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    Re: The cocked hat
    From: George Huxtable
    Date: 2004 Apr 4, 21:50 +0100

    Joel Jacobs wrote-
    
    " ...I have to disagree with your theory that the horizon's visibility is
    the same around
    the clock."
    
    Joel, I wouln't maintain such a "theory". Of course, the visibility can
    vary with azimuth. You are quite right about that.
    
    In his earlier mailing, Joel was referring to "definition" of the horizon,
    where I was concentrating on displacements of the horizon.
    
    Taking a round of star sights at dawn or dusk, if certain parts of the
    horizon are sharp and others are misty, what does the observer do? There
    are plenty of stars in the sky to choose from. He will simply avoid those
    stars where the horizon below is unclear, and instead concentrate on the
    stars above the sharp patches. Having measured a few of those, around the
    horizon wherever he can, it wouldn't improve his average to add in any of
    the others. What a misty horizon will normally do is to make the distant
    horizon appear closer than it really is, not further: so if you choose a
    muzzy horizon as a reference, star altitudes are likely to appear to be too
    great.
    
    If there is nowhere around the horizon where it appears sharp, the observer
    is in a difficult position. Is he seeing a true horizon, or not? If it's
    UNIFORMLY misty, he might surmise that the apparent horizon he can see is
    everywhere at about the same distance from him, even if it may be depressed
    below the true one. In that case, if he measures over a full range of
    azimuths, his resulting cocked-hat, though enlarged, may indeed have,
    roughly, the correct centre, simply because his errors around the horizon
    are uniform.
    
    Perhaps I am making too much of this, by inventing such a strategy. Most
    navigators, including me, would simply put their sextant back in the box,
    in those circumstances. Around the UK, that happens often. Our waters
    weren't really designed for astro navigation.
    
    By the way, when it's misty near the horizon, that's one of the few
    advantages we navigators of small craft have over the merchant seamen on
    their high bridges. With my height-of-eye of 6 feet above sea level, when
    standing in my cockpit, the horizon is much, much closer than it is to a
    merchie, and much less likely to get hazed over. I have read of occasions
    when a ship's boat was lowered, with the navigator aboard, to take a round
    of twilight sights in misty-horizon conditions.
    
    George.
    
    ================================================================
    contact George Huxtable by email at george---.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    ================================================================
    
    
    

       
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