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125-641: 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. 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. UTC
250-460: A clock to count periods of some period changes, which may be either the changes of a natural phenomenon or of an artificial machine. Historically, time standards were often based on the Earth's rotational period. From the late 18 century to the 19th century it was assumed that the Earth's daily rotational rate was constant. Astronomical observations of several kinds, including eclipse records, studied in
375-488: 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 (24 × 60 × 60 = 86400). The current and formal definition in the International System of Units (SI) is more precise: The second [...] is defined by taking the fixed numerical value of the caesium frequency, Δ ν Cs , the unperturbed ground-state hyperfine transition frequency of
500-448: A time zone deviates a fixed, round amount, usually a whole number of hours, from some form of Universal Time , usually UTC. The offset is chosen such that a new day starts approximately while the Sun is crossing the nadir meridian. Alternatively the difference is not really fixed, but it changes twice a year by a round amount, usually one hour, see Daylight saving time . Julian day number
625-548: 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. The International Earth Rotation and Reference Systems Service (IERS) tracks and publishes
750-423: A factor of 100. Therefore a new definition of the second is planned. Atomic clocks now set the length of a second and the time standard for the world. 12960276813 408986496 × 10 − 9 {\displaystyle {\frac {12960276813}{408986496}}\times 10^{-9}} of the tropical year for 1900 January 0 at 12 h ET. 11th CGPM 1960 Resolution 9 CIPM 1967
875-439: A few hundred nanoseconds, which in turn may differ from official UTC by as much as 26 nanoseconds. Conversions for UT1 and TT rely on published difference tables which as of 2022 are specified to 10 microseconds and 0.1 nanoseconds respectively. Definitions: TCG is linearly related to TT as: TCG − TT = L G × (JD − 2443144.5) × 86400 seconds, with the scale difference L G defined as 6.969290134 × 10 exactly. TCB
1000-549: A formula describing a mean tropical year that decreased linearly over time. In 1956, the second was redefined in terms of a year relative to that epoch . The second was thus defined as "the fraction 1 ⁄ 31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time". This definition was adopted as part of the International System of Units in 1960. Even the best mechanical, electric motorized and quartz crystal-based clocks develop discrepancies from environmental conditions; far better for timekeeping
1125-529: A leap second are announced at least six months in advance in "Bulletin C" produced by the International Earth Rotation and Reference Systems Service . The leap seconds cannot be predicted far in advance due to the unpredictable rate of the rotation of Earth. Nearly all UTC days contain exactly 86,400 SI seconds with exactly 60 seconds in each minute. UTC is within about one second of mean solar time (such as UT1 ) at 0° longitude , (at
1250-451: A mean solar day in the mid‑19th century. 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 86,400.0013 s. For this reason, UT is now "slower" than TAI by the difference (or "excess" LOD) of 1.3 ms/day. The excess of
1375-417: A mean solar day to lengthen by one second (at a rate of 2 ms per century). This rate fluctuates within the range of 1.7–2.3 ms/cy. While the rate due to tidal friction alone is about 2.3 ms/cy, the uplift of Canada and Scandinavia by several metres since the last ice age has temporarily reduced this to 1.7 ms/cy over the last 2,700 years. The correct reason for leap seconds, then,
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#17328373647601500-522: A measure of radioactive decay, is measured in inverse seconds and higher powers of second are involved in derivatives of acceleration such as jerk . Though many derivative units for everyday things are reported in terms of larger units of time, not seconds, they are ultimately defined in terms of the SI second; this includes time expressed in hours and minutes, velocity of a car in kilometers per hour or miles per hour, kilowatt hours of electricity usage, and speed of
1625-472: A meridian drifting eastward faster and faster. 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. ITU-R Study Group 7 and Working Party 7A were unable to reach consensus on whether to advance the proposal to the 2012 Radiocommunications Assembly;
1750-565: A meter long; the fastest human sprinters run 10 meters in a second; an ocean wave in deep water travels about 23 meters in one second; sound travels about 343 meters in one second in air; light takes 1.3 seconds to reach Earth from the surface of the Moon, a distance of 384,400 kilometers. A second is directly part of other units, such as frequency measured in hertz ( inverse seconds or s ), speed in meters per second, and acceleration in meters per second squared. The metric system unit becquerel ,
1875-458: A microwave cavity. The fraction of excited atoms is then detected by laser beams. These clocks have 5 × 10 systematic uncertainty, which is equivalent to 50 picoseconds per day. A system of several fountains worldwide contribute to International Atomic Time. These caesium clocks also underpin optical frequency measurements. Optical clocks are based on forbidden optical transitions in ions or atoms. They have frequencies around 10 Hz , with
