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Universal Time (UT) is a time scale based on the rotation of the Earth. It is a modern continuation of Greenwich Mean Time (GMT), i.e., the mean solar time on the Prime Meridian at Greenwich, and GMT is sometimes used loosely as a synonym for UTC. In fact, the expression "Universal Time" is ambiguous, as there are several versions of it, the most commonly used being UTC and UT1 (see below). All of these versions of UT are based on the rotation of the Earth in relation to distant celestial objects (stars and quasars), but with a scaling factor and other adjustments to make them closer to solar time.

Universal Time and standard time [link]

Prior to the introduction of standard time, each municipality around the civilized world set its official clock, if it had one, according to the local position of the Sun (see solar time). This served adequately until the introduction of the steam engine, the telegraph, and rail travel, which made it possible to travel fast enough over long distances to require almost constant re-setting of timepieces as a train progressed in its daily run through several towns. Standard time, where all clocks in a large region are set to the same time, was established to solve this problem. Chronometers or telegraphy were used to synchronize these clocks.

Standard time zones of the world since September 20, 2011. Instructions for converting UTC to or from local times are on the bottom.

Standard time, as originally proposed by Sir Sandford Fleming in 1879, divided the world into twenty-four time zones, each one covering 15 degrees of longitude. All clocks within each zone would be set to the same time as the others, but differed by one hour from those in the neighboring zones. The local time at the Royal Greenwich Observatory in Greenwich, England was chosen as standard at the 1884 International Meridian Conference, leading to the widespread use of Greenwich Mean Time to set local clocks. This location was chosen because by 1884 two-thirds of all charts and maps already used it as their prime meridian. The conference did not adopt Fleming's time zones because they were outside the purpose for which it was called, which was to choose a prime meridian. Nevertheless, by 1929 all major countries had adopted standard time zones. Political considerations^{[clarification needed]} have now increased the number of standard time zones to 40.

In 1928 the term Universal Time was adopted internationally as a more precise term than Greenwich Mean Time, because GMT could refer to either an astronomical day starting at noon or a civil day starting at midnight. However, the term Greenwich Mean Time persists in common usage to this day in reference to civil timekeeping.

Measurement [link]

Time can be measured based on the rotation of the Earth by observing celestial bodies crossing the meridian every day. Astronomers found that it was more accurate to establish time by observing stars as they crossed a meridian rather than by observing the position of the Sun in the sky. Nowadays, UT in relation to International Atomic Time (TAI) is determined by Very Long Baseline Interferometry (VLBI) observations of distant quasars, a method which has an accuracy of microseconds. Most sources of time and celestial coordinate system standards use UT1 as the default meaning of UT, though occasionally UTC may be implied.

The rotation of the Earth and UT are monitored by the International Earth Rotation and Reference Systems Service (IERS). The International Astronomical Union is also involved in setting standards, but the final arbiter of broadcast standards is the International Telecommunication Union or ITU.

The rotation of the Earth is somewhat irregular, and is very gradually slowing due to tidal acceleration. Furthermore, the length of the second was determined from observations of the Moon between 1750 and 1890. All of these factors cause the mean solar day, on the average, to be slightly longer than the nominal 86,400 SI seconds, the traditional number of seconds per day. As UT is slightly irregular in its rate, astronomers introduced Ephemeris Time, which has since been replaced by Terrestrial Time (TT). However, because Universal Time is synchronous with night and day, and more precise atomic-frequency standards drift away from this, UT is still used to produce a correction (called a leap second) to atomic time, in order to obtain a broadcast form of civil time that carries atomic frequency. Thus, civil broadcast standards for time and frequency usually follow TAI closely, but occasionally change discontinuously (or "leap") in order to prevent them from drifting too far from mean solar time.

Barycentric Dynamical Time (TDB), a form of atomic time, is now used in the construction of the ephemerides of the planets and other solar system objects, for two main reasons. First, these ephemerides are tied to optical and radar observations of planetary motion, and the TDB time scale is fitted so that Newton's laws of motion, with corrections for general 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 our solar system.

