The Moon moves around the Earth at an average speed of 1.02 km / s in an approximately elliptical orbit in the same direction in which the vast majority of other bodies in the Solar System move, that is, counterclockwise when viewed from the Moon's orbit from the North Pole of the World. The semi-major axis of the Moon's orbit, equal to the average distance between the centers of the Earth and the Moon, is 384,400 km (approximately 60 Earth radii). Due to the ellipticity of the orbit, the distance to the Moon fluctuates between 356,400 and 406,800 km. The period of revolution of the Moon around the Earth, the so-called sidereal month, is subject to small fluctuations from 27.32166 to 29.53 days, but also to a very small secular reduction. The moon shines only with light reflected from the sun, so one half of it, facing the sun, is illuminated, while the other is plunged into darkness. How much of the illuminated half of the moon is visible to us at a given moment depends on the position of the moon in its orbit around the earth. As the Moon moves in its orbit, its shape is gradually but continuously changing. The various visible shapes of the moon are called its phases.

Ebb and flow is familiar to every surfer. Twice a day, the level of ocean waters rises and falls, and in some places by a very significant amount. Every day the tide comes 50 minutes later than the previous day.

The Moon is kept in its orbit around the Earth for the reason that between these two celestial bodies there are gravitational forces that attract them to each other. The Earth always tries to pull the Moon towards itself, and the Moon pulls the Earth towards itself. Since the oceans are large masses of fluid and can flow, they are easily deformed by the Moon's gravity, taking the shape of a lemon. The ball of solid rock, which is the Earth, remains in the middle. As a result, on the side of the Earth that faces the Moon, a water bulge appears and another similar bulge appears on the opposite side.

As the solid Earth rotates on its axis, tides occur on the shores of the ocean, this occurs twice during every 24 hours and 50 minutes when the shores of the oceans pass through the water mounds. The length of the period is more than 24 hours due to the fact that the Moon itself also moves in its orbit.

Due to the ocean tides, a friction force arises between the surface of the Earth and the waters of the oceans, slowing down the speed of the Earth's rotation around its axis. Our days are gradually getting longer and longer, each century the length of the day increases by about two thousandths of a second. This is evidenced by some types of corals that grow in such a way that each day leaves a clear scar in the body of the coral. Growth varies throughout the year, so that each year has its own stripe, like an annual ring on a tree cut. Studying fossil corals dating back 400 million years, oceanologists discovered that at that time the year consisted of 400 days with a duration of 22 hours. Fossilized remains of even more ancient forms of life indicate that about 2 billion years ago, a day lasted only 10 hours. In the distant future, the length of a day will be equal to our month. The moon will always stand in the same place, since the speed of the Earth's rotation around its axis will exactly coincide with the speed of the Moon's movement in its orbit. Even now, thanks to the tidal forces between the Earth and the Moon, the Moon constantly faces the Earth with the same side, except for small fluctuations. In addition, the speed of the moon in its orbit is constantly increasing. As a result, the Moon is gradually moving away from the Earth at a rate of about 4 cm per year.

The earth casts a long shadow in space, blocking the light of the sun. When the Moon enters the Earth's shadow, a lunar eclipse occurs. If you were on the Moon during a lunar eclipse, you could see the Earth passing in front of the Sun, blocking it. Often, the Moon remains faintly visible, glowing with a dim reddish light. Although it is in shadow, the Moon is illuminated by a small amount of red sunlight, which is refracted by the Earth's atmosphere towards the Moon. A total lunar eclipse can last up to 1 hour 44 minutes. Unlike solar eclipses, lunar eclipses can be observed from any place on Earth where the Moon is above the horizon. Although the Moon passes through its entire orbit around the Earth once a month, eclipses cannot occur monthly due to the fact that the plane of the Moon's orbit is tilted relative to the plane of the Earth's orbit around the Sun. At the most, seven eclipses can occur in a year, of which two or three must be lunar. Solar eclipses only occur on the new moon, when the Moon is exactly between the Earth and the Sun. Lunar eclipses always occur on a full moon when the Earth is between the Moon and the Sun.

Before scientists saw the moon rocks, they had three theories about the origin of the moon, but there was no way to prove any of them were correct. Some believed that the newly formed Earth rotated so fast that it threw off part of the substance that then became the Moon. Others suggested that the moon came from the depths of space and was captured by the force of the earth's gravity. The third theory was that the Earth and the Moon formed independently, almost simultaneously and at about the same distance from the Sun. Differences in the chemical composition of the Earth and the Moon indicate that these celestial bodies are unlikely to have ever been one.

