[LEAPSECS] End of plausible equivalence between UTC and GMT

Zefram zefram at fysh.org
Wed Jan 11 15:53:54 EST 2012

Gerard Ashton wrote:

>The westernmost point of the London Borough of Greenwich

Irrelevant. The names "Greenwich Mean Time" and "Greenwich meridian"
don't refer to the borough per se, but specifically to the transit
instrument at the Royal Observatory, Greenwich. (Historically there
were two meridians so defined, from different transit instruments at
the observatory.) The historically-authentic Greenwich meridian is
well defined to a precision of a centimetre or so on the ground at
Greenwich (600 microarcseconds, or 40 microseconds of angular time).
There's no qualitative difference between being 5 seconds off GMT and
being 6 seconds off GMT.

Side note: wow, IERS's claimed 6 us error on UT1-UTC (in Bulletin A)
corresponds to only 3 mm at the equator. Even more impressive than
the errors on pole location, which are also about 3 mm on the ground
(at the pole).


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