A tide is a recurring rise and fall in the level of the ocean, caused principally by the gravitational influence of the Moon and Sun on Earth. Tides are very long-period waves whose coastal expression includes alternating high and low water. Their timing and magnitude depend on astronomical cycles and the response of individual ocean basins. The difference in height between successive high and low water is called the tidal range. (oceanservice.noaa.gov)
Physical origin
Tides arise from differences in gravitational attraction across Earth, rather than simply from the Moon pulling water upward. The Moon attracts the near side of Earth more strongly than its center, and the far side less strongly. Relative to Earth’s center, these differences produce a stretching effect along the Earth–Moon direction. This differential attraction is the tidal force. It redistributes water and, in an idealized ocean-covered Earth, produces two opposing tidal bulges. (noaa.gov)
For a body much farther away than Earth’s radius, its tide-generating influence scales approximately with its mass divided by the cube of its distance. Although the Sun exerts a much stronger total gravitational attraction on Earth than the Moon, its tide-generating effect is less than half as large because it is much farther away. (oceanservice.noaa.gov)
The two-bulge picture is an explanatory model, not a literal map of observed ocean levels. Continents interrupt the movement of water, and ocean basins respond dynamically to the changing astronomical forcing. Consequently, local high water need not occur when the Moon is directly overhead or on the opposite side of Earth. (noaa.gov)
Daily and monthly cycles
The lunar day—the interval between successive passages of the Moon over a local meridian—averages approximately 24 hours 50 minutes. It exceeds the solar day because the Moon moves along its orbit while Earth rotates. A predominantly lunar semidiurnal tide therefore has successive high waters approximately 12 hours 25 minutes apart, although actual intervals vary locally. (science.nasa.gov)
Coastal tidal patterns fall into three broad categories:
- Semidiurnal: two high waters and two low waters per lunar day, with approximately equal successive heights.
- Diurnal: one high water and one low water per lunar day.
- Mixed semidiurnal: two high waters and two low waters, with appreciably unequal heights.
These patterns reflect the combination of astronomical forcing and regional ocean conditions; some locations change between predominantly diurnal and mixed patterns during the month or year. (oceanservice.noaa.gov)
Around new and full moon, the principal lunar and solar tidal effects reinforce one another, producing spring tides with relatively large ranges. Around first and last quarter, they partially oppose one another, producing neap tides with smaller ranges. “Spring” does not refer to the season. Local response can delay the largest or smallest ranges relative to the corresponding lunar phase. (noaa.gov)
Changing Earth–Moon and Earth–Sun distances also modify tidal strength. A spring tide occurring near lunar perigee, the Moon’s closest approach to Earth, can produce an unusually large range. These variations operate alongside, rather than replace, the daily tidal cycle. (noaa.gov)
Regional dynamics and currents
Coastline shape, seabed depth, and basin configuration alter the propagation of tidal waves. Reflection and interaction within basins can amplify or diminish water-level changes. Earth’s rotation introduces the Coriolis effect, while friction and water inertia contribute to delays in the response. Nearby places can consequently have different tidal ranges and high-water times. (noaa.gov)
The vertical change in water level is distinct from the horizontal flow called a tidal current. In many coastal channels and estuaries, currents reverse between flood flow, generally directed inland, and ebb flow, generally directed seaward. A period of very weak flow during reversal is called slack water. Spring and neap cycles also influence current strength. (oceanservice.noaa.gov)
Measurement and prediction
A tide gauge records water level at a fixed location. Extended observations establish the local pattern and support predictions of high-water and low-water times and heights. Such information is important for shipping, harbor operations, coastal engineering, and scientific investigations. Navigation through shallow waterways also requires information about current speed and direction. (oceanservice.noaa.gov)
A principal prediction technique is harmonic analysis. The observed water-level time series is represented as a sum of periodic components, or tidal constituents. Each has a characteristic astronomical frequency and locally determined amplitude and phase. Combining these constituents reconstructs the predicted astronomical tide. NOAA commonly uses 37 constituents with the greatest influence at its prediction locations. (tidesandcurrents.noaa.gov)
Astronomical predictions are not complete forecasts of actual water level. Winds, atmospheric pressure, storms, and other environmental conditions can produce departures from the predicted tide. Observed sea level therefore includes both tidal and nontidal contributions. (noaa.gov)
Ecological and energetic significance
The intertidal zone lies between high- and low-water levels. Its ecosystems alternate between immersion and exposure, with organisms distributed partly according to their ability to resist drying, withstand waves, and avoid predators. Shell-closing behavior, attachment to rocks, and movement into submerged areas are among the adaptations found there. High and low tides also change feeding opportunities for fish and shorebirds. (oceanservice.noaa.gov)
Tidal motion provides a source of renewable energy. Tidal power systems exploit either the kinetic energy of flowing currents or water-level differences across a barrage to drive turbines and generate electricity. (energy.gov)
Tidal interactions also affect planetary motion. In the Earth–Moon system, tidal friction slows Earth’s rotation while gravitational interaction transfers angular momentum to the lunar orbit, causing the Moon to recede. Related dissipative processes produced the Moon’s tidal locking, in which its rotation period matches its orbital period. (tda.jpl.nasa.gov)