2000-523: A natural linewidth Δ f {\displaystyle \Delta f} of typically 1 Hz, so the Q-factor is about 10 , or even higher. They have better stabilities than microwave clocks, which means that they can facilitate evaluation of lower uncertainties. They also have better time resolution, which means the clock "ticks" faster. Optical clocks use either a single ion, or an optical lattice with 10 – 10 atoms. A definition based on
2125-513: A particular time zone can be determined by adding or subtracting the number of hours and minutes specified by the UTC offset , which ranges from UTC−12:00 in the west to UTC+14:00 in the east (see List of UTC offsets ). 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
2250-421: A refined version of UT, TDT was offset from TAI, by a constant 32.184 seconds. The offset provided a continuity from Ephemeris Time to TDT. TDT has since been redefined as Terrestrial Time (TT). For the calculation of ephemerides, Barycentric Dynamical Time (TDB) was officially recommended to replace ET. TDB is similar to TDT but includes relativistic corrections that move the origin to the barycenter, hence it
2375-562: A resolution to alter UTC with a new system that would eliminate leap seconds by 2035. 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. The compromise that emerged was UTC , which conforms to the pattern for
2500-592: A second is a 1-gigahertz microprocessor that has a cycle time of 1 nanosecond. Camera shutter speeds are often expressed in fractions of a second, such as 1 ⁄ 30 second or 1 ⁄ 1000 second. Sexagesimal divisions of the day from a calendar based on astronomical observation have existed since the third millennium BC, though they were not seconds as we know them today. Small divisions of time could not be measured back then, so such divisions were mathematically derived. The first timekeepers that could count seconds accurately were pendulum clocks invented in
2625-508: A second was selected to correspond exactly to the length of the ephemeris second previously defined. Atomic clocks use such a frequency to measure seconds by counting cycles per second at that frequency. Radiation of this kind is one of the most stable and reproducible phenomena of nature. The current generation of atomic clocks is accurate to within one second in a few hundred million years. Since 1967, atomic clocks based on atoms other than caesium-133 have been developed with increased precision by
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#17328373647602750-547: 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
2875-428: A single day differs from the next by only a small amount; 15 minutes is a cumulative difference over a part of the year. The effect is due chiefly to the obliqueness of Earth's axis with respect to its orbit around the Sun. The difference between apparent solar time and mean time was recognized by astronomers since antiquity, but prior to the invention of accurate mechanical clocks in the mid-17th century, sundials were
3000-466: A source of error). 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, but four hours behind while daylight saving is observed there. In 1928, the term Universal Time ( UT ) was introduced by
3125-442: 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
3250-400: A turntable in rotations per minute. Moreover, most other SI base units are defined by their relationship to the second: the meter is defined by setting the speed of light (in vacuum) to be 299 792 458 m/s, exactly; definitions of the SI base units kilogram , ampere , kelvin , and candela also depend on the second. The only base unit whose definition does not depend on the second
3375-513: A two-digit seconds counter. SI prefixes are frequently combined with the word second to denote subdivisions of the second: milliseconds (thousandths), microseconds (millionths), nanoseconds (billionths), and sometimes smaller units of a second. Multiples of seconds are usually counted in hours and minutes. Though SI prefixes may also be used to form multiples of the second such as kiloseconds (thousands of seconds), such units are rarely used in practice. An everyday experience with small fractions of
3500-418: A value to be chosen for the length of the atomic second that would accord with the celestial laws of motion. 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. In 1961,
3625-456: Is 604,800 seconds; a year (other than leap years ) is 31,536,000 seconds; and a ( Gregorian ) century averages 3,155,695,200 seconds; with all of the above excluding any possible leap seconds . In astronomy, a Julian year is precisely 31,557,600 seconds. Some common events in seconds are: a stone falls about 4.9 meters from rest in one second; a pendulum of length about one meter has a swing of one second, so pendulum clocks have pendulums about
3750-588: Is a count of days elapsed since Greenwich mean noon on 1 January 4713 B.C., Julian proleptic calendar. The Julian Date is the Julian day number followed by the fraction of the day elapsed since the preceding noon. Conveniently for astronomers, this avoids the date skip during an observation night. Modified Julian day (MJD) is defined as MJD = JD - 2400000.5. An MJD day thus begins at midnight, civil date. Julian dates can be expressed in UT1, TAI, TT, etc. and so for precise applications
