View source for Universal Time

{{short description|Time standard based on the slowing rotation of the Earth}}
{{Use dmy dates|date=March 2024}}
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{{Redirect|UT1}}
{{distinguish|Coordinated Universal Time}}
}}
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'''Universal Time''' ('''UT''' or '''UT1''') is a [[time standard]] based on [[Earth's rotation]].<ref name=Seago>{{cite conference|title=Legislative Specifications for Coordinating with Universal Time|url=https://www.ucolick.org/~sla/leapsecs/seago.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.ucolick.org/~sla/leapsecs/seago.pdf |archive-date=9 October 2022 |url-status=live|first1=John H.|last1=Seago|first2=P. Kenneth|last2=Seidelmann|first3=Steve|last3=Allen|book-title=Decoupling civil timekeeping from Earth rotation: proceedings of a colloquium exploring implications of redefining Coordinated Universal Time (UTC)|date=5–7 October 2011|location=Analytical Graphics, Inc., Exton, Pa.|publisher=American Astronautical Society |isbn=978-0877035763}}</ref> 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]] (ERA, which serves as the replacement for [[Greenwich Mean Sidereal Time]]). UT1 is the same everywhere on Earth. UT1 is required to follow the relationship
:ERA = 2π(0.7790572732640 + 1.00273781191135448''&thinsp;·&thinsp;T<sub>u</sub>'') radians
where ''T<sub>u</sub>'' = ([[Julian day|Julian]] UT1 date − 2451545.0).{{Sfn|McCarthy|Seidelmann|2009|pp= 15–17, 62–64, 68–69, 76}}

==History==
Prior to the introduction of [[standard time]], each municipality throughout the clock-using 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 [[rail travel in Britain]], which made it possible to travel fast enough over sufficiently long distances as to require continuous re-setting of [[clock|timepiece]]s as a [[train]] progressed in its daily run through several towns. Starting in 1847, Britain established [[Greenwich Mean Time]], the [[mean solar time]] at [[Greenwich, England]], to solve this problem: all clocks in Great Britain were set to this time regardless of local solar noon.{{efn|Despite its mandatory use on [[Great Western Railway]] stations from 1847 and thus widespread informal adoption, it was not until the [[Statutes (Definition of Time) Act 1880]] that it became law.<ref name=londontime>{{cite web|url=https://londonist.com/london/history/why-britain-sets-its-clocks-to-london | title = Why Britain Sets Its Clocks To London | author = Harry Rosehill | website = Londonist.com | date = 31 May 2017 | access-date= 25 November 2019}}</ref> The act declared that standard time in Great Britain was to be Greenwich [[Solar time#Mean time|Mean Time]] and standard time in Ireland was to be Dublin time.}} Using telescopes, GMT was calibrated to the [[mean solar time]] at [[prime meridian (Greenwich)|the prime meridian]] through the [[Royal Observatory, Greenwich]]. [[Marine chronometer|Chronometer]]s or [[telegraphy]] were used to synchronize these clocks.{{sfn|Howse|1997|loc=ch. 4}}

[[File:World_Time_Zones_Map.png|thumb|upright 2.5|Standard time zones of the world. The number at the bottom of each zone specifies the number of hours to add to UTC to convert it to the local time.]]
As international commerce increased, the need for an international standard of time measurement emerged. Several authors proposed a "universal" or "cosmic" time (see {{section link|Time zone#Worldwide time zones}}). The development of Universal Time began at the [[International Meridian Conference]]. At the end of this conference, on 22 October 1884,{{efn|Voting took place on 13 October.}} 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.{{sfn|Howse|1997|pp=12, 137}} 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 day ''X'' began at noon of civil day ''X''. 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 the [[Royal Navy]], but persisted much later elsewhere because it was mentioned at the 1884 conference. Greenwich was chosen because by 1884 two-thirds of all [[nautical chart]]s and [[map]]s already used it as their [[prime meridian]].{{sfn|Howse|1997|p=[https://archive.org/details/greenwichtimelon0000hows/page/133 133–137]}}

