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    Re: Sextant Positions versus Map Datums?
    From: Bill Noyce
    Date: 2002 Jan 18, 11:25 AM

    > He also goes on (p. 83) to give a formula for the difference between
    > geocentric latitude (assuming a spherical earth) and geographic
    > latitude (using the local vertical; what is plotted on charts).  It
    > reaches a maximum of 11'32" at 45 degrees latitude.
    
    11.5 miles sounds like a lot.  Fortunately, a sextant measures using
    the local vertical, so it would give geographic latitude.  The
    geocentric latitude would only be used for parallax measurements,
    as far as I can see.  Bowditch gives a formula for adjusting the Moon's
    parallax based on latitude and (altitude or azimuth, can't remember),
    but it's quite small, as I recall.
    

       
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