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The gravitational attraction between the Earth and the moon is strongest on the side of the Earth that happens to be facing the moon, simply because it is closer. Together, gravity and inertia are responsible for the creation of two major tidal bulges on the Earth (Ross, D.A., 1995). It is the tendency of moving objects to continue moving in a straight line. Gravity is a major force responsible for creating tides. Gravity, Inertia, and the Two Bulges Two tidal bulges are created on opposite sides of the Earth due to the moon’s gravitational force and inertias counterbalance. Because of these conditions, the sun’s tide-generating force is about half that of the moon (Thurman, H.V., 1994). Thus, its tide-generating force is reduced by 3903, or about 59 million times less than the moon. However, the sun is 390 times further from the Earth than is the moon. If tidal forces were based solely on comparative masses, the sun should have a tide-generating force that is 27 million times greater than that of the moon. Based on its mass, the sun’s gravitational attraction to the Earth is more than 177 times greater than that of the moon to the Earth. Our sun is 27 million times larger than our moon. The effect of distance on tidal forces is seen in the relationship between the sun, the moon, and the Earth’s waters. Gravitational attractive forces only vary inversely to the square of the distance between the objects (Thurman, H.V., 1994). Tidal generating forces vary inversely as the cube of the distance from the tide generating object. With regard to tidal forces on the Earth, the distance between two objects usually is more critical than their masses. Tidal forces are based on the gravitational attractive force. Therefore, the greater the mass of the objects and the closer they are to each other, the greater the gravitational attraction between them (Ross, D.A. Newton’s law of universal gravitation states that the gravitational attraction between two bodies is directly proportional to their masses, and inversely proportional to the square of the distance between the bodies (Sumich, J.L., 1996 Thurman, H.V., 1994). In 1687, Sir Isaac Newton explained that ocean tides result from the gravitational attraction of the sun and moon on the oceans of the earth (Sumich, J.L., 1996). Gravity is one major force that creates tides. The relationship between the masses of the Earth, moon and sun and their distances to each other play critical roles in affecting tides. Near estuary entrances, narrow straits and inlets, the speed of tidal currents can reach up to several kilometers per hour (Ross, D.A., 1995). In the open ocean tidal currents are relatively weak. The weakest currents occur between the flood and ebb currents and are called slack tides. The strongest flood and ebb currents usually occur before or near the time of the high and low tides. The incoming tide along the coast and into the bays and estuaries is called a flood current the outgoing tide is called an ebb current. The difference in height between the high tide and the low tide is called the tidal range.Ī horizontal movement of water often accompanies the rising and falling of the tide. When the highest part, or crest of the wave reaches a particular location, high tide occurs low tide corresponds to the lowest part of the wave, or its trough. Tides originate in the oceans and progress toward the coastlines where they appear as the regular rise and fall of the sea surface. The following pages describe the tremendous forces that cause the world’s tides, and why it is important for us to understand how they work.īasically, tides are very long-period waves that move through the oceans in response to the forces exerted by the moon and sun. As the sun rises in the east and the stars come out at night, we are confident that the ocean waters will regularly rise and fall along our shores. Tides are one of the most reliable phenomena in the world. As the tides rise and fall, they create flood and ebb currents.