• tide
  • The periodic rise and fall of the water resulting from gravitational interaction between the sun, moon and earth. In each lunar day of 24 hours and 49 minutes there are two high tides and two low tides.
Abstract from DBPedia
    Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables can be used for any given locale to find the predicted times and amplitude (or "tidal range"). The predictions are influenced by many factors including the alignment of the Sun and Moon, the (pattern of tides in the deep ocean), the amphidromic systems of the oceans, and the shape of the coastline and near-shore bathymetry (see ). They are however only predictions, the actual time and height of the tide is affected by wind and atmospheric pressure. Many shorelines experience semi-diurnal tides—two nearly equal high and low tides each day. Other locations have a diurnal tide—one high and low tide each day. A "mixed tide"—two uneven magnitude tides a day—is a third regular category. Tides vary on timescales ranging from hours to years due to a number of factors, which determine the lunitidal interval. To make accurate records, tide gauges at fixed stations measure water level over time. Gauges ignore variations caused by waves with periods shorter than minutes. These data are compared to the reference (or datum) level usually called mean sea level. While tides are usually the largest source of short-term sea-level fluctuations, sea levels are also subject to change from thermal expansion, 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 shape of the solid part of the Earth is affected slightly by Earth tide, though this is not as easily seen as the water tidal movements.

    潮汐(ちょうせき)とは、主として月と太陽の引力によって起きる、海面の昇降現象。海岸などでみられる、1日に1~2回のゆっくりした海面の昇降。「潮の干満(しおのかんまん)」、「潮の満ち干(しおのみちひ)」、「潮の満ち引き」とも。大和言葉で「しお」ともいう。漢字では潮と書くが、本来は「潮」は「朝のしお」、「汐」は「夕方のしお」という意味である。原義としてはこれだが一般には海に関するいろいろな意味で「潮」が使われる。 それ以外の要因でも起きており、気圧差や風によるものをという。代表的な気象潮は高潮(たかしお)である。気象潮と区別するため、潮汐力による潮汐を天体潮・天文潮ということがある。 潮汐にともない、表面が下がるところから上がるところへ流体が寄せ集められるために流体の流れが生まれる。これを潮汐流という。日常的な表現としては「潮汐」という言葉がこれを指していることもある。 海面の潮汐である海洋潮汐・海面潮汐が最も認知されているが、実際には湖沼などでも十分に大きなものであれば起こる。地球以外の天体でも、周囲の天体の引力の影響を受け天体の表面の液体が上下する現象は起きうる。

    (Source: http://dbpedia.org/resource/Tide)