Tides and Waves

1.0 What Causes the Tides Tides are periodic rises and falls of large bodies of water. Tides are caused by the gravitational interaction between the earth and the moon. Tides are the cyclic rising and falling of earth's ocean surface caused by the tidal forces of the moon and the sun acting on the earth . Tides cause changes in the depth of the sea and produce oscillating currents known as tidal streams, making prediction of tides important for coastal navigation. The strip of seashore that is submerged at high tide and exposed at low tide, the intertidal zone , is an important ecological product of ocean tides. The relative distance of the Moon from the Earth also affects tide heights. When the moon is at perigee the range is increased and when it is at apogee the range is reduced. The moon's gravity differential field at the surface of the earth is known as the Tide Generating Force . This is the primary mechanism that drives tidal action and explains two bulges, accounting for two high tides per day. Other forces, such as the Sun's gravity, also add to tidal action. The moon exerts its gravitational pull differently on different parts of the earth. The farther away the Moon, the weaker its pull.

Figure 1.0: Imagine a shell of the outer Earth , this diagram shows the Moon's gravity differential over the thickness of the shell.
1.1 Spring Tides and Neap Tides Spring tides are especially strong tides. They occur when the earth, the sun, and the moon are in a line. The gravitational forces of the moon and the sun both contribute to the tides. Spring tides occur during the full moon and the new moon.
The eccentricity of the orbit of the moon in this illustration is greatly exaggerated. The Proxigean Spring Tide is a rare, unusually high tide. This very high tide occurs when the moon is both unusually close to the Earth (at its closest perigee, called the proxigee ) and in the New Moon phase (when the Moon is between the