Transcript chapter11

Tides
Key Concepts

Tides are huge shallow-water waves-the largest waves in the ocean. Tides are caused by a
combination of the gravitational force of the moon and sun and the motion of Earth.

The moon's influence on tides is about twice that of the sun's.

Gravity and inertia cause the ocean surface to bulge. Tides occur as Earth rotates beneath
the bulges.
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The equilibrium theory of tides deals primarily with the position and attraction of the Earth,
moon, and sun. It assumes that the ocean conforms instantly to the forces that affect the
position of its surface, and only approximately predicts the behavior of the tides.
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The dynamic theory of tides takes into account the speed of the long-wavelength tide wave
in water of varying depth, the presence of interfering continents, and the circular
movement or rhythmic back-and-forth rocking of water in ocean basins. It predicts the
behavior of the tides more accurately than the equilibrium theory.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Key Ideas Continued…

Tides caused by the interaction of the gravity of the sun, moon, and Earth are known as
astronomical tides. Meteorological tides, caused by weather, can add to or detract from the
height of tide crests.

The rise and fall of the tides can be used to generate electrical power, and tides are
important in many physical and biological coastal processes.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Tides Are the Longest of All Ocean
Waves
What are the characteristics and causes of tides?
• Tides are caused by the gravitational force of the moon and
sun and the motion of earth.
• The wavelength of tides can be half the circumference of
earth. Tides are the longest of all waves.
• Tides are forced waves because they are never free of the
forces that cause them.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
The Movement of the Moon
Generates Strong Tractive Forces
A Planet orbits the sun in balance between gravity and inertia. (a) If the planet is not
moving, gravity will pull it into the sun. (b) If the planet is moving, the inertia of the planet
will keep it moving in a straight line. (c) In a stable orbit, gravity and inertia together cause
the planet to travel in a fixed path around the sun.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Tides
How do these millions and millions of
objects in space stay in concert with one
another?
Sir Isaac Newton figured it out in the latter
part of the 17th Century
Newton deduced that:
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Every particle of mass in the universe attracts
every other particle of mass. This occurs
with a force that is directly proportional to the
product of their masses, and inversely
proportional to the square of the distance
between the masses. The greater the mass
of the objects and the closer they are
together, the greater the gravitational
attraction.
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Mathematically this can be expressed as:
G(m1m2)
(
r2)
However when considering tides the proportion is
skewed making distance more important than
mass. So:
Tide Generating force =G m1m2
r3
Tides
What is important about the sun as far tides are
concerned?
• Size /Mass
– Moon 7.3 x 10 10
– Sun
2.0 x 10 27
• Earth orbits around it
– Elliptical orbit
– ecliptic
• Distance from earth
– Perihelion 92.2 million miles / winter months
– Aphelion 94.5 million miles / summer months
Distance matters
– Apogee
– Perigee
– Aphelion
– Perihelion
Astronomical tides
Tides
Tides
The Movement of the Moon
Generates Strong Tractive Forces
The moon does not rotate around the center of Earth. Earth and moon
together – the Earth – moon system – rotate around a common center
of mass about 1,650 kilometers (1,023 miles) beneath Earth’s surface.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
The Movement of the Moon
Generates Strong Tractive Forces
The moon’s gravity attracts the ocean toward it. The motion of Earth around the center of
mass of the Earth – moon system throws up a bulge on the side of Earth opposite the
moon. The combination of the two effects creates two tidal bulges.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
The Movement of the Moon
Generates Strong Tractive Forces
The action of gravity and
inertia on particles at five
different locations on Earth.
At points (1) and (2), the
gravitations attraction of the
moon slightly exceeds the
outward-moving tendency of
inertia; the imbalance of
forces causes water to
move along Earth’s surface,
converging at a point toward
the moon. At points (3) and
(4), inertia exceeds
gravitational force, so water
moves along Earth’s
surface to converge at a
point opposite the moon.
Forces are balanced only at
the center of Earth (point
CE).
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
The Movement of the Moon
Generates Strong Tractive Forces
The formation
of tidal bulges
at points toward
and away from
the moon.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
The Movement of the Moon
Generates Strong Tractive Forces
(a) How Earth’s rotation
beneath the tidal bulges
produces high and low
tides. Notice that the
tidal cycle is 24 hrs 50
minutes long because
the moon rises 50
minutes later each day.
(b) A graph of the tides
at the island in (a).
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
The Movement of the Moon
Generates Strong Tractive Forces
A lunar day is longer than a solar day. A lunar day is the time that elapses between the
time the moon is highest in the sky and the next time it is highest in the sky. In a 24-hour
solar day, the moon moves eastward about 12.2°. Earth must rotate another 12.2° - 50
minutes – to again place the moon at the highest position overhead. A lunar day is
therefore 24 hours 50 minutes long. Because Earth must turn an additional 50 minutes
for the same tidal alignment, lunar tides usually arrive 50 minutes later each day.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
The moon moves this
much in 8 hours . . .
. . . and this much in
24 hours
Moon
Rotation
Earth
North
x
Pole
Tidal
bulges
Noon
Start
8
hours
North
x
Pole
North
x
Pole
8:00 P.M.
4:00 A.M.
8
hours
North
x
Pole
North
x
Pole
12:50 P.M.
on Day 2
50
min
1 Solar
1 Lunar
day
day
Noon
8
hours
Stepped Art
Fig. 11-8, p. 303
The Movement of the Moon
Generates Strong Tractive Forces
Tidal bulges follow
the moon. When the
moon’s position is
north of the equator,
