Tony Finch dot at dotat.at
Wed Jan 4 14:40:30 EST 2012

Warner Losh <imp at 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.


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.

f.anthony.n.finch <dot at 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.

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