Tides are predictable periodic variations in sea level and associated water motion caused primarily by the gravitational interactions of Earth with the Moon and Sun, together with Earth's rotation and the geometry of ocean basins. Although commonly observed as the regular rise and fall of coastal water levels, tides are fundamentally a planetary scale response of the ocean and solid Earth to gravitational forcing.
The Moon has the stronger tidal influence because it is much closer to Earth, while the Sun also contributes significantly. Tidal behaviour varies from place to place because continents, coastlines, water depth, basin shape, resonance, friction and Earth's rotation modify the response. Consequently, tidal range is not uniform around the world. Some locations experience relatively small tidal changes, whereas others experience very large ranges because local basin geometry amplifies the underlying astronomical forcing.
The basic gravitational explanation involves the differential force exerted by the Moon and Sun across Earth. Because gravitational acceleration is slightly stronger on the side of Earth facing the Moon and slightly weaker on the opposite side, the ocean develops a pattern of tidal deformation with associated high water regions. Earth's rotation causes locations to move through these patterns, although real ocean tides do not simply follow a perfectly symmetrical pair of water bulges.
Continental barriers, basin geometry and rotational effects profoundly alter tidal propagation. The equilibrium tide is a simplified theoretical model in which the ocean responds directly to astronomical forcing, while real tides are dynamic responses involving ocean basin geometry, friction and wave propagation. The distinction is important because actual tidal systems can differ substantially from the idealised picture.
Spring tides occur when the gravitational effects of the Moon and Sun combine more strongly, particularly near new moon and full moon. Neap tides occur when their tidal influences partially oppose one another, particularly near the first and third quarter phases of the Moon. The terms spring and neap describe the resulting tidal range rather than the season. Tidal constituents are periodic components associated with astronomical frequencies, and harmonic analysis can represent observed tidal behaviour as combinations of these constituents.
The dominant constituents vary geographically, meaning different coastlines can experience different tidal patterns. Some regions have approximately two high tides and two low tides per lunar day, known as semidiurnal behaviour, while others have one high and one low tide per lunar day, known as diurnal behaviour, and mixed regimes can display unequal successive tides.
Tidal currents arise because the horizontal movement associated with changing water levels can become substantial, especially in narrow channels, estuaries and coastal passages. Flood currents generally move water landward during rising tide, while ebb currents move water seaward during falling tide, although the exact patterns depend on local geometry. Tidal currents can influence sediment transport, estuarine mixing, nutrient exchange and the distribution of organisms.
Strong tidal flows can generate turbulence and enhance vertical mixing, bringing deeper water and nutrients into contact with surface waters. In some regions, tidal energy can be harnessed for electricity generation using turbines, although environmental, engineering and economic considerations affect feasibility. Tidal currents can also present hazards to navigation when they become sufficiently strong or interact with complex coastal structures.
Tidal range is strongly controlled by resonance and basin geometry. A partially enclosed basin can amplify tidal motion if its natural oscillation characteristics align with dominant tidal forcing. Shallowing, coastline shape and channel geometry can alter tidal amplitude and phase as the tidal wave propagates. Friction with the seabed dissipates tidal energy and modifies current velocities and timing. In some coastal regions, tides can become highly asymmetric, producing unequal durations or strengths of flood and ebb currents and influencing the direction of sediment transport. Tidal processes therefore contribute significantly to the development and maintenance of estuaries, tidal flats, sandbanks, channels and other coastal landforms.
Tides also occur in the solid Earth and atmosphere, although their magnitudes and consequences differ from ocean tides. The solid Earth undergoes small periodic deformations in response to lunar and solar gravitational forcing, and these Earth tides can be measured using sensitive instruments. Ocean tides are particularly important in geodesy and satellite measurements because changing distributions of seawater mass can affect gravitational observations and Earth's rotation. Tidal friction transfers energy within the Earth Moon system and contributes to long term changes in the rotational and orbital dynamics of the system. Over geological time, tidal effects have therefore had consequences extending beyond daily coastal water level changes.
Tidal observations have practical applications in navigation, coastal engineering, flood assessment, harbour operations, fisheries, offshore construction and renewable energy planning. Accurate tidal predictions require knowledge of local tidal constituents and are generated from observations and numerical models. Tide gauges provide long term records of water level, while satellite altimetry can measure sea surface height over broad oceanic regions.
Tidal predictions must be distinguished from storm surge predictions because tides are primarily astronomically forced and can be forecast accurately, whereas storm surges depend on atmospheric pressure, wind and other variable conditions. In practice, coastal water levels can result from tides, atmospheric effects, waves, seasonal ocean variability and long term sea level change acting simultaneously.
Tides differ from waves in that tides are primarily long period astronomical responses, whereas wind generated waves usually have much shorter periods and are driven primarily by atmospheric energy transferred to the ocean surface. Tides also differ from ordinary ocean currents because tidal currents reverse or vary systematically with the tidal cycle rather than representing a persistent circulation pathway.
A common misconception is that the Moon simply pulls ocean water upward into a single bulge; actual tides are dynamic and strongly modified by Earth's rotation, continents, basin geometry and friction. Tidal science therefore combines celestial mechanics, fluid dynamics, coastal geography and geophysical observation. Its significance extends from explaining everyday coastal water level changes to influencing sedimentary environments, marine ecosystems, navigation, coastal hazards and long term Earth Moon system dynamics.
