Causes the ebb and flow of the earth 4. How the moon causes the tides in the seas and oceans of the earth

The ebb and flow is the periodical rise and fall of the water level in the oceans and seas.

Twice a day, with an interval of about 12 hours and 25 minutes, the water near the ocean or open sea rises and, if there are no obstacles, sometimes floods large areas - this is the tide. Then the water lowers and recedes, exposing the bottom - this is an ebb tide. Why is this happening? Even ancient people thought about this, they noticed that these phenomena are associated with the Moon. I. Newton was the first to point out the main reason for the ebb and flow - this is the attraction of the Earth by the Moon, or rather, the difference between the attraction by the Moon of the entire Earth as a whole and its water shell.

Explaining the ebb and flow of Newton's theory

The attraction of the Earth by the Moon consists of the attraction by the Moon of individual particles of the Earth. Particles in this moment closer to the moon, they 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 gravity would not play any role. But the Earth is not an absolutely rigid body, therefore, the difference in the forces of attraction of particles located near the surface of the Earth and near its center (this difference is called the tidal force) displaces the particles relative to each other, and the Earth, primarily its water shell, is deformed.

As a result, water rises on the side facing the moon and on its opposite side, forming tidal ridges, and excess water accumulates there. Due to this, the water level in other opposite points of the Earth decreases at this time - an ebb tide occurs here.

If the Earth did not rotate, and the Moon remained motionless, then the Earth, together with its aquatic shell would always keep 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, the tidal ledges 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 the height of the tidal wave is different

In the open ocean, the water rises slightly when a tidal wave passes: about 1 m or less, which remains practically invisible 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 ebb and flow.

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

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

Tides and Sun

The sun also has an effect on 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 the Moon oppose, so the tides are smaller.

Tides in the Earth's air shell and in its solid

Tidal phenomena occur not only in water, but also in air envelope Earth. These are called atmospheric ebbs and flows. Tides also occur in the solid 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.

Practical use of the ebb and flow

A tidal power plant is a special type of hydroelectric power plant that uses the energy of tides, and in fact the kinetic energy of the Earth's rotation. 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. Fluctuations in the water level near the coast can reach 18 meters.

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

An experimental TES has been operating in Russia since 1968 in Kislaya Bay on the coast of the Barents Sea.

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

Who wouldn't want to take a walk to the bottom of the sea? "It's impossible! you exclaim. - For this you need at least a caisson! " But don't you know that large areas of the seabed open up for observation twice a day? True, woe to those who decide to stay at this "exhibition" beyond the established time! The seabed opens at low tide. - this is a change of high and low water.

This is one of the mysteries of nature. Many naturalists tried to solve it: Kepler who discovered the law of planetary motion, Newton who established the basic laws of motion, the French scientist Laplace, who studied the emergence of celestial bodies. They all wanted to penetrate the secrets of the life of the oceans.

The wind creates waves on the sea. But the wind is too weak to control the ebb and flow. Even a storm can only help with the tide. What gigantic forces are doing such hard work?

The influence of the moon on ebb and flow

Three giants are fighting for the world's oceans: The sun, the moon and the earth itself... The sun is stronger than all, but it is too far from us to be the winner. The movement of water masses on Earth is mainly controlled by the Moon. Located 384,000 kilometers from Earth, it regulates the "pulse" of the oceans. Like a huge magnet, the Moon pulls masses of water up several meters while the Earth rotates on its axis.

Although the difference between the heights of ebb and flow is on average no more than 4 meters, the work that the moon is doing is enormous. It is equal to 11 trillion horsepower. If this number is written in numbers alone, then it will have 18 zeros and look like this: 11,000,000,000,000,000,000. You cannot collect such a number of horses, even if you drive herds from all "ends" of the globe.

Ebb and flow - sources of energy

After the sun ebb and flow- The biggest energy sources... They could give electricity all over the world. Since time immemorial, man has tried to make the moon serve him. In China and other countries, tidal waters with long ago rotate the millstones.

