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    Re: Refraction
    From: Nicol�s de Hilster
    Date: 2007 Dec 12, 13:52 +0100

    George Huxtable wrote:
    > Nicolas de Hilster wrote-
    >
    > "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."
    >
    > ====================
    >
    >   
    The George answered:
    > I'm a bit puzzled as to why a Sun altitude of -51.6 arc minutes, at its 
    > moment of disappearance, should be thought so unusual.
    >
    > When the last glimpse of the Sun's upper limb can be seen at the horizon, 
    > the apparent altitude of that limb is zero. If we look at the table for 
    > standard refraction in a modern Nautical Almanac, it tells us that in that 
    > case the refraction is -33.8 arc minutes. And if the semidiameter of the Sun 
    > was, say, 16 arc minutes, a rather typical value, that would put the Sun's 
    > expected position (of its centre) at -49.8. Not much of a discrepancy with 
    > the value Nicolas quotes of 51.6.
    >
    >   
    It was probably not the best example I gave, although we have to take 
    into account that the sun was still visible over the top of that 
    hill/mountain and therefore adding some arcminutes (but I would have to 
    find out how much). I found another article (Waerachtighe Beschryvinghe 
    van het Nova-Zembla-effect [true description of the Nova-Zembla 
    -effect]) which was six years older (so 2000) and from the same author, 
    but now with three co-writers (G�nther P. K�nnen, Waldemar H. Lehn, 
    Frits Steenhuisen). It again shows observations from Barentsz, but also 
    from more recent observers as Fridtjof Nansen (1894), Shackleton (1915, 
    he saw the sun coming back twice that day) and Liljequist (1951).
    
    Barents observations were taken at 76�15�.4 N 68�18�.6 O.:
    January 24th, 1597 - It was indicated that they saw a glimpse of the sun
    January 27th, 1597 - They saw the sun "in zijn volle rondicheyt" (in its 
    full roundness)
    
    According to the authors at the first observation the sun's altitude 
    would have been -5�25', at the second -4�41'.
    
    George, could you (or someone else on this list) verify these 
    theoretical altitudes and the refraction given by the Modern Almanac?
    
    Nicol�s
    
    
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