2. Navigation NAU 102 Lesson 24

3. Agenda Tides Definitions Importance Causes Tidal Cycles Reference Planes

4. Tides Why do we care? We don’t want to: Run aground. Hit the bridge. Snap the mooring lines.

5. Tide The vertical rise and fall of the ocean level due to the gravitational and centrifugal forces between the earth and the moon (and the sun).

6. Definitions High tide (or high water) – highest water level reached in a rising tide Low tide (or low water) – lowest water level reached in a falling tide Stand – the brief period at high & low water when no change in water level can be detected. Range – the difference in height between high tide and low tide.

7. Causes Centrifugal Forces Gravitational Forces

8. Causes Centrifugal Force A force that tends to impel a thing outward from the center of rotation.

9. Causes Centrifugal Force The Earth/Moon system revolves around its center of mass, the barycenter. Earth Barycenter Moon

10. Causes Centrifugal Force The Earth/Moon system revolves around its center of mass, the barycenter. Earth Barycenter Moon

11. Causes Gravitational Force Gravity forces the water to flow towards the Moon. Earth Moon

12. Causes Gravitational Force Gravity forces the water to flow towards the Moon. Earth Moon

13. Causes Resultant Force Earth Moon

14. Causes Resultant Force Earth Moon

15. Causes If the Earth was a perfect sphere, completely covered with water: Two High Tides & Two Low Tides Every Lunar Day (24 hours 50 minutes)

16. Daily Rotation of Earth Earth

17. Monthly Orbit of Moon Earth

18. Combination of Motions Earth

19. Orbit of the moon The moon’s orbit is elliptical. Its distance from Earth varies during the month. Earth apogee perigee Tides are 15%-20% higher than average at perigee

20. Effect of the Sun The Sun affects the tides as well. Due to distance, it has only 46% of the Moon’s effect.

21. Spring Tides High tides are higher than normal. Low tides are lower than normal. When the sun, Earth and moon are in line:

22. Neap Tides High tides are lower than normal. Low tides are higher than normal. When the sun, Earth and moon are 90° apart:

23. Tides Spring tides and neap tides each happen twice each month. Greatest range of tide occurs at perigean spring tides.

24. Local Tides If the Earth was a perfect sphere And, it was covered by a uniform amount of water There would be 2 high tides and two low tides each day at every location. The tide range would be about 12 inches at every location.

25. Local Tides Local differences affect both the period of the tide and tide range. Because land shape and water depth varies from place to place, the tides vary with location. In coastal areas, the range of tide can be as much as 50 feet!

26. Local Tides Bay of Fundy Nova Scotia

27. Tidal Cycles Semidiurnal Tide 2 high and 2 low waters each tidal day, with relatively small differences in the respective highs and lows. Tides on the Atlantic coast of the US.

28. Semidiurnal Tides

29. Tidal Cycles Diurnal Tide A single high and single low water occur each tidal day. The northern shore of the Gulf of Mexico, in the Java Sea, the Gulf of Tonkin, and a few other places.

30. Diurnal Tides

31. Tidal Cycles Mixed Tide 2 high and 2 low waters each day, with a large inequality in the high water heights, low water heights or both. Pacific coast of the US.

32. Mixed Tides

33. Reference Planes But, charted depth isn’t necessarily the depth of the water at the moment. How do we know how deep the water is? Look on the chart. It is an average of the depths taken at specific times, depending on the Sounding Datum.

34. Reference Planes Mean Sea Level (MSL) The average level of the ocean at a location. Data is collected over a 18.6 year period called the Nodal Period. Half the time the water is shallower than MSL. Half the time the water is deeper than MSL. Dangerous!

35. Reference Planes Mean low water (MLW) - the average of all low tides at a given place. Mean lower low water (MLLW) - the average lower low waters of each tidal day. (The sounding datum used for U.S. waters) Mean low water springs (MLWS) - the average level of the low waters at spring tides. (Used for some British charts)

36. Reference Planes Mean higher high water (MHHW) - the average higher higher waters of each tidal day. Mean High Water (MHW) - the average of all high tides at a given place. (The vertical datum used for U.S. waters) Mean high water springs (MHWS) - the average level of the High waters at spring tides.

37. Tides

38. Introduction to Navigation Questions?