# 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.

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# 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 timesone 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; 310 ft (13 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

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# Tides

Tides are distortions that occur in the shape of a celestial body. They are caused by the gravitational force of one or more other celestial bodies on that first body. In theory, any two bodies in the universe exert a gravitational force on each other. The most important examples of tidal forces on Earth are ocean tides, which result from the mutual attraction of the Moon and the Sun.

Greek geographer Pytheas (c. 380 b.c.c. 300 b.c.) was perhaps the first careful observer of ocean tides. In about the third century b.c., he traveled outside the Strait of Gibraltar and observed tidal action in the Atlantic Ocean. Pytheas suggested that the pull of the Moon on Earth's oceans caused the tides. Although largely correct, his explanation was not widely accepted by scientists until the eighteenth century, when English physicist and mathematician Isaac Newton (16421727) first succeeded in mathematically describing the tides and what cause them.

## Theories of tidal action

Although the Sun is larger than the Moon, the Moon is closer to Earth and, therefore, has a greater influence on Earth's ocean tides. The Moon's gravity pulls on the ocean water on the near side of Earth. This force causes the water, since it is able to flow, to form a slight bulge outward, making the water in that area slightly deeper.

At the same time, on the opposing side of Earth, a second tidal bulge occurs that is the same size as the first. This second bulge forms because the force of the Moon's gravity pulls the solid body of Earth away from the water on Earth's far side. The result is that two lunar tidal bulges exist on Earth at all timesone on the side of 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-degree angles to the high tide bulges. This belt, which completely encircles Earth, produces the phenomenon known as low tide.

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

## Words to Know

Diurnal: Occurring once every day.

Ebb tide: 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 acted upon by tidal forces.

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

Neap tides: Period of minimum tidal range that occurs about every two weeks when the Moon and Sun are at 90-degree angles to each other (the first and third quarter moons).

Semidiurnal: Occurring twice every day.

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

Spring tides: Period of maximum tidal range that occurs about every two weeks when the Moon and Sun are in line with each other (at the new and full moons).

Tidal current: Horizontal movements of water due to tidal action.

Tidal range: Vertical distance in sea level between high tide and low tide during a single tidal cycle.

Every 14 days, the Moon and Sun are in line with each other (new moon and full moon). Their gravitational forces combine to produce a maximum pull on Earth. 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.

Seven days later, 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.

## The nature of tides

In most places, tides are semidiurnal, meaning there are two tidal cycles each day (a tidal cycle is one high and one low tide). The high water level reached during one of the high tide stages is usually greater than the other high tide 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, a rising tide is called the flood tide. A falling tide is called 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. At this point the water level is static, neither rising nor falling, at least for a short time.

As the Moon revolves around Earth, Earth also rotates on its axis. Consequently, in order to return to the same position relative to the Moon above, Earth must rotate on its axis for 24 hours and 50 minutes (a period known as a lunar day). The additional 50 minutes allows Earth to "catch up" to the Moon. 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 and 25 minutes later than the previous high tide (or low tide).

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. 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. When this occurs, the tidal current may then reach speeds as great as 9 miles (15 kilometers) an hour with crests as high as 10 feet (3 meters) or more.

Most tides rise and fall between 3 and 10 feet (1 and 3 meters). 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, the difference between high and low tides (the tidal range) may be as great as 46 feet (14 meters). In comparison, some large bodies of water, such as the Mediterranean, Baltic, and Caribbean Seas, have areas with tides of less than a foot (0.3 meter). All coastal locations (as well as very large lakes) experience some variation in tidal range due to the affects of neap versus spring tides.

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## tide

tide, alternate and regular rise and fall of sea level in oceans and other large bodies of water. These changes are caused by the gravitational attraction of the moon and, to a lesser extent, of the sun on the earth. More generally, tides are the deformations of celestial bodies from a perfectly spherical shape that result from stresses created by their mutual gravitational attraction (see gravitation). Another way of viewing the tide is as the longest possible ocean wave, one which stretches all the way around the earth. The tide regarded as a wave is sometimes referred to as a tidal wave, although this term has been commonly applied to the shock wave propagated by an underwater earthquake. (To avoid confusion, such shock waves are now called tsunamis, their Japanese name, or seismic sea waves.) Numerous schemes have been proposed to harness the earth's tides, especially in various estuaries, as a practical source of power, but none as yet have proved economically or technologically feasible.

