On Thu 2019-07-04T12:59:45-0700 Paul Eggert hath writ:
- print "# (which measures the true angular orientation of the earth in space)" + print "# (a proxy for Earth's angle in space corrected for polar motion)"
That's a 1950s-lingo explanation from the era when it was not possible to measure the sub-diurnal variations of earth orientation so it was still possible to use simple models that were based on the existence of an axis of rotation relatively fixed to the surface of the earth. Current measurement precision requires general treatment of total body angular momentum and crustal angular velocity with a conventional model that does not match the 1950s notion of polar motion. The aim is for the current model to be continuous in value and rate with the reference frame of Newcomb, even though that was neither inertial nor a long-term measure of mean solar time. -- Steve Allen <sla@ucolick.org> WGS-84 (GPS) UCO/Lick Observatory--ISB 260 Natural Sciences II, Room 165 Lat +36.99855 1156 High Street Voice: +1 831 459 3046 Lng -122.06015 Santa Cruz, CA 95064 https://www.ucolick.org/~sla/ Hgt +250 m