1. CHAPTER 9 Tides

2. Overview Rhythmic rise and fall of sea level
Very long and regular shallow-water waves
Caused by gravitational attraction of Sun, Moon, and Earth
Earth's Circumference at the Equator: miles. 1/20th = 1245 miles

3. Weight
A measure of the pull of gravity

4. Gravity
The force of attraction between two objects. The amount of force is determined by 1. The mass of the objects. The more massive the object the greater the force. 2. The distance between the objects. The greater the distance the less the force.

5. Gravity – Earth, Sun, & Moon
Which is more massive? Sun Which is closer to the Earth? Moon Which has the greater gravitational effect on the earth?

6. Assumptions
The Earth is smooth (like a ball) – no continents. Water uniformly covers the Earth.

7. Gravitational forces Every particle attracts every other particle
Gravitational force proportional to product of masses
Inversely proportional to square of separation distance
Fig. 9.2

8. Earth’s Tidal Bulge
The Moon’s gravity pulls the water out to one side.

9. Centrifugal Force
Centrifugal force is a force outward from the center of rotation

10. Center of Mutual Orbit
If two objects have the same mass they will orbit about a point half way between them.
This point is called the
Center of Mass
Center of Gravity or
Barycenter
Google Barycenter select Center of Mass

11. Earth’s Barycenter
The Earth/Moon center of orbit is a point inside the surface of the Earth. But not the center of the Earth.
It is the point of mutual orbit due to gravity and motion.
Earth's Diameter at the Equator: miles. Barycenter is 1062 miles below Earth’s surface.

12. Earth/Moon Motion This is what the Earth/Moon orbit looks like.
Notice how the Earth “Wobbles”.

13. Equilibrium Bulge
The equilibrium bulge on the opposite side of the Earth from the Moon is caused by centrifugal force from the Earth’s Wobble.

14. The Tide is a Wave
The tide is a wave and it has two crests and two troughs.
The crest is high tide and the trough is low tide.
Trough
Low Tide
High Tide
High Tide
Crest
Crest
Trough
Low Tide

15. Tidal bulges (lunar)
Small horizontal forces push seawater into two bulges
Opposite sides of Earth
One bulge faces Moon
Other bulge opposite side Earth
Fig. 9.6

16. Tidal Day (Lunar Day)
It take 24 hours for the Earth to make one rotation.
But the Moon has moved.
It takes an additional 50 minutes to get back to the Moon

17. Lunar Day Moon orbits Earth
24 hours 50 minutes for observer to see subsequent Moons directly overhead
High tides are 12 hours and 25 minutes apart
Fig. 9.7

18. Tidal bulges (lunar) Tidal bulges (solar)
Moon closer to Earth so lunar tide-producing force greater than that of Sun
Tidal bulges (solar)
Similar to lunar bulges but much smaller
Moon closer to Earth

19. Spring Tide Sun & Moon Pull Together – New & Full Moon
Greatest tidal range (Distance from high to low tide)
Highest High Tide & Lowest Low Tide
Time between spring tides about two weeks

20. Neap Tide Sun & Moon Pull At 90o – Quarter Moons
Least tidal range (Distance from High to Low tide)
Lowest High Tide & Highest Low Tide

21. Idealized tide prediction
Two high tides/two low tides per lunar day
Six lunar hours between high and low tides

22. Complicating factors: Declination
Angular distance of the Moon or Sun above or below Earth’s equator
Sun to Earth: 23.5o N or S of equator
Moon to Earth: 28.5o N or S of equator
Shifts lunar and
solar bulges away from the equator
Unequal tides
Fig. 9.11

23. Declination and tides
Unequal tides (unequal tidal ranges)
Fig. 9.13

24. Complicating factors: Elliptical Orbits
Tidal range greatest at perihelion (January) and perigee
Tidal range least at aphelion (July) and apogee
Perigee and apogee cycle 27.5 days
Fig. 9.12

25. Real tides Earth not covered completely by ocean
Continents and friction with seafloor modify tidal bulges
Tides are shallow water waves with speed determined by depth of water
Tidal bulges cannot form (too slow)
Tidal cells rotate around amphidromic point

26. Tidal cells in world ocean
Cotidal lines
Tide wave rotates once in 12 hours
Counterclockwise in Northern Hemisphere

27. Fig. 9.14

28. Tidal patterns Diurnal – Gulf Coast One high tide/one low tide per day
Semidiurnal – East Coast (Virginia Beach)
Two high tides/two low tides per day
Tidal range about same (same height)
Mixed - West Coast
Tidal range different (not same height)
Most common

30. Tides in coastal waters
Standing waves
Tide waves reflected by coast
Amplification of tidal range
Example, Bay of Fundy maximum tidal range 17 m (56 ft)
Two equal waves coming together from opposite directions to make a standing wave (bottom).

31. Tides in coastal waters
Tidal bore in low-gradient rivers
Fig. 9A

32. Tidal Bore Surfing Wave http://www.youtube.com/watch?v=4ZuZiLuHM1A
Wave

33. Coastal Tidal Currents
Reversing current
High tide, Flood tide, Flood current seawater moves on shore
Low tide, Ebb tide, Ebb current seawater moves offshore
High velocity flow in restricted channels

34. Coastal tidal currents
Whirlpool
Rapidly spinning seawater
Restricted channel connecting two basins with different tidal cycles
Fig. 9.19

35. Tides and marine life
Tide pools and life
Grunion spawning
Fig. 9C

36. Tide-generated power Renewable resource
Does not produce power on demand
Possible harmful environmental effects
Most devices use tidal currents

37. End of CHAPTER 9 Tides