Tides
Tides
Tides (/taɪdz/; from Old English tīd, meaning "time") are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun, and the rotation of the Earth.
Etymology
The term "tides" is derived from the Old English word tīd, which means "time". It is related to the Dutch word tijd and the German word Zeit, both of which also mean "time".
Definition
Tides are the regular rise and fall in water level experienced by seas and oceans in response to the gravitational influences of the Moon and the Sun, and the effects of the Earth's rotation. At any given place, the water level can be higher or lower than the average sea level.
Types of Tides
There are two main types of tides: semidiurnal, where there are two high tides and two low tides each day, and diurnal, where there is one high tide and one low tide each day. A mixed tide – two uneven magnitude tides a day – is a third regular category.
Factors Influencing Tides
Tides are influenced by a number of factors, including the positions of the Moon and the Sun, the shape and depth of the ocean, and the Earth's rotation. The gravitational forces of the Moon and the Sun both contribute to the tides. The Moon's gravitational pull is stronger because it is closer to the Earth.
Related Terms
- Ebb tide: The period when the water level is falling; the period after high water and before low water.
- Flood tide: The period when the water level is rising; the period after low water and before high water.
- Spring tide: A tide that occurs at or near the time of new or full moon and that rises highest and falls lowest from the mean sea level.
- Neap tide: A tide that occurs at or near the time of the first or third quarter of the moon and that rises least and falls least from the mean sea level.
External links
- Medical encyclopedia article on Tides
- Wikipedia's article - Tides
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