Tides 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.
The times and amplitude of tides at a locale are influenced by the alignment of the Sun and Moon, by the pattern of tides in the deep ocean, by the amphidromic systems of the oceans, and the shape of the coastline and near-shore bathymetry (see Timing). Some shorelines experience a semi-diurnal tide - two nearly equal high and low tides each day. Other locations experience a diurnal tide - only one high and low tide each day. A "mixed tide"; two uneven tides a day, or one high and one low, is also possible.
Tides vary on timescales ranging from hours to years due to a number of factors. To make accurate records, tide gauges
at fixed stations measure the 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 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, though this is not as easily seen as the water tidal movements.