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    Re: The cocked hat
    From: Joel Jacobs
    Date: 2004 Apr 4, 18:22 -0400

    George,
    
    You seem to be contradicting yourself or do I misunderstand what you said as
    a prelude to this or perhaps you are making a play on words, ie "definition
    vis-a-vis the "displacement of the horizon"?
    
    "This is exactly contrary to Joel's argument, in which he said-
    >
    > >As everyone knows, the
    > >horizon at sea's definition varies with the direction you are looking due
    to
    > >differences in light, sea conditions and cloud cover."
    
    Many people make too much out of the need for precision. An experienced
    navigator will use what he has available. When necessary, I have taken
    useable sights through the clouds, bringing down only the spot of
    brightness to all sorts of horizons. I have a hunch that Henry Halboth has
    done that an more.
    
    Joel Jacobs
    
    
    ----- Original Message -----
    From: "George Huxtable" 
    To: 
    Sent: Sunday, April 04, 2004 4:50 PM
    Subject: Re: The cocked hat
    
    
    > 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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