| one of the time scales based on the rotation of the Earth
NOTE 1 – In applications in which an imprecision of a few hundredths of a second cannot be tolerated, it is necessary to specify the forms of universal time which should be used:
– UT0 is the mean solar time of the prime meridian obtained from direct astronomical observation;
– UT1 is UT0 corrected for the effects of small movements of the Earth relative to the axis of rotation in a stellar reference frame (polar variation);
– UT2 is UT1 corrected for the effects of small seasonal fluctuation in the rate of rotation of the Earth in a stellar reference frame.
NOTE 2 – The Mean Solar Time of Greenwich called Greenwich Mean Time (GMT) may be regarded as the equivalent of Universal Time, but with a difference of 12 hours.