Where are the ebb and flow. Sea tides and tides. The greatest amplitudes of the tides

MOSCOW STATE UNIVERSITY OF ENVIRONMENTAL ENGINEERING

Essay on "Earth Sciences"

Topic: "Ebb and Flow"

Completed:

Student group H-30

Tsvetkov E.N.

Checked:

Petrova I.F.

Moscow, 2003

Definition.

Ebb and flow, periodic fluctuations in the water level (ups and downs) in the water areas on the Earth, which are due to the gravitational attraction of the Moon and the Sun acting on the rotating Earth. All large water areas, including oceans, seas and lakes, are subject to tides to one degree or another, although they are small on lakes.

The highest water level observed in a day or half a day at high tide is called high tide, the lowest level at low tide is called low tide, and the moment these limit marks are reached is called standing (or stage), respectively, high tide or low tide. The mean sea level is a conditional value, above which the level marks are located during high tides, and below - during low tides. This is the result of averaging large series of urgent observations. The average height of the tide (or low tide) is an average value calculated from a large series of data on the levels of high or low water. Both of these middle levels are linked to the local stock.

Vertical fluctuations in the water level during high and low tides are associated with horizontal movements of water masses in relation to the coast. These processes are complicated by wind surge, river runoff and other factors. Horizontal movements of water masses in the coastal zone are called tidal (or tidal) currents, while vertical fluctuations in the water level are called ebbs and flows. All phenomena associated with ebbs and flows are characterized by periodicity. Tidal currents periodically reverse direction, while ocean currents, moving continuously and unidirectionally, are due to the general circulation of the atmosphere and cover large expanses of the open ocean.

During the transitional intervals from high tide to low tide and vice versa, it is difficult to establish the trend of the tidal current. At this time (not always coinciding with high or low tide) the water is said to "stagnate".

High and low tides alternate cyclically in accordance with the changing astronomical, hydrological and meteorological conditions. The sequence of tidal phases is determined by two maxima and two minima in the daily course.

The essence of the phenomenon.

Although the Sun plays a significant role in tidal processes, the decisive factor in their development is the force of the gravitational attraction of the Moon. The degree of influence of tidal forces on each particle of water, regardless of its location on the earth's surface, is determined by Newton's law of universal gravitation. This law states that two material particles are attracted to each other with a force that is directly proportional to the product of the masses of both particles and inversely proportional to the square of the distance between them. This implies that the greater the mass of bodies, the greater the force of mutual attraction between them (with the same density, a smaller body will create less attraction than a larger one). The law also means that the greater the distance between two bodies, the less the attraction between them. Since this force is inversely proportional to the square of the distance between two bodies, the distance factor plays a much larger role in determining the magnitude of the tidal force than the masses of the bodies.

The gravitational attraction of the Earth, acting on the Moon and keeping it in near-Earth orbit, is opposite to the force of attraction of the Earth by the Moon, which tends to move the Earth towards the Moon and "lifts" all objects on the Earth in the direction of the Moon. The point on the earth's surface, located directly under the Moon, is only 6,400 km away from the center of the Earth and, on average, 386,063 km from the center of the Moon. In addition, the mass of the Earth is 81.3 times the mass of the Moon. Thus, at this point on the earth's surface, the attraction of the Earth, acting on any object, is approximately 300 thousand times greater than the attraction of the Moon. It is a common notion that water on Earth, directly under the Moon, rises in the direction of the Moon, causing water to flow away from other places on the Earth's surface, but since the Moon's pull is so small compared to Earth's, it would not be enough to lift such huge weight.

However, the oceans, seas, and large lakes on Earth, being large liquid bodies, are free to move under the force of lateral displacement, and any slight horizontal shear tendency sets them in motion. All waters that are not directly under the Moon are subject to the action of the component of the Moon's gravitational force directed tangentially (tangentially) to the earth's surface, as well as its component directed outward, and are subject to horizontal displacement relative to the solid earth's crust. As a result, there is a flow of water from the adjacent regions of the earth's surface towards a place under the moon. The resulting accumulation of water at a point under the Moon forms a tide there. The actual tidal wave in the open ocean has a height of only 30–60 cm, but it increases significantly when approaching the coasts of continents or islands.

