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    Re: Sextant Positions versus Map Datums?
    From: Dov Kruger
    Date: 2002 Jan 18, 10:06 AM

    The astronomical positions of VSOP87, and all the derived variants are
    computed first in Heliocentric coordinates, thus the shape of the
    earth is irrelevant to that part of the computation.
    
    Then, it is converted to geocentric coordinates based on the position
    of the center of the earth.  I don't remember seeing anything about
    the ellipsoid here, but Hal if you have a page reference in Meeus, I'd
    like to see it.  What is much more relevant is nutation, because as
    the Earth "nods" up and down due to the gravitational effects of the
    moon on the equatorial bulge, the axis changes, and that changes the
    entire coordinate system.  So the accurate conversion to geocentric
    coordinates takes into effect nutation, and for distant bodies (beyond
    Mars) the time it takes light to arrive.  Those are the only
    corrections I have seen at this stage, and I believe Hal is wrong
    about the ellipse.
    
    You can read all this in Meeus, and there is a program out there
    called Astrolabe which though not the best code, uses VSOP and is
    therefore one of the simplest to read.  By contrast, Steve Moshier has
    some code out there that is supposedly even better (more accurate,
    using the currently most accurate model, DE404) but it is nearly
    impenetrable, filled with lots of different conflicting models, dead
    code left in, and just a more complex algorithm (I think!).  If you
    are an astronomer, you care about accuracy to the arcsecond so you can
    point your telescope.  If you are interested as a navigator, you can
    live with any of these models, they are more than accurate enough,
    particularly this century.  All this so far has nothing to do with the
    ellipsoid or datums, except to contradict any claim that the position
    of bodies in the almanac is somehow corrected for any of that.  It
    can't be -- the position is calculated based on the center of the
    Earth, and a wild and crazy equation for Heliocentric position of the
    other body based on the julian century T.  The apparent position is
    then corrected for lightspeed and nutation.
    
    What you can measure is altitude, and as Trevor corrected me and
    explained to everyone, if there were to be a correction for the
    ellipsoid, you would do it as you convert between GP/AP and altitude
    and azimuth.  If you are using the standard equations, it's obviously
    not happening.  It could be in the 229, but if you look at the notes
    about how the tables are generated, clearly no such correction has
    been done.  The equation they use is spherical, and the only note they
    make is that for angles that would result in bad roundoff error they
    use a different form that is analytically equivalent.  That is an
    issue of computation, not changing the shape of the sphere.
    
    So perhaps Meeus merely mentions somewhere that when they record an
    observation, astronomers convert the other way to determine the
    position of the body?  I assume that would have more to do with
    parallax.  Anyway, I would also like to see the reference.
    >According to Jean Meeus, Astronomical Algorithms, 2nd edition,
    >astronomical observations (which are the basis for the Nautical
    >Almanac) use the IAU 1976 ellipsoid (International Astronomical Union
    >1976).  Those values are
    

       
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