UTC day is adjusted to have 61seconds. The extra second is called a leap second. It accounts for the grand total of the extra length (about 2milliseconds each) of all the mean solar days since the previous leap second. The last minute of a UTC day is permitted to contain 59seconds to cover the remote possibility of the Earth rotating faster, but that has not yet been necessary. The irregular day lengths mean fractional Julian days do not work properly with UTC.
Since 1972, UTC may be calculated by subtracting the accumulated leap seconds from International Atomic Time (TAI), which is a coordinate time scale tracking notional proper time on the rotating surface of the Earth (the geoid). In order to maintain a close approximation to UT1, UTC occasionally has discontinuities where it changes from one linear function of TAI to another. These discontinuities take the form of leap seconds implemented by a UTC day of irregular length. Discontinuities in UTC occurred only at the end of June or December. However, there is provision for them to happen at the end of March and September as well as a second preference.[15][16] The International Earth Rotation and Reference Systems Service (IERS) tracks and publishes the difference between UTC and Universal Time, DUT1 = UT1 − UTC, and introduces discontinuities into UTC to keep DUT1 in the interval (−0.9s, +0.9s).
As with TAI, UTC is only known with the highest precision in retrospect. Users who require an approximation in real time must obtain it from a time laboratory, which disseminates an approximation using techniques such as GPS or radio time signals. Such approximations are designated UTC(k), where k is an abbreviation for the time laboratory.[17] The time of events may be provisionally recorded against one of these approximations; later corrections may be applied using the International Bureau of Weights and Measures (BIPM) monthly publication of tables of differences between canonical TAI/UTC and TAI(k)/UTC(k) as estimated in real-time by participating laboratories.[18] (See the article on International Atomic Time for details.)
Because of time dilation, a standard clock not on the geoid, or in rapid motion, will not maintain synchronicity with UTC. Therefore, telemetry from clocks with a known relation to the geoid is used to provide UTC when required, on locations such as those of spacecraft.
It is impossible to compute the exact time interval elapsed between two UTC timestamps without consulting a table showing how many leap seconds occurred during that interval. By extension, it is not possible to compute the precise duration of a time interval that ends in the future and may encompass an unknown number of leap seconds (for example, the number of TAI seconds between "now" and 2099-12-31 23:59:59). Therefore, many scientific applications that require precise measurement of long (multi-year) intervals use TAI instead. TAI is also commonly used by systems that cannot handle leap seconds. GPS time always remains exactly 19 seconds behind TAI (neither system is affected by the leap seconds introduced in UTC).
Time zones are usually defined as differing from UTC by an integer number of hours,[19] although the laws of each jurisdiction would have to be consulted if sub-second accuracy was required. Several jurisdictions have established time zones that differ by an odd integer number of half-hours or quarter-hours from UT1 or UTC.
The time zone using UTC is sometimes denoted UTC+00:00 or by the letter Z—a reference to the equivalent nautical time zone (GMT), which has been denoted by a Z since about 1950. Time zones were identified by successive letters of the alphabet and the Greenwich time zone was marked by a Z as it was the point of origin. The letter also refers to the "zone description" of zero hours, which has been used since 1920 (see time zone history). Since the NATO phonetic alphabet word for Z is "Zulu", UTC is sometimes known as "Zulu time". This is especially true in aviation, where "Zulu" is the universal standard.[20] This ensures that all pilots, regardless of location, are using the same 24-hour clock, thus avoiding confusion when flying between time zones.[21] See the list of military time zones for letters used in addition to Z in qualifying time zones other than Greenwich.
On electronic devices which only allow the time zone to be configured using maps or city names, UTC can be selected indirectly by selecting cities such as Accra in Ghana or Reykjavík in Iceland as they are always on UTC and do not currently use daylight saving time (which Greenwich and London do, and so could be a source of error).[22]
UTC does not change with a change of seasons, but local time or civil time may change if a time zone jurisdiction observes daylight saving time (summer time). For example, local time on the east coast of the United States is five hours behind UTC during winter,[23] but four hours behind while daylight saving is observed there.[24]
In 1928, the term Universal Time (UT) was introduced by the International Astronomical Union to refer to GMT, with the day starting at midnight.[25] Until the 1950s, broadcast time signals were based on UT, and hence on the rotation of the Earth.