Not so long ago, a fourth theory arose, which is now accepted as the most plausible. This is the giant impact hypothesis. The basic idea is that when the planets that we see now were just forming, some celestial body the size of Mars crashed into the young Earth at a glancing angle with great force. In this case, the lighter substances of the outer layers of the Earth would have to break away from it and scatter in space, forming a ring of debris around the Earth, while the core of the Earth, consisting of iron, would have been preserved intact. Eventually, this ring of debris stuck together to form the Moon.

By studying radioactive substances contained in lunar rocks, scientists were able to calculate the age of the moon. Rocks on the moon became solid about 4.4 billion years ago. The moon had apparently formed not long before; its most probable age is about 4.65 billion years. This is consistent with the age of meteorites, as well as with estimates of the age of the Sun.
The most ancient rocks on the Moon are found in mountainous regions. The age of the rocks taken from the seas of solidified lava is much less. When the Moon was very young, its outer layer was liquid due to the very high temperature. As the moon cooled, its outer covering, or crust, formed, parts of which are now found in mountainous regions. For the next half a billion years, the lunar crust was bombarded continuously by asteroids, that is, small planets, and giant rocks that arose during the formation of the solar system. After the strongest blows, huge dents remained on the surface.

Between 4.2 and 3.1 billion years ago, lava flowed out through holes in the crust, flooding the circular basins left on the surface by colossal impacts. Lava, flooding vast flat areas, created lunar seas, which in our time are solidified oceans of rock.

The seas and oceans move away from the coast twice a day (low tide) and twice approach it (high tide). In some reservoirs, there are practically no tides, while in others the difference between low tide and high tide along the coastline can be up to 16 meters. Basically, the tides are semi-diurnal (twice a day), but in some places they are diurnal, that is, the water level changes only once a day (one low tide and one high tide).

The tides are most noticeable in the coastal strips, but in fact they pass through the entire thickness of the oceans and other bodies of water. In straits and other narrow places, low tides can reach very high speeds - up to 15 km / h. Basically, the phenomenon, like the ebb and flow, is influenced by the Moon, but to some extent the Sun is also involved in this. The Moon is much closer to the Earth than the Sun, so its influence on the planets is stronger even though the natural satellite is much smaller, and both celestial bodies revolve around the star.

The influence of the moon on the tides

If the continents and islands did not interfere with the influence of the Moon on water, and the entire surface of the Earth was covered by an ocean of equal depth, then the tides would look like this. The part of the ocean closest to the Moon, due to the force of gravity, would rise towards the natural satellite, due to the centrifugal force, the opposite part of the reservoir would also rise, it would be a tide. The drop in the water level would have occurred in a line that is perpendicular to the band of influence of the Moon, in that part there would have been a low tide.

The sun can also have some influence on the world's oceans. At the new moon and full moon, when the Moon and the Sun are in a straight line with the Earth, the attractive force of both luminaries adds up, thereby causing the strongest ebbs and flows. If these celestial bodies are perpendicular to each other with respect to the Earth, then the two forces of attraction will oppose each other, and the tides will be the weakest, but still in favor of the Moon.

The presence of various islands makes a great variety in the movement of waters at ebb and flow. In some reservoirs, the channel and natural obstacles in the form of land (islands) play an important role, so the water flows in and out unevenly. The waters change their position not only in accordance with the force of gravity of the moon, but also depending on the terrain. In this case, when the water level changes, it will flow along the path of least resistance, but in accordance with the influence of the night star.