3875-453: Is a dynamical time at the barycenter. TDB differs from TT only in periodic terms. The difference is at most 2 milliseconds. Deficiencies were found in the definition of TDB (though not affecting T eph ), and TDB has been replaced by Barycentric Coordinate Time (TCB) and Geocentric Coordinate Time (TCG), and redefined to be JPL ephemeris time argument T eph , a specific fixed linear transformation of TCB. As defined, TCB (as observed from
Coordinated Universal Time - Misplaced Pages Continue
4000-462: Is a linear transformation of TDB and TDB differs from TT in small, mostly periodic terms. Neglecting these terms (on the order of 2 milliseconds for several millennia around the present epoch), TCB is related to TT by: TCB − TT = L B × (JD − 2443144.5) × 86400 seconds. The scale difference L B has been defined by the IAU to be 1.550519768e-08 exactly. Apparent solar time or true solar time
4125-486: 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. Standardized time measurements are made using
4250-447: Is affected by the leap seconds introduced in UTC). Time zones are usually defined as differing from UTC by an integer number of hours, 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. Current civil time in
4375-739: Is always kept within 0.9 second of UT1) is in common actual use in the UK, and the name GMT is often used to refer to it. (See articles Greenwich Mean Time , Universal Time , Coordinated Universal Time and the sources they cite.) Versions of Universal Time such as UT0 and UT2 have been defined but are no longer in use. Ephemeris time (ET) and its successor time scales described below have all been intended for astronomical use, e.g. in planetary motion calculations, with aims including uniformity, in particular, freedom from irregularities of Earth rotation. Some of these standards are examples of dynamical time scales and/or of coordinate time scales. Ephemeris Time
4500-530: Is an abbreviation for the time laboratory. 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. (See the article on International Atomic Time for details.) Because of time dilation ,
4625-659: Is an unsigned clock depicting Orpheus in the Fremersdorf collection, dated between 1560 and 1570. During the 3rd quarter of the 16th century, Taqi al-Din built a clock with marks every 1/5 minute. In 1579, Jost Bürgi built a clock for William of Hesse that marked seconds. In 1581, Tycho Brahe redesigned clocks that had displayed only minutes at his observatory so they also displayed seconds, even though those seconds were not accurate. In 1587, Tycho complained that his four clocks disagreed by plus or minus four seconds. In 1656, Dutch scientist Christiaan Huygens invented
4750-431: Is approximately +1.7 ms per century. At the end of the 21st century, LOD will be roughly 86,400.004 s, requiring leap seconds every 250 days. Over several centuries, the frequency of leap seconds will become problematic. 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)
4875-410: Is based on TAI, which is a weighted average of hundreds of atomic clocks worldwide. UTC is within about one second of mean solar time at 0° longitude, the currently used prime meridian , and is not adjusted for daylight saving time . 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
5000-406: Is based on the solar day, which is the period between one solar noon (passage of the real Sun across the meridian ) and the next. A solar day is approximately 24 hours of mean time. Because the Earth's orbit around the Sun is elliptical, and because of the obliquity of the Earth's axis relative to the plane of the orbit (the ecliptic) , the apparent solar day varies a few dozen seconds above or below
5125-465: Is generally used for many close but different concepts, including: There have only ever been three definitions of the second: as a fraction of the day, as a fraction of an extrapolated year, and as the microwave frequency of a caesium atomic clock. In early history, clocks were not accurate enough to track seconds. After the invention of mechanical clocks, the CGS system and MKS system of units both defined
Coordinated Universal Time - Misplaced Pages Continue
5250-420: Is irregular and is determined from the positions of distant quasars using long baseline interferometry, laser ranging of the Moon and artificial satellites, as well as GPS satellite orbits. Coordinated Universal Time (UTC) is an atomic time scale designed to approximate UT1. UTC differs from TAI by an integral number of seconds. UTC is kept within 0.9 second of UT1 by the introduction of one-second steps to UTC,
5375-487: Is no longer so; it was initially renamed in 1928 as Universal Time (UT) (partly as a result of ambiguities arising from the changed practice of starting the astronomical day at midnight instead of at noon, adopted as from 1 January 1925). UT1 is still in reality mean time at Greenwich. Today, GMT is a time zone but is still the legal time in the UK in winter (and as adjusted by one hour for summer time). But Coordinated Universal Time (UTC) (an atomic-based time scale which
5500-485: 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