In 1955, the caesiumatomic clock was invented. This provided a form of timekeeping that was both more stable and more convenient than astronomical observations. In 1956, the U.S.National Bureau of Standards and U.S. Naval Observatory started to develop atomic frequency time scales; by 1959, these time scales were used in generating the WWV time signals, named for the shortwave radio station that broadcasts them. In 1960, the U.S.Naval Observatory, the Royal Greenwich Observatory, and the UK National Physical Laboratory coordinated their radio broadcasts so that time steps and frequency changes were coordinated, and the resulting time scale was informally referred to as "Coordinated Universal Time".[26][27]
In a controversial decision, the frequency of the signals was initially set to match the rate of UT, but then kept at the same frequency by the use of atomic clocks and deliberately allowed to drift away from UT. When the divergence grew significantly, the signal was phase shifted (stepped) by 20 ms to bring it back into agreement with UT. Twenty-nine such steps were used before 1960.[28]
In 1958, data was published linking the frequency for the caesium transition, newly established, with the ephemeris second. The ephemeris second is a unit in the system of time that, when used as the independent variable in the laws of motion that govern the movement of the planets and moons in the solar system, enables the laws of motion to accurately predict the observed positions of solar system bodies. Within the limits of observable accuracy, ephemeris seconds are of constant length, as are atomic seconds. This publication allowed a value to be chosen for the length of the atomic second that would accord with the celestial laws of motion.[29]
The coordination of time and frequency transmissions around the world began on 1 January 1960. UTC was first officially adopted in 1963 as CCIR Recommendation 374, Standard-Frequency and Time-Signal Emissions, and "UTC" became the official abbreviation of Coordinated Universal Time in 1967.[2]
In 1961, the Bureau International de l'Heure began coordinating the UTC process internationally (but the name Coordinated Universal Time was not formally adopted by the International Astronomical Union until 1967).[30][31] From then on, there were time steps every few months, and frequency changes at the end of each year. The jumps increased in size to 0.1 seconds. This UTC was intended to permit a very close approximation to UT2.[26]
In 1967, the SI second was redefined in terms of the frequency supplied by a caesium atomic clock. The length of second so defined was practically equal to the second of ephemeris time.[32] This was the frequency that had been provisionally used in TAI since 1958. It was soon decided that having two types of second with different lengths, namely the UTC second and the SI second used in TAI, was a bad idea. It was thought better for time signals to maintain a consistent frequency, and that this frequency should match the SIsecond. Thus it would be necessary to rely on time steps alone to maintain the approximation of UT. This was tried experimentally in a service known as "Stepped Atomic Time" (SAT), which ticked at the same rate as TAI and used jumps of 0.2 seconds to stay synchronised with UT2.[33]
There was also dissatisfaction with the frequent jumps in UTC (and SAT). In 1968, Louis Essen, the inventor of the caesium atomic clock, and G.M.R.Winkler both independently proposed that steps should be of 1second only.[34] to simplify future adjustments. This system was eventually approved as leap seconds in a new UTC in 1970 and implemented in 1972, along with the idea of maintaining the UTC second equal to the TAI second. This CCIR Recommendation 460 "stated that (a) carrier frequencies and time intervals should be maintained constant and should correspond to the definition of the SI second; (b) step adjustments, when necessary, should be exactly 1 s to maintain approximate agreement with Universal Time (UT); and (c) standard signals should contain information on the difference between UTC and UT."[35]
As an intermediate step at the end of 1971, there was a final irregular jump of exactly 0.107758 TAI seconds, making the total of all the small time steps and frequency shifts in UTC or TAI during 1958–1971 exactly ten seconds, so that 1 January 1972 00:00:00 UTC was 1 January 1972 00:00:10 TAI exactly,[36] and a whole number of seconds thereafter. At the same time, the tick rate of UTC was changed to exactly match TAI. UTC also started to track UT1 rather than UT2. Some time signals started to broadcast the DUT1 correction (UT1 − UTC) for applications requiring a closer approximation of UT1 than UTC now provided.[37][38]
The first leap second occurred on 30 June 1972. Since then, leap seconds have occurred on average about once every 19months, always on 30 June or 31 December. As of July2022[update], there have been 27leap seconds in total, all positive, putting UTC 37seconds behind TAI.[42]
A study published in March 2024 in Nature concluded that accelerated melting of ice in Greenland and Antarctica due to climate change has decreased Earth's rotational velocity, affecting UTC adjustments and causing problems for computer networks that rely on UTC.[43]
Rationale
Graph showing the difference DUT1 between UT1 and UTC (in seconds). Vertical segments correspond to leap seconds.
