Tides have been so important for commerce and science for so
many thousands of years that tides have entered our everyday
language: time and tide wait for no one the ebb and flow of events
a high-water mark and turn the tide of battle
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High Tide Bay of Fundy
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Low Tide
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Tides Tides are generated through a combination of gravity and
the motions of the Earth, Moon and Sun.
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Tides It is gravity that tethers the sun, planets, and moon
together and that tugs every particle of water on Earth toward the
Moon and the Sun.
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Does the moon or sun drive the tides? BOTH! But mostly the
moon! Why?
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Tides Although the gravitational attraction between Earth and
the Sun is more than 177 times greater than between Earth and the
Moon, the Moon dominates the tides because the Sun is 390 times
farther from Earth than the moon, so its tide generating force is
reduced to only 46% that of the moon.
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The Earth Moon System Earth and Moon travel through space as a
system You can visualize this by imaging Earth and the Moon as ends
of a sledge hammer, flung into space, tumbling slowly end over
end.
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The Earth Moon System So, as Earth and the Moon orbit the Sun
together, they rotate around the center of mass of the Earth-Moon
system, which is called the barycenter.
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Gravity Newtons law of gravitation states: every particle of
mass in the universe attracts every other particle of mass. The
greater the mass of the object and the closer they are together,
the greater the gravitational attraction
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Earth-Moon System In an idealized case, the Moon creates two
bulges in the ocean surface: one that extends toward the Moon and
the other away from the Moon. As Earth rotates, it carries various
locations into and out of the two tidal bulges
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Tidal Bulges: The Moons Effect Lunar bulges in an ideal ocean
and ideal Earth Tidal periods- time between high tides would be 12
hours Reality high tides occur 12 hours and 25 minutes. WHY?
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Lunar vs. Solar Day During the 24 hours is takes Earth to make
a full rotation, the moon has continued moving another 12.2 degrees
to the east in its orbit around Earth. The Earth must rotate an
additional 50 minutes to catch UP to the Moon Lunar day = 24 hours
and 50 minutes Solar day = 24 hours Lunar tides happen (typically)
twice every lunar day.
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Types of Tides Ideally, we expect two high tides and two low
tides of unequal heights during a lunar day.
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Types of tides Due to modification from varying depths, sizes,
and shapes of ocean basins, tides in many parts of the world
exhibit different patterns: 1. Diurnal have a single high and low
water each lunar day Common in the Gulf of Mexico and the coast of
SE Asia Tidal period of 24h 50m 2. Semidiurnal have two high and
tow low tides during a lunar day. Common along the Atlantic Coast
of the U.S. 12h 25m 3. Mixed tides- Have characteristics of both
diurnal and semidiurnal tides Common throughout the world and is
the type found along the Pacific Coast of the U.S Tidal period of
12h 25m
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How do tides vary during a monthly tidal cycle? Monthly tidal
cycle is 29 days. Moon to complete an orbit around Earth. This
influences the tidal conditions
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Combined Effects of Sun and Moon When the moon is at
quadrature, in either quarter phase, the tide generating force of
the sun is working at right angles to the tide generating force of
the moons. During these times of the month, we experience a minimal
tidal range. This is called neap tide The time that elapses between
successive spring tides ( full moon and new moon) or neap tides (
first and third quarters) is a little more than 2 weeks.
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Combined Effects of Sun and Moon When the sun and the moon are
aligned, either with the moon between earth and the sun(new moon),
or with the moon on the side opposite the sun(full moon), the tide-
generating forces of the sun and moon are added together During
these times of the month, we experience the highest and lowest
tides. This is called spring tides( because the tide surges, not
because of the season.) Syzgy!
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Do tides affect marine life? Tidal pools What organisms live in
tide pools? How do they survive the area they live in?
https://www.youtube.com/watch?v=ZkUL AD8gJT0
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Tide- Generating Force To understand the tide generating force,
you need to understand centripetal force. Centripetal force- pulls
orbiting body toward the center of its orbit.
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Tide- Generating Force The tide generated on Earth is primarily
the result of the rotation of Earth and the Moon about their common
center of mass, the barycenter. For all particles on Earth, the
gravitational attraction of the Moon is directed toward the center
of the Moon. The strength of this gravitation attraction is greater
for particles closer to the Moon. The more distant from Earth the
tide-generating body, the smaller the force. This is why the Moon
controls tides far more than the sun.
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Equilibrium Theory of Tides Factor that affect the equilibrium
theory of tidal bulges: Assume the Moon is aligned with Earths
equator so that the maximum bulge occurs at the equator on opposite
sides of the Earth. Earth requires 24 hours for one complete
rotation and so, at the equator we experience two equal high tides
and two equal low tides each day. This pattern is call the
semidiurnal tides, there are two tidal cycles per 24 hour day.
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Equilibrium Theory of Tides Ignore some of the complexities of
real tides, but provides good model of gross tide phenomena Some of
the simplifying assumptions made by the theory are; 1. Earth has
two equal tidal bulges, one toward the Moon and one away from the
Moon 2. The ocean covers the whole Earth at uniform depth 3. The
continents have no influence
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Gravity The gravitational attraction varies with the square of
distance, so even a small increase in the distance between two
objects significantly decreases the gravitational force between
them. When an object is twice as far away, the gravitational
attraction is only one quarter as strong.
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Lunar vs Solar day - Which is longer?
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Lets get some definitions: Syzygy When the earth, moon and sun
are aligned. Tidal range The vertical difference from high to low
tides. Spring tide Large Tidal range. (Very high highs and very low
lows.) Occurs when the moon is in syzygy. Neap tide Small Tidal
range. (Low highs, and high lows.) Occurs when the moon is in the
quarter position or quadrature
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What about the sun? Does the sun have as big of an effect as
the moon? Recall:
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Size and distance relations.
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How does the Sun measure up? Sun is 27 million times the size
of the moon. Sun is 390 times farther away from the Earth than the
moon. Lots of fancy math later The sun bulge is about 46% of the
lunar bulge.
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Lunar Tide + Solar Tide What happens when two waves are in the
same spot at the same time. Hint it starts with an I told you about
this stuff last time
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Stupid Interference
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What causes Neap and Spring Tides? They are the result of
interference between solar tides and lunar tides.
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Where do we get Neap and Spring Tides?
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We are done with notes for now. But I got a fun assignment You
guys rock!!!
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Earth-Moon System So the bulge or the tide Is it that the tides
move in and out, or that the Earth rotates through hills of water
called tidal bulges? What is the period for tides then?
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The Earth- Moon System The barycenter is not located in the
space between Earth and the Moon. Instead is it located with in
Earths mantle 4700km (2900mi) from planets center. The tidal
pattern we see on Earth primarily results from this sledge hammer
rotation of the Earth-Moon system around its center of mass.
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Earth-Moon System The gravitational forces on objects located
at different places on Earth due to the Moon are shown by arrows.
The length and orientation of the arrows indicate the strength and
direction of the gravitational force.
