# Universal Time

**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 a modern 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}