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Unit 1

Lesson Plan 1

Lesson Plan 2

Lesson Plan 3

Lesson Plan 4

Lesson Plan 5

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Lesson Plan 4




Tide- The periodic rise and fall of the ocean's water level due to the pull of the gravity from the sun and the moon.
Low tide-The two points in the day when the ocean's level is lowest.
High tide-The two points of the day when the ocean's level is highest.


Tides are caused by a gravitational tug-of-war between the sun, moon, and earth. All objects exert gravitational pull on each other. The closer they are, or the larger they are, the greater the pull. All of the planets exert some gravitational pull on the earth. However, the pull of the moon and sun are most noticeable because the moon is so close to us and the sun is so big. It takes the earth 365 days to revolve around the sun. As it revolves around the sun, it spins, or rotates on its axis once every 24 hours. At the same time, the moon revolves around the earth once every 29 days. The gravitational pull of the sun holds the earth in orbit, while the gravitational pull of the earth keeps the moon in orbit.

As a result of this gravitational attraction between the earth and the moon, the side of the earth facing the moon is pulled towards it. Solid objects like the ground and buildings are not distorted as much as liquids like the ocean. A bulge of water occurs on the side of the earth facing the moon. As the earth rotates around the sun, centrifugal force causes an equal bulge of water on the opposite side of the earth. Water is pulled away from these two sides of the earth to form these bulges, or high tides. This leaves a depression, or low spot, in the oceans between. These are the areas of low tides.

Most areas of the earth have two high tides and low tides every day. These high and low tides are slightly more than 6 hours apart. In some areas, the high and the low tides are the same. However, the earth is tilted on its axis, so the bulges are sometimes unequal. Because of this, in the Southern California region, one of the high tides each day is higher and one of the low tides each day is lower than the other. It depends on where you are located in the earth's surface whether your high and low tides are semidiurnal (the same tide twice a day) or semi-diurnal mixed (different tides twice a day).


Concepts/Objectives | Vocabulary/Background | Activity | Extension