During the period between 1848 and 1972, all of the major countries adopted time zones based on the Greenwich meridian.{{sfn|Howse|1997|loc=ch. 6}}

In 1928, the term ''Universal Time'' (''UT'') was introduced by the International Astronomical Union to refer to GMT, with the day starting at midnight.{{sfn|McCarthy|Seidelmann|2009|pp=10–11}} The term was recommended 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.{{sfn|McCarthy|Seidelmann|2009|p=14}} 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|date=February 2024}}

When introduced, broadcast [[time signal]]s were based on UT, and hence on the rotation of the Earth. In 1955 the [[International Time Bureau|BIH]] 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.<ref>{{cite web |last1=Dick |first1=Steven |last2=McCarthy |first2=Dennis | author-link2=Dennis McCarthy (scientist) |title=William Markowitz Obituary |url=https://ad.usno.navy.mil/wds/history/markowitz.html |website=U.S. Naval Observatory |date=15 December 2018|archive-url=https://web.archive.org/web/20181215224810/https://ad.usno.navy.mil/wds/history/markowitz.html |archive-date=15 December 2018 }}</ref><ref>{{cite web |title=On the Determination of Universal Time by the Time Services according to the Decisions of the General Assembly of the I.A.U. in Dublin |url=https://www.ucolick.org/~sla/leapsecs/BHs4n4EN.pdf |publisher=Bulletin Horaire |date=July–August 1955|archive-url=https://web.archive.org/web/20211215230325/https://www.ucolick.org/~sla/leapsecs/BHs4n4EN.pdf |archive-date=15 December 2021 |url-status=live}}</ref>

UT0 and UT2 soon became irrelevant due to the introduction of [[Coordinated Universal Time]] (UTC). Starting in 1956, [[WWV (radio station)|WWV]] broadcast an atomic clock signal stepped by 20 ms increments to bring it into agreement with UT1.{{sfn|Arias|Guinot|Quinn|2003}} 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.<ref name="sla">{{cite web |title=Seasonal Variation of Earth Rotation |url=https://www.ucolick.org/~sla/leapsecs/seasonal.html|first=Steve|last=Allen|website=ucolick.org}}</ref> Starting 1 January 1972, UTC was defined to follow UT1 within 0.9 seconds rather than UT2, marking the decline of UT2.{{sfn|McCarthy|Seidelmann|2009|loc=Ch. 14}}

Modern [[civil time]] generally follows UTC. In some countries, the term ''Greenwich Mean Time'' persists in common usage to this day in reference to UT1, in [[civil time]]keeping as well as in astronomical almanacs and other references. 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]].{{sfn|McCarthy|Seidelmann|2009|loc=Ch. 14}}

