# Universal Time

## Time standard based on the slowing rotation of the Earth / From Wikipedia, the free encyclopedia

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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 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
*· T*) radians_{u}

"UT1" redirects here. For other uses, see UT1 (disambiguation). Not to be confused with Coordinated Universal Time.

where *T _{u}* = (Julian UT1 date − 2451545.0).

^{[2]}