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: Hal Mueller
Date: 2002 Jan 17, 2:23 PM
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@mobilegeographics.com Mobile Geographics LLC http://www.mobilegeographics.com/ 8015 28th Ave NW Seattle, Washington (206) 297-9575 98117 fax (206) 297-9576
