NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: The cocked hat
From: George Huxtable
Date: 2004 Apr 4, 21:50 +0100
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@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. ================================================================