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Earth’s Tides

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Water on Mars (SciShow)

• https://www.youtube.com/watch?v=djcszDBsQkE

• This a You Tube link for a brief SciShow that touches on the new research on Martian water (and a couple of other things as well).

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The Tides

• Earth is unique (as far as we know), compared to other planets, because three-quarters of its surface is covered by water.– Average depth is 3.6 km (over 2 miles).

• Tidal fluctuations occur daily, in some places the difference from low tide to high tide is many meters.– The Bay of Fundy between Maine and New

Brunswick fluctuates by 20 meters (creates an enormous amount of energy).

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Bay of Fundy

https://www.youtube.com/watch?v=5W2sM1Ma7YA&list=PLOia8C-Y_fC0ewutrarifDhEm6Mz6QpK6

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What Causes the Tides?

• Tides are a direct result of the interplay of the moon’s and the sun’s gravitational influence on Earth.

• https://www.youtube.com/watch?v=l37ofe9haMU&list=PLOia8C-Y_fC0ewutrarifDhEm6Mz6QpK6– NASA animation of how the tides change

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Tidal Bulge

• Equilibrium theory of tides:– Under “idealized (the Earth is covered by ocean, w/o

land)” conditions, the moon’s gravity attracts the surface of the ocean toward the moon.

– This creates a tidal bulge on the side of the Earth that faces the moon.

– On the opposite side of the earth, another tidal bulge also is created.

– This is because the centripetal force of the rotation of the earth and moon “throws” water to this side of the Earth to balance the center of mass.

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Lunar Tides and Solar TidesWith the inward

gravitational pull of the

moon and the outward-

flinging force of Earth’s

rotation, tidal bulges form

on the side of the Earth facing the

moon - and the side of the Earth directly opposite the

moon.

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How the Tides Work

• Due to the bulging, the ocean becomes a little deeper in some places and shallower in others.

• The sun also exerts tidal forces. – Instead of just the moon’s tidal bulge, there is a

second which points at the sun. – The interaction of the two account for the changes

in the height of the tides over the course of a month or a year.

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How the Tides Work

• When the Earth, Moon, and Sun are all in a line the lunar and solar tides will add together creating a higher high tide and a lower low tide.

• This tide is very large due to constructive interference as the crests of the lunar and solar tides coincide.

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How the Tides Work

• This is called a spring tide and it occurs at the new and full moons of each lunar month.

• Spring tides are Earth’s most extreme tides – having the greatest difference between high and low tides.

• Neap tides – When the Earth-moon line is perpendicular to the Earth-Sun line (at first and third quarters moons), the daily tides are at their smallest.

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Spring Tide V. Neap Tide

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Dynamic Theory of Tides

• Tides in the actual Earth’s oceans behave a bit differently than in our hypothetical ocean-covered Earth due to:– the placement of landmasses– the shallow depth of water relative to wavelength

of tides.• Wave tides have wave lengths spanning 1,000s of miles

in open ocean – short wave heights of only a meter.– the latitudinal variation of the rotational velocity

of Earth, and the Coriolis Effect.

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Dynamic Theory of Tides

• Landmasses on Earths surface prevent the Earth from simply rotating into and out of tidal bulges.– Some of the energy is reflected back into the

ocean basin and, combined with the Coriolis effect, water is rotated around the ocean basin.

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Coriolis Effect Definition

• the Coriolis effect is a deflection of moving objects when they are viewed in a rotating reference frame.

• The Coriolis effect is caused by the rotation of the Earth and the inertia of the mass experiencing the effect.

• Therefore, the Earth rotates (880 mph at our latitude) and the oceans slosh around like they are in a big bathtub.

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Earth’s Rotation Is Slowing

• Earth’s rotation is slowing.– A piece of evidence for this comes from certain

corals.– As they grow, their growth creates annual rings,

like found in tree trunks.– Their growth rings respond to seasonal changes

and day-night cycle, allowing us to see annual cycles.

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Earth’s Rotation Is Slowing

• Coral growing today shows 365 marks per year.• Ancient corals, however, show more growth

deposits per year.– Corals that are >5 million years old contain coral w/

nearly 400 deposits per year. • Earth’s spin is slowing because of the tidal

effect of the moon.– This is due to friction among the Earth’s crust,

oceans and the Earth/moon interaction.

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Earth’s Rotation Is Slowing

• Due to the friction, the moon’s gravitational pull on Earth’s tidal bulge is slightly offset and this acts to reduce our planet’s spin.

• One effect of this is that the moon is slowly pulling away from Earth by about 4 cm per year.

• Earth’s day is increasing by 2 milliseconds every century.

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Biological Rhythms Tuned to Tides

• Lots of marine organisms, especially those in intertidal areas, have their biological cycles tuned to tides.

• Even terrestrial animals seem to be in tune with tidal cycles – including humans.– Evidence of a common marine ancestor.

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Review Questions

1. Tides are a direct result of what forces? 2. What is a spring tide (what causes it)? 3. What is a neap tide (what causes it)? 4. Spring tides occur at what phases of the moon? 5. How do the tidal bulges affect the depth of the ocean?6. Why is Earth’s rotation slowing? 7. How do we know Earth’s rotation is slowing?8. If we say that the Earth will some day tidally locked,

what does that mean?