[time-nuts] Visiting Greenwich

Dave Martindale dave.martindale at gmail.com
Mon Jul 4 23:14:01 EDT 2016

Wouldn't that be "un pied dans chaque hemisphere" in France?

I visited the Greenwich observatory a number of years ago, but it was after
5 PM and all of the exhibits were closed for the day.  So we only saw the
repeater clock and the meridian line.  One interesting fact:  A GPS
receiver will not agree with the line set into the concrete about where
zero degrees of longitude is located.  The GPS prime meridian is somewhere
nearby, within the park, but not at the marked line.

An explanation for this (that I found at the time) is that the line in the
ground at the observatory is defined as zero longitude in whatever geodetic
ellipsoid and datum the British were using at the time.  The GPS zero
longitude line is at zero in WGS84.  Apparently WGS84 is defined to agree
with the older British datum in longitude *at the equator*, but the two
ellipsoids use different models of the earth's axis and so the two
zero-longitude meridians do not agree at Greenwich's latitude of ~50 N.

Google found this more recent article:
http://www.thegreenwichmeridian.org/tgm/articles.php?article=7 that has
more interesting (and more detailed) information about the difference in
the prime meridian definitions.


On Tuesday, 5 July 2016, jimlux <jimlux at earthlink.net> wrote:

> One must, of course, take a picture with one foot in each hemisphere.
> (Unless, you would follow the French, in which case, the Paris meridian is
> the only true meridian, and then you'd have one meter in each
> hemisphere...<grin>

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