The occurrence of tides. School Encyclopedia
The content of the article
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.
Reversible waterfall
(reversing direction) is another phenomenon associated with tides on rivers. A typical example is a waterfall on the St. John River (New Brunswick, Canada). Here, along a narrow gorge, water at high tide penetrates into a basin located above the low water level, but somewhat below the high water level in the same gorge. Thus, a barrier arises, flowing through which water forms a waterfall. At low tide, the flow of water rushes downstream through a narrowed passage and, overcoming an underwater ledge, forms an ordinary waterfall. At high tide, a steep wave that has penetrated the gorge falls like a waterfall into the overlying basin. The reverse current continues until the water levels on both sides of the threshold are equal and the tide begins to ebb. Then the waterfall is restored again, facing downstream. The average water level difference in the gorge is approx. 2.7 m, however, at the highest tides, the height of a direct waterfall can exceed 4.8 m, and a reverse one - 3.7 m.
The greatest amplitudes of the tides.
The world's highest tide is formed by strong currents in Minas Bay in the Bay of Fundy. Tidal fluctuations here are characterized by a normal course with a semidiurnal period. The water level at high tide often rises by more than 12 m in six hours, and then drops by the same amount over the next six hours. When the action of the spring tide, the position of the Moon at perigee, and the maximum declination of the Moon occur in one day, the tide level can reach 15 m. the top of the bay.
wind and weather.
Wind has a significant effect on tidal phenomena. The wind from the sea drives the water towards the shore, the height of the tide rises above normal, and at low tide the water level also exceeds the average. On the contrary, when the wind blows from the land, the water is driven away from the coast, and the sea level drops.
Due to the increase in atmospheric pressure over a vast area of water, the water level decreases, as the superimposed weight of the atmosphere is added. When atmospheric pressure increases by 25 mm Hg. Art., the water level drops by about 33 cm. A decrease in atmospheric pressure causes a corresponding increase in the water level. Therefore, a sharp drop in atmospheric pressure, combined with hurricane-force winds, can cause a noticeable rise in the water level. Such waves, although they are called tidal waves, are in fact not associated with the influence of tidal forces and do not have the periodicity characteristic of tidal phenomena. The formation of the mentioned waves can be associated either with hurricane force winds or with underwater earthquakes (in the latter case they are called seismic sea waves or tsunami).
The use of tidal energy.
Four methods have been developed to harness the energy of the tides, but the most practical of these is the creation of a system of tidal pools. At the same time, water level fluctuations associated with tidal phenomena are used in the lock system in such a way that the level difference is constantly maintained, which makes it possible to obtain energy. The power of tidal power plants directly depends on the area of the trap pools and the potential level difference. The latter factor, in turn, is a function of the amplitude of the tidal fluctuations. The achievable level difference is by far the most important for power generation, although the cost of facilities depends on the size of the pools. At present, large tidal power plants operate in Russia on the Kola Peninsula and in Primorye, in France in the estuary of the Rance River, in China near Shanghai, and also in other regions of the globe.
| TIDE INFORMATION FOR SOME PORTS IN THE WORLD | ||||
| Port | Interval between tides | Average tide height, m | Spring tide height, m | |
| h | min | |||
| Cape Morris Jesep, Greenland, Denmark | 10 | 49 | 0,12 | 0,18 |
| Reykjavik, Iceland | 4 | 50 | 2,77 | 3,66 |
| R. Coxoak, Hudson Strait, Canada | 8 | 56 | 7,65 | 10,19 |
| St. John's, Newfoundland, Canada | 7 | 12 | 0,76 | 1,04 |
| Barntcoe, Bay of Fundy, Canada | 0 | 09 | 12,02 | 13,51 |
| Portland Maine, USA | 11 | 10 | 2,71 | 3,11 |
| Boston Massachusetts, USA | 11 | 16 | 2,90 | 3,35 |
| New York, pc. New York, USA | 8 | 15 | 1,34 | 1,62 |
| Baltimore, pc. Maryland, USA | 6 | 29 | 0,33 | 0,40 |
| Miami Beach Florida, USA | 7 | 37 | 0,76 | 0,91 |
| Galveston, pc. Texas, USA | 5 | 07 | 0,30 | 0,43* |
| O. Maraca, Brazil | 6 | 00 | 6,98 | 9,15 |
| Rio de Janeiro, Brazil | 2 | 23 | 0,76 | 1,07 |
| Callao, Peru | 5 | 36 | 0,55 | 0,73 |
| Balboa, Panama | 3 | 05 | 3,84 | 5,00 |
| San Francisco, pc. California, USA | 11 | 40 | 1,19 | 1,74* |
