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Universal Time (UT or UT1) is a time standard based on Earth's rotation.[1] 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 current 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 · Tu) radians
where Tu = (Julian UT1 date − 2451545.0).[2]
- ^ Seago, John H.; Seidelmann, P. Kenneth; Allen, Steve (5–7 October 2011). "Legislative Specifications for Coordinating with Universal Time" (PDF). Decoupling civil timekeeping from Earth rotation: proceedings of a colloquium exploring implications of redefining Coordinated Universal Time (UTC). Analytical Graphics, Inc., Exton, Pa.: American Astronautical Society. ISBN 978-0877035763. Archived (PDF) from the original on 9 October 2022.
- ^ McCarthy & Seidelmann 2009, pp. 15–17, 62–64, 68–69, 76.