Tides

History

Theories of tidal action

Variables affecting tidal forces

Tide tables

Semidiurnal and diurnal tides

Effect of the moon’s motion around Earth

Tidal currents

Some side effects of the tides

Tidal power plants

Resources

Tides are deformations in the shape of a body caused by the gravitational force of one or more other bodies. All bodies in the universe exert tidal forces on each other, although the effects are generally too small to observe. As far as Earth is concerned, the most important tidal phenomena are the ocean and ground tides that occur as a result of the moon’s and the sun’s gravity.

History

one of the earliest careful observers of ocean tides was the Greek geographer Pytheas of Massalia. In about the third century BC, Pytheas traveled outside the Straits of Gibraltar and observed tidal action in the Atlantic Ocean. (Tides were essentially absent from the Mediterranean Sea, which was more familiar to the Greeks.) Pytheas proposed an explanation for tidal action: the pull of the moon on Earth’s oceans, he said, caused the tides. This explanation is now known to be basically correct, although it was not widely accepted by scientists until the eighteenth century. English physicist Sir Isaac Newton (1642–1727) first succeeded in mathematically describing the tides in what is known as the equilibrium tidal theory.

Theories of tidal action

The side of Earth that faces the moon experiences a larger gravitational pull, due to the moon’s closer proximity, than other parts of Earth. This force causes Earth itself to bulge slightly toward the moon. This bulge is termed an Earth tide. Since water is free to move horizontally, the oceans tend to flow over Earth’s surface and collect in a secondary bulge on top of the Earth tide; this water bulge is termed an ocean tide.

At the same time, an Earth tide and an ocean tide form on the opposite side of Earth, directly away from the moon. This second bulge forms as follows (focusing on the ocean tide alone, for clarity): the moon and Earth, like all pairs of bodies orbiting each other in space, actually orbit around their common center of mass (that is, the point where, if their individual centers were attached to opposite ends of a rigid stick, the stick could be suspended from a string and remain in balance).

In the case of the Earth-moon system, the common center of mass happens to be inside Earth, about 1,068 miles (663 km) beneath the surface along a line connecting the center of Earth to the center of the moon. As Earth and moon revolve around this point like dancers spinning with linked hands, all points on both bodies experience a centrifugal force. This centrifugal force has the same magnitude and direction at every point on and in Earth (i.e., away from the moon parallel to a line connecting the center of Earth to the center of the moon). Where Earth’s surface is at any angle other than 90° to the line connecting the center of Earth to the center of the moon, water experiences a horizontal component of this centrifugal force. On the half of Earth’s surface facing away from the moon, this horizontal force overcomes the pull of the moon’s gravity and causes water to flow over Earth’s surface to a point on the side of Earth directly opposite the moon-facing tidal bulge. A second tidal bulge thus forms on the side of Earth facing directly away from the moon. This bulge is slightly smaller than the moon-facing bulge because the imbalance between the moon’s gravitation and centrifugal force is smaller at this point. (The moon is closer to the moon-facing bulge, making its gravitation stronger there, whereas the centrifugal force considered here is the same everywhere on Earth.) The larger, moon-facing tide is termed the direct tide; the tide on the opposite side of Earth is termed the opposite tide.

These two tidal bulges—one moon-facing or direct, the other on the opposite side of Earth—are the high tides. Because Earth is spherical, these bulges are actually arcs, meeting at the poles to form a globe-girdling ring or belt of high tide aligned with the moon. (Centrifugal force and the moon’s gravity cancel exactly at the poles, so the high tide is, in this simplified model, highest at the equator and diminishes to zero toward the poles.) Movement of water to this high-tide belt causes a complementary belt of low water to form around Earth at 90° to the line connecting the centers of Earth and moon. This belt produces the phenomenon known as low tide.

The high-tide belt always lies along the line connecting the centers of Earth and moon; however, as Earth rotates daily on its axis, land areas approach this belt, pass through it, and leave it behind. Thus, from the point of view of an observer fixed to the surface of the rotating Earth, the ocean tides are continually sloshing up against some coastlines and draining away from others. As a result, most coastal areas experience two high tides and two low tides each day. One high tide corresponds to the high-tide arc facing the moon, and the other to the high-tide arc facing away from moon.

The sun forms similar tidal bulges in Earth and its oceans, one set due to gravitation and the other to centrifugal force. However, sun’s tidal effect is slightly less than one half that of the moon. (It is both more massive than the moon and more distant; distance wins.) As the moon orbits Earth every 28 days, it twice comes into alignment with Earth and the sun—once when it is directly between Earth and the sun (i.e., when observers on Earth see the shadowed side of the moon) and once when Earth is directly between itself and the the sun (i.e., when observers on Earth see the illuminated or “full” side of the moon). When the moon and the sun are aligned, their tidal forces add up to produce a maximum tidal change. These maximal tides are termed spring tides because the waters of the ocean “spring up” higher (and sink lower) at these times. When the moon and the sun are at right angles to each other (i.e., when the moon is half-illuminated as seen from Earth), the solar and lunar tidal bulges do not add, and the least dramatic tides of the month are observed. These are termed neap tides.

Variables affecting tidal forces

Physicists have derived precise mathematical expressions to describe the gravitational effects of the moon and the sun on Earth. In theory, therefore, it should be possible to make precise predictions of the timing and sizes of all of ocean tides. In fact, absolutely precise predictions are not possible because a large number of factors contribute to the tides at a particular location. Primary among these is that the shape of ocean basins is so irregular that the water in them cannot behave in a simple way. Other variables also complicate the situation. These include variations in Earth’s axial rotation and variations in Earth-moon-sun positioning, including variations in orbital distance and inclination. Estimates of tidal behavior are therefore still based primarily on previous tidal observations, continuous monitoring of coastal water levels, and astronomical tables. (Comparison of predicted with measured tides along United States coasts can be checked at <http://tidesonline.nos.noaa.gov/>, a Website maintained by the U.S. government.)

Tide tables

Data about tidal patterns accumulated in various parts of the world are used to produce tide tables. Tide tables are constructed by examining records to find out, for any given location, the times at which high and low tides have occurred in the past and the levels that those tides have reached. These records are then used to predict the most likely times and levels expected for tides at various times in the future for the same locations. Because of differences in ocean bottoms, coastline shape, and other factors, unique tide tables must be constructed for each specific coastline in the world.

Semidiurnal and diurnal tides

In most places, tides are semidiurnal (twice-daily), meaning that there are two tidal cycles (with one high tide and one low apiece) each day. In other words, during a typical day the tides reach their highest point along the shore and their lowest point twice each day. The high-water level reached during one of the high tide stages is usually higher than the other high point, and the low water level reached during one of the low tide stages is usually lower than the other low tide point. This difference is called the diurnal inequality of the tides.

