The relative location of the Moon and Sun during spring and neap tides. In this diagram, an exaggerated blue ocean around the Earth shows high tides as a thick bulge.
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Windows to the Universe / Lisa Gardiner

Tides of the Ocean

Each day the ocean surface rises and falls – a phenomenon called tides. The highest point of the rise in the ocean surface is called high tide. The lowest point is called low tide. Most places have two high tides and two low tides each day.

Tides happen because of the gravitational pull of the Moon and Sun on ocean water. Because Earth is rotating, the tides change over time. The Moon has a much greater impact on tides than the Sun because it is much closer. The Moon pulls the water that is closest to it creating a high tide on the side of the Earth closest to the Moon. The ocean water forms a high tide on the opposite side of the Earth too. Low tides occur in areas between the high tides.

The difference between high tide and low tide is called the tidal range. Some places have a larger tidal range than others because of differences in the shape of the ocean floor. The difference between high and low tide can be as little as a few centimeters to as much as several meters depending on location. In Canada's Bay of Fundy the tidal range is very large – 16 meters – because of the narrow shape of the bay which funnels the tidewater into a small space.

Tides also differ at different times of the year depending on the location of the Moon and Sun in relation to Earth. At times when the Sun, Earth, and Moon are in line with each other (during full and new moons), the tidal range is larger because both the Sun’s and Moon's gravitational pull create the tide. This is called a spring tide. At times when the Moon and Sun are not in line with Earth, when they are at right angles when viewed from Earth, the tidal range is smaller. This is called a neap tide.

Tides are a powerful force and, because of this, scientists and engineers are working on methods to extract some of that power to produce electricity. This can be done by channeling tidewater to flow through turbines that generate electricity. A tidal power plant located in an estuary in Brittany, France has turbines that turn when the tide is going in and when it is going out. Systems like this may be a danger to wildlife, thus, scientists are studying estuarine ecosystems to understand how it might be affected by turbines.

Last modified September 18, 2008 by Lisa Gardiner.

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