[tz] [PROPOSED] Cite Rec. ITU-R TF.460-6 for leap seconds

[Michael H Deckers]

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.