In 1913, the first "lunar" power station was put into operation in the North Sea near Husum. In England, France, the United States and especially in Argentina, feeling a shortage of fuel, many bold projects for the construction of tidal stations have been created. However, Soviet engineers went farthest, who created a project for the construction of a dam 100 kilometers long and 15 meters high in the Mezen Bay of the White Sea.

At high tide, a reservoir with a capacity of 2 thousand square kilometers is formed behind the dam. Two thousand turbine generators will provide 36 billion kilowatt-hours. This amount of energy was produced in 1929 by France, Italy and Switzerland combined. A kilowatt hour of this energy will cost about a penny. Unfortunately, the "pulse" ebb and flow of the sea beats with unequal strength, like the pulse of a person. The tides do not provide a constant, uniform flow of water, and this makes the project difficult.

The tide is strongest when the Sun and Moon pull masses of water in the same direction. High tides when the water level rises up to 20 meters, are at full and young moon... They are called "syzygy". In the first and last quarter of the month when the moon is at right angles to the sun, tides are lowest and are called "quadrature".

The ebb and flow of the sea are very important for navigation, and therefore their offensive calculated in advance... This calculation is so difficult that it takes many weeks to compile the annual tide calendar. But the inventive mind of man has created a calculating machine, the "electronic brain" of which makes predictions of tides in two days. The tide calendar shows the tide waves travel all over the globe at regular intervals. From the sea shores, they rise into rivers.

The ebb and flow is the periodical rise and fall of the water level in the oceans and seas. Twice a day, with an interval of about 12 hours and 25 minutes, the water near the ocean or open sea rises and, if there are no obstacles, sometimes floods large areas - this is the tide. Then the water lowers and recedes, exposing the bottom - this is an ebb tide. Why is this happening? Even ancient people thought about this, they noticed that these phenomena are associated with the Moon. I. Newton was the first to point out the main reason for the ebb and flow - this is the attraction of the Earth by the Moon, or rather, the difference between the attraction by the Moon of the entire Earth as a whole and its water shell.

Explaining the ebb and flow of Newton's theory

The attraction of the Earth by the Moon consists of the attraction by the Moon of individual particles of the Earth. 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 gravity would not play any role. But the Earth is not an absolutely rigid body, therefore, the difference in the forces of attraction of particles located near the surface of the Earth and near its center (this difference is called the tidal force) displaces the particles relative to each other, and the Earth, primarily its water shell, is deformed.

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, the tidal ledges 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 the height of the tidal wave is different

Tides and Sun

The sun also has an effect on 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 the Moon oppose, so the tides are smaller.

Tides in the Earth's air shell and in its solid

Tidal phenomena occur not only in the water, but also in the air shell of the Earth. These are called atmospheric ebbs and flows. Tides also occur in the solid 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.

There is a rise and fall of water. This is a phenomenon of the ebb and flow of the sea. Already in ancient times, observers noticed that the tide comes some time after the climax 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 located approximately on the same straight line.

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

The Earth revolves around 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 across the Earth, and tidal currents arise. When approaching the shore, the wave height increases as the bottom rises. In the inland seas, the height of the tidal wave is only a few centi-meters, in the open ocean it reaches about one meter. In favorably located narrow bays, the height of the tide increases several times.

The friction of water against the bottom, as well as deformations of the Earth's solid shell, are accompanied by the release of heat, which leads to the dissipation of the energy of the Earth-Moon system. Since the tidal hump is due to the east, the maximum tide occurs after the climax of the Moon, the attraction of the hump causes the Moon to accelerate and slow down the Earth's rotation. 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 2 times, the tide height increases by 8 times. At present, 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 by the same side. After that, the Moon will begin to gradually approach the Earth and in another 2-3 billion years will be torn apart by tidal forces (if, of course, by that time the Solar System will still exist).

