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    Re: Sextant Positions versus Map Datums?
    From: Bill Murdoch
    Date: 2002 Jan 20, 7:39 AM

    The way I look at this, and I am not sure I am right, is best described by
    thinking of a cross section of the earth drawn as a circle and as a ellipse
    with common centers.  The circle would represent a spherical earth and the
    ellipse an ellipsoidal earth.  At the north pole and at the equator, tangents
    to the circle and to the ellipse are parallel to each other.  Lines normal to
    those tangents all pass through the common center.  The normals from the
    poles intersect those from the equators at right angles giving the poles a
    latitude of 90 degrees.  The normals are the orientation of plumb bobs at the
    equator and at the poles.
    
    A line drawn outward from the center of the earth half way between the
    equator and the north pole strikes the circle at a point where a tangent to
    the circle would be at right angles to that line.  A pumb bob at that
    location would be parallel to that line from the center of the earth.  The
    location would be 45 degrees north.
    
    The same line from the center of the earth would not strike the ellipse at
    the point where a normal to a tangent line would be 45 degrees to the
    equator.  That point would be just a little bit farther south.  (I can not do
    the math, but maybe for the slightly flattened earth 11'32").  The plumb bob
    at the location called 45 degrees north latitude on the ellipse would not
    pass through the center of the earth, but would be parallel to a line from
    the center of the earth at 45 degrees to the equator.
    
    Bill Murdoch
    

       
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