Tides 1. What are tides? 2. What force(s) drive the tides? What observations can we make in order to test this hypothesis? p. 234

Portsmouth, New Hampshire at low tide photo by Mark Leckie 9/05

Portsmouth, New Hampshire at low tide; tidal range 2+ m photo by Mark Leckie 9/05

3a. Pick one of the sites. Does high tide occur at the same time every day? What time interval separates successive high tides? 3b. Is this interval (=tidal period) the same as observed in other towns? 3c. According to your ideas about what causes tides, should high and low tides occur at the same times at these sites? Why do they differ so much? p. 45-46

1. Cape Cod experiences twice-daily tides (2 high tides, 2 low tides = ‘semi-diurnal tide’) 2. The time between successive high tides is ~12 hr., 25 min. (this is the tidal period) 3. The timing of the tides can vary considerably along the coast, yet all the towns experience semi-diurnal tides with a period of 12 hr., 25 min.

rotation about its axis It takes 24 hours for the Earth to make one complete rotation about its axis p. 234-235 First Quarter (quadrate) 7 days later Full Moon (conjunction) The Moon revolves around the Earth in the same direction as Earth’s rotation; it takes 29.53 days for the Moon to make one complete revolution around the Earth New Moon (conjunction) Because the Moon is moving as the Earth is spinning, it takes an extra 50 minutes for any place on the Earth to “catch up” to the Moon. Therefore, the Moon rises 50 minutes later each night (24 hr., 50 min.) Third Quarter (quadrate)

Day 4 Day 8 Day 15 Day 23 Day 27 Full Moon New Moon is “invisible” because the face we see at that time is not illuminated by the Sun. It is at Days 1 and 30 in the figure. http://www.graviton.demon.co.uk/phases_of_the_moon.htm

The tides are the result of gravitational forces and centrifugal (or inertial0 forces. The gravity of the Moon pulls the ocean toward it.

The Moon’s gravity also tugs on the Earth itself and makes the Earth wobble in a circular motion as the Moon orbits it. This causes some of the ocean to get “thrown away” from the Moon due to centrifugal or inertial force. This produces a bulge on the opposite side of the Earth.

Taken together, there is one bulge facing the Moon and a second on the opposite side of the Earth. These bulges slowly track the Moon in its monthly orbit. The Earth daily spins through these bulges creating the daily rising and falling of the tides. From the point of view of someone on the Earth the tidal bulges seem to move across the oceans.

Based on this information, what would you predict the pattern of tides to be? p. 234-235 Earth & Moon rotate about a common center of mass. This produces 2 tidal bulges: 1. Gravitational attraction 2. Centrifugal force According to the equilibrium model, there should be two high tides and two low tides per day. Earth rotates under the two tidal bulges

Tides If the moon is the principal cause for the daily or twice-daily rise and fall of the ocean surface, then what would you predict would be the time between successive tides? for a daily tide? for a twice-daily tide? 24 hours, 50 minutes 12 hours, 25 minutes p. 234-235

Now, the Sun is much farther from the Earth than the Moon, but because it is so much more massive than the Moon, it can also have a tidal effect on the oceans. However, the solar effect is only about 46% of the lunar effect. The two solar tidal bulges track the Sun not the Moon, so the solar tides follow a 24 hour clock, not the lunar 24 hr 50 min clock. This means that as the Moon orbits its tidal bulges constructively interfere (work with) or destructively interfere (work against) with the solar tidal bulges.

Twice a month there is constructive interference, that is, the lunar high tides and low tides are respectively on top of solar high tides and low tides. The Moon, Sun and Earth are aligned at these times of New and Full Moon. At these times the high tides are extra high and the low tides are extra low. These Spring Tides happen twice a month. The name has nothing to do with the spring season.

Twice a month there is destructive interference, that is, the lunar high tides are on top of the solar low tides and the lunar low tides are on top of the solar high tides. The smaller solar tides (remember only 46%) partly cancel out the lunar tides. At these times of First Quarter and Third Quarter Moons, the Moon, Earth and Sun form a right angle, and we have Neap Tides. At neap tides the high and low tides are at their smallest, that is, the tidal range is the smallest.

In these tidal records you can make out the daily rhythm of high and low tides, but superimposed on that is the cycle of spring and neap tides causing the tidal range to vary.

Tide-Generating Forces are Complex Tidal behavior on Earth is influenced by: Coriolis effect and friction interference of the continents on the movement of the tidal bulges (waves) size, shape, and depth of the ocean basins, continental shelves, and coastline Tide prediction: >150 variables (tide-generating & tide-altering factors) the 7 most important can be used to mathematically predict the tides (“tide tables”) p. 144

Tidal Movement Around the Ocean Basins 1. Progressive Tide the tidal bulge (wave) moves progressively down the ocean basin (e.g., South Atlantic) 2. Rotary Standing Wave the tidal bulge is confined to an ocean basin (e.g., North Atlantic) the wave rotates around a node that has no tidal range (= amphidromic point) p. 144

Progressive Tides vs. Rotary Standing Waves Amphidromic Points Rotary Standing Waves Progressive Tides Progressive Tides vs. Rotary Standing Waves

Tidal Patterns 1. Diurnal Tide (= daily) 1 high and 1 low tide/lunar day tidal period = 24 hrs., 50 min. common around the Gulf of Mexico 2. Semidirunal Tide (= twice daily) 2 high and 2 low tides/lunar day tidal period = 12 hrs., 25 min. common along the Atlantic coast of the U.S. 3. Mixed Tide either 1 or 2 high tides/lunar day heights of successive tides are significantly different common along the Pacific coast of the U.S. p. 144

Semi-diurnal Diurnal Mixed 12-day record 30-day record Semi-diurnal Diurnal Mixed http://tidesandcurrents.noaa.gov/station_retrieve.shtml?type=Tide+Predictions

Tide Summary Tides are very complex (of the >150 variables that influence tides, 7 are used to accurately predict tides). the Moon has the greatest influence on the tides; the Moon has twice the tide-generating force as the Sun because the Moon is so close to Earth Lunar day: 24 hours, 50 minutes (the moon rises 50 minutes later each night): this is responsible for the phases of moon we observe on Earth. Spring tides - every-other week during New Moon and Full Moon Sun and Moon forces are additive: higher high tides, and lower low tides Neap tides - every-other week during 1st and 3rd quarter Moon Sun and Moon forces are 90° to each other: lower high tides, and higher low tides Continents greatly affect the movement of tides through the ocean basins; therefore, tides move through ocean basins as progressive tides or rotary standing waves (around an amphidromic point). 3 tidal patterns: diurnal tide (1 high tide/day; tidal period: 24 hr. 50 min.) semidiurnal tide (2 high tides/day; tidal period: 12 hr. 25 min.) mixed tide (1 or 2 high tides/day of different amplitude

Tides are shallow water waves p. 144 Tides drawn up by the gravitational attraction of the moon are long wavelength waves L = half the circumference of the Earth Tides are shallow water waves

Harnessing Tiday Energy for Electricity Dam of the tidal power plant on the estuary of the Rance River, Bretagne, France. The world's first commercial axial turbine tidal stream generator — SeaGen — in Strangford Lough, Northern Ireland. The strong wake shows the power in the tidal current. An artistic impression of a tidal barrage, including embankments, a ship lock and caissons housing a sluice and two turbines. http://en.wikipedia.org/wiki/Tidal_power