5625-465: Is not related to TCG directly but rather is a realization of Terrestrial Time (TT), a theoretical timescale that is a rescaling of TCG such that the time rate approximately matches proper time at mean sea level . Universal Time (UT1) is the Earth Rotation Angle (ERA) linearly scaled to match historical definitions of mean solar time at 0° longitude. At high precision, Earth's rotation
5750-407: 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 20th century, this difference was about 1 / 800 of a second per day; therefore, after about 800 days, it accumulated to 1 second (and a leap second was then added). In the graph of DUT1 above, the excess of LOD above
5875-412: Is the mole , and only two of the 22 named derived units, radian and steradian , do not depend on the second either. A set of atomic clocks throughout the world keeps time by consensus: the clocks "vote" on the correct time, and all voting clocks are steered to agree with the consensus, which is called International Atomic Time (TAI). TAI "ticks" atomic seconds. Civil time is defined to agree with
6000-413: Is the natural and exact "vibration" in an energized atom. The frequency of vibration (i.e., radiation) is very specific depending on the type of atom and how it is excited. Since 1967, the second has been defined as exactly "the duration of 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 length of
6125-408: Is the time it takes the Earth to make one revolution with rotation to the stars, approximately 23 hours 56 minutes 4 seconds. A mean solar day is about 3 minutes 56 seconds longer than a mean sidereal day, or 1 ⁄ 366 more than a mean sidereal day. In astronomy , sidereal time is used to predict when a star will reach its highest point in the sky. For accurate astronomical work on land, it
6250-461: Is very slowly decreasing because of tidal deceleration ; this increases the length of the mean solar day . The length of the SI ;second was calibrated on the basis of the second of ephemeris time 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 SI second is close to 1 / 86400 of
6375-567: 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). 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. In 1967,
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#17328373647606500-541: The Gregorian calendar , but Julian day numbers can also be used. Each day contains 24 hours and each hour contains 60 minutes. The number of seconds in a minute is usually 60, but with an occasional leap second , it may be 61 or 59 instead. 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
6625-505: 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 61 seconds. The extra second is called a leap second. It accounts for the grand total of the extra length (about 2 milliseconds each) of all the mean solar days since the previous leap second. The last minute of a UTC day is permitted to contain 59 seconds to cover
6750-586: 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. 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
6875-527: The Rydberg constant would involve fixing the value to a certain value: R ∞ = m e e 4 8 ε 0 2 h 3 c = m e c α 2 2 h {\displaystyle R_{\infty }={\frac {m_{\text{e}}e^{4}}{8\varepsilon _{0}^{2}h^{3}c}}={\frac {m_{\text{e}}c\alpha ^{2}}{2h}}} . The Rydberg constant describes
7000-491: 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. 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
7125-916: The UT1 variant of universal time . See the " Current number of leap seconds " section for the number of leap seconds inserted to date. The first leap second occurred on 30 June 1972. Since then, leap seconds have occurred on average about once every 19 months, always on 30 June or 31 December. As of July 2022, there have been 27 leap seconds in total, all positive, putting UTC 37 seconds behind TAI. 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. Earth's rotational speed
7250-602: 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". In a controversial decision,
7375-473: The caesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz , which is equal to s . This current definition was adopted in 1967 when it became feasible to define the second based on fundamental properties of nature with caesium clocks . Because the speed of Earth's rotation varies and is slowing ever so slightly , a leap second is added at irregular intervals to civil time to keep clocks in sync with Earth's rotation. "Minute" comes from
7500-491: 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
7625-454: The mean time , as opposed to the apparent time displayed by sundials . By that time, sexagesimal divisions of time were well established in Europe. The earliest clocks to display seconds appeared during the last half of the 16th century. The second became accurately measurable with the development of mechanical clocks. The earliest spring-driven timepiece with a second hand that marked seconds
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#17328373647607750-445: The sidereal year at that epoch by the IAU in 1952. This extrapolated timescale brings the observed positions of the celestial bodies into accord with Newtonian dynamical theories of their motion. In 1955, the tropical year , considered more fundamental than the sidereal year, was chosen by the IAU as the unit of time. The tropical year in the definition was not measured but calculated from