| Seattle, Washington, USA | 4 | 29 | 2,32 | 3,45* |
| Nanaimo, British Columbia, Canada | 5 | 00 | ... | 3,42* |
| Sitka, Alaska, USA | 0 | 07 | 2,35 | 3,02* |
| Sunrise, Cook Inlet, pc. Alaska, USA | 6 | 15 | 9,24 | 10,16 |
| Honolulu Hawaii, USA | 3 | 41 | 0,37 | 0,58* |
| Papeete, oh Tahiti, French Polynesia | ... | ... | 0,24 | 0,33 |
| Darwin, Australia | 5 | 00 | 4,39 | 6,19 |
| Melbourne, Australia | 2 | 10 | 0,52 | 0,58 |
| Rangoon, Myanmar | 4 | 26 | 3,90 | 4,97 |
| Zanzibar, Tanzania | 3 | 28 | 2,47 | 3,63 |
| Cape Town, South Africa | 2 | 55 | 0,98 | 1,31 |
| Gibraltar, Vlad. Great Britain | 1 | 27 | 0,70 | 0,94 |
| Granville, France | 5 | 45 | 8,69 | 12,26 |
| Leith, UK | 2 | 08 | 3,72 | 4,91 |
| London, Great Britain | 1 | 18 | 5,67 | 6,56 |
| Dover, UK | 11 | 06 | 4,42 | 5,67 |
| Avonmouth, UK | 6 | 39 | 9,48 | 12,32 |
| Ramsey, oh Maine, UK | 10 | 55 | 5,25 | 7,17 |
| Oslo, Norway | 5 | 26 | 0,30 | 0,33 |
| Hamburg, Germany | 4 | 40 | 2,23 | 2,38 |
| * Daily tide amplitude. | ||||
Literature:
Shuleikin V.V. Physics of the sea. M., 1968
Harvey J. atmosphere and ocean. M., 1982
Drake C., Imbri J., Knaus J., Turekian K. The ocean itself and for us. M., 1982
Two years ago I was vacationing on the coast of the Indian Ocean on the wonderful island of Ceylon. My little hotel was only 50 meters from the ocean. With my own eyes every day I watched all the powerful movement and the stormy life of the ocean. One early morning I was standing on the shore, looking at the waves and thinking about what gives strength to such a powerful oscillation of the ocean, its daily ebb and flow.
What gives strength to the ebb and flow
Gravity equally affects the movement of all objects. But if gravity causes tides in the oceans, and water causes water in Africa, then why are there no tides in lakes? Hmm, what if we assume that everything we know is wrong. Many stupid people scientific world explain it like this. The attraction of the Earth at point A is weaker than at point B. The net effect of the Earth's attraction is stretching the ocean. After which it swells on opposite sides.
Yes, indeed the facts are real and there is a difference in the force of gravity of the Moon at points A and B.
Misunderstanding lies in the explanation of bulges. Maybe they do not appear because of the difference in attraction. And the reasons are less obvious, and they are confused. It's more about the total pressure in different places in the water column. And the Moon at the same time turns the Earth into a hydraulic pump on a planetary scale, and the water swells, clinging to the center. Therefore, even the slightest impact is enough to start a wave movement.
More about tides
But I would like to understand why they are not in another accumulation of water:
- in the human body (it is 80% water);
- in a filled bath;
- in lakes;
- in coffee cups, etc.
Most likely due to less pressure than in the ocean, and poor hydraulics. Unlike the ocean, these are all small accumulations of water. The area of the lake, the cup and the rest is not enough for the minimum pressure on it to change the water level, creating waves.
Large lakes can create pressure for mini tides. But since the winds and surges create big ripples, we just don't notice them. The tides are everywhere, they are just very microscopic.
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 absolutely solid, 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 water rises, forming tidal bulges, 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 water, but also in air shell 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, and in fact kinetic energy rotation of the earth. Tidal power plants are built on the shores of the seas, where gravitational forces 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.
October 15th, 2012
British photographer Michael Martin (Michael Marten) created a series of original shots that capture the coast of Britain from the same angles, but at different times. One shot at high tide and one at low tide.
It turned out very unusual, and positive reviews about the project, literally forced the author to start publishing a book. The book, called "Sea Change", was published in August this year and was released in two languages. It took Michael Marten about eight years to create his impressive series of shots. The time between high and low water averages a little over six hours. Therefore, Michael has to linger in each place longer than just a few clicks of the shutter. The idea of creating a series of such works was nurtured by the author for a long time. He was looking for how to realize the changes of nature on film, without human influence. And I found it by chance, in one of the seaside Scottish villages, where I spent the whole day and found the time of high and low tide.