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 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. 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 waters. 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 (see also 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.
Explanation of the origin of tidal forces. 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 approximately 89 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 fluid 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 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 the 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 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. 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 tides.
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. 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 11/2 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 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 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.
Methods of observation and forecast of tide heights. Tide levels are measured using various types of devices. A footstock is an ordinary rail with a scale in centimeters applied to it, attached vertically to a pier or to a support submerged in water so that the zero mark is below the lowest level of low tide. Level changes are read directly from this scale.
Float stem. These footstocks are used where constant swell or swell make it difficult to determine the level on a fixed scale. Inside a protective well (hollow chamber or pipe) vertically installed on the seabed, a float is placed, which is connected to a pointer fixed on a fixed scale, or a chart recorder pen. Water enters the well through a small hole located well below the minimum sea level. Its tidal changes are transmitted through the float to the measuring instruments.
Hydrostatic sea level recorder. At a certain depth, a block of rubber bags is placed. As the height of the tide (water layer) changes, the hydrostatic pressure changes, which is recorded by measuring instruments. Automatic recording devices (tide gauges) can also be used to obtain a continuous record of tidal fluctuations at any point.
Tide tables. When compiling tide tables, two main methods are used: harmonic and non-harmonic. The non-harmonic method is entirely based on the results of observations. In addition, the characteristics of port water areas and some basic astronomical data (the hourly angle of the Moon, the time of its passage through the celestial meridian, phases, declinations and parallax) are involved. After correcting for these factors, the calculation of the moment of occurrence and the level of the tide for any port is a purely mathematical procedure. The harmonic method is partly analytical and partly based on observations of tide heights over at least one lunar month. To confirm this type of forecast for each port, long series of observations are required, since distortions arise due to such physical phenomena as inertia and friction, as well as the complex configuration of the coasts of the water area and the features of the bottom topography. Since tidal processes are inherently periodic, harmonic analysis is applied to them. The observed tide is considered as the result of the addition of a series of simple components of the tidal waves, each of which is caused by one of the tide-forming forces or one of the factors. For a complete solution, 37 such simple components are used, although in some cases the additional components beyond the 20 main ones are negligible. Simultaneous substitution of 37 constants into the equation and its actual solution is carried out on a computer.
Tides on rivers and currents. The interaction of tides and river currents is clearly visible where large rivers flow into the ocean. The height of the tides in bays, estuaries, and estuaries can increase significantly as a result of an increase in runoff in marginal streams, especially during floods. At the same time, ocean tides penetrate far up rivers in the form of tidal currents. For example, on the Hudson River, a tidal wave comes at a distance of 210 km from the mouth. Tidal currents usually spread upriver to difficult waterfalls or rapids. During high tides, the currents in rivers are faster than during low tides. The maximum speeds of tidal currents reach 22 km/h.
Bor. When water, set in motion by a high tide, is limited in its movement by a narrow channel, a rather steep wave is formed, which moves upstream in a single front. This phenomenon is called a tidal wave, or bore. Such waves are observed on rivers much higher than the mouths, where the combination of friction and the flow of the river to the greatest extent hinders the spread of the tide. Boron formation is known in the Bay of Fundy, Canada. Near Moncton (Prov. New Brunswick), the Ptikodiak River flows into the Bay of Fundy, forming a marginal stream. In low water, its width is 150 m, and it crosses the drying strip. At high tide, a wall of water 750 m long and 60-90 cm high rushes up the river in a hissing and seething whirlwind. The largest known pine forest with a height of 4.5 m is formed on the Fuchunjiang River, which flows into the Hangzhou Bay. See also BOR. Reversing 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 these waves can be associated either with hurricane-force winds or with underwater earthquakes (in the latter case they are called seismic sea waves, or tsunamis).
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.
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

Collier Encyclopedia. - Open Society. 2000 .

See what "ELBOW AND FLOW" is in other dictionaries:

- (Flood tide and ebb tide, ebb and flood) periodic changes in the water level in the sea caused by the action on water particles of the forces of attraction of the Moon and the Sun and the centrifugal forces arising from the circulation of the Earth-Moon, Earth-Sun systems around their common ... ... Marine Dictionary

ebbs and flows- - Telecommunication topics, basic concepts EN tides and currents ... Technical Translator's Handbook

Ebb and flow

high tide and low tide- periodic vertical fluctuations in the level of the ocean or sea, which are the result of changes in the positions of the Moon and the Sun relative to the Earth, coupled with the effects of the Earth's rotation and the features of this relief, and manifested in a periodic horizontal displacement of water masses. Tides cause changes in sea level and periodic currents, known as tidal currents, making tide prediction important for coastal navigation.

The intensity of these phenomena depends on many factors, but the most important of them is the degree of connection of water bodies with the oceans. The more closed the reservoir, the less the degree of manifestation of tidal phenomena.

The yearly recurring tidal cycle remains unchanged due to the exact compensation of the forces of attraction between the Sun and the center of mass of the planetary pair and the forces of inertia applied to this center.

Since the position of the Moon and the Sun in relation to the Earth periodically changes, the intensity of the resulting tidal phenomena also changes.

Low tide at Saint Malo

Story

Ebb tides played a significant role in supplying the coastal population with seafood, allowing food suitable for food to be collected on the exposed seabed.

