UT2R - A smoothed version of UT1, incorporating both the seasonal corrections.UT2 - A smoothed version of UT1, filtering out periodic seasonal variations.The Moon and artificial satellites, as well as theįiltering out periodic variations due to tides. Using long baseline interferometry, laser ranging of UT1 - Universal Time computed from observations of distant quasars.UT0 - Universal Time determined by observing theĭiurnal motion of stars or extragalactic radio sources,Īnd also from ranging observations of the Moon and artificial Earth satellites.Universal Time (UT) has several versions: Than 360 degrees to face the Sun again and moved from position 1 to position 3. A solar day is defined when the Earth has rotated a little bit more.Has rotated 360 degrees to face the same star again A sidereal day is defined when the Earth.UT is used as the reference timeĪs shown in figure on the right, the rotation of the Earth can be measuredīased on the Sun or a far away star resulting 2 definitions for the period of day. UT is more accurate than Greenwich Mean Time (GMT). ![]() Universal Time (UT) is a timescale based on the rotation of the Earth. Finally, use this mean noon time as the GMT noon time, 12pm GMT,.Then, calculate the annual mean value of the actual noon time.īecause of Earth's uneven speed, the mean value may be up to 16 minutesĪway from the actual noon time on some day.The sun reaches its highest point in the sky at Greenwich. First, observe each day the actual noon time, which is the moment when.Greenwich Mean Time (GMT) is calculated roughly like this: The meridian at Greenwich is also the Prime Meridian, where longitude Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich,īefore 1 January 1972, GMT was used worldwide as a reference time independent of locations. UTC is the time standard by which the world regulates clocks and time. ![]() UTC (Coordinated Universal Time) is a timescale based onĪ caesium atomic clock that synchronised with UT. UT (Universal Time) is a timescale based on the rotation of the Earth. Observed at the Royal Observatory in Greenwich, London. GMT (Greenwich Mean Time) is a timescale based on the position of the Sun
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