On Wed 2008-12-31T08:10:17 +0000, Poul-Henning Kamp hath writ: > >But GMT has a > >clear definition tied to - Hush my mouth! - the position of the mean > >sun in the sky. > > No it does not, GMT wandered several seconds when the 0� longitude > were moved away from he observatory.
GMT did not shift by seconds. As with the rest of the discussion here we are talking about two different and necessary-to-distinguish things. 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". Strict GMT is defined by the astronomical longitude of the observatory. Yes, using the international reference frames there was a shift. When BIH chose the statistical weights of the observations from all sites, and as they averaged the differences in the "conventional longitudes" of all the observatories which had been in use from the pre-satellite era the terrestrial reference frame did shift. This shift on the order of centiseconds of time. And yes, it will drift over time at plate tectonic rates. But that shift will not accelerate quadratically in the way that atomic time and universal time will diverge. -- Steve Allen <s...@ucolick.org> WGS-84 (GPS) UCO/Lick Observatory Natural Sciences II, Room 165 Lat +36.99855 University of California Voice: +1 831 459 3046 Lng -122.06015 Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 m _______________________________________________ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs