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    Re: Sextant Positions versus Map Datums?
    From: Steven Wepster
    Date: 2002 Jan 18, 5:28 AM

    Dear Hal,
    
    Do you have a page reference for this?
    
    _Steven.
    
    
    >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
    >
    >a:  6378.14 km (the radius of the earth at the equator)
    >f:  1/298.257 (flattening of the ellipsoid)
    >
    >b/a:  0.99664719 (ratio of polar radius to equatorial radius)
    >
    >b/a = 1-f
    >
    >e = sqrt(2f -f*f) = 0.08181922 (eccentricity of the meridian)
    >
    >I just took a look at the US Naval Observatory's NOVAS package source
    >code, and they're using the same values (and to the same number of
    >significant digits, to boot!)
    >
    >Sextant positions are referenced to the surface of this ellipsoid,
    >not to a sphere.
    >
    >See also http://www.colorado.edu/geography/craft/notes/datum/datum.html
    >
    >
    >Hal
    >
    >----
    >Hal Mueller                hal{at}mobilegeographics.com
    >Mobile Geographics LLC     http://www.mobilegeographics.com/
    >8015 28th Ave NW
    >Seattle, Washington        (206) 297-9575
    >98117                      fax (206) 297-9576
    

       
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