Transcript Slide 1
Why Are There Tides?
The tides are caused by the gravitational pull of the
moon and sun and by the rotations of the earth, moon,
and sun.
The Earth and moon both rotate around a common
point
This rotation produces centrifugal force
Why Are There Tides?
Centrifugal force and the moon’s gravity are not in
perfect balance.
On the side of the earth nearest the moon, the moon’s
gravity is strongest and pulls the water towards the
moon
On the other side away from the moon, centrifugal
force predominates, pushing the water away from the
moon.
Why Are There Tides?
In addition to the rotation of the earth and moon the
earth is spinning on its axis.
High tide occurs when the point is under a bulge
Since the earth takes 24 hours to complete a rotation,
the point will have 2 high tides and 2 low tides every
day.
The moon advances in its orbit too. It takes the point
on earth an extra 50 minutes to catch up and be under
the moon
High tide – low tide
Cont.
A full tidal cycle takes 24 hours and 50 minutes
The sun produces tidal bulges too.
Although the sun is larger than the moon, it is 400
times further away therefore, the pull is about half as
much.
When the sun and moon are in line, which happens at
full and new moons, their effects add together.
Spring Tides
At full and new moons, when the moon and sun are in
line, the difference in eater level between successive
high and low tides is large.
Spring tides occur throughout the year about once
every two weeks
Neap Tides
When the sun and moon are at right angles their
effects partially cancel each other.
During these neap tides the tidal range is small
Neap tides occur when the moon is in first and third
quarter
Tides In The Real World
Tides vary from place to place depending on the
location and on the shape and depth of the basin
As predicted, most places do have two high tides and
two low tides a day : Semidiurnal tides
Some places have mixed semidiurnal tides, with
successive high tides of different height (western US,
Canada)
Cont.
Diurnal tides occur when there is only one high tide
and one low tide every day
Diurnal tides are uncommon. They occur in the Gulf
of Mexico, on the coast of Antarctica, and in parts of
the Caribbean and Pacific.
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