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Earth Science for Kids

Ocean Tides

Tides are the rise and fall of the levels of the ocean. They are caused by the gravitational pull of the Sun and Moon as well as the rotation of the Earth.

Cycles of a Tide

Tides cycle as the Moon rotates around the Earth and as the position of the Sun changes. Throughout the day the sea level is constantly rising or falling.

1. The sea level rises
2. High tide is reached
3. The sea level falls
4. Low tide is reached
5. Back to number 1

This cycle can happen once or twice a day depending on the location of the area to the Moon. Tides that happen once a day are called diurnal. Tides that happen twice a day are called semidiurnal. Because the Earth rotates in the same direction as the Moon, the cycle is actually slightly longer than a day at 24 hours and 50 minutes.

Tides and the Moon

While the Sun and the rotation of the Earth both have some tidal impact, the location of the Moon has the biggest affect on the tide. The gravity of the Moon causes a high tide both on the side of the Earth directly below the Moon (sublunar tide) and the opposite side of the Earth (antipodal). Low tides are on the sides of the Earth 90 degrees away from the Moon. See the picture below.

Low and high tides change with the Moon


Tidal Currents

When the sea level is rising or falling, water is flowing to or from the ocean. This flow causes currents called tidal currents.
Tidal Range

The tidal range is the difference in sea level between low tide and high tide. The tidal range will vary in different locations depending on the location of the Sun and the Moon as well as the topography of the shore line.

In the open ocean the tidal range is typically around 2 feet. However, tidal ranges can be much larger near the shore. The largest tidal range is on the coast of the Bay of Fundy in Canada where the tides can change by as much as 40 feet from high to low tide.



Types of Tides


Interesting Facts about Tides
Activities

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Science >> Earth Science for Kids


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