In a few locations, tides occur only once a day, with a single high tide and a single low tide. Such tidal cycles are known as diurnal (daily) tides. In both diurnal and semidiurnal settings, a rising tide is termed a flood tide and a falling tide is termed an ebb tide. The moment when the water reaches its highest point at high tide (or its lowest point at low tide) is called the slack tide, since the water level is then static, neither rising nor falling, at least for a short time.

Effect of the moon’s motion around Earth

As Earth spins on its axis, completing one turn every 24 hours, the moon orbits around Earth, completing one orbit every 28 days. Consequently, during the time Earth rotates once (24 hours), the moon has moved across about 1/28th of the sky. Earth, consequently, must rotate one day plus 1/28th of a day (about 50 minutes) to bring the moon into the same position overhead. This time period—24 hours, 50 minutes—is termed a lunar day. Since tides are caused by the moon, they recur on a lunar-daily schedule, rather than a 24-hour schedule, and consequently shift their times of occurrence with respect to the 24-hour clock. As a result, on a coast with diurnal tides, each day the high tide (or low tide) will occur 50 minutes later than the day before; on a semidiurnal coast, each high tide (or low tide) will occur 12 hours, 25 minutes later than the previous high tide.

Tidal currents

Any movement of ocean water caused by tidal action is known as a tidal current. In open water, tidal currents are relatively weak and tend to change direction slowly and regularly throughout the day. They form a kind of rotary pattern that sweeps around the ocean like the minute hand on a clock. Closer to land, however, tidal currents tend to change direction rather quickly, flowing toward land during high tide and away from land during low tide. In many cases, this onshore and offshore tidal current flows up the mouth of a river or some other narrow opening. The tidal current may then attain velocities as great as 9 mi (15 km) an hour with crests as high as 10 ft (3 m) or more.

All coastal locations (as well as very large lakes) experience some variation in tidal range during each lunar cycle, due to the affects of neap versus spring tides. Most tides attain less than 10 ft (3 m) in size; 3–10 ft (1–3 m) is common. In some places, however, the tides may be much greater. These locations are characterized by ocean bottoms that act as funnels through which ocean waters rush upward towards or downward away from the shore. In the Bay of Fundy, Canada for example, the tidal range may be as great as 46 ft (14 m). At the opposite extreme, the Mediterranean, Baltic, and Caribbean Seas have tides of less than a foot (0.3 m).

Deep-ocean tidal currents also occur. In fact, it has recently been discovered that about half of the energy input by Earth-moon-sun system as it drags tides around Earth is dissipated in deep-ocean currents, and the rest in shallow-ocean currents. Some 3×10¹² watts of energy are dissipated through friction in deep-ocean circulation alone, with profound long-term effects on Earth’s climate.

Some side effects of the tides

(1) Because Earth’s comparatively rapid rotation is continually dragging the tidal bulges away from their ideal locations, the moon-facing bulge is always slightly ahead of the moon in the direction of its orbit. (The moon orbits in the same sense as Earth spins.) The gravitational pull of the bulge thus tends to accelerate the moon slightly, which causes it to orbit slightly higher above Earth. The moon thus moves about 1.2 inches (3 cm) farther from Earth every year. As it moves away, the magnitude of the tides slowly decreases. (2) Tidal friction is slowing Earth’s axial spin. During the Jurassic period, for example, approximately 180 million years ago, the day was less than 23 hours long; when life first appeared on Earth, it was only about 20 hours long. Tidal friction long ago caused the moon to slow its own axial spin to the point where it now always keeps the same side facing Earth. This is a common effect in bodies orbiting more massive ones, as for example the inner moons of Saturn and Jupiter. (3) Stresses in Earth’s crust caused by the tides tend to trigger earthquakes. Frequency analysis of all recorded earthquakes shows that they have a strong tendency to occur on a semidiurnal basis, in accord with tidal stress.

Tidal power plants

Engineers have long recognized that the movement of tidal currents might be an inexpensive, efficient, and environmentally safe source of power for human use. In general, the plan would be to construct a dam across the entrance of an estuary through which tidal currents flow with significant speed. Then, as tidal currents flow into and out of the estuary twice each day, they could be used to drive turbines which, in turn, could be used to operate electrical generators.

One of the few commercial tidal power stations in operation is located at the mouth of the La Rance River in France. Tides at this location reach a maximum of 44 ft (13.5 m). Each time the tide comes in, a dam at the La Rance station holds water back until it reaches its maximum depth. At that point, gates in the dam are opened and water is forced to flow into the La Rance River, driving a turbine and generator in the process. Gates in the dam are then closed, trapping the water inside the dam. At low tide, the gates open once again, allowing water to flow out of the river, back into the ocean. Again the power of moving water is used to drive a turbine and generator.

The plant is able to produce electricity only four times each day, during each of two high tides and each of two low tides. It generates a modest 250 megawatts in this way with an efficiency about equal to that of a fossil-fuel plant, 25%. With present technology, few other sites exist where tidal power generation is currently considered economically feasible.

See also Alternative energy sources; Gravity and gravitation.

Resources

BOOKS

Pugh, David. Changing Sea Levels: Effects of Tides, Weather and Climate. Cambridge, UK: Cambridge University Press, 2004.

PERIODICALS

Kasahara, Junzo. “Tides, Earthquakes, and Volcanoes.” Science Vol. 297 (July 19, 2002): 348-349.

KEY TERMS

Diurnal —Occurring once per day.

Ebb tide —The period when the water level is falling; the period after high tide and before low tide.

Flood tide —The period when the water level is rising; the period after low tide and before high tide.

High tide —The event corresponding to the largest increase in water level in an area that is induced by tidal forces.

Low tide —The event corresponding to the largest decrease in water level in an area that is induced by tidal forces.

Neap tides —Period of minimum tidal range that occurs about every two weeks when moon and sun are at 90° to each other, that is, at the first and third quarter moons.

Period —The interval of time between two recurring events, such as the high tides in an area.

Semidiurnal —Occuring twice per day.

Slack tide —Period during which the water level is neither rising nor falling.

Spring tides —Period of maximum tidal range; occurs about every two weeks, when Moon and sun are in line with each other, i.e., at the new and full moons.

Tidal current —Horizontal movement of water due to tidal forces.

Tidal range —Vertical distance between high tide and low tide during a single tidal cycle.

Wunsch, Carl. “Moon, Tides and Climate.” Nature 405 (June 15, 2000): 743-744.

OTHER

National Oceanic and Atmospheric Administration, U.S. Department of Commerce. “Our Restless Tides: A Brief Explanation of the Basic Astronomical Factors which Produce Tides and Tidal Currents.” February, 1998. <http://www.co-ops.nos.noaa.gov/restles1.html> (accessed November 27, 2006).

