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    Re: Datum for Nautical Almanac
    From: Charles Seitz
    Date: 2004 Oct 30, 11:46 -0400

    The Naval Observatory has an online calculator for sight reduction:
                         http://aa.usno.navy.mil/   (Data Services)
    They specifically refer to geocentric altitude.
    'Data are shown for the navigational stars and planets only if their
    computed geocentric altitude, Hc, is equal to or greater than +1 degree at
    the place and time specified. '
    This, I assume means with respect to observations from a spherical earth.
    we all know the real earth is better  modeled as a geoid.  That means
    made with respect to the local horizon on the real earth will be slightly
    different than
    those made from a spherical earth.  It follows that circles of equal
    altitude, the
    basis of CN,  are not perfectly circular.
    (I suspect observations of altitude would be nearly identical for both
    reference systems at the poles, equator and near 45 deg lat.  At these
    locations, the center of the earth will be directly under you)
    Since there are numerous geodetic datams, your geographical position does
    indeed depend on your assumed datum.  So, why wouldn't there also be
    some inherent positional discrepancy between a geocentic and geoidal earth
    I suspect that sight reduction tables would be nearly impossible to prepare
    all of the possible complications were considered.  We choose an earth shape
    model that works best for a particular purpose.
                      --- CHAS

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