Tides What are they?

What are they caused by?

Are there different kinds of tides?

• Those of you who have spent some time on the beach have probably noticed that the water level of the ocean does not stay the same all day long. Although waves continually wash the shore, the actual water level changes as the day passes.

• It rises for a period of hours until it reaches its highest level, and then it begins to fall, or recede, to its lowest level. This rise and fall of the ocean is known as the tide. Its highest and lowest levels are, of course called high and low tides.

High Tide Low Tide

Tides – What are they?

• Tides are daily changes in the level of ocean water.

• They are the periodic rise and fall of the water level in the oceans and other large bodies of water.

• I wonder how often this periodic rise and fall happens each day?

What causes tides?

• The dominant force that moves ocean water is the gravitational force of the moon.

Sir Isaac Newton to the Rescue!

• Newton’s Universal Gravitation theory helped to explain how the gravity of the moon (and the sun) were related to tides.

What causes tides?

• The gravity of the moon pulls on everything on the Earth.

• The moon’s gravitational pull on liquids is much more noticeable than on solids.

• Why? Because liquids move more easily than solids.

What causes tides?

• How often tides occur and the difference in tidal levels depend on the position of the moon as it revolves around the Earth.

• The moon’s pull is strongest on the part of the Earth directly facing the moon.

High Tide

• The part of the ocean directly facing the moon, bulges towards the moon.

• There is a corresponding bulge on the opposite side of the Earth.

• These bulges are called high tide.

Low Tide

• When high tides occur, water is drawn away from the area between the high tides, which causes low tides to occur.

How often is there a change in tides?

Try to figure it out . . .

• High tides happen on opposite sides of the Earth.

• The Earth rotates once on its axis every 24 hours.

• How many high tides would any one place in the ocean experience each day?

Answer

• The periodic rise and fall of the water level in the oceans and other large bodies of water happens . . .

• Twice each day!

What effect does the sun have on tides?

• The sun is much larger than the moon.

• The sun is farther away from the Earth than the moon.

Q: Which one (the sun or the moon) do you think has a bigger effect on tides?

A: The moon, just because it is closer.

Spring Tide

• A tide with a large tidal range that occurs two times a month (new moon and full moon)

• Sun, Earth, and the moon are aligned

Neap Tides

• A tide of minimum range that occurs during the first and third quarters of the moon

• The sun, Earth, and moon form a 90 degree angle

Review

Q: What are tides?

• Tide = the periodic rise and fall of the water level in the oceans and other large bodies of water.

Q: What is the dominant force that moves ocean water?

• The gravitational force of the moon!

Review (cont’d)

Q: Where is the moon’s pull is strongest on the Earth?

• The moon’s pull is strongest on the part of the Earth directly facing the moon.

Review (cont’d)

Q: Which one (the sun or the moon) has a bigger effect on tides?

• The moon

Review (cont’d)

Q: How are the sun, Earth, and moon situated in space so that we experience a spring tide?

• Sun, Earth, and the moon are aligned

Review (cont’d)

Q: How are the sun, Earth, and moon situated in space so that we experience a neap tide?

• The sun, Earth, and moon form a 90 degree angle