K. Lee Lerner

Larry Gilman

David E. Newton

Tides

Ocean tides are periodic rises and falls in the level of the sea, and are formed by the gravitational attraction of the Moon and Sun on the water in the ocean. Although the Moon is much smaller than the Sun, it has a greater gravitational attraction for the Earth because the Moon is much closer to Earth. This causes the oceans to bulge out in the direction of the Moon.

Equilibrium Theory of Tides

Two theories help explain tides. The equilibrium theory of tides uses the universal laws of physics, as applied to a water-covered Earth. The dynamic theory of tides studies tides as they occur in the real world, modified by landmasses, geometry of the ocean basins , and Earth's rotation.

The equilibrium tidal theory begins with a hypothetical, water-covered planet and its satellite moon orbiting the Sun. The Moon is held in orbit with Earth by Earth's gravitational force. There is also a centrifugal force pulling the Moon away from Earth and trying to send it spinning out into space.

Earth and the Moon rotate around the common center of mass of the Earth-Moon system; this system is held in orbit by the Sun's gravitational attraction while centrifugal force pulls the center of the mass away from the Sun. Both forces, gravitational and centrifugal, must reach and maintain equilibrium to hold the Earth-Moon system in orbit.

In the Earth-Moon-Sun system, the mass of the Sun is greatest, but its extreme distance renders its gravitational pull nominal. The tidegenerating force of the Moon and Sun vary as the inverse cube of their distances from Earth. The mass of the Moon is very small by comparison, but it is considerably closer, and therefore has a greater attractive effect on water particles than does the Sun.

Lunar Tides.

Water responds to the Moon's gravitational force by flowing toward it, making a bulge on the surface of the ocean. On the side of Earth facing the Moon, gravitational force is applied to water particles toward the Moon. This force produces a lunar bulge in the layer of ocean water. At the same time, the centrifugal force of the Earth-Moon system acting on the water particles at Earth's surface opposite the Moon creates a second bulge.

Two lunar bulges on opposite sides of Earth are created on a planet covered by a uniformly deep ocean. The bulges represent the crests of the two tidal waves (high tide), directly opposite each other, and the low water areas are the two troughs (low tide). The equilibrium tidal theory predicts tides that are semidiurnal, which means two high and two low tides each day.

Earth and the Moon are moving in the same direction along their orbit with the Sun. Earth rotates once during a 24-hour period but Earth must turn an extra 12 degrees, or 50 minutes, for the Moon to be directly over the same place as the day before because of the Moon's rotation. Therefore a tidal day is not 24 hours long but rather 24 hours and 50 minutes, and the tidal period between high tides is 12 hours and 25 minutes. This explains why tides arrive at the same location about an hour later each day. The wavelength of the two tidal waves is one-half the circumference of Earth.

Solar Tides.

The Moon plays the greatest role in tide-building, but the Sun also produces its own tidal bulge. Though of much greater mass, the Sun's distance reduces its tide-raising force to only 46 percent that of the Moon, and the tide period is 24 hours, not 24 hours and 50 minutes. The lunar bulge created by the Moon has greater influence on the ocean and continually moves eastward relative to the solar bulge produced by the Sun.

On land, the tides appear to flood in during a high tide, earning the name flood tide, and then flow back out to sea as an ebb tide. Earth's rotation is responsible for carrying the landmasses into and out of the tidal bulges. It is as if Earth were constantly rotating inside a fluid envelope of ocean whose tidal bulges are supported by both the Moon and Sun.

Spring and Neap Tides

During the 29.5 days it takes for the Moon to orbit Earth, the Sun, Earth, and Moon move in and out of alignment with each other. During the period of the new Moon, the Sun and Moon are lined up on the same side of Earth so that the high tides that are produced independently of each other coincide. The tide level is the result of adding the two waveforms together, producing tides of maximum range between high and low water. These are called spring tides (see part [a] above).

One week later, the Moon is in its first quarter and moves about 12 degrees per day, until it is at a 90-degree angle to the solar bulge (see part [b]). The crests of the lunar bulge will now coincide with the troughs, produced by the Sun, and the same is true of the Sun's crests and Moon's troughs. These tides are called neap tides. Tidal effects of the Moon and Sun tend to cancel each other out, and the range between high and low tide is small.

At the end of another week, the Moon phase is full and the Sun, Earth, and Moon are again in alignment; however, they are on opposite sides of Earth. This again produces spring tides with crests that coincide, but generally slightly less than during the new Moon phase. These are followed again by neap tides, 1 week later, and the 4-week cycle continues with spring tides and neap tides every other week.

Tides are an extreme example of shallow-water waves. The extremely long wavelength of the tidal wave is 20,000 kilometers (12,400 miles) compared to an average ocean depth of 4 kilometer. A shallow wave is one traveling in water depths less than 1/20 of its wavelength; because 4/20,000 is considerable smaller than 1/20, tides are shallow waves. The tidal wave is a progressive wave as it moves through the ocean; however, it is far different than wind-driven, progressive waves. Tidal bulges move as forced waves, with their velocity determined by ocean depth.

Dynamic Theory of Tides

Fortunately for land-dwelling creatures, Earth is not the watery world depicted in the equilibrium tidal theory. To study ocean tides, which are modified by landmasses, the geometry of ocean basins and Earth's rotation and declination, scientists developed the dynamic theory of tides.

Tide Patterns.

Tides behave differently in various parts of the world. Some coastal areas experience a regular pattern of one high tide and one low tide each day, known as a diurnal tide. This pattern is common in shallow inland seas, such the Gulf of Mexico and along the coast of Southeast Asia, and exhibits a tidal period of 24 hours, 50 minutes.

In many areas, including the Atlantic Coast of the United States, there is a high tide to low tide sequence repeated twice a day, termed a semidiurnal tide. These tides usually reach about the same level at high and low tides each day, and have a tidal period of 12 hours, 25 minutes.

The third pattern of tide has two high and two low tides per day, but the tides reach different high and low levels during each daily rhythm. Called a semidiurnal mixed tide, it results from combining a semidiurnal and diurnal tide. Mixed tides commonly have a tidal period of 12 hours, 25 minutes, but may also exhibit diurnal periods. This is the most common tide pattern throughout the world, and is found along the Pacific Coast of the United States.

Declination Tides.

If Earth and the Moon are aligned so that the Moon is north or south of Earth's equator, one tidal bulge will be in the Northern Hemisphere and one in the Southern Hemisphere. A point in the middle latitudes passes through only one crest and one trough during each tidal day. This type of diurnal tide is called a declination tide, because the Moon is said to have declination when it stands above or below the equator and not perpendicular to it.