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

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

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

But we live on Earth and all observations are carried out in a frame of reference connected with the center of the Earth, and not with the center of mass of the Earth - the Moon. To go 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 perform the actions in brackets and take into account that r little in comparison with R and in sums and differences it can be neglected. Then

A 'A = -GM / (R - r) 2 + GM ☾ / R 2 = GM ☾ (-2Rr + r 2) / R 2 (R - r) 2 = -2GM ☾ r / R 3.

Acceleration a’A and a’B equal in magnitude, 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 to the center of the Earth.

Tidal accelerations are the 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 differently attracted by the disturbing body. At points A and B the acceleration of gravity turns out to be 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. Calculations show that the rise of water or tide in the open ocean is about 40 cm. In coastal waters it is much higher, and the record is about 18 m. Newtonian theory cannot explain this.

On the coast of many outer seas, you can see a curious picture: along the coast, not far from the water, there are fishing nets. Moreover, these nets were not supplied for drying, but for fishing. If you stay on the shore and watch the sea, then everything will become clear. Now the water begins to arrive, and where there was a sandbank just a few hours ago, waves splashed. When the water receded, nets appeared, in which the tangled fish sparkled with scales. The fishermen went around the nets and took the catch. Material from the site

This 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 coast in front of me: a trail of ripples, a half-buried seal carcass, rare pieces of fin, fragments of shells. And then there was an even space ... 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. The tide shaft rolled uncontrollably forward across the gray surface. Overtaking each other, the waves ran onto the shore. One after another, the distant rocks sank - and only water is visible all around. She throws salt spray in my face. Instead of a dead plain in front of me lives and breathes water surface».

When a tidal wave enters a funnel-shaped bay in the plan, the shores of the bay compress it, as it were, causing the tide height 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) are in Brittany near the city of Saint-Malo.

Very often, a tidal wave enters the mouths of rivers, raising the water level in them by several meters. For example, near London at the mouth of the Thames River, the tide height is 5 m.

What is ebb and flow

On many sea coasts, you can observe how the water level evenly drops at regular intervals and only sticky soil remains. This process is called ebb. However, after a few hours the water level rises again and the soil on the shore is again covered with water. This process is called tide. The water level changes regularly twice a day.

When the tides give way

Ebb and flow regularly replace each other: ebb tide is followed by ebb tide, followed by the next ebb tide. Highest level water in the sea or ocean at high tide is called full water, and the minimum at low tide is called low water, respectively. The cycle " high water- low tide - low tide - high tide - high water "is 12 hours 25 minutes. This means that the ebb and flow can be observed twice a day.

How does the ebb and flow occur

The gravitational force of the Moon determines the formation of the first tidal ridge in the sea on the side of the Earth facing it. Due to the laws of physics associated with the rotation of the Earth and the emergence of centrifugal force, a second tidal ridge is formed on the opposite side of the Earth, even more powerful than the first. Therefore, the water level rises here as well.

Between these two ridges, it descends, and there is an ebb tide! And the Sun, by the force of its attraction, influences the Earth, as well as the ebb and flow. But the force of the influence of the Sun is much less than that of the Moon, although the mass of the Sun is 30 million times the mass of the Moon. The reason for this lies in the fact that the Sun is 390 times farther from the Earth than the Moon is from the Earth.

First tidal hydroelectric power plant

The ebb and flow, that is, the rise and fall of the sea level, generates a lot of energy. It can be used to generate electricity. The first and largest currently tidal hydroelectric power plant in the world was built in the estuary (narrow bay of the mouth) of the Ranet River (Saint-Malo, France) and was commissioned in 1966. There the difference between ebb and flow is very large (amplitude 8.5 meters).

