TIDESMrs. Walker – Aquatic Science

I. tide: the periodic rise and fall (ebb) of ocean water a. tidal day: 24 hours and 50 minutes b. lunar month (tidal month) the approx. 29.5 days it takes

the moon to complete its orbit around the Earth

II. 3 types of tides: (determined by continental blockage, tilt of the Earth, movement of moon, latitude, length and depth of basin)

1. daily or diurnal tide: one high and low each day (Alaska and our Gulf Coast)

2. semidaily or semidiurnal: two high tides and two low tides each day at are approximately equal (East Coast)

3. Mixed Tides: two high and two low but all at different heights, so hardest to predict (West Coast)

tidal range: difference between the highest and lowest tide level (varies as a result of the changing position of the sun and moon with respect to the Earth)

III. Causes of Tides 1. gravitational pull of the moon a. gravity (attraction between two bodies), draws the moon and Earth toward each other b. water on the Earth in the region directly beneath the moon is pulled by gravitational force toward the moon (direct high tide) 2. centrifugal force of Earth as it rotates a. as the moon and Earth revolve around a central point, a balancing centrifugal force is created to keep them apart b. on the opposite side of the Earth (away from the moon), centrifugal force pulls water away from the Earth, opposing the gravitational force of the moon (indirect high tide)

3. gravitational pull of the sun a. being so far away it exerts less of a pull than the moon and just adds to the moon tide4. relative positions of the moon and sun a. Spring Tide: when the Earth, moon and sun are in a straight line, they exert maximum gravitational pull on the ocean surface (at new and full moon) and have a higher than normal high tide

b. Neap Tide: when the moon is in first and third quarters (two week intervals) and the sun and moon are at a right angle to each other, the two tend to neutralize each other’s tidal pull and you have a lower than normal high tide

5. angle of orbit of the moon around the Earth a. the moon does not maintain a fixed position relative to the Earth and the orientation of the tide generating force moves tooto the left)

6. each basin modifies its tides a. by the shape, water inlets and the Coriolis Effect (tendency of the path of ocean water moving in the Northern Hemisphere to deflect to the right and in the Southern Hemisphere the left)

7. effects of elliptical orbits: a. moon moves through an elliptical orbit about the Earth approximately once a month - Perigee: point nearest the Earth (221,463 miles) - Apogee: point farthest from the Earth (252,710 miles)

b. Earth moves through an elliptical orbit about the sun approximately once a year - Perihelion: Earth nearest the sun (2.7 million miles) - Aphelion: Earth farthest from the sun

8. declination of the moon: distance of moon north or south of the equator a. due to the Earth’s axis tilt, the sun and moon appear to move back and forth across the equator b. equatorial tides: when moon is over the equator, both hemispheres are equally affected by the moon, so the tides in both hemispheres are equal c. diurnal inequality: as the moon moves north or south of the equator, tides in the two hemispheres become unequal

IV. Tidal Currents: horizontal movement of water that accompanies the vertical movement of tides a. can be strong and navigators must know these to move safely out of inlets and bays

b. Types: 1. flood current: water moving in to shore while high tide is coming in (about 6 hours) 2. ebb current: water moving out from shore as tide receded (about 6 hours) 3. slack current: a period of no current while tide is reversing

V. Record Keeping and Tides: to forecast and record tides for safety and commerce 1. record datums: fixed reference (shoreline location) from which we reckon heights and depths a. each datum may be determined in relation to a time period (tidal epoch)

VI. Importance of Tides: 1. accurate forecast of tides is important for the safety of boats entering and leaving shallow harbors 2. Fishing: incoming tides bring new food supplies and fishing is usually best on incoming tides 3. Shellfish: low tide is best time for clam diggers and scallop draggers 4. Daily life cycle of organisms that live on the shore depends on the tides