The Sun also influences declination tides when it is aligned over 23.5 degrees north or south latitude at the summer and winter solstice. The variation causes the bulge created by the Sun to oscillate north to south, making a more diurnal Sun tide during the winter and summer months. The Moon's declination is at 28.5 degrees north to south latitude, and because the orbit is inclined 5 degrees to the Earth and Sun orbit, it takes 18.6 years for the Moon to complete its cycle of maximum declination.

Also, the Moon does not move around Earth in a perfectly circular orbit and Earth does not circle the Sun at a constant distance. In the Northern Hemisphere, Earth is closer to the Sun in the winter months, so the solar tides play a greater role as a tide producer in the winter than summer.

Coriolis Effect

Another influence on waves, tides, and ocean currents is the Coriolis effect. Earth is constantly rotating towards the east and the speed of Earth's rotation varies greatly at different latitudes. It travels fastest on the equator and slows in speed at the farther latitudes. Because of this, the Coriolis effect forces moving objects on Earth to follow curved paths. In the Northern Hemisphere, an object will follow a path to the right of its intended course, and in the Southern Hemisphere, an object will follow a path to the left of its intended course.

The Coriolis effect acts on all objects moving freely over the surface of Earth, and has a dramatic effect on atmospheric circulation and ocean currents and tides. Most ocean basins in the Northern Hemisphere will exhibit a circular current flow, known as a gyre, that rotates in a clockwise direction. The opposite is true in the Southern Hemisphere where ocean gyres predominately rotate counterclockwise.

Landforms and Tides

There are over 150 factors that can affect tide behavior along any given coast or ocean basin. The greatest influences on the tides are the Coriolis effect, landforms, and ocean depth.

An idealized tidal wave would move across Earth at 1,600 kilometers per hour (1,000 miles per hour) at the equator. Because tides are an extreme example of a shallow-water wave, friction with the ocean floor slows tides to a speed of about 700 kilometers per hour (435 miles per hour). Continents further restrict tide movement. The tidal waves cannot keep up with the rotation speed of Earth and they break up into a number of smaller tidal cells.

There are about twelve cells worldwide with five in the Pacific Ocean. In the middle of each cell is a node located near the center of an ocean basin, termed an amphidromic point, a no-tide point in the ocean around which the tidal crests and troughs rotate through each tidal cycle. Owing to the shape and location of landmasses surrounding the ocean basins, the tidal crests and troughs cancel each at these points.

The tidal wave crests sweep around each amphidromic point, like spokes on a bicycle wheel. Because a large volume of water moves with the tidal wave, it is easily influenced by the Coriolis effect. The tidal waves move counterclockwise around the amphidromic points in the Northern Hemisphere and clockwise around amphidromic points in the Southern Hemisphere. The farther from the amphidromic point, the higher the tide level becomes. The farthest point away from the central node is the antinode, where maximum vertical movement between crest and trough is found.

Tides in Bays and Estuaries.

As the tides rise along the coastline, they enter the bays, harbors, and estuaries and travel inland as far as elevation will allow. This point is termed the head of tidewater. The time of high tide becomes progressively delayed the farther inland it must travel. As tides enter coastal waters, they are affected by reflection just as waves are. In certain circumstances, constructive interference can result in tides with extreme highs and lows.

A classic example of constructive interference is the Bay of Fundy in Nova Scotia, Canada. The bay opens into the Atlantic Ocean and extends inland in two narrow arms for 258 kilometers (160 miles), curving to the north. Due to its length, the time it requires for the tide to reach the head of tidewater is nearly equal to that of the tidal period. This forces a buildup of tidal water in the northern end of the bay.

Coupled with the extra energy of the Coriolis effect (the bay bends to the right), during maximum spring tides, the Bay of Fundy receives the highest tidal range in the world at 17 meters (56 feet). This type of constructive interference can also occur as tides flow into an estuary against an outflowing current. Standing waves, termed tidal bores, will form and progress upstream at heights of several feet.

Because of the effects of friction, as the tidal wave approaches shallow water, a reversing current forms where the water flows with force, in and out of restrictive passages. Reversing current is of concern to navigators due to their high velocities, reaching 44 kilometers (28 miles) per hour between the coastal islands of British Columbia.

Tide Prediction and Tide Tables

In the uniform tidal system (semi and diurnal), the greatest height to which the tide rises on any day is known as high water and the lowest point is low water. In a mixed system, it refers to higher high and lower high water and higher low and lower low waters. Tidal observations made over a period of time are used to calculate the average or mean tide levels.

Because the depth of coastal waters is important for navigation, an average low-water reference is established. Water depths are measured from this level and recorded on navigational charts. The low water reference point is usually established at the mean low-water level, and a zero reference or tidal datum is established at this point. In mixed tidal areas, mean low water is used as the tidal datum. Sometimes, the low-tide level may fall below the mean value used as the tidal datum, producing a minus tide.

Tidal predictions are based on recorded high measurement from past records, and then are used to predict the future. But because of all the complex combination of possibilities, it is difficult to predict Earth's tides from knowledge of physical processes and the historical record. Yet with a combination of actual local measurements with known astronomical data, scientists can derive very accurate tide predictions.

Tide gage recording stations are installed at numerous coastal sites, which track the rise and fall of ocean waters.^* A minimum of 19 years of records is needed to allow for the long 18.6-year period of declination of the Moon. Tide tables are published annually by the National Oceanic and Atmospheric Administration and give the dates, times, phases of the Moon, and ocean and water levels for high and low tide at numerous locations along the coast and inland on some bays and estuaries to the head of tidewater.

see also Beaches; Coastal Ocean; Energy from the Ocean; Estuaries; Ocean Currents; Waves.

Ron Crouse

Bibliography

Garrison, Tom. Oceanography, An Invitation to Marine Science. New York: WadsworthPublishing Company, 1996.

Prager, Ellen J., with Sylvia A Earle. The Oceans. New York: McGraw-Hill, 2000.

Summerhayes, C. P., and S. A. Thorpe. Oceanography, An Illustrated Guide. New York:John Wiley & Sons, 1996.

Thurman, Harold V., and Alan P. Trujillo. Essentials of Oceanography. Upper SaddleRiver, NJ: Prentice Hall, 1999.

Internet Resources

Tides Online. National Oceanic and Atmospheric Administration. <http://tidesonline.nos.noaa.gov>

^* See "Sea Level" for a photograph of a tide gage station.

Tides

Tides are deformations in the shape of a body caused by the gravitational force of one or more other bodies. All bodies in the universe exert tidal forces on each other, although the effects are generally too small to observe. As far Earth is concerned, the most important tidal phenomena are the ocean and ground tides that occur as a result of the Moon's and the Sun's gravity.

