1. 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

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

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

4. 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

5. 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.

6. rotation about its axis
It takes 24 hours
for the Earth to
make one complete
rotation about its axis p
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 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)

7. 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.

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

9. 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.

10. 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.

11. Based on this information, what would you predict
the pattern of tides to be? p
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

12. 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

13. 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.

14. 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.

15. 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.

16. 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.

17. 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

18. 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

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

21. 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

22. Semi-diurnal Diurnal Mixed
12-day record
30-day record
Semi-diurnal
Diurnal
Mixed

25. 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

26. 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

27. 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.