Tides. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Key Concepts Tides are huge shallow-water waves-the largest waves in the ocean. Tides

Tides

© 2006 Brooks/Cole, a division of Thomson Learning, Inc.

Key ConceptsKey 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.

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.

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.


Key Ideas Continued…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.


Tides Are the Longest of All Tides Are the Longest of All Ocean WavesOcean 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.


The Movement of the Moon The Movement of the Moon Generates Strong Tractive ForcesGenerates 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.

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

TidesNewton deduced that:

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.

Tides

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

TidesWhat 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 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.



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.



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).



The formation of tidal bulges at points toward and away from the moon.



(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).



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.

Fig. 11-8, p. 303

The moon moves this much in 8 hours . . .

. . . and this much in 24 hours

North x

Pole

8:00 P.M.

8 hours

North x

Pole

12:50 P.M. on Day 2

50 min

1 Lunar day

Start

North x

Pole

Moon

Earth

Tidal bulges

Noon

Ro

tati

on

Stepped Art

North x

Pole

4:00 A.M.

8 hours

North x

Pole

Noon

1 Solar day

8 hours



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.



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.


Sun and Moon Influence Tides Sun and Moon Influence Tides TogetherTogether

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.


Sun and Moon Influence Tides Sun and Moon Influence Tides TogetherTogether

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.


The Dynamic Theory of TidesThe 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.


Tidal Patterns Center on Tidal Patterns Center on Amphidromic PointsAmphidromic 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).


Tidal Patterns Vary with Ocean Tidal Patterns Vary with Ocean Basin Shape and SizeBasin 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.


Tidal Patterns Vary with Ocean Tidal Patterns Vary with Ocean Basin Shape and SizeBasin 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.


Chapter in PerspectiveChapter 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.

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

Tides

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

Tides

• Wachapreague, Wachapreague Channel, Virginia

http://www.mobilegeographics.com:81/locations/6811.html

http://www.mobilegeographics.com:81/locations/6811.html

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

Documents

Tides. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Key Concepts Tides are huge shallow-water waves-the largest waves in the ocean. Tides