===Adoption in various countries===
The table shows the dates of adoption of time zones based on the Greenwich meridian, including half-hour zones.
{{Col-begin}}
{{Col-break}}
{|
! width=60 align=left | Year 
!  align=left | Countries<ref>{{harvnb|Howse|1980|pp=154–5}}. Names have not been updated.</ref>
|-
| 1847 || Great Britain<ref name=londontime/>
|-
| 1880 || Ireland (entire island)
|-
| 1883 || Canada, United States{{efn|legal in 1918 ([[Standard Time Act]])}}
|-
| 1884 || Serbia 
|-
| 1886 || New Zealand<ref>{{Cite news |last=Stafford |first=E. W. |date=30 October 1868 |title=The New Zealand Gazette |url=https://library.victoria.ac.nz/databases/nzgazettearchive/pubs/gazettes/1868/1868%20ISSUE%20061.pdf }}</ref>
|-
| 1888 || Japan 
|-
| 1892 || Belgium, the Netherlands,{{efn|Legal time reverted to Amsterdam time 1909; to Central European Time 1940.}} S. Africa{{efn|except Natal}} 
|-
| 1893 || Italy, Germany, Austria-Hungary (railways) 
|-
| 1894 || Bulgaria, Denmark, Norway, Switzerland, Romania, Turkey (railways) 
|-
| 1895 || Australia, Natal 
|-
| 1896 || Formosa (Taiwan)
|-
| 1899 || Puerto Rico, Philippines 
|-
| 1900 || Sweden, Egypt, Alaska 
|-
| 1901 || Spain 
|-
| 1902 || Mozambique, Rhodesia 
|-
| 1903 || Ts'intao, Tientsin 
|-
| 1904 || China Coast, Korea, Manchuria, N. Borneo 
|-
| 1905 || Chile 
|-
| 1906 || India (except Calcutta), Ceylon (Sri Lanka), Seychelles 
|-
| 1907 || Mauritius, Chagos 
|-
| 1908 || Faroe Is., Iceland 
|-
| 1911 || France, Algeria, Tunis, many French overseas possessions, British West Indies 
|-
| 1912 || Portugal and overseas possessions, other French possessions, Samoa, Hawaii, Midway and Guam, Timor, Bismarck Arch., Jamaica, Bahamas Is. 
|-
| 1913 || British Honduras, Dahomey 
|-
| 1914 || Albania, Brazil, Colombia 
|-
| 1916 || Greece, Poland, Turkey 
|}
{{Col-break}}
{|
! width=60 align=left | Year 
!  align=left | Countries
|-
| 1917 || Iraq, Palestine 
|-
| 1918 || Guatemala, Panama, Gambia, Gold Coast 
|-
| 1919 || Latvia, Nigeria 
|-
| 1920 || Argentina, Uruguay, Burma, Siam 
|-
| 1921 || Finland, Estonia, Costa Rica 
|-
| 1922 || Mexico 
|-
| 1924 || Java, USSR 
|-
| 1925 || Cuba 
|-
| 1928 || China Inland 
|-
| 1930 || Bermuda 
|-
| 1931 || Paraguay 
|-
| 1932 || Barbados, Bolivia, Dutch East Indies 
|-
| 1934 || Nicaragua, E. Niger 
|-
| By 1936 || Labrador, Norfolk I. 
|-
| By 1937 || Cayman Is., Curaçao, Ecuador, Newfoundland 
|-
| By 1939 || Fernando Po, Persia 
|-
| By 1940 || Lord Howe I. 
|-
| 1940 || The Netherlands 
|-
| By 1948 || Aden, Ascension I., Bahrain, British Somaliland, Calcutta, Dutch Guiana, Kenya, Federated Malay States, Oman, Straits Settlements, St. Helena, Uganda, Zanzibar 
|-
| By 1953 || Rarotonga, South Georgia 
|-
| By 1954 || Cook Is. 
|-
| By 1959 || Maldive I. Republic 
|-
| By 1961 || Friendly Is., Tonga Is. 
|-
| By 1962 || Saudi Arabia 
|-
| By 1964 || Niue Is. 
|-
| 1972 || Liberia 
|}
{{col-end}}

Apart from [[Nepal Standard Time]] (UTC+05:45), the [[Chatham Standard Time Zone]] (UTC+12:45) used in New Zealand's [[Chatham Islands]]{{sfn|HM Nautical Almanac Office|2015}} and the officially unsanctioned [[Eucla, Western Australia#Time zone|Central Western Time Zone]] (UTC+8:45) used in [[Eucla, 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|date=February 2024}}

==Measurement==
{{see also|Earth's rotation#Measurement}}

Historically, Universal Time was computed from observing the position of the [[Sun]] 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 to [[International Atomic Time]] (TAI) is determined by [[Very Long Baseline Interferometry]] (VLBI) observations of the positions of distant celestial objects ([[star]]s and [[quasar]]s), a method which can determine UT1 to within 15 microseconds or better.{{sfn|McCarthy|Seidelmann|2009|pages=68–9}}{{sfn|Urban|Seidelmann|2013|page=175}}

[[File:Universal Dial Plate or Times of all Nations, 1854.jpg|thumb|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.{{sfn|McCarthy|Seidelmann|2009|loc=Ch. 18}}

