Warner Losh <i...@bsdimp.com> wrote: > > GMT can == UTC > UTC != GMT
What Warner says is mostly correct. Historically GMT was the local mean solar time maintained at Greenwich, then after WWII at Herstmonceux. Because it was originally a local mean solar time it was not quite equivalent to the modern UT1 or UT2. For a concrete example of the difference, go to Greenwich with a GPS and observe the difference between the Airy meridian and the WGS84 meridian. Even while GMT existed it used different meridians: before 1851 the Bradley meridian was used (0.02 seconds of time west of Airy) which is still the meridian used for Ordnance Survey maps; there was also a small error transferring time from Greenwich to Herstmonceux. And before 1925 GMT changed date at noon. While the RGO was at Herstmonceux the Greenwich time signal used by the BBC was generated there, and its timescale was referred to as GMT even after the switch to UTC in 1972. So we are discussing a long-standing and officially sanctioned ambiguity. GMT was slow to die: the RGO's PZT transit instrument was decommissioned in 1984, its time department was renamed "space geodesy" in 1987, and the time signal was discontinued in 1990 when the RGO moved to Cambridge. http://www.lib.cam.ac.uk/deptserv/manuscripts/RGO_history/RGO_GAW-1948-1990_v1.pdf So GMT either refers to various historic local mean solar timescales, or to UT1 which is the closest modern equivalent, or to UTC for the purposes of civil time. Tony. -- f.anthony.n.finch <d...@dotat.at> http://dotat.at/ Viking: Southwest 5 to 7 becoming cyclonic then northwest 7 to severe gale 9, perhaps storm 10 later. Rough or very rough becoming very rough or high. Rain then squally wintry showers. Good, occasionally poor. _______________________________________________ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs