On Wed 2015-04-29T08:33:48 -0700, Tom Van Baak hath writ: > BTW, this excellent question came to time-nuts yesterday; does anyone here > have a definitive answer for Mike?
The definitive answer is that UT1 is published by the IERS using all of the models and methods which are described in the IERS Conventions. http://www.iers.org/IERS/EN/DataProducts/Conventions/conventions.html Currently that's the 2010 version. I recommend reading it and the papers that it refences, but that's me. Among other things those models include a solar system model based on aparameterized post Newtonian approximation of General Relativity, an ephemeris that uses that model with a state vector based on centuries of astronomical observations, a spherical harmonic expansion of the gravity field of the earth, a tectonic model for the expected motions of the stations that are using various techniques to measure observables of the models. The input observations themselves depend on those stations who have been able to procure funding to keep their instrumentation and personnel in good working order. So the place where UT1 is defined is a huge linear system of equations based on linearized approximations of all those models filled with whatever observational data are available. A problem with the question of where to stand when measuring the prime meridian is that the wanderings of the pole are measurable on sub-diurnal intevals, so the meridian moves around every day. The value of UT1 and the value of longitude of any particular marker on the surface of the earth became the subjects of least squares approximations as soon as the BIH gained the authority to publish the value of Heure Definitive, and that authority was firmly in place by the time that the Greenwich meridian circle was shut down during World War 2. In the published volumes of Bulletin Horaire are numerous occasions when the BIH revised the values of Heure Definitive, thus redefining what time it had been, and by the same token, what longitude things were at. Figuring out exactly who slid the prime meridian away from the site of the Greenwich meridian circle and when is a subject that has been looked at and abandoned without definitive answer, in part because the merdian circle was moved from Greenwich to Herstmonceux during that interval, but also because all the other meridian circles have been abandoned in favor of more precise modern techniques. -- Steve Allen <[email protected]> WGS-84 (GPS) UCO/Lick Observatory--ISB Natural Sciences II, Room 165 Lat +36.99855 1156 High Street 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] https://pairlist6.pair.net/mailman/listinfo/leapsecs