Terminology

Low water (Brittany, France)

The maximum level of the water surface at high tide is called full water, and the minimum at low tide - low water. In the ocean, where the bottom is even, and the land is far away, high water manifests itself as two “bloatings” of the water surface: one of them is located on the side of the moon, and the other is at the opposite end of the globe. There may also be two more smaller swellings on the side directed towards the Sun and opposite to it. An explanation of this effect can be found below, in the section tide physics.

Since the Moon and the Sun move relative to the Earth, water humps move with them, forming tidal waves and tidal currents. In the open sea, tidal currents are rotational in nature, and near the coast and in narrow bays and straits, they are reciprocating.

If the whole Earth were covered with water, we would observe two regular high and low tides daily. But since the unimpeded propagation of tidal waves is prevented by land areas: islands and continents, and also due to the action of the Coriolis force on moving water, instead of two tidal waves, there are many small waves that slowly (in most cases with a period of 12 hours 25.2 minutes ) run around a point called amphidromic, where the tide amplitude is zero. The dominant component of the tide (the lunar tide M2) forms about a dozen amphidromic points on the surface of the World Ocean with wave motion clockwise and about the same counterclockwise (see map). All this makes it impossible to predict the time of the tide only on the basis of the positions of the Moon and the Sun relative to the Earth. Instead, they use the "yearbook of tides" - a reference tool for calculating the time of the onset of tides and their height at various points on the globe. Tide tables are also used, with data on the moments and heights of low and high waters, calculated a year ahead for major tidal ports.

Tide component M2

If we connect points on the map with the same tide phases, we get the so-called cotidal lines radiating from the amphidromic point. Typically, cotidal lines characterize the position of the crest of the tidal wave for each hour. In fact, the cotidal lines reflect the speed of propagation of the tidal wave in 1 hour. Maps that show lines of equal amplitudes and phases of tidal waves are called cotidal cards.

high tide- the difference between the highest water level at high tide (high tide) and its lowest level at low tide (low tide). The height of the tide is a variable value, however, its average indicator is given when characterizing each section of the coast.

Depending on the relative position of the Moon and the Sun, small and large tidal waves can reinforce each other. For such tides, special names have historically developed:

• Quadrature tide- the smallest tide, when the tide-forming forces of the Moon and the Sun act at right angles to each other (this position of the luminaries is called quadrature).
• spring tide- the greatest tide, when the tide-forming forces of the Moon and the Sun act along the same direction (this position of the luminaries is called syzygy).

The smaller or larger the tide, the smaller or, respectively, the greater the ebb.

The highest tides in the world

It can be observed in the Bay of Fundy (15.6-18 m), which is located on the east coast of Canada between New Brunswick and Nova Scotia.

On the European continent, the highest tides (up to 13.5 m) are observed in Brittany near the city of Saint Malo. Here the tidal wave is focused by the coastline of the Cornwall (England) and Cotentin (France) peninsulas.

Tide physics

Modern wording

In relation to the planet Earth, the cause of tides is the presence of the planet in the gravitational field created by the Sun and the Moon. Since the effects they create are independent, the impact of these celestial bodies on the Earth can be considered separately. In this case, for each pair of bodies, we can assume that each of them revolves around a common center of gravity. For the Earth-Sun pair, this center is located in the depths of the Sun at a distance of 451 km from its center. For the Earth-Moon pair, it is located deep in the Earth at a distance of 2/3 of its radius.

Each of these bodies experiences the action of tidal forces, the source of which is the gravitational force and internal forces that ensure the integrity of the celestial body, in the role of which is the force of its own attraction, hereinafter referred to as self-gravity. The emergence of tidal forces is most clearly seen in the example of the Earth-Sun system.

The tidal force is the result of the competing interaction of the gravitational force directed towards the center of gravity and decreasing inversely with the square of the distance from it, and the fictitious centrifugal force of inertia due to the rotation of a celestial body around this center. These forces, being opposite in direction, coincide in magnitude only at the center of mass of each of the celestial bodies. Due to the action of internal forces, the Earth revolves around the center of the Sun as a whole with a constant angular velocity for each element of its mass. Therefore, as this element of mass moves away from the center of gravity, the centrifugal force acting on it grows in proportion to the square of the distance. A more detailed distribution of tidal forces in their projection onto a plane perpendicular to the plane of the ecliptic is shown in Fig.1.

Fig.1 Scheme of the distribution of tidal forces in the projection onto a plane perpendicular to the Ecliptic. A gravitating body is either on the right or on the left.