Tidal Effect on the Earth

Tides are raised in the earth's solid crust and atmosphere as well as in the oceans. Every body in the universe has some tidal effect, however small, on every other body. This effect is directly proportional to the mass of the body causing the tide but inversely proportional to the cube of the distance between the bodies. The earth's nearby moon is about 2.17 times as effective as the more massive sun in raising tides on the earth, even though the sun exerts a much greater total force on the earth than does the moon. Thus, the moon's proximity explains its dominant role in creating tides.

Direct and Indirect Tides

At any given time, there are two high tides on the earth, the direct tide on the side facing the moon and the indirect tide on the opposite side. As the earth rotates on its axis, the location of the two diametrically opposed tidal bulges varies on the earth's surface. The earth's rotation and the moon's revolution, which have the same direction, bring each point on the earth opposite the moon once every 24 hr and 50 min. Therefore, the average interval between direct and indirect high tides is about 12 hr and 25 min. In many places along the Atlantic coasts of N America and Europe, the two daily low tides are of nearly equal duration and magnitude, called semidiurnal tides.

In certain shallow seas and narrow estuaries, the tides differ from this simple pattern. For example, in certain regions such as the Pacific coast of N America, one of the two daily tides is appreciably higher than the other or the interval between successive tides is unequal; these are called mixed tides. In other regions, such as the Gulf of Mexico, there is only one high tide per day called a diurnal tide, with a period of 24 hr and 50 min.

The Magnitude and Effects of Tidal Ranges

The range of the tides is the difference in sea level between high and low tides. Spring tide, having the maximum range, occurs during the full moon when the earth is between the moon and the sun, and new moon when the moon is between the earth and the sun. At these times in the lunar cycle when the moon, earth, and sun are aligned the condition is known as syzygy. The term king tide is used in some regions to describe the highest tides of the year. Neap tide, having the minimum range, occurs during the moon's first and last quarters, when the moon, earth, and sun form a right angle. The typical tidal range in the open ocean is 2 ft (0.61 m) but is much greater near the coast. Tidal ranges vary around the world and average about 6 to 10 ft (2 to 3 m). The world's widest tidal range occurs in the Bay of Fundy, in E Canada, where the sea level changes by 40 ft (12 m) during the day, while the Mediterranean, Baltic, and Caribbean Seas are relatively tideless.

As the tides change, currents must flow to redistribute the ocean's water. Near the coast, the direction of the current changes every 61/4 hr from toward the shore (flood current) to away from the shore (ebb current). In the open ocean, the tidal currents are rotary, shifting through all directions of the compass in a period matching that of the local tide. When tidal currents flow into the mouth of a river, they speed up. In extreme cases, the tidal rise advances up the river as a solid wall of water often several feet high, a rare phenomenon called a tidal bore. During times of high tide accompanied by high wind and low pressure, as during a hurricane, a tidal surge can occur, causing coastal erosion, flooding, and damage to coastal cities.

The Prediction of Tides

Detailed prediction of ocean tides from theories of classical mechanics and hydrodynamics has not been entirely successful, largely because of complications introduced by the irregular shape of the ocean basins and coastlines. Useful results are obtained empirically by analyzing records of previous tides at a particular location to predict future tides. The importance of tides for maritime activities has prompted the compilation of tide tables for harbors, which give the time and height of high water and low water based on past observations and corrected for the varying positions of celestial bodies.

Bibliography

See A. C. Redfield, Introduction to Tides (1982); D. Arnold, Tides and Currents (1987); G. Marchuk and B. A. Kagan, Dynamics of Ocean Tides (1989).

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## tide

tide / tīd/ • n. the alternate rising and falling of the sea, usually twice in each lunar day at a particular place, due to the attraction of the moon and sun: the changing patterns of the tides | they were driven on by wind and tide. ∎  the water as affected by this: the rising tide covered the wharf. ∎ fig. a powerful surge of feeling or trend of events: he drifted into sleep on a tide of euphoria we must reverse the growing tide of racism sweeping the country. • v. [intr.] archaic drift with or as if with the tide. ∎  (of a ship) float or drift in or out of a harbor by taking advantage of favoring tides. PHRASES: turn the tide reverse the trend of events: the air power that helped to turn the tide of battle.PHRASAL VERBS: tide someone over help someone through a difficult period, esp. with financial assistance: she needed a small loan to tide her over.DERIVATIVES: tide·less adj.

