[time-nuts] Visiting Greenwich

Tom Van Baak tvb at LeapSecond.com
Tue Jul 5 00:49:10 EDT 2016

Hi Dave,

Last year a more up to-date and highly-technical version of the meridian mystery was published:

"Why the Greenwich meridian moved"

See especially figure 3. The PDF is here:

When I was in Greenwich last year I made these plots to show the old/tourist meridian (red x) and the true meridian (green x):

I also brought a laptop and 3 GPS receivers with me and collected 3 x 20 minutes of NMEA data while sitting on the old/tourist line. Sure enough, the mean error approached 102 meters:


----- Original Message ----- 
From: "Dave Martindale" <dave.martindale at gmail.com>
To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Sent: Monday, July 04, 2016 8:14 PM
Subject: Re: [time-nuts] Visiting Greenwich

> 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.
> Dave
> 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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