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    Extremely poor conditions, details.
    From: Alexandre Eremenko
    Date: 2012 Mar 21, 09:30 -0400

    Frank,
    You wrote:
    On Tue, 20 Mar 2012, Frank Reed wrote:
    
    > The conditions that produce this dense, cold,
    > low-hanging fog are notorious for abnormal refraction, too.
    > I think you have seen a really fine example of this!
    
    Have you ever had similar kind of error? Or can you refer to some
    source saying they can be that large? You say "notorious", so there
    must be some reference. There was no visible fog.
    
    Let me give more detail on one observation from the series which
    looks the best:
    Bill, Astra, height of the eye 12 feet.
    GMT 21:44:39 Sextant Reading: 22d22'2
    I take dip correction -3.4 from the Almanac.
    Combined LL and refraction correction is +13'7, from the Almanac.
    Sun semidiameter is 16'1.
    So the combined correction for the dip and refraction (excluding LL)
    according to the Almanac
    would be -3'4+13'7-16'1=-5'8.
    The computed altitude of the Sun center is 22d43'2.
    Thus the actual correction due to dip and refraction was
    22d43'2-16'1-22d22'2=4'9.
    
    I summarize: Almanac shows -5.8 and in reality it was +4'9,
    for the effect of dip and refraction combined, 10'7 difference.
    Can someone show me a picture of how the two rays,
    one from the sun lower limb and another from the horizon must have
    been distorted? And why could such thing happen?
    
    I repeat: everything looked quite nice. The horizon looked crisp, except
    the part under the Sun where there was glare.
    There was only one somewhat strange thing that I noticed.
    There was a very strong glare on the water under the Sun, usual thing.
    So strong that it was uncomfortable to look at it without sunglasses.
    I had no sunglasses, but I tried to look. And what I saw, loked like a
    "bump"
    on the horizon under Sun. I thought that this must be some optical
    illusion caused by reaction of the eye on the glare ("irradiation" or
    whatever). Of course, the horizon filter
    filtered the glare, and there was no visible "bump" when looking
    through the sextant horizon glass with a filter.
    
    But the bump which I think I saw with my naked eye was wide
    and perhaps wider than the sextant field of view.
    If this bump was really "present" it could cause this error.
    But what mechanism could cause it?
    
    So far I found no clear statement in the literature showing that
    the combined effect of dip and refraction can be under certain conditions
    10' greater than the Almanac says.
    
    Alex.
    
    
    
    
    
    

       
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