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    Re: Sextant Positions versus Map Datums?
    From: Michael Wescott
    Date: 2002 Jan 18, 9:26 AM

    >> Sextant positions are referenced to the surface of this ellipsoid,
    >> not to a sphere.
    > But was I also wrong in suggesting that common sight-reduction
    > procedures assume a spherical Earth?
    There are two places in sight reductions where one can find the
    spherical model assumed. The first is the relation that 1 minute of
    arc is 1 nautical mile, i.e. the offset from the assumed position. The
    second place is in the correction for parallax of the moon.  The
    common element for both is in assuming a certain fixed relation
    between distance and arc. In both cases the errors are small and the
    calculations complex.
    > Or are we doing spherical trig with celestial positions calculated for
    > an ellipsoid?
    Neither. It might as well be called spatial or 3D trig since it is
    really about the angles between planes and lines in 3 dimensions. It's
    just much easier to draw (and imagine) the problem as it intersects a
    sphere than the full 3D image. This point is made, often too subtly,
    when texts on navigation show the calculations with respect to the
    imaginary celestial sphere rather than the surface of the earth.
            Mike Wescott

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