History

One of the earliest careful observers of ocean tides was the Greek geographer Pytheas of Massalia. In about the third century b.c., Pytheas traveled outside the Straits of Gibraltar and observed tidal action in the Atlantic Ocean. (Tides were essentially absent from the Mediterranean Sea, which was more familiar to the Greeks.) Pytheas proposed an explanation for tidal action: the pull of the Moon on the Earth's oceans, he said, caused the tides. This explanation is now known to be basically correct, although it was not widely accepted by scientists until the eighteenth century. English physicist Sir Isaac Newton (1642–1727) first succeeded in mathematically describing the tides in what is known as the equilibrium tidal theory.

Theories of tidal action

The side of the Earth that faces the Moon experiences a larger gravitational pull, due to the Moon's closer proximity, than other parts of the Earth. This force causes the Earth itself to bulge slightly toward the Moon. This bulge is termed an Earth tide. Since water is free to move horizontally, the oceans tend to flow over the Earth's surface and collect in a secondary bulge on top of the Earth tide; this water bulge is termed an ocean tide.

At the same time, an Earth tide and an ocean tide form on the opposite side of the Earth, directly away from the Moon. This second bulge forms as follows (focusing on the ocean tide alone, for clarity): The Moon and the Earth, like all pairs of bodies orbiting each other in space , actually orbit around their common center of mass (that is, the point where, if their individual centers were attached to opposite ends of a rigid stick, the stick could be suspended from a string and remain in balance).

In the case of the Earth-Moon system, the common center of mass happens to be inside Earth, about 1068 miles (663 km) beneath the surface along a line connecting the center of the Earth to the center of the Moon. As the Earth and Moon revolve around this point like dancers spinning with linked hands, all points on both bodies experience a centrifugal force. This centrifugal force has the same magnitude and direction at every point on and in the Earth (i.e., away from the Moon parallel to a line connecting the center of the Earth to the center of the Moon). Where Earth's surface is at any angle other than 90° to the line connecting the center of the Earth to the center of the Moon, water experiences a horizontal component of this centrifugal force. On the half of Earth's surface facing away from the Moon, this horizontal force overcomes the pull of the Moon's gravity and causes water to flow over the Earth's surface to a point on the side of the Earth directly opposite the Moon-facing tidal bulge. A second tidal bulge thus form on the side of the Earth facing directly away from the Moon. This bulge is slightly smaller than the Moon-facing bulge because the imbalance between the Moon's gravitation and centrifugal force is smaller at this point. (The Moon is closer to the Moon-facing bulge, making its gravitation stronger there, whereas the centrifugal force considered here is the same everywhere on the Earth.) The larger, Moon-facing tide is termed the direct tide; the tide on the opposite side of the Earth is termed the opposite tide.

These two tidal bulges—one Moon-facing or direct, the other on the opposite side of the Earth—are the high tides. Because the Earth is spherical, these bulges are actually arcs, meeting at the poles to form a globe-girdling ring or belt of high tide aligned with the Moon. (Centrifugal force and the Moon's gravity cancel exactly at the poles, so the high tide is, in this simplified model, highest at the equator and diminishes to zero toward the poles.) Movement of water to this high-tide belt causes a complementary belt of low water to form around the Earth at 90° to the line connecting the centers of the Earth and Moon. This belt produces the phenomenon known as low tide.

The high-tide belt always lies along the line connecting the centers of the Earth and Moon; however, as the Earth rotates daily on its axis, land areas approach this belt, pass through it, and leave it behind. Thus, from the point of view of an observer fixed to the surface of the rotating Earth, the ocean tides are continually sloshing up against some coastlines and draining away from others. As a result, most coastal areas experience two high tides and two low tides each day. One high tide corresponds to the high-tide arc facing the Moon, and the other to the high-tide arc facing away from the Moon.

The Sun forms similar tidal bulges in the Earth and its oceans, one set due to gravitation and the other to centrifugal force. However, the Sun's tidal effect is slightly less than one half that of the Moon. (It is both more massive than the Moon and more distant; distance wins.) As the Moon orbits the Earth every 28 days, it twice comes into alignment with the Earth and Sun—once when it is directly between the Earth and the Sun (i.e., when observers on Earth see the shadowed side of the Moon) and once when the Earth is directly between itself and the Sun (i.e., when observers on Earth see the illuminated or "full" side of the Moon). When the Moon and Sun are aligned, their tidal forces add up to produce a maximum tidal change. These maximal tides are termed spring tides because the waters of the ocean "spring up" higher (and sink lower) at these times. When the Moon and Sun are at right angles to each other (i.e., when the Moon is half-illuminated as seen from Earth), the solar and lunar tidal bulges do not add, and the least dramatic tides of the month are observed. These are termed neap tides.

Variables affecting tidal forces

Physicists have derived precise mathematical expressions to describe the gravitational effects of the Moon and the Sun on the Earth. In theory, therefore, it should be possible to make precise predictions of the timing and sizes of all of ocean tides. In fact, absolutely precise predictions are not possible because a large number of factors contribute to the tides at a particular location. Primary among these is that the shape of ocean basins is so irregular that the water in them cannot behave in a simple way. Other variables also complicate the situation. These include variations in the Earth's axial rotation and variations in Earth-Moon-Sun positioning, including variations in orbital distance and inclination. Estimates of tidal behavior are therefore still based primarily on previous tidal observations, continuous monitoring of coastal water levels, and astronomical tables. (Comparison of predicted with measured tides along U.S. coasts can be checked at <http://tidesonline.nos.noaa.gov/>, a Website maintained by the U.S. government.)

Tide tables

Data about tidal patterns accumulated in various parts of the world are used to produce tide tables. Tide tables are constructed by examining records to find out, for any given location, the times at which high and low tides have occurred in the past and the levels which those tides have reached. These records are then used to predict the most likely times and levels to be expected for tides at various times in the future for the same locations. Because of differences in ocean bottoms, coastline shape, and other factors, unique tide tables must be constructed for each specific coastline in the world.

Semidiurnal and diurnal tides

In most places, tides are semidiurnal (twice-daily), meaning that there are two tidal cycles (with one high tide and one low apiece) each day. In other words, during a typical day the tides reach their highest point along the shore and their lowest point twice each day. The high-water level reached during one of the high tide stages is usually higher than the other high point, and the low water level reached during one of the low tide stages is usually lower than the other low tide point. This difference is called the diurnal inequality of the tides.

In a few locations, tides occur only once a day, with a single high tide and a single low tide. Such tidal cycles are known as diurnal (daily) tides. In both diurnal and semidiurnal settings, a rising tide is termed a flood tide and a falling tide is termed an ebb tide. The moment when the water reaches its highest point at high tide (or its lowest point at low tide) is called the slack tide, since the water level is then static, neither rising nor falling, at least for a short time.