Periodic fluctuations in the water level (ups and downs) in water areas on Earth are called tides.
The highest water level observed in a day or half a day at high tide is called high water, the lowest level at low tide is called low water, 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.
Vertical fluctuations in the water level during high and low tides are associated with horizontal movements 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 change direction to the opposite, in contrast to them, ocean currents moving continuously and unidirectionally are due to the general circulation of the atmosphere and cover large expanses of the open ocean.
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.
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 the law gravity Newton.
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, however, 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 tendency to shear horizontally 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 shores 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 fluctuations, 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-daily 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 side North Pole ecliptic, it is obvious that the Moon revolves around the Earth in the same direction in which the Earth rotates around its axis - counterclockwise. With each next turn given point the earth's surface again takes a position directly under the moon a little 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.
Crescent Inequality. This main type of variations is characterized by a periodicity of approximately 143/4 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 vectors of the Sun's and Moon's attractive forces are mutually perpendicular (during quadratures, i.e. when the Moon is in the first or last quarter), their tidal forces counteract, since 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.
Lunar parallax inequality. The period of fluctuations in the heights of the tides, which occurs due to lunar parallax, is 271/2 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.
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 celestial equator, two high tides on a given day (as well as two low tides) differ slightly, 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.
Diurnal inequality does not significantly affect the heights of two successive low tides in Atlantic Ocean, and even its effect on the heights of the tides is small compared to the total amplitude of the oscillations. However, in the Pacific Ocean, the diurnal irregularity manifests itself in the levels of low tides three times more 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 half month. Thus, during the equinoxes, there are periods when the declinations of both the Sun and the Moon are approximately equal to 0. The total tidal 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, there are 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, more can be expected. high levels tides and more low levels low tides. Similarly, if the passage of aphelion coincides with the apogee, less high tides and shallower low tides occur.
The greatest amplitudes of the tides. The world's highest tide is formed by strong currents in Minas Bay in the Bay of Fundy. Tidal fluctuations here are characterized by a normal course with a semidiurnal period. The water level at high tide often rises by more than 12 m in six hours, and then drops by the same amount over the next six hours. When the action of the spring tide, the position of the Moon at perigee, and the maximum declination of the Moon occur in one day, the tide level can reach 15 m. the top of the bay. The causes of tides, which have been the subject of constant study for many centuries, are among the problems that have given rise to many conflicting theories even in relatively recent times.
C. Darwin wrote in 1911: "There is no need to search for ancient literature for the sake of grotesque theories of tides." However, sailors manage to measure their height and use the possibilities of tides without having an idea of the real causes of their occurrence.
I think that we can especially not bother about the causes of the origin of the tides. Based on long-term observations, special tables are calculated for any point in the water area of the earth, which indicate the time of high and low water for each day. I am planning my trip, for example, to Egypt, which is just famous for its shallow lagoons, but try to guess in advance so that full water falls in the first half of the day, which will allow you to fully ride most of the daylight hours.
Another issue related to tides of interest to the kiter is the relationship between wind and water level fluctuations.
A folklore says that the wind increases at high tide, and on the contrary, it turns sour at low tide.
The influence of wind on tidal phenomena is more clearly understood. The wind from the sea drives the water towards the shore, the height of the tide rises above normal, and at low tide the water level also exceeds the average. On the contrary, when the wind blows from the land, the water is driven away from the coast, and the sea level drops.
The second mechanism operates by increasing atmospheric pressure over a vast area of water, lowering the water level as the superimposed weight of the atmosphere is added. When atmospheric pressure increases by 25 mm Hg. Art., the water level drops by about 33 cm. A high pressure zone or anticyclone is usually called good weather, but not for a kiter. Calm in the center of the anticyclone. A decrease in atmospheric pressure causes a corresponding rise in the water level. Therefore, a sharp drop in atmospheric pressure, combined with hurricane-force winds, can cause a noticeable rise in the water level. Such waves, although they are called tidal waves, are in fact not associated with the influence of tidal forces and do not have the periodicity characteristic of tidal phenomena.
But it is quite possible that low tides can also affect the wind, for example, a decrease in the water level in coastal lagoons leads to greater warming of the water, and as a result, to a decrease in the temperature difference between the cold sea and the heated land, which weakens the breeze effect.
Photo by Michael Marten
There is a rise and fall of water. This phenomenon sea tides and ebbs. 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 jurassic(190-130 million years ago), the tides would be much higher, and the day would be 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 east coast North America the height of the tide 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
