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.
• 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
Tides – What are they?
• Tides are daily changes in the level of
ocean 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?
• The dominant force that moves ocean
water is the gravitational force of the
moon.
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.
What causes tides?
• The gravity of the moon pulls on
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.
High Tide
• The part of the ocean directly facing the
moon, bulges towards the moon.
• There is a corresponding bulge on the
opposite side of the Earth.
• These bulges are called high tide.
Low Tide
• When high tides occur, water is drawn
away from the area between the high
tides, which causes low tides to occur.
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!
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.
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
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 force that moves ocean
water?
• The gravitational force of the moon!
Review (cont’d)
Q: Where is the moon’s pull is 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
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
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