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    Re: Sextant Positions versus Map Datums?
    From: Dov Kruger
    Date: 2002 Jan 15, 12:21 PM

    > Okay, a sextant position is based on the almanac which is based
    
    >Okay, a sextant position is based on the almanac which is >based on...ergh...
    
    All the astronomical models like VSOP87 are accurate to far more than we need 
    for celestial navigation and do it over a timespan of 6000 years.  In order 
    to plot star position to the nearest arcsecond, they need to take the slight 
    bulge of the earth into account.  It would be a second order effect, as the 
    parallax of even different sides of earth orbit is infinitesimal for a star, 
    so we are only talking about a very slight change in angle of the surface.  
    It is a small enough percentage that for accuracy to the arc-minute it may 
    not be needed for stars, and certainly it isn't in the sight reduction 
    procedure.  This leads me to believe it is irrelevant for navigational 
    accuracy.
    
    However, if you look in the nautical almanac, you will see in the appendix on 
    methods of sight reduction, the correction for a lunar observation due to 
    parallax includes a small correction (on the order of 0.1 to 0.2 minutes due 
    to parallax depending on latitude.  I hope I am not off on that but I don't 
    have the almanac handy at the moment, so that number is from memory.  So, for 
    lunar measurements, the oblateness of the earth is relevant, and for anything 
    else it is not.
    
    >If NAD27 and WGS84 disagree by a mile, then wouldn't
    Do they in fact disagree by a mile?  That would surprise me.
    
    > on...ergh...How exactly does that match up to a map datum? Should
    > (*) OK, the various datums use slightly different >ellipsoids, but I
    >believe that the horizontal difference is not significant >for
    >this discussion. The difference is more evident in the  -- >Peter
    
    I think Peter hits it on the head.
    

       
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