Due to the movement of water from neighboring regions towards a point under the Moon, corresponding outflows of water occur at two other points remote from it at a distance equal to a quarter of the circumference of the Earth. It is interesting to note that the lowering of the ocean level at these two points is accompanied by a rise in sea level not only on the side of the Earth facing the Moon, but also on the opposite side. This fact is also explained by Newton's law. Two or more objects located at different distances from the same source of gravity and, therefore, subjected to acceleration of gravity of different magnitudes, move relative to each other, since the object closest to the center of gravity is most strongly attracted to it. Water at a sublunar point experiences a stronger attraction to the Moon than the Earth below it, but the Earth, in turn, is more strongly attracted to the Moon than water on the opposite side of the planet. Thus, a tidal wave arises, which on the side of the Earth facing the Moon is called direct, and on the opposite side it is called reverse. The first of them is only 5% higher than the second.

Due to the rotation of the Moon in its orbit around the Earth, approximately 12 hours and 25 minutes pass between two successive high tides or two low tides in a given place. The interval between the climaxes of successive high and low tides is approx. 6 h 12 min. The period of 24 hours and 50 minutes between two successive high tides is called a tidal (or lunar) day.

Tide inequalities. Tidal processes are very complex, so many factors must be taken into account in order to understand them. In any case, the main features will be determined by: 1) the stage of tide development relative to the passage of the Moon; 2) the amplitude of the tide; and 3) the type of tidal fluctuation, or the shape of the water level curve. Numerous variations in the direction and magnitude of tidal forces give rise to differences in the magnitudes of morning and evening tides in a given port, as well as between the same tides in different ports. These differences are called tide inequalities.

semi-permanent effect. Usually during the day, due to the main tidal force - the rotation of the Earth around its axis - two complete tidal cycles are formed. When viewed from the North Pole of the ecliptic, it is obvious that the Moon rotates around the Earth in the same direction in which the Earth rotates around its axis - counterclockwise. With each next revolution, this point on the earth's surface again takes a position directly under the Moon, somewhat later than during the previous revolution. For this reason, both high and low tides are late every day by about 50 minutes. This value is called the lunar delay.

Semi-monthly inequality. This main type of variation is characterized by a periodicity of approximately 14 3/4 days, which is associated with the rotation of the Moon around the Earth and the passage of successive phases, in particular syzygies (new moons and full moons), i.e. moments when the sun, earth and moon are in a straight line. So far, we have dealt only with the tidal action of the Moon. The Sun's gravitational field also acts on the tides, but although the Sun's mass is much larger than the Moon's, the distance from the Earth to the Sun is so much greater than the distance to the Moon that the Sun's tidal force is less than half that of the Moon. However, when the Sun and the Moon are on the same straight line, both on the same side of the Earth, and on different sides (on a new moon or a full moon), their attractive forces add up, acting along one axis, and the solar tide is superimposed on the lunar tide. Similarly, the attraction of the Sun increases the ebb caused by the influence of the Moon. As a result, the tides are higher and the tides are lower than if they were caused only by the pull of the moon. Such tides are called spring tides.

When the gravitational force vectors of the Sun and Moon are mutually perpendicular (during quadratures, i.e. when the Moon is in the first or last quarter), their tidal forces counteract as the tide caused by the attraction of the Sun is superimposed on the ebb caused by the Moon. Under such conditions, the tides are not as high, and the tides are not as low, as if they were due only to the gravitational force of the Moon. Such intermediate tides are called quadrature. The range of high and low water levels in this case is reduced by approximately three times compared to the spring tide. In the Atlantic Ocean, both spring tides and quadrature tides are usually a day late compared to the corresponding phase of the moon. In the Pacific Ocean, such a delay is only 5 hours. In the ports of New York and San Francisco and in the Gulf of Mexico, spring tides are 40% higher than quadrature ones.

lunar The period of fluctuations in the heights of the tides, which occurs due to lunar parallax, is 27 1/2 days. The reason for this inequality is the change in the distance of the Moon from the Earth during the rotation of the latter. Due to the elliptical shape of the lunar orbit, the Moon's tidal force is 40% higher at perigee than at apogee. This calculation is valid for the port of New York, where the effect of the moon being at apogee or perigee is usually delayed by about 1 1/2 days from the corresponding phase of the moon. For the port of San Francisco, the difference in tide heights due to the moon being at perigee or apogee is only 32%, and they follow the corresponding phases of the moon with a delay of two days.

daily inequality. The period of this inequality is 24 hours 50 minutes. The reasons for its occurrence are the rotation of the Earth around its axis and the change in the declination of the Moon. When the Moon is near the celestial equator, the two high tides on a given day (as well as two low tides) differ little, and the heights of the morning and evening high and low waters are very close. However, as the Moon's north or south declination increases, morning and evening tides of the same type differ in height, and when the Moon reaches its greatest north or south declination, this difference is greatest. Tropical tides are also known, so called because the Moon is almost over the Northern or Southern tropics.

