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Re: Datum for Nautical Almanac
From: Herbert Prinz
Date: 2004 Oct 30, 04:40 -0400
From: Herbert Prinz
Date: 2004 Oct 30, 04:40 -0400
Michael Dorl wrote: > are the following statements concerning the NA true? > > Decl and GHA/SHA are with respect to earth's mean equator and equinox. It has to be equator and equinox of date, because this is what you are able to measure. > Diurnal; aberrations effects are not included. Correct. > Diurnal parallax effects are included based a spherical earth of (I > don't know radius). I would think it is the equatorial radius, but the N.A. doesn't say. In an astronomical almanac, this is what I would expect. But we should not take it for granted. In nautical almanacs, there is all sorts of fudging going on with semidiameters, parallaxe, etc. The Connaissance des temps at the end of the 18th cy used the radius through Paris, but by the mid 19th cy they had switched to the equatorial radius. Consequently, they called it then by its correct name "Equatorial horizontal parallaxe". The difference between the Equ.H.P. and the HP at the pole is 11". So, it does matter within the nominal accuracy of the N.A. Herbert Prinz