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
The tides are the regular rising and falling of
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
Let's look at the effect of the moon,
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
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
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
The Earth rotates
once every 24
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
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
Three types of tides:
1. Semidiurnal: two high tides and two low
tides each day, both about the same
2. Mixed: two high tides and two low tides
each day, but one high tide is higher than
3. Diurnal: one high tide and one low tide
In macrotidal areas, the high tide may
come in as a visible wave called the
A tide crest encounter a narrow river
passage, and it rushes forward to
cause the bore (f.11.32)
• 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
• 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
• 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
• 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