The diurnal inequality does not significantly affect the heights of two consecutive low tides in the Atlantic Ocean, and even its effect on the heights of the tides is small compared to the overall amplitude of the oscillations. However, in the Pacific Ocean, the diurnal irregularity manifests itself in the levels of low tides three times stronger than in the levels of the tides.

Semi-annual inequality. Its cause is the revolution of the Earth around the Sun and the corresponding change in the declination of the Sun. Twice a year, for several days during the equinoxes, the Sun is near the celestial equator, i.e. its declination is close to 0. The moon is also located near the celestial equator approximately during the day every fortnight. Thus, during the equinoxes, there are periods when the declinations of both the Sun and the Moon are approximately 0. The total tide-forming effect of the attraction of these two bodies at such moments is most noticeable in areas located near the earth's equator. If at the same time the Moon is in the phase of a new moon or a full moon, so-called. equinoctial spring tides.

solar parallax inequality. The period of manifestation of this inequality is one year. Its cause is a change in the distance from the Earth to the Sun in the process of the Earth's orbital motion. Once for each revolution around the Earth, the Moon is at the shortest distance from it at perigee. Once a year, around January 2, the Earth, moving in its orbit, also reaches the point of closest approach to the Sun (perihelion). When these two moments of closest approach coincide, causing the greatest net tidal force, higher tide levels and lower tidal levels can be expected. Similarly, if the passage of aphelion coincides with the apogee, less high tides and shallower low tides occur.

Change in time.

The phenomenon of tides has not changed in time, since the movement of both the Moon and the Sun remain the same as a thousand years ago - namely, the movement of these two celestial bodies affect the tides on Earth.

Distribution and scale of manifestation.

The magnitude and nature of the tides in various parts of the coast of the World Ocean depend on the configuration of the shores, the angle of inclination of the seabed, and a number of other factors. Most typically they appear on the open coast of the ocean. The penetration of tidal waves into inland seas is difficult, and therefore the amplitude of the tides in them is small.

The narrow, shallow Danish Straits reliably shield the Baltic Sea from the tides. Theoretical calculations show that the amplitude of the water level fluctuation in the Baltic is approximately 10 centimeters, but it is almost impossible to see these tides, since they are completely erased by water level fluctuations under the influence of wind or changes in atmospheric pressure. Even more reliably protected from the tidal wave are our southern seas - the Black and Azov, which communicate with the waters of the World Ocean through a series of narrow straits, and the internal Aegean and Mediterranean seas. If the difference in water level during high and low tide on the Atlantic coast of Spain near Gibraltar reached 3 meters, then in the Mediterranean Sea near the strait it is only 1.3 meters. In other parts of the sea, the tides are even less significant and usually do not exceed 0.5 meters. In the Aegean Sea and the Bosporus and Dardanelles, the tidal wave attenuates even more. Therefore, in the Black Sea, water level fluctuations under the influence of tides are less than 10 centimeters. In the Sea of ​​Azov, connected to the Black Sea only by the narrow Kerch Strait, the amplitude of the tides is close to zero.

For the same reason, the tides are also very small in the Sea of ​​Japan - here they barely reach 0.5 meters.

If in the inland seas the magnitude of the tides is reduced in comparison with the open coast of the ocean, then in the bays and bays, which have a wide connection with the ocean, it increases. The tidal wave enters such bays freely. Water masses rush forward, but, constrained by narrowing banks and finding no way out, they rise up and flood the land to a considerable height.

At the entrance to the White Sea, in the so-called Funnel, the tides are almost the same as on the coast of the Barents Sea, that is, they are 4–5 meters. At Cape Kanin Nos, they do not even exceed 3 meters. However, entering the gradually narrowing Funnel of the White Sea, the tidal wave becomes higher and higher in the Mezen Bay already reaches a height of ten meters.

