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