Earth's rotational speed is very slowly decreasing because of tidal deceleration; this increases the length of the mean solar day. The length of the SIsecond was calibrated on the basis of the second of ephemeris time[29][32] and can now be seen to have a relationship with the mean solar day observed between 1750 and 1892, analysed by Simon Newcomb. As a result, the SIsecond is close to 1/86400 of a mean solar day in the mid‑19th century.[44] In earlier centuries, the mean solar day was shorter than 86,400 SI seconds, and in more recent centuries it is longer than 86,400 seconds. Near the end of the 20th century, the length of the mean solar day (also known simply as "length of day" or "LOD") was approximately 86,400.0013s.[45] For this reason, UT is now "slower" than TAI by the difference (or "excess" LOD) of 1.3ms/day.
The excess of the LOD over the nominal 86,400s accumulates over time, causing the UTC day, initially synchronised with the mean sun, to become desynchronised and run ahead of it. Near the end of the 20th century, with the LOD at 1.3ms above the nominal value, UTC ran faster than UT by 1.3ms per day, getting a second ahead roughly every 800 days. Thus, leap seconds were inserted at approximately this interval, retarding UTC to keep it synchronised in the long term.[46] The actual rotational period varies on unpredictable factors such as tectonic motion and has to be observed, rather than computed.
Just as adding a leap day every four years does not mean the year is getting longer by one day every four years, the insertion of a leap second every 800 days does not indicate that the mean solar day is getting longer by a second every 800 days. It will take about 50,000 years for a mean solar day to lengthen by one second (at a rate of 2ms per century). This rate fluctuates within the range of 1.7–2.3ms/cy. While the rate due to tidal friction alone is about 2.3ms/cy, the uplift of Canada and Scandinavia by several metres since the last ice age has temporarily reduced this to 1.7ms/cy over the last 2,700years.[47] The correct reason for leap seconds, then, is not the current difference between actual and nominal LOD, but rather the accumulation of this difference over a period of time: Near the end of the 20thcentury, this difference was about 1/800 of a second per day; therefore, after about 800 days, it accumulated to 1second (and a leap second was then added).
In the graph of DUT1 above, the excess of LOD above the nominal 86,400s corresponds to the downward slope of the graph between vertical segments. (The slope became shallower in the 1980s, 2000s and late 2010s to 2020s because of slight accelerations of Earth's rotation temporarily shortening the day.) Vertical position on the graph corresponds to the accumulation of this difference over time, and the vertical segments correspond to leap seconds introduced to match this accumulated difference. Leap seconds are timed to keep DUT1 within the vertical range depicted by the adjacent graph. The frequency of leap seconds therefore corresponds to the slope of the diagonal graph segments, and thus to the excess LOD. Time periods when the slope reverses direction (slopes upwards, not the vertical segments) are times when the excess LOD is negative, that is, when the LOD is below 86,400 s.
As the Earth's rotation continues to slow, positive leap seconds will be required more frequently. The long-term rate of change of LOD is approximately +1.7ms per century. At the end of the 21st century, LOD will be roughly 86,400.004s, requiring leap seconds every 250 days. Over several centuries, the frequency of leap seconds will become problematic.[48] A change in the trend of the UT1 – UTC values was seen beginning around June 2019 in which instead of slowing down (with leap seconds to keep the difference between UT1 and UTC less than 0.9 seconds) the Earth's rotation has sped up, causing this difference to increase. If the trend continues, a negative leap second may be required, which has not been used before. This may not be needed until 2025.[49][50]
Some time in the 22nd century, two leap seconds will be required every year. The current practice of only allowing leap seconds in June and December will be insufficient to maintain a difference of less than 1 second, and it might be decided to introduce leap seconds in March and September. In the 25thcentury, four leap seconds are projected to be required every year, so the current quarterly options would be insufficient.
