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    Re: Leap seconds. was: [6802] Longest year since 1992
    From: George Huxtable
    Date: 2008 Dec 28, 21:33 -0000

    > Greg R, in [6805], wrote-
    > "Besides, the almanacs have been on UT since when - mid 70s? (and thus
    > pretty much "disconnected" from "sun time"). ".
    to which I commented-
    > On the contrary, though, the leap seconds are inserted just so that UT,
    > incrementing at a constant rate compatible with atomic time, keeps in step
    > with the Mean Sun, to within 0.6 seconds, as do the almanacs. They remain
    > closely connected with Sun time.
    And Greg has replied-
    "I think you misinterpreted what I was trying to say - i.e. that our time
    reference (based on a multiple of the period of the Cesium atom) is
    something that's not related to the motion of the earth (and by reference,
    also the sun) any longer (which is exactly why there's a need for leap
    seconds periodically, in an effort to keep the two roughly "in sync").
    My point was that since we're now on this atomic timescale (that really
    isn't in sync with "sun time"), as long as the almanacs and our timekeepers
    were on the same time scale (so that we could accurately mark the date/time
    of a celestial event), does it really matter what actual "time" the event
    occurred? It could be some random number not even related to "sun time", but
    as long as we could use it as a reference point for starting our
    calculations (i.e. GHA/Dec) the actual value is actually irrelevant."
    Next from George-
    Well, all I had to go on was Greg R's words, in [6805], which I quoted. It
    was those words that I was questioning.
    But really , it's more complicated than Greg makes out. There really ARE two
    time-scales, that run at irreconcilable rates, with the differences
    unpredictable in advance. It's a fact of life. And they have different
    effects. Take the implications for astronomers. The way they point their
    telescopes towards a star depends on the rotation of the Earth. So that
    depends on a time-scale that's gradually slowing, and unpredictably. But now
    think about the motions of , say, Mars, in the sky, relative to the Sun and
    stars. That is completely unaffected by the revolutions of the Earth. If the
    Earth stopped spinning on its axis, Mars' motion round the Sun would
    continue unaffected. It's governed by the physical laws of gravity and
    relativity, for which, unchanging atomic time is relevant.
    Yes, Greg R can use some sort of formula to convert one time-scale to the
    other, knowing the history of the way Earth rotation speed has varied in the
    past. But he CAN'T do that into the future, because of the unpredictability
    of the slowing. And that's really the basic reason why efforts to constrain
    two such time-scales to run in step are, in the long-term, doomed to fail.
    Navigation List archive: www.fer3.com/arc
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