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## tide

tide1
A. †portion of time, season, age; †hour; (arch.) point of time, due time; definite time of day or of the year (surviving in eventide, noontide, springtide); church anniversary or festival (arch. except as in Eastertide, Shrovetide, Whitsuntide) OE.;

B. swelling of the sea or its alternate rising and falling XIV. OE. tīd = OS. tīd (Du. tijd), OHG. zīt (G. zeit), ON. tíō :- Gmc. *tīdiz, f. *tī- :- IE. *dī- *dā(i)- divide, cut up, repr. by Gr. daiesthai divide, distribute, Skr. dâti, dyâti cuts, harvests, shares. In B prob. after MLG. (ge)tīde, tīe, MDu. ghetīde (Du. (ge)tij), a special development of the sense ‘fixed time’.
So tide vb. (arch.) happen, befall. OE. tīdan, earlier ġetīdan, f. the sb.

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## tide

tide
1. The periodic rise and fall of the Earth's oceans, caused by the relative gravitational attraction of the Sun, Moon, and Earth. The effect of the Moon is about twice that of the Sun, giving rise to the spring-neap cycle of tides. Variation in tides is caused by: (a) changes in the relative positions of the Sun, Moon, and Earth; (b) uneven distribution of water on the Earth's surface; and (c) variation in the sea-bed topography. Semi-diurnal tides are those with two high and two low waters (period 12 hours and 25 minutes) during a tidal day (24 hours and 50 minutes). Diurnal tides have one high and one low water during a tidal day.

2. See EARTH TIDES.

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## tide

tide The periodic rise and fall of the Earth's oceans, caused by the relative gravitational attraction of the Sun, Moon, and Earth. The effect of the Moon is about twice that of the Sun, giving rise to the springneap cycle of tides. Variation in tides is caused by: (a)changes in the relative positions of the Sun, Moon, and Earth;(b)uneven distribution of water on the Earth's surface; and(c)variation in the seabed topography. Semidiurnal tides are those with two high and two low waters (period 12 hours and 25 minutes) during a tidal day (24 hours and 50 minutes). Diurnal tides have one high and one low water during a tidal day.

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"tide." A Dictionary of Ecology. . Encyclopedia.com. 24 May. 2017 <http://www.encyclopedia.com>.

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## tide

tide a particular time, season, or festival of the Christian Church; tide meaning ‘time, period, era’ is recorded from Old English (in form tīd) and is of German origin, ultimately related to German Zeit.

From late Middle English, the word has also meant (now the current meaning) the alternate rising and falling of the sea, usually twice in each lunar day at a particular place, due to the attraction of the moon and sun.

See also happy as a clam at high tide, a rising tide lifts all boats, save the tide, time and tide wait for no man.

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## tide

tide Periodic rise and fall of the surface level of the oceans caused by the gravitational attraction of the Moon and Sun. Tides follow the Moon's cycle of 28 days, so they arrive at a given spot 50 minutes later each day. When the Sun and Moon are in conjunction or opposition, the greatest tidal range occurs, called spring tides. When they are in quadrature, when the Moon is half-full, tidal ranges are lowest and are called neap tides.

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# Tide

a stream; a current of things or emotions.

Examples : tide of blood; of emigration, 1830; of emotions; of events; of feelings; of upright freedom, 1519; of popular prejudice, 1777; of sorrows, 1738.

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## tide

tide2 flow or carry along like the tide XVI; get over, surmount XVII. f. TIDE1 B.

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## tide

tideabide, applied, aside, astride, backslide, beside, bestride, betide, bide, bride, chide, Clyde, cockeyed, coincide, collide, confide, cried, decide, divide, dried, elide, five-a-side, glide, guide, hide, hollow-eyed, I'd, implied, lied, misguide, nationwide, nide, offside, onside, outride, outside, pan-fried, pied, pie-eyed, popeyed, pride, provide, ride, Said, shied, side, slide, sloe-eyed, snide, square-eyed, starry-eyed, statewide, Strathclyde, stride, subdivide, subside, tide, tried, undyed, wall-eyed, wide, worldwide •carbide • unmodified •overqualified, unqualified •dignified, signified •unverified • countrified •unpurified • unclassified •unspecified • sissified • unsanctified •self-satisfied, unsatisfied •unidentified • unquantified •unfortified • unjustified • uncertified •formaldehyde • oxhide • rawhide •cowhide • allied • landslide • bolide •paraglide • polyamide • bromide •thalidomide • selenide • cyanide •unoccupied