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    Re: Dip
    From: Dan Allen
    Date: 2004 Nov 27, 17:13 -0700

    Many texts describe the large uncertainty of refraction near the horizon.
    There is no refraction straight overhead.  In the middle altitudes it is
    small and predictable and that's where most sights are shot (or at local
    noon).  Few sights in practice are shot of bodies at the horizon, and the
    large uncertainty in the dip is just one of the reasons for this.
    
    Dan
    
    -----Original Message-----
    From: Navigation Mailing List
    [mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM]On Behalf Of Alexandre
    Eremenko
    Sent: Saturday, November 27, 2004 4:41 PM
    To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
    Subject: Dip
    
    
    Dear fellow list members,
    
    I recently found an atricle which seems
    very disturbing to me:
    "Refraction near the horizon" by Schaefer and Liller,
    Publ. Astr. Soc. Pacific.
    You can read the paper on
    http://www.math.purdue.edu/~eremenko/dip.pdf
    The main point of the article is that the refraction
    near the horizon (and thus the dip of the horizon)
    varies unpredictably and the variation is enormous.
    
    The authors do not discuss Cel Nav in this paper,
    their main concern is refuring "Archaeoastronomy".
    
    I don't care about Archaeoastronomy, but their main
    conclusions seem to imply that the refraction
    near the horizon (and thus the dip) is uncertain
    to more than 1/2 of a degree.
    
    Then how is traditional CelNav based on altitude measurements
    possible at all?
    
    Any comments?
    
    Alex.
    
    
    

       
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