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    Re: Sextant Positions versus Map Datums?
    From: Hal Mueller
    Date: 2002 Jan 17, 2:23 PM

    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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