Effect of the Moon's motion around Earth

As the Earth spins on its axis, completing one turn every 24 hours, the Moon orbits around Earth, completing one orbit every 28 days. Consequently, during the time Earth rotates once (24 hours), the Moon has moved across about 1/28th of the sky. Earth, conseqently, must rotate one day plus 1/28th of a day (about 50 minutes) to bring the Moon into the same position overhead. This time period—24 hours, 50 minutes—is termed a lunar day. Since tides are caused by the Moon, they recur on a lunar-daily schedule, rather than a 24-hour schedule, and consequently shift their times of occurrence with respect to the 24-hour clock. As a result, on a coast with diurnal tides, each day the high tide (or low tide) will occur 50 minutes later than the day before; on a semidiurnal coast, each high tide (or low tide) will occur 12 hours, 25 minutes later than the previous high tide.

Tidal currents

Any movement of ocean water caused by tidal action is known as a tidal current. In open water, tidal currents are relatively weak and tend to change direction slowly and regularly throughout the day. They form a kind of rotary pattern that sweeps around the ocean like the minute hand on a clock. Closer to land, however, tidal currents tend to change direction rather quickly, flowing toward land during high tide and away from land during low tide. In many cases, this onshore and offshore tidal current flows up the mouth of a river or some other narrow opening. The tidal current may then attain velocities as great as 9 mi (15 km) an hour with crests as high as 10 ft (3 m) or more.

All coastal locations (as well as very large lakes) experience some variation in tidal range during each lunar cycle, due to the affects of neap versus spring tides. Most tides attain less than 10 ft in size; 3–10 ft (1–3 m) is common. In some places, however, the tides may be much greater. These locations are characterized by ocean bottoms that act as funnels through which ocean waters rush upward towards or downward away from the shore. In the Bay of Fundy, Canada, for the tidal range may be as great as 46 ft (14 m). At the opposite extreme, the Mediterranean, Baltic, and Caribbean Seas have tides of less than 1 ft (0.3 m).

Deep-ocean tidal currents also occur. In fact, it has recently been discovered that about half of the energy put by Earth-Moon-Sun system into dragging tides around the Earth is dissipated in deep-ocean currents, and the rest in shallow-ocean currents. Some 3 × 1012 watts of energy are dissipated through friction in deep-ocean circulation alone, with profound long-term effects on Earth's climate.

Some side effects of the tides

(1) Because the Earth's comparatively rapid rotation is continually dragging the tidal bulges away from their ideal locations, the Moon-facing bulge is always slightly ahead of the Moon in the direction of its orbit. (The Moon orbits in the same sense as the Earth spins.) The gravitational pull of the bulge thus tends to accelerate the Moon slightly, which causes it to orbit slightly higher above the Earth. The Moon thus moves about 1.2 inches (3 cm) farther from the Earth every year. As it moves away, the magnitude of the tides slowly decreases. (2) Tidal friction is slowing the Earth's axial spin. During the Jurassic period, for example, approximately 180 million years ago, the day was less than 23 hours long; when life first appeared on Earth, it was only about 20 hours long. Tidal friction long ago caused the Moon to slow its own axial spin to the point where it now always keeps the same side facing Earth. This is a common effect in bodies orbiting more massive ones, as for example the inner moons of Saturn and Jupiter . (3) Stresses in Earth's crust caused by the tides tend to trigger earthquakes. Frequency analysis of all recorded earthquakes shows that they have a strong tendency to occur on a semidiurnal basis, in accord with tidal stress.

Tidal power plants

Engineers have long recognized that the movement of tidal currents might be an inexpensive, efficient, and environmentally safe source of power for human use. In general, the plan would be to construct a dam across the entrance of an estuary through which tidal currents flow with significant speed. Then, as tidal currents flow into and out of the estuary twice each day, they could be used to drive turbines which, in turn, could be used to operate electrical generators.

One of the few commercial tidal power stations in operation is located at the mouth of the La Rance River in France. Tides at this location reach a maximum of 44 ft (13.5 m). Each time the tide comes in, a dam at the La Rance station holds water back until it reaches its maximum depth. At that point, gates in the dam are opened and water is forced to flow into the La Rance River, driving a turbine and generator in the process. Gates in the dam are then closed, trapping the water inside the dam. At low tide, the gates open once again, allowing water to flow out of the river, back into the ocean. Again the power of moving water is used to drive a turbine and generator.

The plant is able to produce electricity only four times each day, during each of two high tides and each of two low tides. It generates a modest 250 megawatts in this way with an efficiency about equal to that of a fossil-fuel plant, 25%. With present technology, few other sites exist where tidal power generation is currently considered economically feasible.

See also Alternative energy sources; Gravity and gravitation.

Resources

periodicals

Kasahara, Junzo. "Tides, Earthquakes, and Volcanoes." Science 297 (July 19, 2002): 348-349.

Wunsch, Carl. "Moon, Tides and Climate." Nature 405 (June 15, 2000): 743-744.

other

National Oceanic and Atmospheric Administration, U.S. Department of Commerce. "Our Restless Tides: A Brief Explanation of the Basic Astronomical Factors which Produce Tides and Tidal Currents." Center for Operational Oceanographic Products and Services. February 1998 [December 29, 2002]. <http://www.co-ops.nos.noaa.gov/restles1.html>.

K. Lee Lerner
Larry Gilman
David E. Newton

KEY TERMS

Diurnal

—Occurring once per day.

Ebb tide

—The period when the water level is falling; the period after high tide and before low tide.

Flood tide

—The period when the water level is rising; the period after low tide and before high tide.

High tide

—The event corresponding to the largest increase in water level in an area that is induced by tidal forces.

Low tide

—The event corresponding to the largest decrease in water level in an area that is induced by tidal forces.

Neap tides

—Period of minimum tidal range that occurs about every two weeks when the Moon and Sun are at 90° to each other, that is, at the first and third quarter moons.

Period

—The interval of time between two recurring events, such as the high tides in an area.

Semidiurnal

—Occuring twice per day.

Slack tide

—Period during which the water level is neither rising nor falling.

Spring tides

—Period of maximum tidal range; occurs about every two weeks, when the Moon and Sun are in line with each other, i.e., at the new and full moons.

Tidal current

—Horizontal movement of water due to tidal forces.

Tidal range

—Vertical distance between high tide and low tide during a single tidal cycle.

Tides

Introduction

Tides are the variation of sea level at coastal locations. This variation is caused by periodic vertical movements— alternating rising and falling of sea level—of ocean waters, such as those within the open sea, bays, and gulfs, with respect to land masses. More specifically, tides result from the distortion in the shape of Earth (specifically, its large water bodies) caused by the force of gravity exerted by the Moon and, to a lesser extent, by the sun, and the centrifugal forces produced by the revolution of the Moon-Earth system and, to a lesser extent, by the revolution of the Sun-Earth system around their common centers of gravity.

