Apparent Time(AT), or Solar Time, is time calculated dependent on the motion of the Sun, as on a sundial. A clock reads Apparent Time only if it is located at the proper longtitude within a time zone.

Mean Time(MT), sometimes called Civil Times, is calculated using the Mean Solar day(precesily twenty-four hours).It assumes Earth's orbital speed is constant throughout the year. We set our watches and clocks by MT.

Sidereal Time(ST) is the time calculated relative to fixed stars rather than to the Sun. The Sidereal Day is approximately 23 hours and 56 minutes. It's less than 24 hours because of Earth's motion around the Sun.

Universal Time (UT) is the reference time used for scientific calculations based on atomic clocks that keep time from midnight to midnight. Greenwich Mean Time(GMT) is calculated at 0 longitude, on which rests the Royal Observatory in Greenwich, England. On January 1,1792, UT replaced GMT as the reference time for scientific calculations.