Versions [link]

There are several versions of Universal Time:

• UT0 is Universal Time determined at an observatory by observing the diurnal motion of stars or extragalactic radio sources, and also from ranging observations of the Moon and artificial Earth satellites. It is uncorrected for the displacement of Earth's geographic pole from its rotational pole. This displacement, called polar motion, causes the geographic position of any place on Earth to vary by several metres, and different observatories will find a different value for UT0 at the same moment. It is thus not, strictly speaking, Universal.
• UT1 is the principal form of Universal Time. While conceptually it is mean solar time at 0° longitude, precise measurements of the Sun are difficult. Hence, it is computed from observations of distant quasars using long baseline interferometry, laser ranging of the Moon and artificial satellites, as well as the determination of GPS satellite orbits. UT1 is the same everywhere on Earth, and is proportional to the rotation angle of the Earth with respect to distant quasars, specifically, the International Celestial Reference Frame (ICRF), neglecting some small adjustments. The observations allow the determination of a measure of the Earth's angle with respect to the ICRF, called the Earth Rotation Angle (ERA, which serves as a modern replacement for Greenwich Mean Sidereal Time). UT1 is required to follow the relationship

ERA = 2π(0.7790572732640 + 1.00273781191135448T_u) radians

where T_u = (Julian UT1 date - 2451545.0)^[1]

• UT1R is a smoothed version of UT1, filtering out periodic variations due to tides. It includes 62 smoothing terms, with periods ranging from 5.6 days to 18.6 years.^[2]
• UT2 is a smoothed version of UT1, filtering out periodic seasonal variations. It is mostly of historic interest and rarely used anymore. It is defined by the equation:

Failed to parse (Missing texvc executable; please see math/README to configure.): UT2 = UT1 + 0.022\cdot\sin(2\pi t) - 0.012\cdot\cos(2\pi t) - 0.006\cdot\sin(4\pi t) + 0.007\cdot\cos(4\pi t)\;\mbox{seconds}

where t is the time as fraction of the Besselian year.[3]

• UT2R is a smoothed version of UT1, incorporating both the seasonal corrections of UT2 and the tidal corrections of UT1R. It is the most smoothed form of Universal Time. Its non-uniformities reveal the unpredictable components of Earth rotation, due to atmospheric weather, plate tectonics, and currents in the interior of the Earth.
• UTC (Coordinated Universal Time) is an atomic timescale that approximates UT1. It is the international standard on which civil time is based. It ticks SI seconds, in step with TAI. It usually has 86,400 SI seconds per day, but is kept within 0.9 seconds of UT1 by the introduction of occasional intercalary leap seconds. As of 2012^[update] these leaps have always been positive, with a day of 86401 seconds. When an accuracy better than one second is not required, UTC can be used as an approximation of UT1. The difference between UT1 and UTC is known as DUT1.

See also [link]

• List of international common standards
• Unix time

Notes [link]

References [link]

• "Date and Time Definitions". United States Naval Observatory. https://www.usno.navy.mil/USNO/earth-orientation/eo-info/general/date-time-def. 
• "Earth Rotation Variations Due to Zonal Tides". Paris: Earth Orientation Center. https://hpiers.obspm.fr/eop-pc/models/UT1/UT1R_tab.html. Retrieved 2 October 2011. 
• Galison, Peter (2003). Einstein's clocks, Poincaré's maps: Empires of time. New York: W.W. Norton & Co. ISBN 0-393-02001-0.  Discusses the history of time standardization.
• McCarthy, Dennis D. (July 1991). "Astronomical Time". Proceedings of the IEEE 79 (7): 915–920. DOI:10.1109/5.84967. https://www.cl.cam.ac.uk/~mgk25/volatile/astronomical-time.pdf. 
• McCarthy, D. D. & Seidelmann, P. K. (2009). TIME—From Earth Rotation to Atomic Physics. Weinheim:Wiley-VCH Verlag GmbH & Co. KGaA. ISBN 978-3-527-40780-4.
• O'Malley, Michael (1996). Keeping watch: A history of American time. Washington DC: Smithsonian. ISBN 1-56098-672-7. 
• Seidelmann, P. Kenneth (1992). Explanatory supplement to the Astronomical Almanac. Mill Valley, California: University Science Books. ISBN 0-935702-68-7. 
• "What is TT?". Naval Oceanography Portal. United States Naval Observatory. https://www.usno.navy.mil/USNO/astronomical-applications/astronomical-information-center/what-is-tt/?searchterm=terrestrial_time. 

 This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C".

External links [link]

• Time Lord by Clark Blaise: a biography of Stanford Fleming and the idea of standard time

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