NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
On the Mischief of Geographical Position.
From: Herbert Prinz
Date: 2002 Jan 21, 9:18 AM
From: Herbert Prinz
Date: 2002 Jan 21, 9:18 AM
Nine out of ten popular books on celestial navigation define the Geographical Position of a celestial body as that point on the Earth�s surface where the latter is intersected by a line from the body to the center of the Earth. If this definition were correct, the body would not be in the zenith of its Geographical Position on a non-spherical Earth. The navigator would be in a worse situation than the proverbial man who hunts his own shadow: the sub-stellar point of his zenith, i.e. his own position, could be as far as 12 nm away from him. To remedy this situation the author will (sometimes after admitting that the Earth is in reality a spheroid) tacitly assume that the celestial navigator uses a spherical Earth at any rate and will in particular present the concept of latitude as if there were only geocentric latitude. Hence the author has to make believe that it is the latter that is related to the declination of the stars and therefore of interest to the navigator. Even the most reputable books such as Dutton�s engage in this kind of deceptive oversimplification, leaving many readers wondering, why it all works on an elliptical Earth. Indeed, it would not. But as we are, in fact, knowingly or unknowingly, performing all sight reduction on the CELESTIAL sphere, which is a sphere by definition, our sight reduction methods based on spherical trigonometry are strictly rigorous. It is the ZENITH that the navigator is attempting to locate in the celestial sphere. Once that has been accomplished, he projects it down via the local vertical, simply by renaming declination latitude, and GHA longitude, and calls it his �position�. This works on an egg, a pear, a lemon or an onion. Herbert Prinz (from 1368950/-4603950/4182550 ECEF)