7875-449: The " leap second ". To date these steps (and difference "TAI-UTC") have always been positive. The Global Positioning System broadcasts a very precise time signal worldwide, along with instructions for converting GPS time (GPST) to UTC. It was defined with a constant offset from TAI: GPST = TAI - 19 s. The GPS time standard is maintained independently but regularly synchronized with or from, UTC time. Standard time or civil time in
8000-418: The 13th General Assembly in 1967 (Trans. IAU, 1968). Time zones around the world are expressed using positive, zero, or negative offsets from UTC , as in the list of time zones by UTC offset . 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
8125-400: The 14th century, had displays that divided the hour into halves, thirds, quarters and sometimes even 12 parts, but never by 60. In fact, the hour was not commonly divided in 60 minutes as it was not uniform in duration. It was not practical for timekeepers to consider minutes until the first mechanical clocks that displayed minutes appeared near the end of the 16th century. Mechanical clocks kept
8250-419: The 17th century. Starting in the 1950s, atomic clocks became better timekeepers than Earth's rotation, and they continue to set the standard today. A mechanical clock, which does not depend on measuring the relative rotational position of the Earth, keeps uniform time called mean time , within whatever accuracy is intrinsic to it. That means that every second, minute and every other division of time counted by
8375-404: The 19th century, raised suspicions that the rate at which Earth rotates is gradually slowing and also shows small-scale irregularities, and this was confirmed in the early twentieth century. Time standards based on Earth rotation were replaced (or initially supplemented) for astronomical use from 1952 onwards by an ephemeris time standard based on the Earth's orbital period and in practice on
8500-424: The 2010s held the accuracy record: it gains or loses less than a second in 15 billion years, which is longer than the estimated age of the universe. Such a clock can measure a change in its elevation of as little as 2 cm by the change in its rate due to gravitational time dilation . There have only ever been three definitions of the second: as a fraction of the day, as a fraction of an extrapolated year, and as
8625-574: The 25th century, four leap seconds are projected to be required every year, so the current quarterly options would be insufficient. In April 2001, Rob Seaman of the National Optical Astronomy Observatory proposed that leap seconds be allowed to be added monthly rather than twice yearly. In 2022 a resolution was adopted by the General Conference on Weights and Measures to redefine UTC and abolish leap seconds, but keep
8750-514: The Advancement of Science (BAAS) in 1862 stated that "All men of science are agreed to use the second of mean solar time as the unit of time." BAAS formally proposed the CGS system in 1874, although this system was gradually replaced over the next 70 years by MKS units. Both the CGS and MKS systems used the same second as their base unit of time. MKS was adopted internationally during the 1940s, defining
8875-595: The DUT1 correction (UT1 − UTC) for applications requiring a closer approximation of UT1 than UTC now provided. The current version of UTC is defined by International Telecommunication Union Recommendation (ITU-R TF.460-6), Standard-frequency and time-signal emissions , and is based on International Atomic Time (TAI) with leap seconds added at irregular intervals to compensate for the accumulated difference between TAI and time measured by Earth's rotation . Leap seconds are inserted as necessary to keep UTC within 0.9 seconds of
9000-545: 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. 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
9125-528: The Earth's surface) is of divergent rate relative to all of ET, T eph and TDT/TT; and the same is true, to a lesser extent, of TCG. The ephemerides of Sun, Moon and planets in current widespread and official use continue to be those calculated at the Jet Propulsion Laboratory (updated as from 2003 to DE405 ) using as argument T eph . Second The second (symbol: s ) is a unit of time , historically defined as 1 ⁄ 86400 of
9250-539: The International Astronomical Union to refer to GMT, with the day starting at midnight. Until the 1950s, broadcast time signals were based on UT, and hence on the rotation of the Earth. In 1955, the caesium atomic 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
9375-492: The JPL relativistic coordinate time scale T eph ). For applications at the Earth's surface, ET's official replacement was Terrestrial Dynamical Time (TDT), which maintained continuity with it. TDT is a uniform atomic time scale, whose unit is the SI second. TDT is tied in its rate to the SI second, as is International Atomic Time (TAI), but because TAI was somewhat arbitrarily defined at its inception in 1958 to be initially equal to
9500-503: The LOD over the nominal 86,400 s 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 1.3 ms above the nominal value, UTC ran faster than UT by 1.3 ms 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
9625-477: The Latin pars minuta prima , meaning "first small part" i.e. first division of the hour - dividing into sixty, and "second" comes from the pars minuta secunda , "second small part", dividing again into sixty. Analog clocks and watches often have sixty tick marks on their faces, representing seconds (and minutes), and a "second hand" to mark the passage of time in seconds. Digital clocks and watches often have
