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    Re: Datum for Nautical Almanac
    From: Michael Dorl
    Date: 2004 Oct 29, 10:32 -0500

    In reply to my msg, George Huxtable ; wrote:
    >The ellipsoidal shape of the Earth is allowed for already in our charts.
    >Instead of taking the Earth as a sphere, in which case all the lines of
    >latitude would be equally spaced, they are redefined as lines where the
    >direction of gravity makes an equal angle with the Earth's axis. This
    >"fiddle" allows to forget about the Earth's ellipsoidal shape, though it
    >complicates any exact calculation of the distance from A to B on the
    >Earth's surface.
    I understand that declination is independent of the reference spheroid
    provided the spin axes agree.
    are the following statements concerning the NA true?
       Decl and GHA/SHA are with respect to earth's mean equator and equinox.
       Diurnal; aberrations effects are not included.
       Diurnal parallax effects are included based a spherical earth of (I
    don't know radius).
    The suppositions concerning diurnal effects are made since the NA contains
    no corrections based on position;  the NA corrections depend
    only on observed altitude.

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