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Tides• We experience

tides on Earth when the ocean level rises and falls.

• We experience 2 high tides and 2 low tides in a little over one day.

• But what causes tides?

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Tides and Gravity• The gravitational pull of the

Moon is the most important reason we experience tides

• The Sun is also important.• Tides occur when the Earth

gets “stretched” by the Moon’s gravity.

• The parts of the Earth closest to the Moon get pulled more than parts in the opposite direction.

• It is the difference in the strength of gravity with distance that causes tides.

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The Tidal Bulge• Since water can flow more

easily than rock it “piles up” into a bulge pointing towards the Moon and a bulge pointing away from the Moon.

• The bulge on the side of Earth farthest from the Moon is due to the Earth being pulled “out from under” the water on the far side.

• This varying amount of gravity is the “stretching force” that causes the tidal bulges to form along the Earth-Moon line.

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Two Tides a Day

• When we are beneath one of the tidal bulges we experience “high tide”

• In the space between the bulges we experience “low tide”

• As the Earth spins beneath these tidal bulges we get carried into and then away from two high and low tides each day.

• Because the Moon moves in its orbit the tidal bulges move as well so it takes a little more than 12 hours for two high tides to occur.

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The Sun’s Role• Certainly the Sun contributes

to tides as well but to a lesser degree.

• When the Moon and the Sun are nearly aligned (that is at New Moon and Full Moon) the tidal forces are the strongest and we get more extreme high and low tides. These are called Spring Tides.

• When the Moon is at 1st and 3rd Quarter the Earth experiences less extreme tides. These are called Neap Tides.

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Tidal Friction & Tidal Braking• As the Earth spins it is constantly

running up against the tidal bulges.

• The Earth forces the bulges to move slightly off of alignment with the Moon. This force is called Tidal Friction.

• The tidal bulges themselves exert a gravitational influence on the Moon causing it to speed up and move to a slightly higher orbit.

• The pull of the Moon on the bulges slows the Earth’s rotation slightly. This is called Tidal Braking.

The Moon is moving away from the Earth at 1 inch/year.The Earth is slowing down by 0.002 seconds/century

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• The Moon has one face always towards the Earth due to tidal braking.

• Tides on the Moon have caused its spin to perfectly match its orbital period.

• Pluto and Charon also experienced tidal braking and they are locked into keeping one side always facing each other.

• Most of the moons in the solar system are tidally braked as well.

Tidal Braking & Spin Orbit Resonance

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Other Examples of Tides

Tides are responsible for:

• ripping interacting galaxies apart

• pulling gas from one star to another in binary systems

• and melting the interior of Io as it interacts with Jupiter and the other Galilean satellites.