9750-502: The Sun (a year) was much more stable than Earth's rotation. This led to proposals as early as 1950 to define the second as a fraction of a year. The Earth's motion was described in Newcomb's Tables of the Sun (1895), which provided a formula for estimating the motion of the Sun relative to the epoch 1900 based on astronomical observations made between 1750 and 1892. This resulted in adoption of an ephemeris time scale expressed in units of
9875-454: 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." As an intermediate step at
10000-513: The abbreviations of the variants of Universal Time (UT0, UT1, UT2, UT1R, etc.). 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
10125-469: The achievement of accuracy in measurement. In former times, before the distribution of accurate time signals, it was part of the routine work at any observatory to observe the sidereal times of meridian transit of selected 'clock stars' (of well-known position and movement), and to use these to correct observatory clocks running local mean sidereal time; but nowadays local sidereal time is usually generated by computer, based on time signals. Mean solar time
10250-550: 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. This ensures that all pilots, regardless of location, are using
10375-552: The atoms move very fast, causing Doppler shifts. The radiation needed to cool the hydrogen – 121.5 nm – is also difficult. Another hurdle involves improving the uncertainty in QED calculations, specifically the Lamb shift in the 1s-2s transition of the hydrogen atom. A redefinition must include improved optical clock reliability. TAI must be contributed to by optical clocks before the BIPM affirms
10500-402: The caesium atom used to realize the definition. In a laboratory sufficiently small to allow the effects of the non-uniformity of the gravitational field to be neglected when compared to the uncertainties of the realization of the second, the proper second is obtained after application of the special relativistic correction for the velocity of the atom in the laboratory. It is wrong to correct for
10625-527: The center of Earth's mass. TCG is a theoretical ideal, and any particular realization will have measurement error . International Atomic Time (TAI) is the primary physically realized time standard. TAI is produced by the International Bureau of Weights and Measures (BIPM), and is based on the combined input of many atomic clocks around the world, each corrected for environmental and relativistic effects (both gravitational and because of speed, like in GNSS ). TAI
10750-539: The chairman of Study Group 7 elected to advance the question to the 2012 Radiocommunications Assembly (20 January 2012), but consideration of the proposal was postponed by the ITU until the World Radio Conference in 2015. This conference, in turn, considered the question, but no permanent decision was reached; it only chose to engage in further study with the goal of reconsideration in 2023. A proposed alternative to
10875-440: 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. This will result in
11000-473: The clock has the same duration as any other identical division of time. But a sundial , which measures the relative position of the Sun in the sky called apparent time , does not keep uniform time. The time kept by a sundial varies by time of year, meaning that seconds, minutes and every other division of time is a different duration at different times of the year. The time of day measured with mean time versus apparent time may differ by as much as 15 minutes, but
11125-473: The difference between UTC and Universal Time, DUT1 = UT1 − UTC, and introduces discontinuities into UTC to keep DUT1 in the interval (−0.9 s, +0.9 s). 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
11250-482: 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, 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
11375-457: The energy levels in a hydrogen atom with the nonrelativistic approximation E n ≈ − R ∞ c h n 2 {\displaystyle E_{n}\approx -{\frac {R_{\infty }ch}{n^{2}}}} . The only viable way to fix the Rydberg constant involves trapping and cooling hydrogen. This is difficult because it is very light and
11500-442: The ephemeris second was defined as "the fraction 1 ⁄ 31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time". This definition was adopted as part of the International System of Units in 1960. Most recently, atomic clocks have been developed that offer improved accuracy. Since 1967, the SI base unit for time is the SI second, defined as exactly "the duration of 9,192,631,770 periods of
11625-499: The first pendulum clock. It had a pendulum length of just under a meter, giving it a swing of one second, and an escapement that ticked every second. It was the first clock that could accurately keep time in seconds. By the 1730s, 80 years later, John Harrison 's maritime chronometers could keep time accurate to within one second in 100 days. In 1832, Gauss proposed using the second as the base unit of time in his millimeter–milligram–second system of units . The British Association for