the gravitational
bulge toward the
moon is also located
north of the equator
and the opposite
inertia bulge is below
the equator.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
The Movement of the Moon
Generates Strong Tractive Forces
How the changing position of the moon relative to Earth’s equator
produces higher and lower high tides. Sometimes the moon is below
the equator, and sometimes it is above.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Sun and Moon Influence Tides
Together
Relative positions of the sun, moon, and Earth during spring and neap tides. (a) At the new and full
moons, the solar and lunar tides reinforce each other, making spring tides, the highest high and
lowest low tides. (b) At the first-and third-quarter moons, the sun, Earth, and moon form a right angle,
creating neap tides, the lowest high and the highest low tides.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Sun and Moon Influence Tides
Together
Tidal records for a typical month at (a) New York and (b) Port Adelaide,
Australia. Note the relationship of spring and neap tides to the phases of the
moon.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
The Dynamic Theory of Tides
What are some key ideas and terms describing tides?
• The dynamic theory of tides explains the characteristics of
ocean tides based on celestial mechanics (the gravity of the
sun and moon acting on Earth) and the characteristics of fluid
motion.
• Semidiurnal tides occur twice in a lunar day
• Diurnal tides occur once each lunar day
• Mixed tides describe a tidal pattern of significantly different
heights through the cycle
• Amphidromic points are nodes at the center of ocean
basins; these are no-tide points.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Tidal Patterns Center on
Amphidromic Points
The development of amphidromic circulation
(a) A tide wave crest enters an ocean basin in the Northern Hemisphere. The wave trends to the right
because of the Coriolis effect (b), causing a high tide on the basin’s eastern shore. Unable to continue turning
to the right because of the interference of the shore, the crest moves northward, following the shoreline (c)
and causing a high tide on the basin’s northern shore. The wave continues its progress around the basin in a
counterclockwise direction (d), forming a high tide on the western shore and completing the circuit. The point
around which the crest moves is an amphidromic point (AP).
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Tidal Patterns Vary with Ocean
Basin Shape and Size
How do tides behave in confined basins?
The tidal range is determined by basin configuration. (a) An imaginary amphidromic system in a broad,
shallow basin. The numbers indicate the hourly positions of tide crests as a cycle progresses. (b) The
amphidromic system for the Gulf of St. Lawrence between New Brunswick and Newfoundland,
southeastern Canada. Dashed lines show the tide heights when the tide crest is passing.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Tidal Patterns Vary with Ocean
Basin Shape and Size
Tides in a narrow basin. (a) True amphidromic systems do not develop in
narrow basins because there is no space for rotation. (b) Tides in the Bay of
Fundy, Nova Scotia, are extreme because water in the bay naturally resonates
(seiches) at the same frequency as the lunar tide.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Chapter in Perspective
In this chapter you learned that tides have the longest wavelengths of the ocean's
waves. They are caused by a combination of the gravitational force of the moon and the
sun, the motion of the Earth, and the tendency of water in enclosed ocean basins to rock
at a specific frequency. Unlike the other waves, these huge shallow-water waves are
never free of the forces that cause them and so act in unusual but generally predictable
ways. Basin resonances and other factors combine to cause different tidal patterns on
different coasts. The rise and fall of the tides can be used to generate electrical power,
and are important in many physical and biological coastal processes.
In the next chapter you will learn how the interaction of wind, waves, and weather
affects the edges of the land – the coasts. Coasts are complex, dynamic places where
the only constant is change.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Tides
The sun and earth relationship …and the seasons
• Our sun is a star in a galaxy of stars called the Milky
Way
• There are billions of stars in the galaxy… and there
millions of galaxies
• Probably billions of satellites of the stars in the
universe
• All orbit in concert around through each other
Tides
So what does all that mean:
Tides
The Moon
• The moon has much greater influence than the sun
• Moon exerts over twice the force as the sun
• Why?
– Distance
– orbit
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Two theories of tide
generation
Equilibrium Theory
Dynamic Theory
Tides
Declination
• Earth revolves around the sun in an
elliptical pattern called the ecliptic
• Earth orbits around it
– Elliptical orbit
• Further earth is not perpendicular on
the elliptic but leans 23.5 degrees
Tides
• Declination of the moon and sun
• Earth requires 24 hours to make one
revolution around the sun
• You would assume that 2 highs and two
lows a day
• Tidal period = 12 hours
• They actually occur every 12h 50 minutes
Called lunar day
• Lunar hour therefore is 1 hour 2 minutes
Neap Tides
– Moon, earth an sun are at right angles
– High not very high and lows not very low
– Occur at two weak intervals
Spring Tides
– moon, earth and sun aligned
– High tides are very high and low tides are very
low
– Occur at two week intervals
Dynamic theory of tides
– Amphidromic Points
Node towsrd center of the ocean
No tides
Tidal crest rotates
About a dozen amphidromic points word
wide
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• Wachapreague, Wachapreague Channel,
Virginia
Tides
• See figure 11.5 260
• The moon however does not have a circular orbit it is some
what elliptical 5 degrees
• Distance of sun to earth 93 million miles
• Moos distance to earth 234,000 miles
• Barycenter
• Sun is only 46% gravitational pull
• Centripetal force – a center seeking force tens to make
rotating bodies move toward the center of the rotation
• Centrifugal force – a force that seems to make
• An object move away from the center of a curved path it is
following. Results from the application of a centripetal force
acting against inertia the object