Even more significant is the rise in the water level in the northernmost part of the Sea of ​​Okhotsk. So, at the entrance to Shelikhov Bay, the sea level at high tide rises to 4–5 meters, in the apex (the most remote from the sea) part of the bay it rises to 9.5 meters, and in Penzhina Bay it reaches almost 13 meters!

The tides are very high in the English Channel. In English, its coast in the small Lime Bay, the water in syzygy rises to 14.4 meters, and in French, near the town of Granville, even 15 meters.

The tides reach their extreme values ​​in some parts of the Atlantic coast of Canada. In the Frobisher Strait (it is located at the entrance to the Hudson Strait) - 15.6 meters, and in the Bay of Fundy (near the US border) - as much as 18 meters.

Sometimes the influence of sea tides is also seen on rivers. The tidal wave comes to the mouth area from open areas of the ocean or sea. As we approach the coast, the level rises, and the profile of the tidal wave is deformed under the influence of a decrease in depth and features of the coast configuration. On the seashore, its front slope becomes steeper than the back one. From the estuarine coast, the tidal wave penetrates into the channel system of the river. More saline water along the bottom of the river channel, like a wedge, is rapidly moving against the current. The collision of two oncoming streams, sea and river, causes the formation of a steep shaft, called the bora. In the Cantangjiang River, which flows into the East China Sea south of Shanghai, the bore reaches a height of 7 - 8 meters, and the steepness of the wave is 70 degrees. This terrible water wall at a speed of 15 - 16 kilometers per hour sweeps up the river, washing away the banks and threatening to sink any ship that did not take refuge in a calm backwater in time. The Amazon, the greatest river in South America, is also famous for its powerful forest. There, a wave 5-6 meters high propagates up the river for three thousand kilometers from the ocean. On the Mekong, tidal waves spread up to 500 km, on the Mississippi - up to 400 km, on the Northern Dvina - up to 140 km. The tide carries saline water into the river. At the same time, either complete or partial mixing of river and salty sea waters occurs in the mouth section of the river, or a stratified state occurs, when a sharp difference in salinity of surface and underlying waters is observed. Salt water penetrates into the mouth of the river the further, the greater the depth of the channel and the density (salinity) of sea water and the lower the flow of river water.

Myths and legends.

For a long time, the causes of hot flashes remained incomprehensible. In ancient times, they were explained by the breath of the deity of the Ocean living in the sea, or by the result of the breath of the planet. Other fantastic assumptions about the nature of the tides have also been made. (also see p. Research History)

There is a rise and fall of water. This is the phenomenon of sea tides. Already in antiquity, observers noticed that the tide comes some time after the culmination of the Moon at the place of observation. Moreover, the tides are strongest on the days of new and full moons, when the centers of the Moon and the Sun are approximately on the same straight line.

Given this, I. Newton explained the tides by the action of gravity from the Moon and the Sun, namely, that different parts of the Earth are attracted by the Moon in different ways.

The Earth rotates on its axis much faster than the Moon revolves around the Earth. As a result, the tidal hump (the relative position of the Earth and the Moon is shown in Figure 38) moves, a tidal wave runs along the Earth, and tidal currents arise. When approaching the shore, the height of the wave increases as the bottom rises. In the inland seas, the height of the tidal wave is only a few centimeters, while in the open ocean it reaches about one meter. In well-located narrow bays, the height of the tide increases several times more.

The friction of the water against the bottom, as well as the deformation of the solid shell of the Earth, are accompanied by the release of heat, which leads to the dissipation of the energy of the Earth-Moon system. Since the tide hump is due east, the maximum tide occurs after the Moon's culmination, the attraction of the hump causes the Moon to accelerate and the Earth's rotation to slow. The moon is gradually moving away from the earth. Indeed, geological data show that in the Jurassic period (190-130 million years ago), the tides were much higher, and the day was shorter. It should be noted that when the distance to the Moon decreases by a factor of 2, the height of the tide increases by a factor of 8. Currently, the day is increasing by 0.00017 s per year. So in about 1.5 billion years their length will increase to 40 modern days. The month will be the same length. As a result, the Earth and the Moon will always face each other with the same side. After that, the Moon will begin to gradually approach the Earth and in another 2-3 billion years it will be torn apart by tidal forces (if, of course, the Solar System still exists by that time).