11750-458: The footnote was added at the 86th (1997) meeting of the CIPM GCPM 1998 7th Edition SI Brochure A future re-definition of the second would be justified if these idealized conditions can be achieved much easier than with the current definition. The definition of the second should be understood as the definition of the unit of proper time: it applies in a small spatial domain that shares the motion of
11875-416: 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. In 1958, data was published linking the frequency for
12000-596: The hour like the modern second (= hour / 60×60 ). Sundials and water clocks were among the earliest timekeeping devices, and units of time were measured in degrees of arc. Conceptual units of time smaller than realisable on sundials were also used. There are references to "second" as part of a lunar month in the writings of natural philosophers of the Middle Ages, which were mathematical subdivisions that could not be measured mechanically. The earliest mechanical clocks, which appeared starting in
12125-554: The leap second is the leap hour or leap minute, which requires changes only once every few centuries. 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. Time standard A time standard
12250-408: The local gravitational field. The reference to an unperturbed atom is intended to make it clear that the definition of the SI second is based on an isolated caesium atom that is unperturbed by any external field, such as ambient black-body radiation. The second, so defined, is the unit of proper time in the sense of the general theory of relativity. To allow the provision of a coordinated time scale,
12375-434: The long term. 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
12500-405: The mean value of 24 hours. As the variation accumulates over a few weeks, there are differences as large as 16 minutes between apparent solar time and mean solar time (see Equation of time ). However, these variations cancel out over a year. There are also other perturbations such as Earth's wobble, but these are less than a second per year. Sidereal time is time by the stars. A sidereal rotation
12625-431: The microwave frequency of a caesium atomic clock, which have each realized a sexagesimal division of the day from ancient astronomical calendars. Civilizations in the classic period and earlier created divisions of the calendar as well as arcs using a sexagesimal system of counting, so at that time the second was a sexagesimal subdivision of the day (ancient second = day / 60×60 ), not of
12750-448: The motion of the Moon. The invention in 1955 of the caesium atomic clock has led to the replacement of older and purely astronomical time standards, for most practical purposes, by newer time standards based wholly or partly on atomic time. Various types of second and day are used as the basic time interval for most time scales. Other intervals of time (minutes, hours, and years) are usually defined in terms of these two. The term "time"
12875-494: The nominal 86,400 s 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
13000-528: The official abbreviation of Coordinated Universal Time in 1967. 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
13125-437: The only reliable timepieces, and apparent solar time was the only generally accepted standard. Fractions of a second are usually denoted in decimal notation, for example 2.01 seconds, or two and one hundredth seconds. Multiples of seconds are usually expressed as minutes and seconds, or hours, minutes and seconds of clock time, separated by colons, such as 11:23:24, or 45:23 (the latter notation can give rise to ambiguity, because
13250-404: The radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom" (at a temperature of 0 K and at mean sea level ). The SI second is the basis of all atomic timescales, e.g. coordinated universal time, GPS time, International Atomic Time, etc. Geocentric Coordinate Time (TCG) is a coordinate time having its spatial origin at
13375-553: The realization of the second based on the Cs hyperfine transition frequency, but some can be reproduced with superior stability. SI Brochure 9 In 2022, the best realisation of the second is done with caesium primary standard clocks such as IT-CsF2, NIST-F2, NPL-CsF2, PTB-CSF2, SU–CsFO2 or SYRTE-FO2. These clocks work by laser-cooling a cloud of Cs atoms to a microkelvin in a magneto-optic trap. These cold atoms are then launched vertically by laser light. The atoms then undergo Ramsey excitation in
13500-483: 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
13625-421: The rotation of the Earth with respect to the Sun, and does not contain any leap seconds. UT1 always differs from UTC by less than a second. While they are not yet part of any timekeeping standard, optical lattice clocks with frequencies in the visible light spectrum now exist and are the most accurate timekeepers of all. A strontium clock with frequency 430 THz , in the red range of visible light, during
13750-409: The rotation of the Earth. The international standard for timekeeping is Coordinated Universal Time (UTC). This time scale "ticks" the same atomic seconds as TAI, but inserts or omits leap seconds as necessary to correct for variations in the rate of rotation of the Earth. A time scale in which the seconds are not exactly equal to atomic seconds is UT1, a form of universal time . UT1 is defined by
13875-628: The same 24-hour clock , thus avoiding confusion when flying between time zones. 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
