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Re: New Moon, Perigee, and Solstice
From: Trevor Kenchington
Date: 2003 Dec 24, 20:58 +0000
From: Trevor Kenchington
Date: 2003 Dec 24, 20:58 +0000
Frank, I'm afraid you were badly off-base with this latest message. You wrote: > It's a very simple issue. The ocean basins are a complex set of > interacting oscillators. There are MANY resonant frequencies. A small > difference in the frequency of the driving force *can* lead to a large > difference in the amplitude of the response AND the phase lag of the > response. Northwest Europe has a SPECIFIC phase lag that Bowditch copied > from European sources (or maybe simply "knew" from the common culture of > European sea lore). The specific phase lag for Europe leads to a > specific lag of the Spring Tides there. That phase difference applies to > much of northwest Europe, but it does not apply to the Americas generally. Northwest Europe most certainly doesn't have a "specific phase lag": the times of HW literally span right round the clock just within the Southern Bight of the North Sea, for example). Besides, if there was one single phase lag for the entire region, it would not explain a roughly-constant "age" of the tides throughout. That would need a roughly-constant difference in the phase lags of the lunar and solar semi-diurnal tides. Maybe that difference is roughly constant from Ushant to the Shetlands but, if so, that seems a bit of a surprising outcome from the complex of resonant systems over there. You continued: > Another error in early accounts of the tides: In western Europe, the > tide "wave" progresses up the coast starting in Spain and working its > way north towards Ireland. Early observers "guessed" that this was a > general property of the tides worldwide, but it's not. For example, on > the East Coast of the US, the tide cycle is almost simultaneous all up > and down the coast --if it's high tide in New Jersey, it's high tide in > South Carolina. Any notion that the tides of northwest European seas are dominated by progressive waves (whether passing Spain or otherwise) should have been laid to rest by 1900. On both sides of the Atlantic, the tides are mostly dominated by amphidromic systems. It just so happens that the system centred somewhere off Newfoundland produces a co-tidal line roughly parallel to the coast from Cape Cod to Florida. Certain parts of the Irish coast experience much the same thing, though on a far more localized scale. And finally: > No. Laplace's theories were only barely beginning to address the real > ocean tides and would not have provided the kind of practical knowledge > that Bowditch required for his navigation manual. The early hints from > Laplace could explain differences along an idealized rectangular > east-west channel (hence the reference to the English Channel you > quoted), but they could not yet address the overall phase lag. You are right that LaPlace's tidal theories have little relevance to the real tides in the oceans. However, you are way off base in supposing that they have anything to do with the tides in the English Channel. Besides much else, LaPlace modelled the tidal waves as progressing westerly, following the apparent motion of Moon and Sun. The times of HW in the England Channel progress eastwards from Scilly to Dover, Ushant to Calais. Trevor Kenchington -- Trevor J. Kenchington PhD Gadus@iStar.ca Gadus Associates, Office(902) 889-9250 R.R.#1, Musquodoboit Harbour, Fax (902) 889-9251 Nova Scotia B0J 2L0, CANADA Home (902) 889-3555 Science Serving the Fisheries http://home.istar.ca/~gadus