Universal Time (UT) and Greenwich Mean Time (GMT)

Since the earth's orbit around the sun is not a perfect circle and its speed changes with its distance from the sun, apparent solar time cannot be a uniform time scale.   The length of the solar day is also affected by the inclination of the earth's spin axis to the plane of the earth's orbit.  To correct for the non-uniformities, astronomers calculated the effects of the earth's non-circular orbit and the polar inclination on apparent solar time.  Mean solar time is apparent solar time corrected for these two effects.  Universal Time (UT0) is the mean solar time at the Greenwich Meridian and is called Greenwich Mean Time (GMT).

There are two refinements of UT0 called UT1 and UT2.  As better clocks were developed, astronomers began to notice a discrepancy in UT0 measured at different locations.  This was later found caused by a wobble in the axis of the earth.  After careful measurements at various observatories throughout the world, the effect of the wobble was eliminated by the introduction of a new time designation UT1.

With further improvements in the precision of clocks, it was discovered that UT1 had periodic variations caused by the seasonal fluctuation in the rate of rotation of the earth.  Corrections were made to remove this effect and an even more uniform time UT2 was obtained.