Universal Time (UT) is the mean solar time referenced to the prime meridian
. It counts from 0 o'clock midnight and refers to an average solar day and does not take into account the rotational changes during the Earth's revolutions. Two times are defined with UT0 and UT1. UT0 refers as rotation time to a special observation point. It is derived from the everyday motion of stars or extraterrestrial radiation sources. UT1 is UT0 corrected by computer calculations. it varies with the irregularities due to the Earth's rotation.
Although Universal Time replaced Greenwich Mean Time (GTM) as early as 1928, the term Greenwich Mean Time is still commonly used today. Universal Time is the reference time for Universal Time Coordinated (UTC), the coordinated world time.