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    Re: Refraction and humidity
    From: Marcel Tschudin
    Date: 2007 Mar 28, 18:23 +0300

    On 3/28/07, Alexandre E Eremenko  wrote:
    > ...
    > I have is the chapter of Chauvenet where he explains
    > the work of Bessel, and attaches tables computed by Bessel's
    > formula with refraction in fractions of a SECOND.
    
    Alex, I would be cautious with such an information. It's certainly
    true right overhead in zenith direction where there is no refraction.
    Things become more difficult when approaching the horizon. It still
    might be true under "normal" observer conditions down to somewhere
    around 70 deg ZD. I think that what Frank explained is about
    appropriate for navigational purposes under "normal" observation
    conditions. Under "normal" conditions I mean an observer somewhere at
    mid-latitudes, under average (stable) weather conditions and possibly
    also not too high up in the sky.
    
    Bill, yes, all the parameters you mentioned are of concern when
    calculating refraction at high zenith distances; even the linear
    relationship of refraction from pressure and temperature  fails there,
    they become highly exponential.
    
    Andy Young's Observatory paper, which Richard mentioned is highly
    recommendable. For those who don't have access to it, they can find a
    lot of its content in Andy's labyrinth of over hundred Web-pages on
    this subject. There are two entries into the labyrinth
    - The Tables of Contents: http://mintaka.sdsu.edu/GF/bibliog/toc.html
    - The Alphabetical Index: http://mintaka.sdsu.edu/GF/bibliog/alphindex.html
    
    Marcel
    
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