Tides are the rising of Earth's ocean surface caused by the tidal forces of the Moon and the Sun acting on the oceans. Tides cause changes in the depth of the marine and estuarine water bodies and produce oscillating currents known as tidal streams, making prediction of tides important for coastal navigation (see 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 (see Intertidal ecology).

The changing tide produced at a given location is the result of the changing positions of the Moon and Sun relative to the Earth coupled with the effects of Earth rotation and the bathymetry of oceans, seas and estuaries. More generally, tidal phenomena can occur in other systems besides the ocean, whenever a gravitational field that varies in time and space is present (see Other tides).

The daily sea-level fluctuations, as they actually occur — especially in shallow seas and near coasts — are not solely caused by the astronomical tidal forcing, but may also be strongly affected by meteorology (wind), for instance resulting in storm surges.

Characteristics

• Over several hours the water rises or advances up a beach in the flood
• The water reaches its highest level, called high water.
• The sea level lowers or falls over several hours during the ebb tide.
• The level stops falling at low water.

The moment that the tidal current ceases, is called slack water or slack tide. Then the tide reverses direction and is said to be turning. Slack water usually occurs near high water and low water. But there are locations where the moments of slack tide differ significantly from those of high and low water.

Tides may be semidiurnal (two high waters and two low waters each day), or diurnal (one tidal cycle per day). In most locations, tides are semidiurnal. Because of the diurnal contribution, there is a difference in height (the daily inequality) between the two high waters on a given day; these are differentiated as the higher high water and the lower high water in tide tables. Similarly, the two low waters each day are referred to as the higher low water and the lower low water. The daily inequality changes with time and is generally small when the Moon is over the equator.

The various frequencies of orbital forcing which contribute to tidal variations are called tidal constituents. In most locations, the largest is the "principal lunar semidiurnal" constituent, also known as the M2 (or M₂) tidal constituent. Its period is about 12 hours and 25.2 minutes, exactly half a tidal lunar day, the average time separating one lunar zenith from the next, and thus the time required for the Earth to rotate once relative to the Moon. This is the constituent tracked by simple tide clocks.