Hi Dave, Last year a more up to-date and highly-technical version of the meridian mystery was published:
"Why the Greenwich meridian moved" http://link.springer.com/article/10.1007/s00190-015-0844-y See especially figure 3. The PDF is here: http://link.springer.com/content/pdf/10.1007%2Fs00190-015-0844-y.pdf 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): http://leapsecond.com/pages/meridian/map-100.gif http://leapsecond.com/pages/meridian/map-101.gif 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: http://leapsecond.com/pages/meridian/simul-5.gif /tvb ----- Original Message ----- From: "Dave Martindale" <[email protected]> To: "Discussion of precise time and frequency measurement" <[email protected]> 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 <[email protected]> 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> >> _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
