[LEAPSECS] An example

[Michael Deckers]

    On 2010-11-02 18:55, Poul-Henning Kamp wrote, first quoting
    Steve Allen:


>  >  The existing international agreement for the meaning

>  >  of "day" is "mean solar day".

>

>  You mean "one of the existing..." ?

>

>  The astronomical meaning of the word day may indeed be what you say,

>  but the equally internationally agreed standard for computer operating

>  systems define a day as 86400 SI seconds.

>

>  The question is which one ITU-R adopts.


    Isn't a day always exactly 86400 s, in whatever time scale you
    are considering? If you consider UT, you get mean solar days,
    if you consider TCB you get a day of coordinate time for the solar
    system, and if you consider UTC you either get the same as
    you would get with TAI, or 1 s more of TAI.

    Upon comparison, these intervals of days are different (and the
    difference may even depend on the method of comparison), but the
    time scales nonetheless use the same units of days, hours, minutes,
    seconds, hours. It is the very basis of relativity that two time
    scales both measured in SI seconds do not necessarily give the same
    intervals of days everywhere. Other time scales with the same
    property (which, accordingly, they must have!) add no further
    complication, in my opinion.

    For height above the geoid (altitude) we have a similar
    situation: there are several notions of geodesic height, and
    their height diffenrences do not agree, but all can be expressed
    in the same unit of SI meters (or feet).

    Anyway, thanks for the illuminating exchange of ideas about
    the fate of UTC!

    Michael Deckers.