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Rodney,

You wrote:

>>>A practical interface for this question would be tide tables for a
>>>mid-Pacific Island, such as Canton and Enderbury. That should give a
>>>good handle on the phase of the tidal bulge as it would be in a
>>>uniformly water-covered planet. I don't have such a tide table.
>>>
>>
>>Neither do I but I don't think it would show what you expect. The tides
>>of the mid-Pacific are dominated by amphidromic systems just as much as
>>those of the North Sea are. They do not resemble a "tidal bulge" on a
>>planet that lacked land masses. (I'm not sure that a planet without land
>>would have recognizable bulges anyway, unless the ocean was also
>>extremely deep and covering a very small solid core.)
>>
>>
> Myabe it wouldn't show the bulge. The bottom is anything but uniform.
> But I think the bulge would show in a quite recognizable form in an
> ocean over a sperical core, even if the ocean were as shallow as ours.
> The friction (hence, the "Q") would be different.


I'd not want to be dogmatic on this but my limited understanding is that
this is not a matter of friction but of water depth. The wavelength of a
semi-diurnal tidal "bulge" would clearly have to extend over 180 degrees
of longitude. From tropical to mid-temperate latitudes, that means that
the wave would be so long that even if the ocean were a uniform 6000
metres deep, the "bulge" would respond as a shallow-water wave. (Even
tsunamis do that in the real ocean and their periods are only 15 minutes
or so.) Thus, the speed of propagation of the tidal "bulge" would be
determined by water depth, not by the rate of rotation of the Earth
under the Sun and Moon. The tide generating forces would, therefore, not
be able to drag a "bulge" around with them and instead would set up the
sort of complex of resonance patterns that we see in the real open oceans.

It is not too hard (even for me!) to figure out how deep the ocean would
have to be to allow a tidal "bulge" to keep up with the Moon. At the
Equator, it would need an ocean nearly as deep as the radius of the
planet. (And I don't want to even contemplate the physics of wave
propagation when the circuit of the seabed is almost zero and that of
the surface is 22,000 miles.)


Corrections from the physicists on the list would be appreciated.


Trevor Kenchington


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Trevor J. Kenchington PhD                         Gadus@iStar.ca
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