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    Re: Position bias from UTC
    From: Paul Hirose
    Date: 2020 Jan 7, 13:46 -0800

    On 2020-01-04 15:01, Robin Stuart wrote:
    > When I came to think things through starting from first principles recently 
    I was operating under the assumption you couldn’t possibly create an almanac 
    based on a time scale that is not known in advance.
    To some extent the Nautical Almanac really is constructed from a time
    scale not known in advance. GHA is affected by the revolution of solar
    system bodies in their orbits, which is calculated from TT (Terrestrial
    Time). Since the almanac time argument is UT1, that's a problem. For
    each line in the daily pages, the tabulated UT1 must be converted to TT
    via the application of ∆T (= TT - UT1, currently about 69 seconds).
    The true ∆T is only known after the fact, so the Almanac must be based
    on a prediction. Fortunately the GHA of a body is mainly affected by
    Earth rotation angle, which is a function of UT1 and therefore can be
    calculated from the tabulated time with utmost exactness. The orbital
    motion contribution is greatest for the Moon: 0.5″ per second, or 1/30
    the 15″ per second due to Earth rotation. For other bodies the ratio is
    even more favorable.
    In other words, a 1 second error in ∆T affects Moon GHA only .008′.
    But if leap seconds are discontinued, change will be forced on celestial
    navigation. One option is to retain UT1 as the NA time basis. Without
    leap seconds, UT1 and UTC will diverge, eventually reaching the point
    where longitude error becomes intolerable without a DUT1 correction.
    With the correction, celestial navigation will be as accurate as before.
    Another option is to tabulate the NA in UTC, which has the advantage
    that users need not change their procedures. But it does put the
    computers of the Almanac in the wrong side of the 30:1 ratio. That is,
    the 0.5″ per second (maximum) GHA contribution from TT will be precisely
    known, while the 15″ per second from UT1 will depend on the ∆T estimate.
    A few years ago a paper by a USNO astronomer said it would be no small
    challenge to maintain the present day NA accuracy with our current
    modeling of ∆T.

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