What Causes 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. 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.
Ocean tides are caused mainly by the moon’s gravitational force, which tugs at our planet. The moon pulls up the water on the part of the ocean that is turned toward it. At the same time, a second tidal bulge forms on the Earth’s opposite side. As the moon orbits the Earth, the bulge of water travels over the ocean.
When it reaches the coast, the tidal bulge raises the waterline along the shore, causing what is known as high tide. The water is then pulled away, causing low tide. In most places in the world, tides go in and out this way twice each day.
The difference between high tide and low tide is called the tidal range. The biggest tidal range is found in the Bay of Fundy, Canada where sea level rises and falls as much as 16 m (53 feet) in just over 6 hours. The smallest tidal ranges are less than 1 m (3 feet).
The highest tides, called spring tides, are formed when the earth, sun and moon are lined up in a row. This happens every two weeks during a new moon or full moon. Smaller tides, called neap tides, are formed when the earth, sun and moon form a right angle. This causes the sun and moon to pull the water in two different directions. Neap tides happen during a quarter or three-quarter moon.
The width of the shoreline strip that is affected by waves depends on the tidal range. A large tidal range means that a wide strip of land might be subjected to the force of the waves. If the tidal range is very small, all of the wave’s energy will be concentrated in the same place.
The rise and fall of tides causes water to move in and out of estuaries, bays and harbors. This movement is called a tidal current. When the tide is rising, water flows from the ocean into the bay creating a flood current. When the tide falls, water flows from the bay back into the ocean creating ebb current.