Universal Time (UT orUT1) is atime standard based onEarth's rotation.[1] While originally it wasmean 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 theInternational Celestial Reference Frame (ICRF), called theEarth Rotation Angle (ERA, which serves as the replacement forGreenwich Mean Sidereal Time). UT1 is the same everywhere on Earth. UT1 is required to follow the relationship
whereTu = (Julian UT1 date − 2451545.0).[2]
Prior to the introduction ofstandard time, each municipality throughout the clock-using world set its official clock, if it had one, according to the local position of the Sun (seesolar time). This served adequately until the introduction ofrail travel in Britain, which made it possible to travel fast enough over sufficiently long distances as to require continuous re-setting oftimepieces as atrain progressed in its daily run through several towns. Starting in 1847, Britain establishedGreenwich Mean Time, themean solar time atGreenwich, England, to solve this problem: all clocks in Great Britain were set to this time regardless of local solar noon.[a] Using telescopes, GMT was calibrated to themean solar time atthe prime meridian through theRoyal Observatory, Greenwich.Chronometers ortelegraphy were used to synchronize these clocks.[4]
As international commerce increased, the need for an international standard of time measurement emerged. Several authors proposed a "universal" or "cosmic" time (seeTime zone § Worldwide time zones). The development of Universal Time began at theInternational Meridian Conference. At the end of this conference, on 22 October 1884,[b] the recommended base reference for world time, the "universal day", was announced to be the local mean solar time at the Royal Observatory in Greenwich, counted from 0 hours at Greenwich mean midnight.[5] This agreed with the civil Greenwich Mean Time used on the island of Great Britain since 1847. In contrast, astronomical GMT began at mean noon, i.e. astronomical dayX began at noon of civil dayX. The purpose of this was to keep one night's observations under one date. The civil system was adopted as of 0 hours (civil) 1 January 1925. Nautical GMT began 24 hours before astronomical GMT, at least until 1805 in theRoyal Navy, but persisted much later elsewhere because it was mentioned at the 1884 conference. Greenwich was chosen because by 1884 two-thirds of allnautical charts andmaps already used it as theirprime meridian.[6]
During the period between 1848 and 1972, all of the major countries adopted time zones based on the Greenwich meridian.[7]
In 1928, the termUniversal Time (UT) was introduced by the International Astronomical Union to refer to GMT, with the day starting at midnight.[8] The term was recommended as a more precise term thanGreenwich Mean Time, becauseGMT could refer to either anastronomical day starting at noon or a civil day starting at midnight.[9] As the general public had always begun the day at midnight, the timescale continued to be presented to them as Greenwich Mean Time.[citation needed]
When introduced, broadcasttime signals were based on UT, and hence on the rotation of the Earth. In 1955 theBIH adopted a proposal by William Markowitz, effective 1 January 1956, dividing UT into UT0 (UT as formerly computed), UT1 (UT0 corrected for polar motion) and UT2 (UT0 corrected for polar motion and seasonal variation). UT1 was the version sufficient for "many astronomical and geodetic applications", while UT2 was to be broadcast over radio to the public.[10][11]
UT0 and UT2 soon became irrelevant due to the introduction ofCoordinated Universal Time (UTC). Starting in 1956,WWV broadcast an atomic clock signal stepped by 20 ms increments to bring it into agreement with UT1.[12] The up to 20 ms error from UT1 is on the same order of magnitude as the differences between UT0, UT1, and UT2. By 1960, the U.S. Naval Observatory, the Royal Greenwich Observatory, and the UK National Physical Laboratory had developed UTC, with a similar stepping approach. The 1960 URSI meeting recommended that all time services should follow the lead of the UK and US and broadcast coordinated time using a frequency offset from cesium aimed to match the predicted progression of UT2 with occasional steps as needed.[13] Starting 1 January 1972, UTC was defined to follow UT1 within 0.9 seconds rather than UT2, marking the decline of UT2.[14]
Moderncivil time generally follows UTC. In some countries, the termGreenwich Mean Time persists in common usage to this day in reference to UT1, incivil timekeeping as well as in astronomical almanacs and other references. Whenever a level ofaccuracy better than one second is not required, UTC can be used as an approximation of UT1. The difference between UT1 and UTC is known asDUT1.[14]
The table shows the dates of adoption of time zones based on the Greenwich meridian, including half-hour zones.
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Apart fromNepal Standard Time (UTC+05:45), theChatham Standard Time Zone (UTC+12:45) used in New Zealand'sChatham Islands[17] and the officially unsanctionedCentral Western Time Zone (UTC+8:45) used inEucla, Western Australia and surrounding areas, all time zones in use are defined by an offset from UTC that is a multiple of half an hour, and in most cases a multiple of an hour.[citation needed]
Historically, Universal Time was computed from observing the position of theSun in the sky. But astronomers found that it was more accurate to measure the rotation of the Earth by observing stars as they crossed the meridian each day. Nowadays, UT in relation toInternational Atomic Time (TAI) is determined byVery Long Baseline Interferometry (VLBI) observations of the positions of distant celestial objects (stars andquasars), a method which can determine UT1 to within 15 microseconds or better.[18][19]
The rotation of the Earth and UT are monitored by theInternational Earth Rotation and Reference Systems Service (IERS). TheInternational Astronomical Union also is involved in setting standards, but the final arbiter of broadcast standards is theInternational Telecommunication Union or ITU.[20]
The rotation of the Earth is somewhat irregular and also is very gradually slowing due totidal acceleration. Furthermore, the length of the second was determined from observations of theMoon between 1750 and 1890. All of these factors cause the modernmean solar day, on the average, to be slightly longer than the nominal 86,400SI seconds, the traditional number of seconds per day.[f] As UT is thus slightly irregular in its rate, astronomers introducedEphemeris Time, which has since been replaced byTerrestrial Time (TT). Because Universal Time is determined by the Earth's rotation, which drifts away from more precise atomic-frequency standards, an adjustment (called aleap second) to this atomic time is needed since (as of 2019[update]) 'broadcast time' remains broadly synchronised with solar time.[g] Thus, the civil broadcast standard for time and frequency usually follows International Atomic Time closely, but occasionally step (or "leap") in order to prevent them from drifting too far from mean solar time.[citation needed]
Barycentric Dynamical Time (TDB), a form of atomic time, is now used in the construction of the ephemerides of theplanets and otherSolar System objects, for two main reasons.[21] First, these ephemerides are tied to optical andradar observations of planetary motion, and the TDB time scale is fitted so thatNewton's laws of motion, with corrections forgeneral relativity, are followed. Next, the time scales based on Earth's rotation are not uniform and therefore, are not suitable for predicting the motion of bodies in the Solar System.[citation needed]
UT1 is the principal form of Universal Time.[1] However, there are also several other infrequently used time standards that are referred to asUniversal Time, which agree within 0.03 seconds with UT1:[22]
There are two other forms of Universal Time you are unlikely to encounter unless you dig into some very technical literature. UT1R [...]
This article incorporatespublic domain material fromFederal Standard 1037C.General Services Administration. Archived fromthe original on 22 January 2022.
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