In 2022 a resolution was adopted by the General Conference on Weights and Measures to redefine UTC and abolish leap seconds, but keep the civil second constant and equal to the SI second, so that sundials would slowly get further and further out of sync with civil time. The leap seconds will be eliminated by 2035. The resolution does not break the connection between UTC and UT1, but increases the maximum allowable difference. The details of what the maximum difference will be and how corrections will be implemented is left for future discussions.[3] This will result in a shift of the sun's movements relative to civil time, with the difference increasing quadratically with time (i.e., proportional to elapsed centuries squared). This is analogous to the shift of seasons relative to the yearly calendar that results from the calendar year not precisely matching the tropical year length. This would be a change in civil timekeeping, and would have a slow effect at first, but becoming drastic over several centuries. UTC (and TAI) would be more and more ahead of UT; it would coincide with local mean time along a meridian drifting eastward faster and faster.[52] Thus, the time system will lose its fixed connection to the geographic coordinates based on the IERS meridian. The difference between UTC and UT would reach 0.5 hours after the year 2600 and 6.5 hours around 4600.[53]
ITU-R Study Group7 and Working Party7A were unable to reach consensus on whether to advance the proposal to the 2012 Radiocommunications Assembly; the chairman of Study Group7 elected to advance the question to the 2012 Radiocommunications Assembly (20 January 2012),[54] but consideration of the proposal was postponed by the ITU until the World Radio Conference in 2015.[55] This conference, in turn, considered the question,[56] but no permanent decision was reached; it only chose to engage in further study with the goal of reconsideration in 2023.[57][needs update]
A proposed alternative to the leap second is the leap hour or leap minute, which requires changes only once every few centuries.[58]
ITU World Radiocommunication Conference 2023 (WRC-23), which was held in Dubai (United Arab Emirates) from 20 November to 15 December 2023 formally recognized the Resolution 4 of the 27th CGPM (2022) which decides that the maximum value for the difference (UT1-UTC) will be increased in, or before, 2035.[59]
International Atomic Time is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid. TAI is a weighted average of the time kept by over 450 atomic clocks in over 80 national laboratories worldwide. It is a continuous scale of time, without leap seconds, and it is the principal realisation of Terrestrial Time. It is the basis for Coordinated Universal Time (UTC), which is used for civil timekeeping all over the Earth's surface and which has leap seconds.
A day is the time period of a full rotation of the Earth with respect to the Sun. On average, this is 24 hours. As a day passes at a given location it experiences morning, noon, afternoon, evening, and night. This daily cycle drives circadian rhythms in many organisms, which are vital to many life processes.
In precise timekeeping, ΔT is a measure of the cumulative effect of the departure of the Earth's rotation period from the fixed-length day of International Atomic Time. Formally, ΔT is the time difference ΔT = TT − UT between Universal Time and Terrestrial Time. The value of ΔT for the start of 1902 was approximately zero; for 2002 it was about 64 seconds. So Earth's rotations over that century took about 64 seconds longer than would be required for days of atomic time. As well as this long-term drift in the length of the day there are short-term fluctuations in the length of day which are dealt with separately.
The term ephemeris time can in principle refer to time in association with any ephemeris. In practice it has been used more specifically to refer to:
a former standard astronomical time scale adopted in 1952 by the IAU, and superseded during the 1970s. This time scale was proposed in 1948, to overcome the disadvantages of irregularly fluctuating mean solar time. The intent was to define a uniform time based on Newtonian theory. Ephemeris time was a first application of the concept of a dynamical time scale, in which the time and time scale are defined implicitly, inferred from the observed position of an astronomical object via the dynamical theory of its motion.
a modern relativistic coordinate time scale, implemented by the JPL ephemeris time argument Teph, in a series of numerically integrated Development Ephemerides. Among them is the DE405 ephemeris in widespread current use. The time scale represented by Teph is closely related to, but distinct from, the TCB time scale currently adopted as a standard by the IAU.
Greenwich Mean Time (GMT) is the local mean time at the Royal Observatory in Greenwich, London, counted from midnight. At different times in the past, it has been calculated in different ways, including being calculated from noon; as a consequence, it cannot be used to specify a particular time unless a context is given. The term "GMT" is also used as one of the names for the time zone UTC+00:00 and, in UK law, is the basis for civil time in the United Kingdom.
