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    Re: Refraction
    From: Nicol�s de Hilster
    Date: 2007 Dec 11, 11:00 +0100

    Marcel Tschudin wrote:
    > This graph may
    > help you to understand why one shouldn't use observations below about
    > 3 to 5 degrees above the horizon for navigational purposes. The reason
    > for the large scatter of observed refractions are the (diurnal)
    > changes in the planetary boundary layer, i.e. the lowest part of the
    > atmosphere where the heat exchange between the earth (or sea) and the
    > atmosphere takes place. Those effects are high over land, less near
    > the coast and minimal over the sea.
    >   
    In the Dutch magazine Cornelis Douwes no. 165, Siebren van der Werf has 
    written an article on sun's observations on Nova Zembla. There is a very 
    nice series of pictures in it showing the sun setting on 30 March 2004 
    behind Sheringham Point (as seen from Resolute Bay). The sun only 
    disappeared when its altitude was already -51.6 arc minutes! This 
    remarkable effect was caused by what they called the Toboggan effect (a 
    Toboggan is a small sledge) as the light follows a cold layer along the 
    slopes of Sheringham Point. Why was this researched? Simply because in 
    1597 Willem Barentsz and his crew established their longitude by 
    observing the Jupiter/Moon conjunction at the same spot on January 24th 
    and 25th of that year. Due to the Toboggan effect they saw this 
    conjunction no less than two hours after the real conjunction, resulting 
    in a 30 degree (!) error in longitude.
    
    Nicol�s
    
    
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