The influence of the moon on the tide

Consider, following Newton, in more detail the tides caused by the attraction of the Moon, since the influence of the Sun is significantly (2.2 times) less.

Let us write down the expressions for the accelerations caused by the attraction of the Moon for different points of the Earth, taking into account that these accelerations are the same for all bodies at a given point in space. In the inertial frame of reference associated with the center of mass of the system, the acceleration values ​​will be:

A A \u003d -GM / (R - r) 2, a B \u003d GM / (R + r) 2, a O \u003d -GM / R 2,

where a A, aO, a B are the accelerations caused by the attraction of the Moon at the points A, O, B(Fig. 37); M is the mass of the moon; r is the radius of the Earth; R- the distance between the centers of the Earth and the Moon (for calculations, it can be taken equal to 60 r); G is the gravitational constant.

But we live on the Earth and all observations are carried out in a reference system associated with the center of the Earth, and not with the Earth-Moon center of mass. To pass to this system, it is necessary to subtract the acceleration of the center of the Earth from all accelerations. Then

A’ A = -GM ☾ / (R - r) 2 + GM ☾ / R 2 , a’ B = -GM ☾ / (R + r) 2 + GM / R 2 .

Let's do the parentheses and take into account that r little compared to R and can be neglected in sums and differences. Then

A’ A \u003d -GM / (R - r) 2 + GM ☾ / R 2 \u003d GM ☾ (-2Rr + r 2) / R 2 (R - r) 2 \u003d -2GM ☾ r / R 3.

Accelerations a’A and a’B identical in modulus, opposite in direction, each directed from the center of the Earth. They're called tidal accelerations. At points C and D tidal accelerations, smaller in magnitude and directed towards the center of the Earth.

Tidal accelerations are called accelerations arising in the frame of reference associated with the body due to the fact that, due to the finite dimensions of this body, its different parts are attracted differently by the perturbing body. At points A and B the acceleration of gravity is less than at the points C and D(Fig. 37). Therefore, in order for the pressure at the same depth to be the same (as in communicating vessels) at these points, the water must rise, forming the so-called tidal hump. The calculation shows that the rise of water or the tide in the open ocean is about 40 cm. In coastal waters it is much larger, and the record is about 18 m. The Newtonian theory cannot explain this.

On the coast of many outer seas one can see a curious picture: fishing nets are stretched along the coast not far from the water. Moreover, these nets were set up not for drying, but for catching fish. If you stay on the shore and watch the sea, then everything will become clear. Now the water begins to rise, and where only a few hours ago there was a sandbank, waves splashed. When the water receded, nets appeared in which the entangled fish sparkled with scales. The fishermen, bypassing the nets, took off the catch. material from the site

Here is how an eyewitness describes the onset of the tide: “We got to the sea,” a fellow traveler told me. I looked around in bewilderment. There really was a shore in front of me: a trail of ripples, a half-buried skeleton of a seal, rare pieces of a fin, fragments of shells. And beyond that stretched a flat expanse... and no sea. But three hours later, the motionless line of the horizon began to breathe, became agitated. And now the sea swell sparkled behind her. A wave of tide rolled uncontrollably forward across the gray surface. Overtaking each other, the waves ran ashore. One after another, distant rocks sank - and all around you can see only water. She throws salt spray in my face. Instead of a dead plain, the water surface lives and breathes in front of me.

When a tidal wave enters a funnel-shaped bay, the shores of the bay seem to compress it, which causes the height of the tide to increase several times. So, in the Bay of Fundy off the eastern coast of North America, the tide height reaches 18 m. In Europe, the highest tides (up to 13.5 meters) occur in Brittany near the city of Saint-Malo.

Very often the tidal wave comes into the mouth

The influence of the Moon on the earthly world exists, but it is not pronounced. It is almost impossible to see it. The only phenomenon that visibly demonstrates the effect of the moon's gravity is the effect of the moon on the tides. Our ancient ancestors associated them with the Moon. And they were absolutely right.

