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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@huxtable.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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