Tide

• 1. Astronomical Origins

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  The Moon pulls on Earth's oceans, creating two tidal bulges. One bulge forms on the side facing the satellite. The second bulge appears on the opposite side of the planet. This happens because gravity weakens with distance. Water on the near side feels a stronger pull than water at Earth's center. Water on the far side feels a weaker pull than the center. The solid Earth resists deformation while fluid moves freely to match this imbalance. The Sun also exerts a gravitational force, but it is only 46% as strong as the lunar effect. During spring tides, the Moon contributes 69% and the Sun contributes 31%. These forces stretch the ocean surface into an equilibrium shape that constantly changes. Earth rotates relative to this shape, causing daily cycles of rising and falling water.

• 2. Historical Theories

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  Seleucus of Seleucia theorized around 150 BC that tides were caused by the Moon. He believed the interaction was mediated by pneuma. Bede linked semidurnal tides to the Moon in 725 AD. He noted that tides rise and fall 4/5 of an hour later each day. Abu Ma'shar al-Balkhi taught that ebb and flood tides were caused by the Moon circa 886. Simon Stevin dismissed misconceptions about ebb and flood in his 1608 work. Johannes Kepler correctly suggested that the gravitation of the Moon caused the tides in 1609. Galileo attributed tides to the sloshing of water caused by Earth's movement around the Sun in 1632. Isaac Newton published his explanation of tides in the Principia in 1687. Daniel Bernoulli, Leonhard Euler, Colin Maclaurin and Antoine Cavalleri shared a prize from the Académie Royale des Sciences in Paris in 1740. Pierre-Simon Laplace formulated partial differential equations relating ocean flow to surface height. Arthur Thomas Doodson developed the first modern development of tide-generating potential in harmonic form in 1921.

• 3. Harmonic Analysis

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  William Thomson led the first systematic harmonic analysis of tidal records starting in 1867. The main result was building a tide-predicting machine using pulleys to add six harmonic time functions. This machine was programmed by resetting gears and chains to adjust phasing and amplitudes. Similar machines were used until the 1960s. A.T. Doodson extended this work, introducing the Doodson Number notation to organize hundreds of resulting terms. Fourier series use sinusoidal functions as a basis set with frequencies that are zero, one, two, three times the fundamental cycle. Tidal patterns decompose into many sinusoids having many fundamental frequencies. These correspond to combinations of Earth, Moon, and Sun motions. Careful Fourier data analysis over a nineteen-year period uses frequencies called tidal harmonic constituents. Nineteen years is preferred because relative positions repeat almost exactly in the Metonic cycle. The resulting amplitudes and phases predict expected tides. These are usually dominated by constituents near 12 hours or 24 hours. Longer term constituents include 14 day or fortnightly, monthly, and semiannual cycles.

• 4. Coastal Dynamics

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  Measurements made in November 1998 at Burntcoat Head in the Bay of Fundy recorded a maximum range of 53 feet. Similar measurements in March 2002 at Leaf Basin, Ungava Bay gave values allowing for measurement errors. The Bay of Fundy has the world's highest tides due to its shape and bathymetry. Southampton in the United Kingdom has a double high water caused by interaction between M2 and M4 tidal constituents. Portland has double low waters for the same reason. The M4 tide is found all along the south coast of the United Kingdom. Its effect is most noticeable between the Isle of Wight and Portland. Extremely small tides occur in the Gulf of Mexico and Sea of Japan. In Cook Strait between New Zealand islands, one side's high water is simultaneous with the other's low water. Strong currents result with almost zero tidal height change in the strait's center. Near Cape Terawhiti in the middle of Cook Strait, tidal height variation is almost nil while current reaches maximum. Cotidal lines differ by one hour from neighbors in the northern hemisphere. They rotate counterclockwise around amphidromic points there. In the southern hemisphere this direction is clockwise.

• 5. Energy Extraction

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  Tidal energy can be extracted by inserting a water turbine into a current or building ponds that release water through a turbine. La Rance at Saint Malo, France is the most famous installed system. Impoundment dams are expensive to construct and disrupt natural water cycles. Ship navigation becomes difficult when dams block channels. Turbines lose efficiency at lower operating rates. Power available from a flow is proportional to the cube of flow speed. Times during which high power generation is possible are brief. Engineers face challenges withstanding corrosion and biological fouling. Environmental issues arise alongside engineering difficulties. Tidal power proponents note that unlike wind systems, levels can be reliably predicted save for weather effects. Most turbines obstruct ships if placed in best currents. Multiple ponds allow power generation at chosen times but require complex infrastructure.

• 6. Marine Ecology

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  Intertidal ecology studies ecosystems between low- and high-water lines along a shore. At low water, the zone is exposed. At high water, it is underwater. Vertical zonation divides communities into distinct horizontal bands of specific species. A species' ability to cope with desiccation determines its upper limit. Competition with other species sets its lower limit. The approximately 12-hour and fortnightly tidal cycle affects intertidal organisms. Their biological rhythms tend to occur in rough multiples of these periods. Vertebrates display similar circatidal rhythms including gestation and egg hatching. Humans use intertidal regions for food and recreation. Overexploitation damages intertidals directly. Invasive species and climate change have large negative effects. Marine Protected Areas protect these areas and aid scientific research. The menstrual cycle lasts roughly a lunar month, an even multiple of the tidal period. Such parallels hint at common descent from a marine ancestor.