Tides, to a smaller degree, are also found in large lakes, such as the Great Lakes of North America, and other bodies of waters, such as rivers, that are raised and lowered by ocean waters. The highest tides on Earth occur at the Bay of Fundy in Nova Scotia, Canada. The range of tides there can exceed 40 ft (12 m).

Historical Background and Scientific Foundations

Tides are produced due to the force of gravity exerted by the Moon and, to a lesser degree, by the sun onto Earth,

and centrifugal forces produced by the revolution of the Earth and Moon (and Earth and sun) around their common centers of gravity. These two forces are balanced (zero) at the center of mass of Earth. However, at points on the surface, these forces are not zero, which is what produces tides.

The centrifugal force always has a constant magnitude (size) and is directed away from the center of revolution of two bodies. Consequently, the centrifugal force is continuously directed away from the Moon in the Moon-Earth system, and away from the sun in the sun-Earth system.

The gravitational force produced by the Moon or sun is of a different magnitude at different points on Earth. This variation occurs because the magnitude of gravity changes with distance between bodies based on the Universal Law of Gravity developed by British physicist Isaac Newton (1642–1727). It states that gravity is much weaker the farther away two massive bodies are to each other.

When ocean waters face the Moon, the magnitude of Moon's gravitational force on these waters is larger than for the waters farthest away because the near-waters are about 8,000 mi (12,870 km) closer to the Moon— the approximate diameter of Earth. These near-waters have a large amount of gravitational force pulling at them but with a smaller centrifugal force in the opposite direction. The subtraction of the two forces still produces a positive tide-raising force.

Conversely, when waters are facing away from the Moon, the gravitational force is smaller than felt by the waters on the near side, but the centrifugal force—which is still of the same magnitude—is in the same direction as the gravitational force. Thus, the two forces are added together to, again, produce another tide-raising force.

When a particular side of Earth is facing the Moon, its water has a tendency to be pulled toward it. This action produces high tides—also called direct tides. The water away from the Moon, because it feels less gravity, tends to move away from the lunar body, rather than to rotate with Earth. This action also produces high tides—called opposite tides. The water that is perpendicular from these points, conversely, experiences low tides. The cycle of two high tides and two low tides is called a lunar day, which lasts about 24 hours, 50 minutes.

The sun also produces tides on Earth. However, because it is so far away—approximately 93 million mi (150 million km)—its gravitational force on tides is less than the gravitational force from the Moon, which is only about 238,000 mi (383,000 km) away.

WORDS TO KNOW

CENTRIFUGAL FORCE: The inertial reaction that causes a body to move away from a center about which it revolves.

ECLIPSE: The partial or total apparent darkening of the sun when the moon comes between the sun and Earth (solar eclipse), or the darkening of the moon when the full moon is in Earth's shadow (lunar eclipse).

GRAVITATIONAL FORCE: The mutual force of attraction that exists between all particles or bodies with mass. A force dependent upon mass and the distance between objects. The English physicist and mathematician Sir Isaac Newton (1642–1727) set out the classical theory of gravity in his Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy). According to classical theory, gravitational force, always attractive between two objects, increases directly and proportionately with mass of the objects but is inversely proportional to the square of the distance between the objects.

LAW OF GRAVITY: Mathematical statement of the force of attraction between all material objects. The law of gravity states that the force pulling two objects together is proportional to the product of their masses divided by the square of the distance between them. Gravity keeps Earth's atmosphere from drifting away and is a crucial force in all circulations of wind and water.

LUNAR DAY: The length of time between one sunrise on the surface of Earth's moon and the next: the length of a day on the moon. A lunar day is approximately 28 days long; that is, the moon rotates once on its axis in the time it takes it to circle Earth, which means that one face of the moon is always kept facing Earth.

TSUNAMI: Ocean wave caused by a large displacement of mass under the surface of the water, such as an earthquake or volcanic eruption.

Impacts and Issues

The relative positions of these three bodies (Earth, Moon, and sun) do not change appreciably over time, so the variation of tides based on their interactions is fairly regular. However, scientists have found that the 1,500-year climate cycle on Earth could be affected by tides. Certain alignments of the Moon, sun, and Earth— such as during an eclipse—make for strong tides. Thus, the warming and cooling patterns experienced on Earth could be affected by these strong tides.

Tides are not caused only by the gravitational tugging from the Moon and sun, but also by physical processes within Earth. Earthquakes, hurricanes, undersea volcanic eruptions, and underwater landslides can cause what are called tidal waves or tsunamis, which can cause severe damage to shorelines and loss of life to coastal residents.

In addition, the circulation of the atmosphere creates weather phenomena that dramatically change the amplitude (height) of tides. Such phenomena include hurricanes and cyclones.

Scientists estimate that sea level is roughly 4 in (102 mm) higher than it was about one hundred years ago. Recent satellite measurements show sea level is going up about .08 in (2 mm) each year. As sea levels continue to rise, larger tides are created that potentially could cause more destruction to coastal areas and displacement, injury, and death to people living and working nearby.

Scientists are not sure whether rising sea levels are related to global warming or just a part of normal weather fluctuations. Either way, rising sea levels and increased tides have created adverse problems for coastal areas.

On the positive side, tides can be used to produce electricity, which is considered an alternative energy source to petroleum. To harness electricity from their motion, tides are allowed to flow through a dam that drives a turbine (a machine with rotating blades) to produce electricity. After flowing through, the water is trapped. After the tides on the other side retreat, the trapped water is then released. This action causes it to reverse direction back through the dam, again driving the turbines to produce electricity. The first tide-producing electrical power plant in the world is the 2,500-ft (750-m) Rance tidal power plant, which is in operation on the Rance River in northwestern France.

See Also Coastlines, Changing; Hurricanes; Ocean Circulation and Currents; Oceans and Seas; Sea Level Rise; Sea Temperatures and Storm Intensity.

BIBLIOGRAPHY

Books

Cartwright, David Edgar. Tides: A Scientific History. Cambridge and New York: Cambridge University Press, 2000.

McCully, James Greig. Beyond the Moon: A Conversational Common Sense Guide to Understanding the Tides. Hackensack, NJ: World Scientific, 2006.

Pugh, David T. Changing Sea Levels: Effects of Tides, Weather, and Climate. Cambridge and New York: Cambridge University Press, 2004.

Web Sites

“It's High Tide for Ice Age Climate Change.” Science News, April 5, 2000. <http://www.sciencenews.org/pages/pdfs/data/2000/157-16/15716-08.pdf> (accessed November 9, 2007).

