Chapter 9 Tides

1. Chapter 9 Tides

2. Look For The Following Key Ideas In Chapter 9 • 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.

3. 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.

4. Tides and the Forces That Generate Them 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.

5. 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.

6. 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.

7. 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.

8. 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).

9. The Movement of the Moon Generates Strong Tractive Forces The formation of tidal bulges at points toward and away from the moon.

10. The Movement of the Moon Generates Strong Tractive Forces How Earth’s rotation beneath the tidal bulges produces high and low tides. Notice that the tidal cycle is 24 hours 50 minutes long because the moon rises 50 minutes later each day.

11. 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.

12. Moon Earth North x Pole North x Pole North x Pole North x Pole North x Pole Rotation Tidal bulges Noon 8:00 P.M. 4:00 A.M. Noon 12:50 P.M. on Day 2 50 min 8 hours 8 hours 8 hours Start 1 Solar day 1 Lunar day The moon moves this much in 8 hours . . . . . . and this much in 24 hours Stepped Art Fig. 10-8, p. 232

13. The Movement of the Moon Generates Strong Tractive Forces The 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.

14. 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. Follow the flag eastward around Earth, and see a “high” high tide followed by a “low” high tide.

15. Sun and Moon Influence the Tides Together top: The positions of the Sun, the moon and Earth during a spring tide. bottom: The positions of the Sun, the moon and Earth during a neap tide.

16. Sun and Moon Influence the 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. MHW = mean high water, MLW = mean low water

17. 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.

18. Tidal Patterns Center on Amphidromic Points Tide curves for the three common types of tides. (a) A mixed tide pattern at Los Angeles, California. (b) A diurnal tide pattern at Mobile, Alabama. (c) A semidiurnal tide pattern at Cape Cod, Massachusetts. (d) The worldwide geographical distribution of the three tidal patterns. Most of the world’s ocean coasts have semidiurnal tides.

19. Tidal Patterns Center on Amphidromic Points The development of amphidromic circulation

20. Tidal Patterns Vary with Ocean Basin and Size How do tides behave in confined basins? The tidal range is determined by basin configuration. (above) An imaginary amphidromic system in a broad, shallow basin.

21. Tidal Patterns Vary with Ocean Basin and Size Tides in narrow, restricted basins. Note that a true amphidromic system does not develop because space for rotation is not available.

22. Chapter 09 Summary 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.

23. End of Chapter 09