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    Re: Global oceanic tides,
    From: Geoffrey Butt
    Date: 2003 Aug 26, 03:33 +0100

     Jared Sherman 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.
     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
     " ... 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.
     Thanks Trevor for the following clarification. I had probably not understood
     this to be the origin of the inequality in the alternate semi-diurnal
     " >  I haven't done the calculations myself but have read (somewhere) that the
     " >  two causes for ocean raising have slightly different magnitudes - which
     " >  explains why plotting sequential tidal ranges from tide tables results in
     " >  the 'odd' tides following a slightly different curve from that for the
     " >  'even' tides.
     " That, in contrast, is false. In Newtonian theory the two bulges are of
     " exactly equal magnitude -- as is actually true of the tide generating
     " forces.
     " The Newtonian explanation for the diurnal tides (which in most areas
     " appear only as the diurnal inequality between the heights of successive
     " semi-diurnal high waters and ditto for low waters) is that the Moon
     " (and the Sun) rarely have zero declination. When the Moon has a northern
     " declination, for example, the "bulge" under the Moon will lie north of
     " the Equator, while the antipodean "bulge" will lie at some southern
     " latitude. Now imagine an observer at any temperate northern latitude.
     " When the Moon passes his meridian, the observer will be quite close to
     " the "bulge" and so will experience most of its height. When the Moon
     " passes a meridian 180 degrees away, the observer will again experience
     " high tide but the peak of the "bulge" will be far to his southward so
     " his water level won't rise as high as it did 12 hours (and about 25
     " minutes) earlier. The difference in height of the two high waters is the
     " diurnal inequality and is the result of superimposing the diurnal tide
     " on the semi-diurnal.
     " Again: These "bulges" don't exist but the forces that try to raise them
     " do and the magnitude of those forces varies with the same pattern as
     " Newtonian theory pretends that the "bulges" vary.

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