Universal Time: Difference between revisions

{{short description|Time standard based on the slowing rotation of the Earth}}

{{Redirect|UT1}}

{{distinguish|Coordinated Universal Time}}

{{pp-sock|small=yes}}

:ERA = 2π(0.7790572732640 + 1.00273781191135448'' · T_u'') radians

 

==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 long distances 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]] on the [[Prime meridian (Greenwich)|Prime Meridian]] at [[Greenwich, England]], to solve this problem: all clocks in 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 [[Time in the United Kingdom#History|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>}} Using telescopes, GMT was calibrated to the [[mean solar time]] at the [[Royal Observatory, Greenwich]] in the UK. [[Marine chronometer|Chronometer]]s or [[telegraphy]] were used to synchronize these clocks.{{sfn|Howse|1997|loc=ch. 4}}

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, Greenwich|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]}}

 

 

 

 

 

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}}

{|

! width=60 align=left | Year

|-

|-

| 1880 || Ireland (entire island)

|-

|-

| 1884 || Serbia

|-

| 1888 || Japan

|-

|-

| 1893 || Italy, Germany, Austria-Hungary (railways)

| 1894 || Bulgaria, Denmark, Norway, Switzerland, Romania, Turkey (railways)

|-

|-

| 1896 || Formosa (Taiwan)

| By 1948 || Aden, Ascension I., Bahrain, British Somaliland, Calcutta, Dutch Guiana, Kenya, Federated Malay States, Oman, Straits Settlements, St. Helena, Uganda, Zanzibar

|-

|-

| By 1954 || Cook Is.

{{col-end}}

 

 

==Measurement==

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

 

 

[[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}}

 

 

 

 

 

* '''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>

* {{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 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 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 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 }}