Glossary term: Universal Time (UT)

Description: Universal Time, abbreviated UT, is an umbrella term for several ways of defining the measurement of time. UT1 is the mean solar time at longitude zero (the historical location of the Royal Greenwich Observatory). Mean solar time defines the length of a day as the average duration between one noon (highest position of the Sun in the sky) and the next. In practice, the necessary measurements are made not with the Sun, but with distant astronomical objects such as quasars at night. UTC, Universal Time Coordinated, is a time standard based on the timekeeping of a specific large set of atomic clocks ("international atomic time" or TAI), but with occasional extra seconds ("leap seconds") added to some days to ensure UTC and UT1 never diverge by more than 0.9 seconds.

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