William Arthur Atkins

Tides

Tides are deformations in the shape of a body caused by the gravitational force of one or more other bodies. At least in theory, any two bodies in the Universe exert such a force on each other, although obvious tidal effects are generally too small to observe. By far the most important examples of tidal forces as far as humans are concerned are ocean tides that occur on Earth as a result of the Moon and Sun's gravitational attraction.

The side of Earth facing the Moon, due to the Moon's proximity, experiences a larger gravitational pull, or force, than other areas. This force causes ocean water , since it is able to flow, to form a slight bulge, making the water in that area slightly deeper. At the same time, another bulge forms on the opposing side of the Earth. This second bulge, which is perhaps a bit harder to understand, forms due to centrifugal force. Contrary to popular belief, the Moon does not revolve around the Earth, but rather the Earth and Moon revolve about a common point that is within the Earth, but nowhere near its center (2880 miles or 4640 km away). When you twirl a ball above your head at the end of a piece of string, the ball pulls against the string. This pull is known as centrifugal force.

When the Earth-Moon system revolves around its common axis, the side of Earth that is farthest from the Moon experiences a centrifugal force, like a ball spinning at the end of a string. This force causes a second tidal bulge to form, which is the same size as the first. The result is that two lunar tidal bulges exist on Earth at all times—one on the side of the Earth facing the Moon and another directly opposite to it. These bulges account for the phenomenon known as high tide.

The formation of these two high tide bulges causes a belt of low water to form at 90° to the high tide bulges. This belt, which completely encircles the Earth, produces the phenomenon known as low tide.

As Earth rotates on its axis, land areas slide underneath the bulges, forcing the oceans up over some coastlines and beneath the low tide belt, forcing water out away from other coastlines. In a sense, as Earth rotates on its axis, the high tide bulges and the low tide belt remains stationary and the continents and ocean basins move beneath them. As a result, most coastal areas experience two high tides and two low tides each day.

In addition to the lunar bulges, the Sun forms its own tidal bulges, one due to gravitational force and the other due to centrifugal force. However, due to the Sun's much greater distance from the Earth, its tidal effect is approximately one half that of the Moon.

When the Moon and Sun are in line with each other (new Moon and full Moon), their gravitational, or tidal forces, combine to produce a maximum pull. The tides produced in such cases are known as spring tides. The spring high tide produces the highest high tide and the spring low tide produces the lowest low tide of the fortnight. This is the same as saying the spring tides have the greatest tidal range, which is the vertical difference between high tide and low tide.

When the Moon and Sun are at right angles to each other (first and third quarter Moon), the two forces act in opposition to each other to produce a minimum pull on the oceans. The tides in this case are known as neap tides. The neap high tide produces the lowest high tide and the neap low tide produces the highest low tide, or the smallest tidal range, of the fortnight.

It is now possible to write very precise mathematical equations that describe the gravitational effects of the Moon and the Sun. In theory, it should be possible to make very precise predictions of the time, size, and occurrence of tides. In fact, however, such predictions are not possible because a large number of factors contribute to the height of the oceans at high and low tide at a particular location. Primary among these is that the shape of ocean basins is so irregular that water does not behave in the "ideal" way that mathematical equations would predict. However, a number of other variables also complicate

the situation. These include variations in the Earth's axial rotation , and variations in Earth-Moon-Sun positioning, including variations in orbital distance and inclination.

Scientists continue to improve their predictions of tidal variations using mathematical models based on the equilibrium theory of tides. However, for the present, estimates of tidal behavior are still based on previous tidal observations, continuous monitoring of coastal water levels, and astronomical tables. This more practical approach is referred to as the dynamical theory of tides, which is based on observation rather than mathematical equations.

The accumulated information about tidal patterns in various parts of the world is used to produce tide tables. Tide tables are constructed by looking back over past records to find out for any given location the times at which tides have occurred for many years in the past and the height to which those tides have reached at maximum and minimum levels. These past records are then used to predict the most likely times and heights to be expected for tides at various times in the future for the same locations. Because of differences in ocean bottoms, coastline, and other factors, unique tide tables must be constructed for each specific coastline every place in the world. They can then be used by fishermen, those on ocean liners, and others who need to know about tidal actions.

In most places, tides are semidiurnal, that is, there are two tidal cycles (high and low tides) each day. In other words, during a typical day, the tides reach their highest point along the shore and their lowest point twice each day. The high water level reached during one of the high tide stages is usually greater than the other high point, and the low water level reached during one of the low tide stages is usually less than the other low tide point. This consistent difference is called the diurnal inequality of the tides.

In a few locations, tides occur only once a day, with a single high tide stage and a single low tide stage. These are known as diurnal tides. In both diurnal and semidiurnal settings, when the tide is rising, it is called the flood tide. When the tide is falling, it is the ebb tide. The point when the water reaches its highest point at high tide, or its lowest point at low tide, is called the slack tide, since the water level is static, neither rising nor falling, at least for a short time.

As the Moon revolves around the Earth, the Earth also rotates on its axis. Consequently, the Earth must rotate on its axis for 24 hours, 50 minutes, known as a lunar day, to return to the same position relative to the Moon above. The additional 50 minutes allows Earth to "catch up" to the Moon, so to speak. In other words, if the Moon was directly overhead at Boston, Massachusetts, at noon yesterday, it will again be above Boston at 12:50 PM today. As a result, on a coast with diurnal tides, each day the high tide (or low tide) will occur 50 minutes later than the day before. Whereas, on a semidiurnal coast, each high tide (or low tide) will occur 12 hours, 25 minutes later than the previous high one.

The movement of ocean water as a result of tidal action is known as a tidal current. In open water, tidal currents are relatively weak and tend to change direction slowly and regularly throughout the day. They form, therefore, a kind of rotary current that sweeps around the ocean like the minute hand on a clock. Closer to land, however, tidal currents tend to change direction rather quickly, flowing toward land during high tide and away from land during low tide. In many cases, this onshore and offshore tidal current flows up the mouth of a river or some other narrow opening. The tidal current may then attain velocities as great as 9 mi (15 km) an hour with crests as high as 10 ft (3 m) or more.

Most tides attain less than 10 ft in size; 3–10 ft (1–3 m) is common. In some locations, however, the tides may be much greater. These locations are characterized by ocean bottoms that act as funnels through which ocean waters rush upward towards or downward away from the shore at very rapid speeds. In the Bay of Fundy, between Nova Scotia and New Brunswick, for example, the difference between high and low tides, the tidal range, may be as great as 46 ft (14 m). In comparison, some large bodies of water, such as the Mediterranean, Baltic, and Caribbean Seas , have areas with tides of less than 1 ft (0.3 m). All coastal locations (as well as very large lakes ) experience some variation in tidal range during a fortnight due to the affects of neap versus spring tides.

See also Celestial sphere: The apparent movements of the Sun, Moon, planets, and stars; Gravity and the gravitational field; Marine transgression and marine regression