Tides
What causes Tides Reading from class:
what_causes_the_tides_-_slg.docx | |
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Tides
Figure 1: Where is the intertidal zone in this picture?
Tides are daily changes in the level of ocean water. They occur all around the globe. High tides occur when the water reaches its highest level in a day. Low tides occur when the water reaches its lowest level in a day. Tides keep cycling from high to low and back again. In most places the water level rises and falls twice a day. So there are two high tides and two low tides approximately every 24 hours. In Figure 1 , you can see the difference between high and low tides. This is called the tidal range.
Why Tides Occur
Figure 2: High and low tides are due mainly to the pull of the moon
Figure 2 shows why tides occur. The main cause of tides is the pull of the moon’s gravity on Earth. The pull is greatest on whatever is closest to the moon. Although the gravity pulls the land, only the water can move. As a result:
Water on the side of Earth facing the moon is pulled hardest by the moon’s gravity. This causes a bulge of water on that side of Earth. That bulge is a high tide.
Earth itself is pulled harder by the moon’s gravity than is the ocean on the side of Earth opposite the moon. As a result, there is bulge of water on the opposite side of Earth. This creates another high tide.
With water bulging on two sides of Earth, there is less water left in between. This creates low tides on the other two sides of the planet.
Water on the side of Earth facing the moon is pulled hardest by the moon’s gravity. This causes a bulge of water on that side of Earth. That bulge is a high tide.
Earth itself is pulled harder by the moon’s gravity than is the ocean on the side of Earth opposite the moon. As a result, there is bulge of water on the opposite side of Earth. This creates another high tide.
With water bulging on two sides of Earth, there is less water left in between. This creates low tides on the other two sides of the planet.
Spring Tides and Neap Tides
Figure 3: The sun and moon both affect Earth
The sun’s gravity also pulls on Earth and its oceans. Even though the sun is much larger than the moon, the pull of the sun’s gravity is much less because the sun is much farther away. The sun's gravity strengthens or weakens the moon’s influence on tides. Figure 3 shows the position of the moon relative to the sun at different times during the month. The positions of the moon and sun relative to each other determines how the sun affects tides. This creates spring tides or neap tides.
Spring tides occur during the new moon and full moon. The sun and moon are in a straight line either on the same side of Earth or on opposite sides. Their gravitational pull combines to cause very high and very low tides. Spring tides have the greatest tidal range.
Neap tides occur during the first and third quarters of the moon. The moon and sun are at right angles to each other. Their gravity pulls on the oceans in different directions so the highs and lows are not as great. Neap tides have the smallest tidal range.
This animation shows the effect of the Moon and Sun on the tides: http://www.onr.navy.mil/focus/ocean/motion/tides1.htm.
Chapter section taken from: http://www.ck12.org/flexbook/chapter/19418
Spring tides occur during the new moon and full moon. The sun and moon are in a straight line either on the same side of Earth or on opposite sides. Their gravitational pull combines to cause very high and very low tides. Spring tides have the greatest tidal range.
Neap tides occur during the first and third quarters of the moon. The moon and sun are at right angles to each other. Their gravity pulls on the oceans in different directions so the highs and lows are not as great. Neap tides have the smallest tidal range.
This animation shows the effect of the Moon and Sun on the tides: http://www.onr.navy.mil/focus/ocean/motion/tides1.htm.
Chapter section taken from: http://www.ck12.org/flexbook/chapter/19418
As you read: Answer the following questions
1. What is a tide?
2. What is the difference between a High Tide and a Low Tide?
3. What is the main cause of tides on Earth?
4.*Why do tides occur only in water not on land? (Refer to: Why Tides Occur)
5. What causes low tides?
6. **If the tidal cycle was actually 12 hours then high tides would occur at the same time every day. In reality, high tides occur about every 12 hours and 25 minutes. Can you think of why this would be the case? (Refer to: Why Tides Occur)
7. *How does the Sun effect Earth’s tides? (Refer to: Spring Tides and Neap Tides)
8. When do Spring tides occur?
9. When do Neap Tides occur?
10. **Assume that a spring tide occurs on September 1. Predict when the next neap tide will occur. When will the next spring tide occur? Explain your answers. (Refer to: Spring Tides and Neap Tides)
2. What is the difference between a High Tide and a Low Tide?
3. What is the main cause of tides on Earth?
4.*Why do tides occur only in water not on land? (Refer to: Why Tides Occur)
5. What causes low tides?
6. **If the tidal cycle was actually 12 hours then high tides would occur at the same time every day. In reality, high tides occur about every 12 hours and 25 minutes. Can you think of why this would be the case? (Refer to: Why Tides Occur)
7. *How does the Sun effect Earth’s tides? (Refer to: Spring Tides and Neap Tides)
8. When do Spring tides occur?
9. When do Neap Tides occur?
10. **Assume that a spring tide occurs on September 1. Predict when the next neap tide will occur. When will the next spring tide occur? Explain your answers. (Refer to: Spring Tides and Neap Tides)
Tides Reading Questions.doc | |
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File Type: | doc |