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Template:Redirect2 Universal Time (UT) is a time standard 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

Prior to the introduction of standard time, each municipality throughout 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.Template:Sfn

Standard time zones of the world since September 20, 2011, instructions for converting UTC to or from local times are on the bottom, using addition or subtraction

Standard time, as originally proposed by Scottish-Canadian 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 nautical charts and maps already used it as their prime meridian.Template:Sfn 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 basis for universal time (as well as a prime meridian).

During the period between 1848 to 1972, all of the major countries adopted time zones based on the Greenwich meridian.Template:Sfn

In 1935, the term Universal Time was recommended by the International Astronomical Union 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.Template:Sfn The term Greenwich Mean Time persists, however, in common usage to this day in reference to civil timekeeping.

Measurement

Based on the rotation of the Earth, time can be measured 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 can determine UT1 to within 4 milliseconds.Template:SfnTemplate:Sfn

An 1853 "Universal Dial Plate" showing the relative times of "all nations" before the adoption of universal time

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

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). Because Universal Time is synchronous with night and day, and that more precise atomic-frequency standards drift away from this, however, 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 International Atomic Time 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

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. The location of the observatory is considered to have fixed coordinates in a terrestrial reference frame (such as the International Terrestrial Reference Frame) but the position of the rotational axis of the Earth wanders over the surface of the Earth; this is known as polar motion. UT0 does not contain any correction for polar motion. The difference between UT0 and UT1 is on the order of a few tens of milliseconds. The designation UT0 is no longer in common use.Template:Sfn
• 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)Template:Sfn

• 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.Template:Sfn
• 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

$UT2=UT1+0.022\cdot \sin(2\pi t)-0.012\cdot \cos(2\pi t)-0.006\cdot \sin(4\pi t)+0.007\cdot \cos(4\pi t)\;{\mbox{seconds}}$

where t is the time as fraction of the Besselian year.

• 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.Potter or Ceramic Artist Truman Bedell from Rexton, has interests which include ceramics, best property developers in singapore developers in singapore and scrabble. Was especially enthused after visiting Alejandro de Humboldt National Park.
• 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. Template:As of, these leaps have always been positive (the days which contained a leap second were 86,401 seconds long). Whenever a level of 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.Template:Sfn

Adoption in various countries

The table shows the dates of adoption of time zones based on the Greenwich meridian, including half-hour zones.

Year	Countries	Year	Countries	Year	Countries
1848	Great Britain	1906	India, Ceylon, Seychelles	1930	Bermuda
1879	Sweden	1907	Mauritius, Chagos	1931	Paraguay
1883	Canada, USA	1908	Faroe Is., Iceland	1932	Barbados, Bolivia, Dutch East Indies
1884	Serbia	1911	France, Algeria, Tunis, British West Indies	1934	Nicaragua, E. Niger
1888	Japan	1912	Portugal, other French possessions, Samoa, Hawaii, Midway and Guam, Timor, Bismarck Arch., Jamaica, Bahamas Is.	By 1936	Labrador, Norfolk I.
1892	Belgium, Holland, S. Africa	1913	British Honduras, Dahomey	By 1937	Cayman Is., Curaçao, Ecuador, Newfoundland
1893	Italy, Germany, Austria-Hungary (railways)	1914	Albania, Brazil, Colombia	By 1939	Fernando Po, Persia
1894	Bulgaria, Denmark, Norway, Switzerland, Romania, Turkey (railways)	1916	Greece, Ireland, Poland, Turkey	1940	The Netherlands
1895	Australia, New Zealand, Natal	1917	Iraq, Palestine	By 1940	Lord Howe I.
1896	Formosa (Taiwan)	1918	Guatemala, Panama, Gambia, Gold Coast	By 1948	Aden, Ascension I., Bahrein, British Somaliland, Calcutta, Dutch Guiana, Kenya, Federated Malay States, Oman, Straits Settlements, St. Helena, Uganda, Zanzibar
1899	Puerto Rico, Philippines	1919	Latvia, Nigeria	By 1953	Raratonga, South Georgia
1900	Sweden, Egypt, Alaska	1920	Argentine, Uruguay, Burma, Siam	By 1954	Cook Is.
1901	Spain	1921	Finland, Estonia, Costa Rica	By 1959	Maldive I. Republic
1902	Mozambique, Rhodesia	1922	Mexico	By 1961	Friendly Is., Tonga Is.
1903	Ts'intao, Tientsin	1924	Java, USSR	By 1962	Saudi Arabia
1904	China Coast, Korea, Manchuria, N. Borneo	1925	Cuba	By 1964	Niue Is.
1905	Chile	1928	China Inland	1972	Liberia

Apart from Nepal Time Zone (+5h 45m) and Chatham Isle (+12h 45m), all countries were keeping time within an even hour or half-hour of Greenwich.

See also

• List of international common standards
• Unix time

Notes

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References

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• 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.
  
  My blog: http://www.primaboinca.com/view_profile.php?userid=5889534
• 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.
  
  My blog: http://www.primaboinca.com/view_profile.php?userid=5889534
• 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.
  
  My blog: http://www.primaboinca.com/view_profile.php?userid=5889534
• 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.
  
  My blog: http://www.primaboinca.com/view_profile.php?userid=5889534
• Template:Cite web
• Template:Cite web

External links

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