[LEAPSECS] Leap smear

[Michael Deckers]

    On 2011-09-20 17:08, Steve Allen wrote:


>  At the CCIR Plenary Assembly in 1978 when CCIR Rec. 460-2 were

>  published the supporting documentation literally rejoices at the 1975

>  CGPM resolution which says that UTC is mean solar time.


    Interesting. Did they really think the CGPM said that
           "UTC is mean solar time"?

    Of course, the CGPM never said that. They "considered" in 1975
    that the "system called 'Coordinated Universal Time' (UTC)"
    -- they did not call UTC a time scale -- "makes available
    to the users"
        "an approximation to Universal Time (or,
         if one prefers, mean solar time)".

    Note that the definitive French version is unambiguous; in
    French the quote reads:
         "une approximation du Temps universel (ou,
          si l’on préfère, du temps solaire moyen)"
    implying an "approximation .. to mean solar time".

    This statement of CGPM is just a conjunction of two statements,
    which were true at the time:
      -  UTC is approximation of UT1
      -  UT1 could be considered as mean solar time.

    The first statement remains true even with the proposed
    redefinition of UTC (just with a worse approximation than the
    one used in 1975). The second statement may not be true with
    the current definition of UT1, simply because no mean sun has
    been defined for the current UT1.

    If one prefers to call the current version of UT1 a mean solar
    time then this is only justified as far as UT1 has agreed in
    phase and rate until 2003 with the mean solar time as defined
    in 1978 and before.

    The current UT1 is better called Earth rotation time. It is
    well-known that the "non-rotating origins" of UT1 rotate and
    thus the current UT1 differs secularly from mean solar
    time. The current definition of UT1 makes it not only
    dependent on the attitude of the Earth in space, and on
    (a bounded function of) the position in its orbit, but also
    on the past movements of the pole of rotation, on which
    mean solar time in any reasonable definition does not depend.

    But then -- UT1 will certainly be redefined, at the latest
    when the increased accuracy of its determination makes the
    "non-rotating origins" impracticable. It might even become
    mean solar time again.

    Michael Deckers.