ReplacedĮT (Ephemeris Time) in 1984, was replaced by TT (Terrestial Time) Was used 1984-2000 as a time-scale ofĮphemerides from the Earth's surface. Jan 1 can be regarded as a continuous time-scale. For most purposes, ET up to 1983 Dec 31 and TDT from 1984 Was used 1960-1983, and was replaced by TDT and TDB Seconds to UT1, where t is the fraction of the year (zero at 1 Jan).ĮT = Ephemeris Time. UT2 = UT1 corrected for seasonal variations in the Earth's rotational UT1 = UT0 corrected for polar wandering - usually one means UT1 when Or from more modern methods involving GPS satellites. UT0 = 'raw', uncorrected UT as derived from meridian circle observations There are severalĭifferent definitions of UT, but the difference between them is always Defined by the Earth's rotation, formerly determinedīy astronomical observations but today GPS satellites are used To keep the difference between UTC and UT less than 0.9 s. When needed, leap seconds are introduced in UTC TAI-UT1 was approximately 0 on 1958 Jan 1. TAI = International Atomic Time (Temps Atomique International = TAI) isĭefined as the weighted average of the time kept by about 200Ītomic clocks in over 50 national laboratories worldwide. Added a link to Steve Allen's detailed history of the time scales. Thanks to John Stockton for a number of suggestions to this page. Major overhaul: corrected several small errors, updated tables, added new links. Replaced obsolete links with working ones Time Scales: UT1, UTC, TAI, ET, TT, GPS time T I M E S C A L E Sġ993? First version as an ASCII document.Ģ004-09-? First HTML version, for an article in the Swedish ham radio magazine QTC

Nuclear time scale