What other factors affect the ebb and flow

In addition to the forces of gravity, cosmic bodies, the Moon and the Sun, other factors affect the ebb and flow: the rotation of the Earth slows down the tides, the shores do not allow water to rise. In addition, high and low tides are affected by severe storms, which make it difficult to outflow from the coast sea water... Therefore, its level in such places is much higher than at a normal high tide. The ebb and flow of the tide is also affected by the force of the wind: if it blows from the shore, the water level drops significantly below normal.

Is the ebb and flow always visible

They say that in some seas, for example in the Mediterranean or Baltic, there is no ebb and flow. Of course, this is not the case, because they are found in all seas. However, in the Mediterranean and Baltic Seas, the difference between full and low water (the amplitude of high and low tide) is so insignificant that it is practically invisible. In the North Sea, on the other hand, the ebb and flow are very clearly distinguished.

Tidal waves arise in the oceans and move to the marginal seas. If the marginal sea is connected to the ocean only by a narrow strait, such as the Mediterranean Sea, the tidal waves either do not reach it, or are very weakened. The North Sea is connected with the Atlantic Ocean by a wide strait, so the tidal waves easily reach the coast and the tide in this place is perfectly visible.

What is syzygy tide

Particularly strong ebb and flow can be observed during 14 days, when the Moon and the Sun during the full moon and new moon (syzygy) are in line with the Earth. At this time, the tidal forces of both celestial bodies, acting in the same direction, add up and intensify the tide. The so-called syzygy tide begins when the high water rises at its highest. Accordingly, at low tide, the water drops to the lowest level.

What is the amplitude of ebb and flow

The difference between full and low tide during high tide and low tide is called the amplitude. In this case, the forces of attraction of the Sun and the Moon play a role: when they strengthen each other, the amplitude increases (syzygy tide), and when the forces of attraction weaken, the amplitude, on the contrary, decreases (quadrature tide). In the open sea, the tide amplitude does not exceed 50 centimeters. On the banks, on the other hand, it is much larger.

So, on the coast of the North Sea of Germany, for example, it is 2-3 meters, on the English coast of the North Sea - up to 8 meters, and in the Bay of Saint-Malo (France) in the English Channel - up to 11 meters. This can be explained by the fact that in shallow waters, tidal waves, like all others, lose speed and slow down, as a result of which the water level rises.

What is Quadrature Tide

For seven days after the full moon and new moon, the Sun, Earth and Moon are no longer on the same straight line. When the tidal forces of the Moon and the Sun interact at right angles to each other, a quadrature tide begins: the full water rises slightly, and the low water level practically does not fall.

What are tidal currents

Tides not only cause water levels to rise and fall. As the sea rises and falls, the water moves back and forth. In the open sea this is hardly noticeable, but in straits and bays, where the movement of water is limited, tidal currents can be observed. In the first case (tidal current) it is directed towards the coast, in the second (ebb current) - in the opposite direction. Specialists usually call the change of tidal currents a turn. At the moment of turning, the water is in a calm state, and this phenomenon is called the "blind spot" of the tide.

Where are the greatest amplitudes of ebb and flow

The Bay of Fundy, on the east coast of Canada, is home to some of the largest high and low tide amplitudes on the planet. This means that the difference between full and low water at high tide and low tide is here the maximum. With a syzygy tide, it reaches 21 meters. Previously, fishermen installed nets at high tide, and collected fish from them at low tide: unusual way fishing!

How a storm tide occurs

A stormy tide is called a tide when the water rolls onto the coast especially high. It arises from strong winds that blow towards land and come with the syzygy tide. Let us remind you: during it, full water rises especially high, and low water sinks especially low. This occurs during the full moon and new moon periods.

The strength of the winds and their duration lead to storm tides when the water rises more than a meter above the mid-tide point. Distinguish between a strong storm tide, in which the water rises by 2.5 meters, and super strong - when the water rises by more than 3 meters.

What speed can tidal currents reach?

Deep in the oceans, tidal currents reach speeds of about a kilometer per hour. In narrow straits, it can be 15 to 20 kilometers per hour.