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WEDNESDAY SEPTEMBER 27 - INTRODUCTION TO TIDES
Waves depend on the wind and thus can be unpredictable. But tides result
from the highly predictable gravitational pull of the moon and the sun on
the earth. So high and low tides are known with great precision.
To understand how tides form, consider that the moon's pull on the earth is
slightly stronger on the side facing the moon. This causes the Earth to
toward the moon. Because water is more flexible
than rock, most of that stretching occurs in the oceans. This raises a
small "tidal bulge" on the side of
earth facing the moon and a second tidal
bulge on the opposite side of earth. Now, imagine the earth rotating, but
keep the tidal bulges in place: any location on earth will move alternately
into deeper and then shallower water, giving two high tides and two low
tides each day.
The sun is so far away that its tide-generating potential is less than
half the moon's, but the sun does change the details of the tides. For
example, when the moon is full or new, the sun's and moon's gravitational
pulls combine. This creates the so-called
"spring tides," with higher high
tides and lower low tides.