The rotation of the Earth is somewhat irregular and also 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 modern [[mean solar day]], on the average, to be slightly longer than the nominal 86,400 [[SI]] seconds, the traditional number of seconds per day.{{efn|24 hours of 60 minutes of 60 seconds}} As UT is thus slightly irregular in its rate, astronomers introduced [[Ephemeris Time]], which has since been replaced by [[Terrestrial Time]] (TT). Because Universal Time is determined by the Earth's rotation, which drifts away from more precise atomic-frequency standards, an adjustment (called a [[leap second]]) to this atomic time is needed since ({{as of|2019|lc=on}}) 'broadcast time' remains broadly synchronised with solar time.{{efn|Continuation of this principle is under active debate in standards bodies. See [[Leap second#Future of leap seconds]]}} 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|date=February 2024}}

[[Barycentric Dynamical Time]] (TDB), a form of atomic time, is now used in the construction of the ephemerides of the [[planet]]s and other solar system objects, for two main reasons.<ref>{{harvnb|Urban|Seidelmann|2013|page=7}}. Strictly speaking, a major producer of ephemerides, the [[Jet Propulsion Laboratory]], uses a time scale they derive, T<sub>eph</sub>, which is functionally equivalent to TDB.</ref> 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.{{citation needed|date=February 2024}}

==Alternate versions<span class="anchor" id="Versions"></span>==

'''UT1''' is the principal form of Universal Time.<ref name=Seago/> However, there are also several other infrequently used time standards that are referred to as ''Universal Time'', which agree within 0.03 seconds with UT1:<ref>{{cite web |last=Schlyter |first=Paul |title=Time Scales: UT1, UTC, TAI, ET, TT, GPS time |url=https://stjarnhimlen.se/comp/time.html |website=stjarnhimlen.se |access-date=24 May 2022}}</ref>

* '''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 while UT1 does include them. 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.{{sfn|Urban|Seidelmann|2013|page=81}}
* '''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.{{sfn|IERS|n.d.}} UT1R is still in use in the technical literature but rarely used elsewhere.<ref>{{cite web |title=A Brief Note on Time Systems |url=https://www.astronomy.ohio-state.edu/pogge.1/Ast350/timesys.html |website=Department of Astronomy, [[Ohio State University]] |access-date=24 May 2022|quote=There are two other forms of Universal Time you are unlikely to encounter unless you dig into some very technical literature. UT1R [...]}}</ref>
* '''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
::<math>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}</math>
::where ''t'' is the time as fraction of the [[Year#Besselian year|Besselian year]].<ref>{{cite web|url=https://maia.usno.navy.mil/information/eo-values|title=Common Units and Conversions in Earth Orientation|website=IERS Rapid Service / Prediction Center, U.S. Naval Observatory}}</ref>

==See also==
{{portal|border=no|World}}
* [[Airy mean time]] on Mars
* [[Earth orientation parameters]]
* [[List of international common standards]]
* [[Unix time]]

