The gravitational pull from the moon on the earth's waters causes tides. Tides are the rise and fall of sea levels caused by the combined effects of the rotation of the Earth and the gravitational forces exerted by the Moon and the Sun. The tides occur with a period of approximately 12 and a half hours and are influenced by the shape of the near-shore bottom.
Most coastal areas experience two daily high (and two low) tides. This is because at the point right "under" the Moon (the sub-lunar point), the water is at its closest to the Moon, so it experiences stronger gravity and rises. On the opposite side of the Earth (the antipodal point), the water is at its farthest from the moon, so it is pulled less; at this point the Earth moves more toward the Moon than the water does—causing that water to "rise" (relative to the Earth) as well. In between the sub-lunar and antipodal points, the force on the water is diagonal or transverse to the sub-lunar/antipodal axis (and always towards that axis), resulting in low tide.
Tide prediction is important for coastal navigation. The intertidal zone, the strip of seashore that high tide submerges and low tide exposes, is an important ecological product of ocean tides.
While tides are usually the largest source of short-term sea-level fluctuations, sea levels are also subject to forces such as wind and barometric pressure changes, resulting in storm surges, especially in shallow seas and near coasts.
Tidal phenomena are not limited to the oceans, but can occur in other systems whenever a gravitational field that varies in time and space is present. For example, the solid part of the Earth is affected by tides.
Most coastal areas experience two daily high (and two low) tides. This is because at the point right "under" the Moon (the sub-lunar point), the water is at its closest to the Moon, so it experiences stronger gravity and rises. On the opposite side of the Earth (the antipodal point), the water is at its farthest from the moon, so it is pulled less; at this point the Earth moves more toward the Moon than the water does—causing that water to "rise" (relative to the Earth) as well. In between the sub-lunar and antipodal points, the force on the water is diagonal or transverse to the sub-lunar/antipodal axis (and always towards that axis), resulting in low tide.
Tide prediction is important for coastal navigation. The intertidal zone, the strip of seashore that high tide submerges and low tide exposes, is an important ecological product of ocean tides.
While tides are usually the largest source of short-term sea-level fluctuations, sea levels are also subject to forces such as wind and barometric pressure changes, resulting in storm surges, especially in shallow seas and near coasts.
Tidal phenomena are not limited to the oceans, but can occur in other systems whenever a gravitational field that varies in time and space is present. For example, the solid part of the Earth is affected by tides.