On 2019-07-04 16:29, Steve Allen wrote:
On Thu 2019-07-04T00:35:32-0700 Paul Eggert hath writ:
Subject: [tz] [PROPOSED] Cite Rec. ITU-R TF.460-6 for leap seconds In addition to the listed changes I suggest being more specific about the meaning of UT1 in leapseconds.awk by defining what is meant by "space".
print "# The International Earth Rotation and Reference Systems Service" print "# periodically uses leap seconds to keep UTC to within 0.9 s of UT1" print "# (which measures the true angular orientation of the earth in space)"
to
print "# The International Earth Rotation and Reference Systems Service" print "# periodically uses leap seconds to keep UTC to within 0.9 s of UT1" print "# (which is a measure of the angular orientation of the earth's crust" print "# with respect to a reference frame designed to match Newcomb's" print "# Tables of the Sun in Astronomical Papers Prepared for the Use of" pring "# the American Ephemeris and Nautical Almanac, Volume VI, 1895)"
as is nicely explained with full references https://en.wikipedia.org/wiki/Newcomb%27s_Tables_of_the_Sun
Certainly the parameters in the modern definition of UT1 were chosen so that UT1 is continuous in phase and rate with Newcomb's definition of UT based on his tables of the Sun. But I find it a bit misleading to imply that only this match was the goal of the modern definition of UT1. The rotating body, the reference for measuring rotation angle, and the rotation axis have all been changed significantly since Newcomb's time around 1900. For one thing, the rotation described by modern UT1 is not that of the Earths's crust but that of the Earth-centered reference system ITRS; the Earth's crustal plates are moving with respect to the ITRS both secularly and with short periods (Earth tides). The exact coordinates of Greenwich in the ITRS change subdaily and secularly. Furthermore, the modern definition of UT1 is independent of the movement of the Sun, it only depends on the rotation of the reference system ITRS around an axis with respect to a celestial sphere defined by distant quasars taken as non-moving references (although that sphere is still affected by very small relativistic rotations that indeed do depend on the moving Earth). Finally, the axis of the rotation described by UT1 is not the instantaneous direction of the Earth's rotational angular momentum (in some reference system) but of a mean over 2 days (2 rotations). This is a practical measure to ensure that UT1 does not depend on short period movements of the terrestrial pole that are not induced by location-independent effects. Michael Deckers.