==Notes==
{{notelist}}

==Citations==
{{reflist}}

==References==
*{{cite web|url=https://maia.usno.navy.mil/information/eo-values|title=Common Units and Conversions in Earth Orientation|website=IERS Rapid Service / Prediction Center, U.S. Naval Observatory|access-date=16 June 2022}}
* {{Cite conference | conference = ITU-R Special Rapporteur Group Colloquium on the UTC Time Scale | date = 29 May 2003 | url = http://www.ucolick.org/~sla/leapsecs/torino/arias_3.pdf | title = Rotation of the Earth and Time scales | last1 = Arias | first1 = E. F. | last2 = Guinot | first2 = B. | last3 = Quinn | first3 = T. J. }}
*{{Anchor|ERC}}{{cite web|title= Earth Rotation Variations Due to Zonal Tides | publisher = Earth Orientation Center | location = Paris | access-date = 2 October 2011 | url = http://hpiers.obspm.fr/eop-pc/models/UT1/UT1R_tab.html }}
*{{cite book|last=Galison|first=Peter|author-link=Peter Galison|title=Einstein's clocks, Poincaré's maps: Empires of time|location=New York|publisher=W.W. Norton & Co.|date=2003|isbn=0-393-02001-0|url=https://archive.org/details/einsteinsclocksp00gali}} Discusses the history of time standardization.
*{{cite journal|last=Guinot| first = Bernard | date = July 2011 | title = Solar time, legal time, time in use | journal = Metrologia | volume = 48 | doi = 10.1088/0026-1394/48/4/S08 | issue = 4 | pages = S181–S185 | bibcode = 2011Metro..48S.181G | s2cid = 121852011 }}
*{{cite web | author = HM Nautical Almanac Office | title = World Time Zone Map | url = http://astro.ukho.gov.uk/nao/miscellanea/WMTZ/ | date = April 2015 | access-date = 11 April 2015 | archive-date = 3 May 2014 | archive-url = https://web.archive.org/web/20140503200102/http://astro.ukho.gov.uk/nao/miscellanea/WMTZ/ | url-status = dead }}
*{{cite book|title=Greenwich Time and the discovery of the longitude|first=Derek|last=Howse|date=1980|pages=154–5|publisher=Oxford Univ Press }}. Names have not been updated.
*{{cite book | last = Howse | first = Derek | title = Greenwich Time and the Longitude | date = 1997 | publisher = Phillip Wilson | isbn = 0-85667-468-0 | url-access = registration | url = https://archive.org/details/greenwichtimelon0000hows }}
*{{cite journal|first=Dennis D.|last=McCarthy| author-link1=Dennis McCarthy (scientist)|url=http://www.cl.cam.ac.uk/~mgk25/volatile/astronomical-time.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.cl.cam.ac.uk/~mgk25/volatile/astronomical-time.pdf |archive-date=9 October 2022 |url-status=live|title=Astronomical Time|journal=Proceedings of the IEEE |volume=79 |issue=7|date=July 1991 |pages=915–920|doi=10.1109/5.84967 }}
*{{cite book | last1 = McCarthy | first1=Dennis | last2= Seidelmann| first2= P. Kenneth| date= 2009| title=TIME—From Earth Rotation to Atomic Physics| place= Weinheim | publisher=Wiley-VCH Verlag GmbH & Co. KGaA. | isbn=978-3-527-40780-4 }}
*{{cite book|last=O'Malley|first=Michael|title=Keeping watch: A history of American time|location=Washington DC|publisher=Smithsonian|date=1996|isbn=1-56098-672-7|url=https://archive.org/details/keepingwatchhist00mich}}
*{{cite book|last=Seidelmann|first=P. Kenneth|title=Explanatory supplement to the Astronomical Almanac|location=Mill Valley, California|publisher=University Science Books|date=1992|isbn=0-935702-68-7 }}
*{{cite book| editor1-last = Urban |editor1-first = Sean | editor2-last = Seidelmann | editor2-first = P. Kenneth | date = 2013 | title = Explanatory Supplement to the Astronomical Almanac | edition = 3rd | location = Mill Valley, California | publisher = University Science Books}}
*{{cite web|title=UT1R| url= http://hpiers.obspm.fr/eop-pc/models/UT1/UT1R_tab.html | publisher = International Earth Rotation and Reference System Service | access-date = 6 March 2013 |ref={{harvid|IERS|n.d.}}}}
*{{cite web|title=Terrestrial Time (TT)|url=https://aa.usno.navy.mil/faq/TT|work=Astronomical Applications Department|publisher=[[United States Naval Observatory]] | access-date= 16 June 2022}}
{{FS1037C}}

==External links==
*[http://www.penguinrandomhouse.com/books/14490/time-lord-by-clark-blaise/ ''Time Lord''] by Clark Blaise: a biography of Sanford Fleming and the idea of standard time

{{Time Topics}}
{{Time measurement and standards}}

[[Category:Time scales]]
[[Category:Time in astronomy]]