How does the moon affect the tides

The tides are so strong in some places that the water recedes hundreds of meters from the coast, exposing the bottom, where the peoples living on the coast collected seafood. But with inexorable precision, the water receding from the shore rolls again. If you do not know how often the tides occur, you can be far from the coast and even die under the advancing water mass. The coastal peoples perfectly knew the timetable for the arrival and departure of the waters.

This phenomenon occurs twice a day. Moreover, ebbs and flows exist not only in the seas and oceans. All water sources are influenced by the moon. But far from the seas, this is almost imperceptible: sometimes the water rises a little, then it falls a little.

The influence of the moon on liquids

Fluid is the only natural element that moves behind the moon, making oscillations. A stone or a house cannot be attracted to the moon because they have a solid structure. The malleable and plastic water clearly demonstrates the effect of the lunar mass.

What happens during high tide or low tide? How does the moon raise water? The Moon most strongly affects the waters of the seas and oceans from that side of the Earth, which at the moment is directly facing it.

If you look at the Earth at this moment, you can see how the Moon draws the waters of the oceans towards itself, lifts them, and the water column swells, forming a “hump”, or rather, two “humps” appear - high from the side where the Moon is located , and less pronounced on the opposite side.

"Humps" precisely follow the movement of the Moon around the Earth. Since the world ocean is a single whole and the waters in it communicate, the humps move from the coast, then to the coast. Since the Moon passes twice through points located at a distance of 180 degrees from each other, we observe two high tides and two low tides.

Ebb and flow according to the phases of the moon

• The greatest ebb and flow occur on the ocean shores. In our country - on the shores of the Arctic and Pacific Oceans.
• Less significant tides are characteristic of inland seas.
• Even weaker this phenomenon is observed in lakes or rivers.
• But even on the shores of the oceans, the tides are stronger at one time of the year and weaker at another. This is already connected with the remoteness of the Moon from the Earth.
• The closer the Moon is to the surface of our planet, the stronger the ebbs and flows will be. The further - the, naturally, weaker.

Water masses are influenced not only by the Moon, but also by the Sun. Only the distance from the Earth to the Sun is much greater, so we do not notice its gravitational activity. But it has long been known that sometimes the tides become very strong. This happens whenever there is a new moon or a full moon.

This is where the power of the Sun comes into play. At this moment, all three planets - the Moon, the Earth and the Sun - line up in a straight line. Two forces of attraction already act on the Earth - both the Moon and the Sun.

Naturally, the height of the rise and fall of the waters increases. The strongest will be the combined influence of the Moon and the Sun, when both planets are on the same side of the Earth, that is, when the Moon is between the Earth and the Sun. And more water will rise from the side of the Earth facing the Moon.

This amazing property of the Moon is used by people to get free energy. On the shores of the seas and oceans, tidal hydroelectric power stations are now being built, which generate electricity thanks to the "work" of the moon. Tidal hydroelectric power plants are considered the most environmentally friendly. They act according to natural rhythms and do not pollute the environment.

Ebb and flow is the periodic rise and fall of the water level in the oceans and seas.

Twice during the day, with an interval of about 12 hours and 25 minutes, the water near the coast of the ocean or the open sea rises and, if there are no barriers, sometimes floods large spaces - this is a tide. Then the water goes down and recedes, exposing the bottom - this is the ebb. Why is this happening? Even ancient people thought about this, they noticed that these phenomena are associated with the moon. The main cause of the tides was first pointed out by I. Newton - this is the attraction of the Earth by the Moon, or rather, the difference between the attraction of the Moon of the entire Earth as a whole and its water shell.

Ebb and flow explained by Newton's theory

The attraction of the Earth by the Moon is made up of the attraction of the individual particles of the Earth by the Moon. Particles that are currently closer to the Moon are attracted by it more strongly, and more distant ones are weaker. If the Earth were absolutely solid, then this difference in the force of attraction would not play any role. But the Earth is not an absolutely solid body, therefore the difference in the attractive forces of particles located near the surface of the Earth and near its center (this difference is called the tide-forming force) displaces the particles relative to each other, and the Earth, primarily its water shell, is deformed.

As a result, on the side that faces the Moon, and on its opposite side, the water rises, forming tidal protrusions, and excess water accumulates there. Due to this, the water level in other opposite points of the Earth at this time decreases - there is a low tide here.