According to the Newtonian paradigm, the reproduction of changes in the shape of the bodies subjected to their action, achieved as a result of the action of tidal forces, can be achieved only if these forces are fully compensated by other forces, which may include the force of universal gravitation.

Fig.2 Deformation of the Earth's water shell as a result of the balance of tidal force, self-gravity force and the force of water reaction to the compressive force

As a result of the addition of these forces, tidal forces arise symmetrically on both sides of the globe, directed in different directions from it. The tidal force directed towards the Sun is of a gravitational nature, while that directed away from the Sun is a consequence of a fictitious inertial force.

These forces are extremely weak and cannot be compared with the forces of self-gravity (the acceleration they create is 10 million times less than the acceleration of free fall). However, they cause a shift in the particles of water in the oceans (resistance to shear in water at low speeds is practically zero, while compression is extremely high), until the tangent to the surface of the water becomes perpendicular to the resulting force.

As a result, a wave arises on the surface of the oceans, occupying a constant position in systems of mutually gravitating bodies, but running along the surface of the ocean together with the daily movement of its bottom and coasts. Thus (neglecting ocean currents) each particle of water makes an oscillatory movement up and down twice during the day.

The horizontal movement of water is observed only near the coast as a result of the rise in its level. The speed of movement is greater, the more gently the seabed is located.

Tidal potential

(the concept of acad. Shuleikin)

Neglecting the size, structure and shape of the Moon, we write down the specific force of attraction of a test body located on the Earth. Let be the radius vector directed from the test body towards the Moon, be the length of this vector. In this case, the force of attraction of this body by the Moon will be equal to

where is the selenometric gravitational constant. We place the test body at the point . The force of attraction of a test body placed at the center of mass of the Earth will be equal to

Here, and are understood as the radius vector connecting the centers of mass of the Earth and the Moon, and their absolute values. We will call the tidal force the difference between these two gravitational forces

In formulas (1) and (2), the Moon is considered to be a ball with a spherically symmetric mass distribution. The force function of the attraction of the test body by the Moon is no different from the force function of the attraction of the ball and is equal to The second force is applied to the center of mass of the Earth and is a strictly constant value. To obtain the force function for this force, we introduce a time coordinate system. We draw the axis from the center of the Earth and direct it towards the Moon. We leave the directions of the other two axes arbitrary. Then the force function of the force will be equal to . Tidal potential will be equal to the difference of these two force functions. Let's designate it , we will receive Constant we will define from a condition of normalization according to which the tidal potential in the center of the Earth is equal to zero. At the center of the Earth , It follows that . Therefore, we obtain the final formula for the tidal potential in the form (4)

Insofar as

For small values ​​of , , the last expression can be represented in the following form

Substituting (5) into (4), we obtain

Deformation of the surface of the planet under the influence of ebbs and flows

The perturbing effect of the tidal potential deforms the level surface of the planet. Let us evaluate this effect, assuming that the Earth is a sphere with a spherically symmetric mass distribution. The unperturbed gravitational potential of the Earth on the surface will be equal to . For a dot. , located at a distance from the center of the sphere, the gravitational potential of the Earth is . Reducing by the gravitational constant, we get . Here the variables are and . Let us denote the ratio of the masses of the gravitating body to the mass of the planet by a Greek letter and solve the resulting expression for :

Since with the same degree of accuracy we get

Given the smallness of the ratio, the last expressions can be written as

Thus, we have obtained the equation of a biaxial ellipsoid, in which the axis of rotation coincides with the axis, i.e. with a straight line connecting the gravitating body with the center of the Earth. The semiaxes of this ellipsoid are obviously equal

At the end we give a small numerical illustration of this effect. Let's calculate the tidal "hump" on the Earth, caused by the attraction of the Moon. The radius of the Earth is km, the distance between the centers of the Earth and the Moon, taking into account the instability of the lunar orbit, is km, the ratio of the mass of the Earth to the mass of the Moon is 81:1. Obviously, when substituting into the formula, we get a value approximately equal to 36 cm.

see also

Notes

Literature

• Frish S. A. and Timoreva A. V. Course of General Physics, Textbook for the Physics and Mathematics and Physics and Technology Departments of State Universities, Volume I. M .: GITTL, 1957
• Shchuleykin V.V. Physics of the sea. M.: Publishing House "Nauka", Department of Earth Sciences of the Academy of Sciences of the USSR 1967
• Voight S.S. What are tides. Editorial Board of Popular Science Literature of the Academy of Sciences of the USSR

Links

• WXTide32 is a free tide charting program.

Moon
Peculiarities

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.