NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Datum for Nautical Almanac
From: Charles Seitz
Date: 2004 Oct 30, 11:46 -0400
From: Charles Seitz
Date: 2004 Oct 30, 11:46 -0400
The Naval Observatory has an online calculator for sight reduction: http://aa.usno.navy.mil/ (Data Services) They specifically refer to geocentric altitude. 'Data are shown for the navigational stars and planets only if their computed geocentric altitude, Hc, is equal to or greater than +1 degree at the place and time specified. ' This, I assume means with respect to observations from a spherical earth. But, we all know the real earth is better modeled as a geoid. That means observations made with respect to the local horizon on the real earth will be slightly different than those made from a spherical earth. It follows that circles of equal altitude, the basis of CN, are not perfectly circular. (I suspect observations of altitude would be nearly identical for both reference systems at the poles, equator and near 45 deg lat. At these locations, the center of the earth will be directly under you) Since there are numerous geodetic datams, your geographical position does indeed depend on your assumed datum. So, why wouldn't there also be some inherent positional discrepancy between a geocentic and geoidal earth model? I suspect that sight reduction tables would be nearly impossible to prepare if all of the possible complications were considered. We choose an earth shape model that works best for a particular purpose. --- CHAS