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From: Trevor J. Kenchington (no email)
Date: Tue Aug 26 2003 - 09:48:23 EDT
Geoffrey Butt wrote:
> " < * .. so there are two raised ocean levels opposite one another and as the
> " Earth rotates beneath one experiences locally two tides per day>
> "
> " All well and good, but how do you factor in those places where there is
> " only 1 tide per day? Or at least, where the locals think there is only one
> " tide per day? "
>
> The above referred to the idealised 'ocean only' global model which
> approximates to the deep ocean 'driver' for local shallow water tides.
> As George Huxtable wrote recently the actual tide experienced locally
> depends on the configuration of resonant basins around coasts - some of
> which will resonate with apparantly one tide per day.
Sorry Geoff but the Newtonian model _doesn't_ approximate to deep-ocean
tides either. As I have already noted in this thread, the ocean isn't
deep enough for a wave to run around the planet in 24 hours and wouldn't
be even in the absence of any land masses (though I should have noted
that that is true at low latitudes -- clearly, it is not near the
poles). Even if Earth were a sphere of water, with no seabed at all, the
tides still wouldn't be able to follow the Newtonian model.
The deep oceans tides show a series of inter-linked resonant amphidromic
systems, no different in their basic nature from the ones long known in
the North Sea and similar basins.
You continued:
> Trevor J. Kenchington wrote:
>
> " > * .. so there are two raised ocean levels opposite one another and as the
> " > Earth rotates beneath one experiences locally two tides per day "
> "
> " All of which is correct, as far as it goes into detail, save for the
> " very last part: As I wrote earlier today, these two "raised" areas do
> " not really exist.
>
> I thought early observations from satellites had demonstrated that they do
> exist.
I don't know what early observations you are thinking of but it is
certain that no such "raised" areas exist. Satellite altimetry data has
been crucial to modern understanding of ocean tides but it has allowed
modelling of the amphidromic systems, has not confirmed something which
is been known not to exist. [If anyone wants more details, try a Web
search for "TOPEX/POSEIDON". The models and resulting charts appeared in
the mid-1990s.]
> " ... but even Medieval knowledge of the tides of the English Channel
> " was sufficient to prove to any thinking person that there is not some
> " bulge moving westwards down channel at 15 degrees of longitude per hour.
>
> No, as above, the nature of flow of tides in the English Channel is a
> function primarily of the channel dimensions and configuration of the sea
> bed and not of the mechanism which instigates the driving impulses.
Again, there is no fundamental difference between the mechanisms in
narrow waters and those in the open ocean, though I will grant you that
that was not established until comparatively recently. It was late in
the 19th century that the notion of progressive tidal waves moving
around the Southern Ocean and propagating northwards up the Atlantic was
laid to rest. It then took another hundred years (plus the advent of
massive computing power and a whole lot of satellite data) for the
details of the standing-wave patterns in open ocean to be worked out.
Still, the idea of Newtonian "bulges" moving around the world has
_never_ been taken seriously by anyone but those who have trusted
over-simplified written accounts over common sense and personal
experience. Sir Isaac himself didn't think that such bulges existed. Nor
did any of his contemporaries. They just were not able to link Newton's
understanding of tide generating forces (which was and is correct) to
the observed tides. Fortunately, it later proved possible to combine
theory and observation into precise tidal predictions long before the
linking mechanism was worked out.
My stress on Medieval knowledge was a reaction (on over-reaction,
perhaps) to the renowned textbooks used by North American yachtsmen
which assure them that high-water springs always occurs when the Moon
crosses your meridian (or 180 degrees away). Chaucer's shipman knew that
that was not true and had assorted means of remembering the vulgar
establishments of all the ports he frequented. It is tragic that, so
many centuries later, common knowledge has actually regressed while
specialists have moved so far forward.
Trevor Kenchington
--
Trevor J. Kenchington PhD
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
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