NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Sextant Positions versus Map Datums?
From: Steven Wepster
Date: 2002 Jan 18, 5:28 AM
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@mobilegeographics.com >Mobile Geographics LLC http://www.mobilegeographics.com/ >8015 28th Ave NW >Seattle, Washington (206) 297-9575 >98117 fax (206) 297-9576
