Editing Universal Time (section)

{{short description|Time standard based on the slowing rotation of the Earth}}
{{Use dmy dates|date=March 2024}}
{{hatnote group|
{{Redirect|UT1}}
{{distinguish|Coordinated Universal Time}}
}}
{{pp-sock|small=yes}}

'''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}}