14000-403: The same notation is used to denote hours and minutes). It rarely makes sense to express longer periods of time like hours or days in seconds, because they are awkwardly large numbers. For the metric unit of second, there are decimal prefixes representing 10 to 10 seconds. Some common units of time in seconds are: a minute is 60 seconds; an hour is 3,600 seconds; a day is 86,400 seconds; a week
14125-412: The second as 1 ⁄ 86,400 of a mean solar day. Sometime in the late 1940s, quartz crystal oscillator clocks with an operating frequency of ~100 kHz advanced to keep time with accuracy better than 1 part in 10 over an operating period of a day. It became apparent that a consensus of such clocks kept better time than the rotation of the Earth. Metrologists also knew that Earth's orbit around
14250-416: The second as 1 ⁄ 86,400 of a mean solar day . MKS was adopted internationally during the 1940s. In the late 1940s, quartz crystal oscillator clocks could measure time more accurately than the rotation of the Earth. Metrologists also knew that Earth's orbit around the Sun (a year) was much more stable than Earth's rotation. This led to the definition of ephemeris time and the tropical year , and
14375-424: The signals of different primary clocks in different locations are combined, which have to be corrected for relativistic caesium frequency shifts (see section 2.3.6). The CIPM has adopted various secondary representations of the second, based on a selected number of spectral lines of atoms, ions or molecules. The unperturbed frequencies of these lines can be determined with a relative uncertainty not lower than that of
14500-423: The size of the current SI second referred to atomic time. This Ephemeris Time standard was non-relativistic and did not fulfil growing needs for relativistic coordinate time scales. It was in use for the official almanacs and planetary ephemerides from 1960 to 1983, and was replaced in official almanacs for 1984 and after, by numerically integrated Jet Propulsion Laboratory Development Ephemeris DE200 (based on
14625-622: The sun). It has been superseded by Universal Time . Greenwich Mean Time was originally mean time deduced from meridian observations made at the Royal Greenwich Observatory (RGO). The principal meridian of that observatory was chosen in 1884 by the International Meridian Conference to be the Prime Meridian . GMT either by that name or as 'mean time at Greenwich' used to be an international time standard, but
14750-638: The time from satellite signals. UTC is also the time standard used in aviation , e.g. for flight plans and air traffic control . In this context it is frequently referred to as Zulu time, as described below. Weather forecasts and maps all use UTC to avoid confusion about time zones and daylight saving time. The International Space Station also uses UTC as a time standard. Amateur radio operators often schedule their radio contacts in UTC, because transmissions on some frequencies can be picked up in many time zones. UTC divides time into days, hours, minutes, and seconds . Days are conventionally identified using
14875-489: The timescale should be specified, e.g. MJD 49135.3824 TAI. Barycentric Coordinate Time (TCB) is a coordinate time having its spatial origin at the center of mass of the Solar System , which is called the barycenter. Conversions between atomic time systems (TAI, GPST, and UTC) are for the most part exact. However, GPS time is a measured value as opposed to a computed "paper" scale. As such it may differ from UTC(USNO) by
15000-489: 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
15125-415: Was a time standard used especially at sea for navigational purposes, calculated by observing apparent solar time and then adding to it a correction, the equation of time , which compensated for two known irregularities in the length of the day, caused by the ellipticity of the Earth's orbit and the obliquity of the Earth's equator and polar axis to the ecliptic (which is the plane of the Earth's orbit around
15250-407: 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 1 second only. 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
15375-494: Was from 1952 to 1976 an official time scale standard of the International Astronomical Union ; it was a dynamical time scale based on the orbital motion of the Earth around the Sun, from which the ephemeris second was derived as a defined fraction of the tropical year. This ephemeris second was the standard for the SI second from 1956 to 1967, and it was also the source for calibration of the caesium atomic clock ; its length has been closely duplicated, to within 1 part in 10 , in
15500-407: Was thought better for time signals to maintain a consistent frequency, and that this frequency should match the SI second. 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. There
15625-407: Was usual to observe sidereal time rather than solar time to measure mean solar time, because the observations of 'fixed' stars could be measured and reduced more accurately than observations of the Sun (in spite of the need to make various small compensations, for refraction, aberration, precession, nutation and proper motion). It is well known that observations of the Sun pose substantial obstacles to
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