On Mon 2011-02-07T12:21:08 -0500, Finkleman, Dave hath writ: > Neiher Gravity nor the Geoid are "standardized."
The SI definition for TAI says "rotating geoid". Resolution B1.9 (Re-definition of Terrestrial Time TT) from the XXIVth IAU GA recognizes both that the geoid changes and that it is hard to specify the geoid, therefore the rate of TT was changed from the rotating geoid to a constant potential offset from the coordinate time of the geocenter. Bernard Guinot has stated his belief that TAI is supposed to be a realization of TT, therefore the CCTF should incorporate the IAU definition for TT into TAI, but that has not happened. I suspect that CCTF has not acted because as a practical matter it would make no difference. Until there is an ensemble of cesium chronometers not on the surface of the earth there is no easy way to measure the potential depth to 1e-10, so the corrections currently being used to compensate for the NIST and PTB chronometers being about a mile high are as good as things can get. If it turns out that an actual measurement finds the potential of the geoid to differ then that would require a notable rate change in TAI. So the fact is that we should all be preparing our children for the day when the rates of TT and TAI differ. Alternatively, the IAU might redefine TT again. It's hard to say which would cause less grief. -- Steve Allen <[email protected]> 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 [email protected] http://six.pairlist.net/mailman/listinfo/leapsecs