If the Earth did not rotate, and the Moon remained motionless, then the Earth, together with its water shell, would always retain the same elongated shape. But the Earth rotates, and the Moon moves around the Earth in about 24 hours and 50 minutes. With the same period, tidal protrusions follow the Moon and move along the surface of the oceans and seas from east to west. Since there are two such protrusions, a tidal wave passes over each point in the ocean twice a day with an interval of about 12 hours and 25 minutes.

Why is the height of the tidal wave different

In the open ocean, the water rises slightly during the passage of a tidal wave: about 1 m or less, which remains almost imperceptible to sailors. But off the coast, even such a rise in the water level is noticeable. In bays and narrow bays, the water level rises much higher during high tides, since the coast prevents the movement of the tidal wave and water accumulates here during the entire time between low tide and high tide.

The largest tide (about 18 m) is observed in one of the bays on the coast in Canada. In Russia, the highest tides (13 m) occur in the Gizhiginskaya and Penzhinskaya bays of the Sea of ​​Okhotsk. In inland seas (for example, in the Baltic or Black), the tides are almost imperceptible, because masses of water moving along with the ocean tidal wave do not have time to penetrate into such seas. But all the same, in every sea or even lake, independent tidal waves arise with a small mass of water. For example, the height of the tides in the Black Sea reaches only 10 cm.

In the same area, the height of the tide is different, since the distance from the Moon to the Earth and the greatest height of the Moon above the horizon change over time, and this leads to a change in the magnitude of tide-forming forces.

Tides and Sun

The sun also influences the tides. But the tidal forces of the Sun are 2.2 times less than the tidal forces of the Moon.

During the new moon and full moon, the tidal forces of the sun and moon act in the same direction - then the highest tides are obtained. But during the first and third quarters of the moon, the tidal forces of the sun and moon counteract, so the tides are smaller.

Tides in the air shell of the Earth and in its solid body

Tidal phenomena occur not only in the water, but also in the air shell of the Earth. They are called atmospheric tides. Tides also occur in the solid body of the Earth, since the Earth is not absolutely solid. Vertical oscillations of the Earth's surface due to tides reach several tens of centimeters.

The practical use of ebb and flow

A tidal power plant is a special type of hydroelectric power plant that uses the energy of the tides, but in fact the kinetic energy of the rotation of the Earth. Tidal power plants are built on the shores of the seas, where the gravitational forces of the Moon and the Sun change the water level twice a day. Water level fluctuations near the coast can reach 18 meters.

In 1967, a tidal power station was built in France at the mouth of the Rance River.

In Russia, since 1968, an experimental TPP has been operating in Kislaya Bay on the coast of the Barents Sea.

There are PES and abroad - in France, Great Britain, Canada, China, India, the USA and other countries.

The world ocean lives by its own rules, which are harmoniously combined with the laws of the universe. For a long time, people have noticed that they are actively moving, but they could not understand what these fluctuations in sea level are connected with. Let's find out what is high tide, low tide?

Ebb and flow: mysteries of the ocean

Sailors knew perfectly well that the tides were a daily occurrence. But neither ordinary inhabitants nor learned minds could understand the nature of these changes. As early as the fifth century BC, philosophers tried to describe and characterize how the oceans move. seemed to be something fantastic and unusual. Even reputable scientists considered the tides to be the breath of the planet. This version has existed for several millennia. Only at the end of the seventeenth century, the meaning of the word "tide" was associated with the movement of the moon. But it has not been possible to explain this process from a scientific point of view. Hundreds of years later, scientists figured out this mystery and gave an exact definition of the daily change in water level. The science of oceanology, which appeared in the twentieth century, established that the tide is the rise and fall of the water level of the oceans due to the gravitational influence of the moon.

Are the tides the same everywhere?

The influence of the moon on the earth's crust is not the same, so it cannot be said that the tides are identical all over the world. In some parts of the world, daily sea level drops reach up to sixteen meters. And the inhabitants of the Black Sea coast practically do not notice the tides at all, since they are the most insignificant in the world.

Usually the change occurs twice a day - in the morning and in the evening. But in the South China Sea, the tide is the movement of water masses, which occurs only once every twenty-four hours. Most of all, changes in sea level are noticeable in straits or other bottlenecks. If you observe, then with the naked eye it will be noticeable how quickly the water leaves or comes. Sometimes in a few minutes it rises to five meters.

