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Re: Tides and tidal bulge



James McLean wrote:
Anyhow, the calendar for the San Diego area shows approximately 2 high
tides per day, as expected. However, on closer inspection, the time of the
high tides *does not* coincide with when the moon is at zenith. In fact, I
found 2 days for which the tides were almost identical, one on a new moon
and one on a full moon!
Jim, this is to be expected. The tidal stresses are due to the *gradient* of
the gravitational field--NOT the field itself--in the vicinity of the earth
due to the other heavenly bodies. Since nearly all of this is due to the sun
and the moon and since both distance earth-sun distance and the earth-moon
distance is, to a first approximation (ignoring elliptical orbit and relative
orbital plane tilt effects), the same for a new moon as for a full moon, it is
easy to show that the gradient of the combined gravitational field of the moon
and of the sun is nearly the same in these two (new and full phase) cases.
Remember, new and full phases of the moon induce Spring (strong) tides and 1st
and 3rd quarter phases of the moon induce Neap (weak) tides. Of course there
may be a phase shift at San Diego such that the actual arrival of the Spring/
Neap tides at San Diego may not coincide with the lunar phases which induce
them.

David Bowman
gtc.georgetown.ky.us