[LEAPSECS] Question about UT1 and the IERS Reference Meridian

Steve Allen sla at ucolick.org
Wed Apr 29 12:37:46 EDT 2015

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.
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                 <sla at ucolick.org>               WGS-84 (GPS)
UCO/Lick Observatory--ISB   Natural Sciences II, Room 165   Lat  +36.99855
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Santa Cruz, CA 95064        http://www.ucolick.org/~sla/    Hgt +250 m

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