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    Re: Sextant Positions versus Map Datums?
    From: Dan Allen
    Date: 2002 Jan 20, 3:16 PM

    For more information about geodesy, which is the topic at hand, there is a
    NIMA book online at:
    which is pretty interesting.  You can also download a copy as a PDF file
    from here:
    Dan Allen
    -----Original Message-----
    From  Navigation Mailing List [mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM]On
    Behalf Of WSMurdoch@AOL.COM
    Sent: Sunday, January 20, 2002 6:39 AM
    Subject: Re: Sextant Positions versus Map Datums?
    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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