[tz] Potted history of timekeeping from the BBC

[Steve Allen]

On Tue 2017-04-04T07:35:21 -0700, Randal L. Schwartz hath writ:
> And you should also note that the location where they charge you to take
> that photograph is about 20 meters to the west of where it should be.
> At least, I presume my modern smartphone's GPS is fairly accurate, as
> well as having looked it up on Google Earth later.

The meridian is exactly as it has always been.  The GPS is telling
where the meridian is for a satellite looking down to the center of
the earth.  The Observatory is where the meridian is for someone
looking up at the stars.  It is the same meridian because the meridian
is a direction, not a place, and throughout its history the
International Time Bureau did a good job of keeping it constant

Full analysis is in this paper from 2015
https://link.springer.com/article/10.1007/s00190-015-0844-y
by Malys, Seago, Pavlis, Seidelmann, and Kaplan

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