As we have already found out, the change in sea level is caused by the impact on the earth's crust of its invariable satellite, the Moon. But how does this process take place? To understand what a tide is, it is necessary to understand in detail the interaction of all the planets in the solar system.

The moon and the earth are in constant dependence on each other. The Earth attracts its satellite, and that, in turn, tends to attract our planet. This endless rivalry allows you to maintain the required distance between the two cosmic bodies. The Moon and the Earth move in their orbits, now moving away, now approaching each other.

At that moment, when the Moon comes closer to our planet, the earth's crust arches towards it. This causes a wave of water on the surface of the earth's crust, as if it tends to rise higher. The separation of the earth's satellite causes a drop in the level of the World Ocean.

The interval of high and low tides on Earth

Since the tide is a regular phenomenon, it must have its own specific interval of movement. Oceanologists have been able to calculate the exact time of the lunar day. This term is usually called the revolution of the moon around our planet, it is slightly longer than our usual twenty-four hours. Every day the tides shift by fifty minutes. This time interval is necessary for the wave to "catch up" with the Moon, which moves thirteen degrees over the Earth's day.

Effect of ocean tides on rivers

We have already figured out what the tide is, but few people know about the effect of these oceanic oscillations on our planet. Surprisingly, even rivers are affected by ocean tides, and sometimes the result of this intervention is incredibly frightening.

During high tides, a wave that has entered the mouth of a river meets a stream of fresh water. As a result of the mixing of water masses of different densities, a powerful shaft is formed, which begins to move at great speed against the flow of the river. This stream is called boron, and it is capable of destroying almost all living things in its path. A similar phenomenon in a few minutes washes away coastal settlements and erodes the coastline. Bor stops as suddenly as it started.

Scientists have recorded cases when a powerful boron turned rivers back or completely stopped them. It is not difficult to imagine how catastrophic these phenomenal tidal events have become for all the inhabitants of the river.

How do tides affect marine life?

Not surprisingly, the tides have a huge impact on all organisms that live in the depths of the ocean. The hardest part is for small animals that live in coastal zones. They have to constantly adapt to changing water levels. For many of them, tides are a way to change habitat. During high tides, small crustaceans move closer to the shore and find food for themselves, the ebb wave pulls them deeper into the ocean.

Oceanologists have proven that many marine life is closely related to tidal waves. For example, in some species of whales, metabolism slows down during low tides. In other deep-sea inhabitants, reproductive activity depends on the height of the wave and its amplitude.

Most scientists believe that the disappearance of phenomena such as fluctuations in the level of the oceans will lead to the extinction of many living beings. Indeed, in this case, they will lose their source of nutrition and will not be able to adjust their biological clock to a certain rhythm.

The speed of rotation of the Earth: is the influence of tides great?

For many decades, scientists have been studying everything related to the term "tide". This is the process that brings more and more mysteries every year. Many experts attribute the speed of the Earth's rotation to the action of tidal waves. According to this theory, under the influence of the tides, they are formed on their way, they constantly overcome the resistance of the earth's crust. As a result, almost imperceptibly to humans, the speed of the planet's rotation slows down.

Studying sea corals, oceanologists found out that several billion years ago, the earth's day was twenty-two hours. In the future, the rotation of the Earth will slow down even more, and at some point it will simply equal the amplitude of the lunar day. In this case, as scientists predict, the ebbs and flows will simply disappear.

Human activity and the amplitude of oscillations of the World Ocean

It is not surprising that man is also subject to the action of the tides. After all, it is 80% liquid and cannot but respond to the influence of the moon. But man would not be the crown of the creation of nature if he had not learned to use practically all natural phenomena to his advantage.

The energy of the tidal wave is incredibly high, so for many years there have been various projects for the construction of power plants in areas with a large amplitude of movement of water masses. There are already several such power plants in Russia. The first was built in the White Sea and was an experimental version. The power of this station did not exceed eight hundred kilowatts. Now this figure seems ridiculous, and new tidal wave power plants are generating energy that powers many cities.

Scientists see the future of Russian energy in these projects, because they allow us to treat nature more carefully and cooperate with it.

Ebb and flow are natural phenomena that not so long ago were completely unexplored. Each new discovery by oceanologists leads to even greater questions in this area. But perhaps someday scientists will be able to unravel all the mysteries that the ocean tide presents to mankind every day.