Prai Jei wrote in
:

dlzc (or somebody else of the same name) wrote thusly in message
.com:

On Feb 13, 11:31 am, wrote:
Can someone please explain why the gravity of the moon
(which passes over our beach once a day) causes two
high tides per day? Is it due to the natural frequency of
the combined large bodies of water being approx double
the frequency of the moon's appearance? Or is it
something else?

More than one way to look at this. Consider "chunks" of water on the
Earth, looking down on the North pole, with Moon at 12:00. The chunks
at 12:00 and 6:00 have the Moon acting in concert with Earth, the
"lines of action" are parallel. The chunks at 3:00 and 9:00 are free
to move towards/away from the Moon, because they are not getting (very
much) farther from the Earth by doing so. So for a lowest harmonic
solution with two minima (3:00 and 9:00), yields two maxima. Of
course this is only sloppy, so...

http://en.wikipedia.org/wiki/Tide
... down to Tidal Physics

David A. Smith

OK, now could somebody explain why the time of high tide varies so much
with longitude e.g. six hours between Cardiff and London despite the
longitude difference being only about 4 degrees so moonrise is only
about 15 minutes later in Cardiff than in London.

This is explained in the wiki article cited by David in the port you
replied to.

Klazmon.