The Tides

The tides are the periodic rising and falling of the surface of the oceans, and of bodies of water connected with the ocean such as bays and gulfs. The tides are caused by the gravitational attraction of the moon and sun, occurring unequally on different parts of the earth. In each lunar day of 24 hours and 51 minutes there are two high tides and two low tides, alternating at equal intervals of flood and ebb.

Tides cause changes in the depth of the sea and estuary water bodies, producing oscillating currents known as tidal streams. The strip of seashore that is submerged at high tide and exposed at low tide is an important ecological product of ocean tides. Tidal phenomena can also occur in other systems besides the ocean, whenever a gravitational field that varies in time and space is present.