Download What are they? What are they caused by? Are there different kinds of

2/16/2015
Tides
What are they?
What are they caused by?
Are there different kinds of tides?
• Those of you who have spent some time on
the beach have probably noticed that the
water level of the ocean does not stay
the same all day long. Although waves
continually wash the shore, the actual water
level changes as the day passes.
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• It rises for a period of
hours until it reaches
its highest level, and
then it begins to fall,
or recede, to its
lowest level. This
rise and fall of the
ocean is known as
the tide. Its highest
and lowest levels are,
of course called high
and low tides.
High Tide
Low Tide
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Tides – What are they?
• Tides are changes in the level of ocean
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water.
They are the periodic rise and fall of the
water level in the oceans and other large
bodies of water.
I wonder how often this periodic rise and
fall happens each day?
What causes tides?
What moves ocean water?
• Wind
• Earthquakes
• Plate tectonics
• The dominant force that moves ocean
water is the gravitational forces from
both the sun and the moon.
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Early thoughts about what
caused the tides.
• Some believed the earth was a living
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animal and the tides a result of its
breathing.
Others, who thought the ocean’s waters
were the earth’s blood, decided the tides
were its pulse, caused by the beating of
its great heart.
Early thoughts about what
caused the tides.
• Early followers of the religious prophet
Muhammad believed that the rise and fall of the
waters were caused when the “Angel of the Sea”
placed his foot in and out of the ocean.
• Primitive natives thought the tides were a sign
of a sea god’s anger and made human sacrifices
to please him.
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The Lure of the Moon
• The relationship between the phases of
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the moon and the tides was discovered
more than 2,000 years ago!
People of the time knew the moon had
something to do with the tides, but they
couldn’t explain why.
Sir Isaac Newton to the Rescue!
• Newton’s Universal
Gravitation theory
helped to explain
how the gravity of
the moon (and the
sun) were related
to tides.
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What causes tides?
• The gravity of the moon pulls on
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everything on the Earth.
The moon’s gravitational pull on liquids is
much more noticeable than on solids.
Why? Because liquids move more easily
than solids.
What causes tides?
• How often tides occur and the difference
in tidal levels depend on the position of
the moon as it revolves around the Earth.
• The moon’s pull is strongest on the part of
the Earth directly facing the moon.
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High Tide
• The part of the ocean directly facing the
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moon, bulges towards the moon.
There is a corresponding bulge on the
opposite side of the Earth.
These bulges are called high tide.
Tides newsflash.mpg
Low Tide
• When high tides occur, water is drawn
away from the area between the high
tides, which causes low tides to occur.
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How often is there a change in tides?
Try to figure it out . . .
• High tides happen on opposite sides of the
Earth.
• The Earth rotates once on its axis every
24 hours.
• How many high tides would any one place
in the ocean experience each day?
Answer
• The periodic rise and
fall of the water level
in the oceans and
other large bodies of
water happens . . .
• Twice each day!
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But then why are tides about 50
minutes later each day?
Try to figure it out . . .
• In which direction does the Earth rotate?
• In which direction does the Moon orbit the
Earth?
Answer
• In one day, the Moon has
completed about 3.6% of its full
orbit of Earth. It has moved slightly
ahead of the Earth (from position 1
to position 2).
• So the Earth has to rotate more
than a full day to put Anchorage
directly in line with the Moon again.
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What effect does the sun
have on tides?
• The sun is much larger than the moon.
• The sun is farther away from the Earth
than the moon.
Q: Which one (the sun or the moon) do you
think has a bigger effect on tides?
A: The moon, just because it is closer.
Tidal Variations
• Tidal Range = the difference in levels of
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ocean water at high tide and low tide.
Tidal ranges vary greatly due to the
combined forces of the sun and the moon
on the Earth
It all depends on the position of the Earth,
moon, and the sun in space.
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Spring Tide
• A tide with a large
tidal range that
occurs two times a
month (new moon
and full moon)
• Sun, Earth, and the
moon are aligned
Neap Tides
• A tide of minimum
range that occurs
during the first and
third quarters of the
moon
• The sun, Earth, and
moon form a 90
degree angle
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When the moon is new
or full, the gravitational
forces of the sun and
moon are pulling at the
same side of the earth.
(See the diagram
below.) This occurrence
creates the extra large
"spring" tides.
When the moon is at first
and third quarter, the
gravitational forces of the
sun and moon are pulling
at 90 degrees from each
other. (see the diagram
below.) This occurrence
yeilds little net tides called
neap tides.
Earth Revealed tides.mpg
Tide-producing forces
• Gravity and
motions among
Earth, Moon,
and Sun
Fig. 10-2
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Month tidal cycle
• Spring tides
– New Moon, Full
Moon
– Earth, Moon,
Sun syzygy
– Higher than
usual high tides
Fig. 10-12
Neap tide
– First Quarter,
Last Quarter
– Earth, Moon,
Sun quadrature
– Lower than
usual high tide
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Unequal tidal range
Fig. 10-15
Tidal bores
• Wave created by
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tide rushes
upstream
Large tidal range
Low-lying coastal
river
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P1000235.MOV
P1000238.MOV
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Weird Science!
• The moon also creates tides in our
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atmosphere
They are called lunar winds
They can be detected by studying slight
periodic fluctuations in weather patterns
Very strange, indeed!
Review
Q: What are tides?
• Tide = the periodic rise and fall of the water
level in the oceans and other large bodies of
water.
Q: What is the dominant tidal forces that move
ocean water?
• The gravitational forces from both the sun and
the moon.
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Review (cont’d)
Q: Where is the moon’s pull strongest on
the Earth?
• The moon’s pull is strongest on the part of
the Earth directly facing the moon.
Review (cont’d)
Q: Which one (the sun or the moon) has a
bigger effect on tides?
• The moon
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Review (cont’d)
Q: What is the difference in levels of ocean
water at high tide and low tide called?
• Tidal Range
Q: Tidal ranges vary greatly due to the
combined forces of the _________ and
the ________ on the Earth
• Sun, moon
Review (cont’d)
Q: How are the sun,
Earth, and moon
situated in space so
that we experience a
spring tide?
• Sun, Earth, and the
moon are aligned
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Review (cont’d)
Q: How are the sun,
Earth, and moon
situated in space so
that we experience a
neap tide?
• The sun, Earth, and
moon form a 90
degree angle
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