# Tides

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```					Tides

Chap 11
Tides
   Tides in the eastern Bay of Fundy on
the Atlantic coast of Canada. (f.11.31)
   Tidal range is 15 meters (50 feet).
   Water rises 1 meter (3.3 ft.) in 23
minutes.
   The tides are the regular rising and falling of
sea level.
   They are caused by the gravitational
attraction of the moon and the sun.
   These celestial bodies pull on the Earth.
   The solid Earth moves only a tad toward the
moon and sun, but the liquid ocean moves
more.
   Let's look at the effect of the moon,
first.
   It has a much stronger effect on the
tides because it is so much closer:
   The pull of the moon causes the ocean
to 'bulge' out away from Earth and
toward the moon:
   The water level is also low along the equator
in the direction toward you and away from
you.
   Where the bulge occurs is high tide; where
the low points occur (toward you and away
from you) is low tide.
   The Earth rotates every 24 hours, so points
on Earth rotate into the high tide, then into
the low tide.
   Now here comes one of the many bizarre
aspects of the tides: there is also a tidal
bulge on the opposite side of the planet:
   How is this possible? Centrifugal force.
   Swing an object attached to a string in a
circle.
   Your hand is pulling the object toward you,
but another force must be pushing the object
away from you! That force is centrifugal
force (centrifugal means 'fleeing the center').
   The tide is a
standing wave: it
has two crests
(high tide), and
two troughs (low
tide). (f.11.24)
   The Earth rotates
once every 24
hours.
   If you were to plot sea level over this 24
hour period, it would look like, f.11.25.
   Such a tide is called semidiurnal,
because there are 2 high tides and 2
low tides every day.
   The difference in height between high
tide and low tide is called the tidal
range.
   How high the high tide gets, and how low the
low tide gets, depends on the other celestial
body we have ignored up until now: the sun.
   The effects of the sun are to raise high
tides to a maximum, when the sun's gravity
pulls in the same direction as the moon's,
and to lower high tides to a minimum, when
the sun's gravity pulls in opposition to the
moon's.
Three types of tides:
1.   Semidiurnal: two high tides and two low
tides each day, both about the same
height
2.   Mixed: two high tides and two low tides
each day, but one high tide is higher than
the other
3.   Diurnal: one high tide and one low tide
each day.
Tidal Bore
   In macrotidal areas, the high tide may
come in as a visible wave called the
tidal bore.
   A tide crest encounter a narrow river
passage, and it rushes forward to
cause the bore (f.11.32)
Tidal Currents
•   When a point on Earth passes through
the tidal crest, high tide is coming, and
water moves onshore.
•   It may move up rivers as a visible bore,
or it may just rise almost imperceptibly.
•   Either way, the water moves onshore
and this is called the flood tide or flood
current.
•   At the center of the crest,
water stops moving
onshore and stands still
for a little while.
•   This is a stage of the tide
called slack water or slack
tide.
•   Now the point rotates into
the trough of the tide, and
low tide is coming.
•   Water changes direction
now and moves offshore,
back out to sea.
•   This is called the ebb tide or ebb
current.
•   Once the center of the trough is reached,
there is no more water movement
offshore and the water lingers again.
•   This is also called slack water or slack
tide.

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