[LEAPSECS] what happened to GMT

[Steve Allen]

On Sat 2014-01-11T18:49:58 +0000, Michael Spacefalcon hath writ:

> But does this ``Zulu'' mean GMT (a natural phenomenon

> whose meaning is independent of human whim)


If only it were so, but looking back at the 1884 International
Meridian Conference it appears that the reason Simon Newcomb vanished
from the meetings was that he agreed when Janssen said it wasn't as
simple as that.  "A circle has no end" so somehow there must be a
human convention for where the "point de depart", the origin, will be.

During WW2 when the Airy transit at Greenwich was not in operation the
BIH "Heure Definitive" continued to exist via the other observatories.
The final Airy observation was 1954-03-30.  By 1956 the UK transit
astronomers then moved to Herstmonceux noted that the current offset
of UT from GMT was small, and admitted they would not even try to
establish the exact offset from Greenwich
http://adsabs.harvard.edu/full/1957Obs....77...31A
It was far too late in 1971 when their successors again wrote about it
http://adsabs.harvard.edu/abs/1971Obs....91..155O
basically saying "We did what?"  because by then the DOPPLER satellite
tracking had begun to supersede optical transit measures.  The BIH
change in weighting from meridian transit observations to satellite,
lunar, and VLBI is evident in these 1984 plots from Martine Feissel:
http://www.ucolick.org/~sla/leapsecs/MFatt2.pdf
Hiding in those charts is the shift of all terrestrial longitudes from
their nonrelativistic Greenwich-based local values to their
relativistic geocentric global values.

--
Steve Allen                 <sla at ucolick.org>                WGS-84 (GPS)
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