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    Re: Datum for Nautical Almanac
    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
    
    
    

       
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