In a confined area, such as a narrow, rocky inlet or bay, the tidal range could be many meters. The tidal range may be a few centimeters. Along a smooth, wide beach, the water can spread over a large area. The shape of a seacoast and the shape of the ocean floor both make a difference in the range and frequency of the tides. Because the Earth’s surface is not uniform, tides do not follow the same patterns in all places. Although the sun is almost 390 times farther away from the Earth than is the moon, its high mass still affects the tides. Each month, the range changes in a regular pattern as a result of the sun’s gravitational force on the Earth. The vertical difference between high and low tide is called the tidal range. The Earth continues spinning, the tide ebbs, another low tide occurs, and the cycle (24 hours long) begins again. As the Earth keeps rotating, another high tide occurs in the same area when it is on the side of the Earth opposite the moon (low high tide). As the Earth rotates, that area moves away from the moon’s influence and the tide ebbs. For example, when an area covered by the ocean faces the moon, the moon’s gravitational force on the water causes a high high tide. Most tides are semidiurnal, which means they take place twice a day. The flow of water from high tide to low tide is called an ebb tide. Between these high tides are areas of lower water levels-low tides. Low Tides and Ebb Tides One high tide always faces the moon, while the other faces away from it. Along the seashore, the water rises and spreads onto the land. In the open ocean, the water bulges out toward the moon. The high tide caused by the bulge on the opposite side of the Earth is called the low high tide. The high tide on the side of the Earth facing the moon is called the high high tide. These bulges in the ocean waters are known as high tides. The bulge may be understood as the moon’s tidal force pulling the planet (not the ocean) toward it. Here, the ocean bulges in the opposite direction of the moon, not toward it. The other bulge occurs on the opposite side of the Earth. This is the moon’s direct tidal force pulling the ocean toward it.

One bulge occurs on the side of the Earth facing the moon. These differences in gravitational force allow the ocean to bulge outward in two places at the same time. It is weakest on the side of the Earth facing the opposite direction. High Tides The tidal force exerted by the moon is strongest on the side of the Earth facing the moon. Water is liquid and can respond to gravity more dramatically. The moon’s tidal force has a much greater effect on the surface of the ocean, of course. Volcanologists study terrestrial tides because this movement in the Earth’s crust can sometimes trigger a volcanic eruption. Terrestrial tides are important for radio astronomy and calculating coordinates on a global positioning system (GPS). Terrestrial tides can change an object’s precise location. These movements are called terrestrial tides. Land surfaces do move, however, up to 55 centimeters (22 inches) a day. This has little effect on Earth’s land surfaces, because they are less flexible. The moon exerts a tidal force on the whole planet. The moon’s ability to raise tides on the Earth is an example of a tidal force. Although the sun and moon both exert gravitational force on the Earth, the moon’s pull is stronger because the moon is much closer to the Earth than the sun is. The closer objects are, the greater the gravitational force is between them. The major tidal constituent is the moon’s gravitational pull on the Earth. The Earth’s rotation is a tidal constituent.

Causes of Tides Forces that contribute to tides are called tidal constituents. Some lakes and rivers can also have tides. When the water falls to its lowest level, it is at low tide. When the water has risen to its highest level, covering much of the shore, it is at high tide. Along coasts, the water slowly rises up over the shore and then slowly falls back again. The regular rise and fall of the ocean’s waters are known as tides.

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