A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time and imprecise observed solar time (UT1), which varies due to irregularities and long-term slowdown in the Earth's rotation. The UTC time standard, widely used for international timekeeping and as the reference for civil time in most countries, uses TAI and consequently would run ahead of observed solar time unless it is reset to UT1 as needed. The leap second facility exists to provide this adjustment. The leap second was introduced in 1972. Since then, 27 leap seconds have been added to UTC, with the most recent occurring on December 31, 2016. All have so far been positive leap seconds, adding a second to a UTC day; while it is possible for a negative leap second to be needed, one has not happened yet.
The second is a unit of time, historically defined as 1⁄86400 of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds each.
Terrestrial Time (TT) is a modern astronomical time standard defined by the International Astronomical Union, primarily for time-measurements of astronomical observations made from the surface of Earth. For example, the Astronomical Almanac uses TT for its tables of positions (ephemerides) of the Sun, Moon and planets as seen from Earth. In this role, TT continues Terrestrial Dynamical Time, which succeeded ephemeris time (ET). TT shares the original purpose for which ET was designed, to be free of the irregularities in the rotation of Earth.
A time standard is a specification for measuring time: either the rate at which time passes or points in time or both. In modern times, several time specifications have been officially recognized as standards, where formerly they were matters of custom and practice. An example of a kind of time standard can be a time scale, specifying a method for measuring divisions of time. A standard for civil time can specify both time intervals and time-of-day.
Universal Time is a time standard based on Earth's rotation. While originally it was mean solar time at 0° longitude, precise measurements of the Sun are difficult. Therefore, UT1 is computed from a measure of the Earth's angle with respect to the International Celestial Reference Frame (ICRF), called the Earth Rotation Angle. UT1 is the same everywhere on Earth. UT1 is required to follow the relationship
Sidereal time is a system of timekeeping used especially by astronomers. Using sidereal time and the celestial coordinate system, it is easy to locate the positions of celestial objects in the night sky. Sidereal time is a "time scale that is based on Earth's rate of rotation measured relative to the fixed stars".
The International Earth Rotation and Reference Systems Service (IERS), formerly the International Earth Rotation Service, is the body responsible for maintaining global time and reference frame standards, notably through its Earth Orientation Parameter (EOP) and International Celestial Reference System (ICRS) groups.
Solar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of solar time is the day, based on the synodic rotation period. Traditionally, there are three types of time reckoning based on astronomical observations: apparent solar time and mean solar time, and sidereal time, which is based on the apparent motions of stars other than the Sun.
Unix time is a date and time representation widely used in computing. It measures time by the number of non-leap seconds that have elapsed since 00:00:00 UTC on 1 January 1970, the Unix epoch. For example, at midnight on January 1 2010, Unix time was 1262304000.
In modern usage, civil time refers to statutory time as designated by civilian authorities. Modern civil time is generally national standard time in a time zone at a fixed offset from Coordinated Universal Time (UTC), possibly adjusted by daylight saving time during part of the year. UTC is calculated by reference to atomic clocks and was adopted in 1972. Older systems use telescope observations.
DUT1 is a time correction equal to the difference between Universal Time (UT1), which is defined by Earth's rotation, and Coordinated Universal Time (UTC), which is defined by a network of precision atomic clocks.
unit|base unit]] of time in the International System of Units (SI), and by extension most of the Western world, is the second, defined as about 9 billion oscillations of the caesium atom. The exact modern SI definition is "[The second] is defined by taking the fixed numerical value of the cesium frequency, ΔνCs, the unperturbed ground-state hyperfine transition frequency of the cesium 133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s−1."
Earth's rotation or Earth's spin is the rotation of planet Earth around its own axis, as well as changes in the orientation of the rotation axis in space. Earth rotates eastward, in prograde motion. As viewed from the northern polar star Polaris, Earth turns counterclockwise.
A tropical year or solar year is the time that the Sun takes to return to the same position in the sky – as viewed from the Earth or another celestial body of the Solar System – thus completing a full cycle of astronomical seasons. For example, it is the time from vernal equinox to the next vernal equinox, or from summer solstice to the next summer solstice. It is the type of year used by tropical solar calendars.
