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    Re: Star-star distances for arc error
    From: Douglas Denny
    Date: 2009 Jun 19, 01:57 -0700

    I've done this. It provides a method of checking a sextant but is not easy 
    with accuracy unless you have some clamping system to hold the sextant still 
    and be able to rotate the sextant into alignment.
    
    If you use horizontal angles with stars approximately of the same declination 
    the refraction element is negligable if they are above say 30 degrees.
    Most sextants only read with accuracy to about a half of a minute of arc anyway.
    
    Star distances can be calculated from declinations/ RA (or hour angles) with simple spherical trig.
    
    Douglas Denny.
    Chichester.  England.
    
    
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