Quoting Steve Allen <s...@ucolick.org>: > The geodetic longitude of the Greenwich transits is 5 arc seconds west > of the "International Meridian" defined by the global, > satellite-geocentered, VLBI-oriented reference frames. That's less > than one second of time -- but it's mostly irrelevant to GMT. > > Of those 5 arcseconds difference, the major component is due to the > E/W component of the deflection of vertical at Greenwich. That is to > say, at Greenwich "up" is 5 arc seconds different than "geocentrically > radial".
Steve: The 5 arcseconds is the difference between two geodetic longitudes, so I believe deflection of the vertical is not a component of this difference, it has already been taken into account. The drift from the Airy meridian is due to i) The merging of transit measurements from many global observatories by the BIH. ii) The the shift of the Greenwich transit observations to PZT measurements at Herstmonceux. iii) The introduction of the satellite Doppler technique (Transit) to help in establishing the BIH meridian; the Doppler datum turned out to have a different longitude origin. iv) Misalignment of the series of WGS reference meridians with that of BIH. Further details here: <http://www.nmm.ac.uk/explore/astronomy-and-time/astronomy-facts/history/the-longitude-of-greenwich> <http://en.wikipedia.org/wiki/Prime_Meridian> =============================================================================== Richard B. Langley E-mail: l...@unb.ca Geodetic Research Laboratory Web: http://www.unb.ca/GGE/ Dept. of Geodesy and Geomatics Engineering Phone: +1 506 453-5142 University of New Brunswick Fax: +1 506 453-4943 Fredericton, N.B., Canada E3B 5A3 Fredericton? Where's that? See: http://www.city.fredericton.nb.ca/ =============================================================================== _______________________________________________ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