The IERS Reference Meridian (IRM), also called the International Reference Meridian, is the prime meridian maintained by the International Earth Rotation and Reference Systems Service (IERS). It passes about 5.3 arcseconds east of George Biddell Airy's 1851 transit circle which is 102 metres (335 ft) at the latitude of the Royal Observatory, Greenwich. Thus it differs slightly from the historical Greenwich Meridian.
References
Notes
↑ The pips are no longer broadcast from Greenwich, but from the National Physical Laboratory in Teddington, Surrey, which uses Coordinated Universal Time (UTC) – the successor of GMT – for its reading.[1]
↑ McCarthy & Seidelmann 2009, p.230. (Average for period from 1 January 1991 through 1 January 2009. Average varies considerably depending on what period is chosen.)
↑ "Plots for UT1-UTC – Bulletin A All". International Earth Rotation and Reference Systems Service. 16 September 2021. Archived from the original on 23 October 2021. Retrieved 16 September 2021.
Arias, E. F.; Guinot, B.; Quinn, T. J. (29 May 2003). Rotation of the Earth and Time scales(PDF). ITU-R Special Rapporteur Group Colloquium on the UTC Time Scale.
"Aviation Time". AOPA's Path to Aviation. Aircraft Owners and Pilots Association. 2006. Archived from the original on 27 November 2006. Retrieved 17 July 2011.
Blair, Byron E., ed. (1974), Time and Frequency: Theory and Fundamentals(PDF), National Bureau of Standards, National Institute of Standards and Technology since 1988, p.32, archived(PDF) from the original on 1 March 2021, retrieved 30 June 2019
Chester, Geoff (15 June 2015). "Wait a second... 2015 will be a little longer". CHIPS: The Department of the Navy's Information Technology Magazine. Department of the Navy. Archived from the original on 12 February 2022. Retrieved 12 March 2021.
Finkleman, David; Allen, Steve; Seago, John; Seaman, Rob; Seidelmann, P. Kenneth (2011). "The Future of Time: UTC and the Leap Second". American Scientist. 99 (July–August 2011): 312. arXiv:1106.3141. Bibcode:2011arXiv1106.3141F. doi:10.1511/2011.91.1.
Horzepa, Stan (17 September 2010). "Surfin': Time for Ham Radio". American Radio Relay League. Archived from the original on 23 September 2010. Retrieved 24 October 2011.
"How NTP Works". NTP: The Network Time Protocol. 28 July 2011. Archived from the original on 20 June 2014. Retrieved 18 June 2022. See heading "NTP Timescale and Data Formats".
"IAU resolutions adopted at the XVIth General Assembly, Grenoble, France, 1976"(PDF). 1976. Archived(PDF) from the original on 2 May 2019. Retrieved 18 June 2022. Resolution no. 3 by Commissions 4 (Ephemerides/Ephémérides) and 31 (Time/L'Heure) (near the end of the document) "recommend that the following notations be used in all languages", UT0(i), UT1(i), UT2(i), UTC, UTC(i), UT, where (i) is institution "i".
Markowitz, Wm. (1988). "Comparisons of ET (Solar), ET (Lunar), UT and TDT". In Babcock, A. K.; Wilkins, G. A. (eds.). The Earth's Rotation and Reference Frames for Geodesy and Geophysics: Proceedings of the 128th Symposium of the International Astronomical Union, held in Coolfont, West Virginia, U.S.A., 20–24 October 1986. International Astronomical Union Symposia. Vol.128. Dordrecht: Kluwer Academic Publishers. pp.413–418. Bibcode:1988IAUS..128..413M. ISBN978-90-277-2657-5.
McCarthy, D.; Guinot, B. (2013). "Time". In Urban, Sean E.; Seidelmann, P. Kenneth (eds.). Explanatory Supplement to the Astronomical Almanac (3rded.). Mill Valley, CA: University Science Books.
United States Naval Observatory. "Universal Time". Archived from the original on 22 July 2011. Retrieved 10 October 2013.
"Universal Time". Oxford Dictionaries: British and World English. Oxford University Press. Archived from the original on 12 July 2013. Retrieved 6 August 2014.
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