What is called ebb and flow. The Mystery of Ocean Tides

Student of group N-30

Tsvetkov E.N.

Checked:

Petrova I.F.

Moscow, 2003

Main part…………………………………………………….
Definition..……………......……………………………...
The essence of the phenomenon………………………………………………………...
Change over time………………………………………………………
Distribution and scale of manifestation………………...
Myths and legends …………………………………………….
History of the study………………………………………………………
Environmental consequences………………………………...
Impact on economic activity…………………
Human influence on this process…………………….
Possibility of forecasting and management…………….
Bibliography………………………………………………..

Definition.

Ebbs and flows, periodic fluctuations in water levels (rises and falls) in water areas on Earth, which are caused by the gravitational attraction of the Moon and 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 in lakes they are small.

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

Vertical fluctuations in water level during high and low tides are associated with horizontal movements of water masses in relation to the shore. 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 water levels 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 determined by the general circulation of the atmosphere and cover large areas of the open ocean.

During transition intervals from high tide to low tide and vice versa, it is difficult to establish the trend of the tidal current. At this time (which does not always coincide with the high or low tide), the water is said to “stagnate.”

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

October 15th, 2012

British photographer Michael Marten created a series of original photographs capturing 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 to be quite unusual, and positive reviews of the project literally forced the author to start publishing the 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 photographs. The time between high and low water averages just over six hours. Therefore, Michael has to linger in each place for longer than just the time of a few shutter clicks. The author had been nurturing the idea of creating a series of such works for a long time. He was looking for how to realize changes in nature on film, without human influence. And I found it by chance, in one of the coastal Scottish villages, where I spent the whole day and caught the time of high and low tide.

Periodic fluctuations in water levels (rises and falls) in water areas on Earth are called tides.

Vertical fluctuations in water level during high and low tides are associated with horizontal movements of water masses in relation to the shore. 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 water levels 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, ocean currents, moving continuously and unidirectionally, are caused by the general circulation of the atmosphere and cover large areas of the open ocean.

Although the Sun plays a significant role in tidal processes, the decisive factor in their development is the gravitational pull 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 attract each other with a force directly proportional to the product of the masses of both particles and inversely proportional to the square of the distance between them. It is understood that the greater the mass of the bodies, the greater the force of mutual attraction that arises 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 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 located on the Earth in the direction of the Moon.

The point on the earth's surface located directly below the Moon is only 6,400 km 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 Earth’s gravity acting on any object is approximately 300 thousand times greater than the Moon’s gravity.

It is a common idea that water on Earth directly below 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 gravity is so small compared to the Earth's, it would not be enough to lift so much water. huge weight.
However, the oceans, seas and large lakes on Earth, being large liquid bodies, are free to move under the influence of lateral displacement forces, and any slight tendency to move 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, water flows from adjacent areas of the earth's surface towards a place located under the Moon. The resulting accumulation of water at a point under the Moon forms a tide there. The tidal wave itself 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 areas towards a point under the Moon, corresponding ebbs of water occur at two other points removed from it at a distance equal to a quarter of the Earth’s circumference. It is interesting to note that the decrease in sea 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 the 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 the sublunar point experiences a stronger pull towards the Moon than the Earth below it, but the Earth in turn has a stronger pull towards 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 - 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 hours 12 minutes The period of 24 hours 50 minutes between two successive tides is called a tidal (or lunar) day.

Tide inequalities. Tidal processes are very complex and many factors must be taken into account to understand them. In any case, the main features will be determined:
1) the stage of development of the tide relative to the passage of the Moon;
2) tidal amplitude 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 magnitude 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-diurnal effect. Usually within a 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 subsequent revolution, a given 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 the ebb and flow of the tides are delayed by approximately 50 minutes every day. This value is called lunar delay.

Half-month inequality. This main type of variation is characterized by a periodicity of approximately 143/4 days, which is associated with the rotation of the Moon around the Earth and its passage through successive phases, in particular syzygies (new moons and full moons), i.e. moments when the Sun, Earth and Moon are located on the same straight line.

So far we have touched only on the tidal influence of the Moon. The gravitational field of the Sun also affects the tides, however, although the mass of the Sun is much greater than the mass of the Moon, the distance from the Earth to the Sun is so greater than the distance to the Moon that the tidal force of the Sun is less than half that of the Moon.

However, when the Sun and Moon are on the same straight line, either on the same side of the Earth or on opposite sides (during the new moon or full moon), their gravitational forces add up, acting along the same axis, and the solar tide overlaps with the lunar tide.

Likewise, the attraction of the Sun increases the ebb caused by the influence of the Moon. As a result, the tides become higher and the tides lower than if they were caused by the Moon's gravity alone. Such tides are called spring tides.

When the gravitational force vectors of the Sun and the Moon are mutually perpendicular (during quadratures, i.e. when the Moon is in the first or last quarter), their tidal forces oppose, 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 ebbs and flows are called quadrature.

The range of high and low water marks in this case is reduced by approximately three times compared to the spring tide.

Lunar parallactic inequality. The period of fluctuations in tidal heights, 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 latter’s rotation. Due to the elliptical shape of the lunar orbit, the tidal force of the Moon at perigee is 40% higher 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 a 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 the two low tides) differ slightly, and the heights of 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 above the Northern or Southern tropics.

The diurnal inequality does not significantly affect the heights of two successive 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 fluctuations. However, in the Pacific Ocean, diurnal variability is three times greater in low tide levels than in high tide levels.

Semiannual inequality. Its cause is the rotation 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 for approximately one 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 new moon or full moon phase, the so-called. equinoctial spring tides.

Solar parallax inequality. The period of manifestation of this inequality is one year. Its cause is the change in the distance from the Earth to the Sun during the orbital movement of the Earth. Once for each revolution around the Earth, the Moon is at its 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 tidal levels and lower tidal levels can be expected. Likewise, if the passage of aphelion coincides with apogee, lower tides and shallower tides occur.

Greatest tidal amplitudes. The world's highest tide is generated by strong currents in Minas Bay in the Bay of Fundy. Tidal fluctuations here are characterized by a normal course with a semi-diurnal 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 effect of spring tide, the position of the Moon at perigee and the maximum declination of the Moon occur on the same day, the tide level can reach 15 m. This exceptionally large amplitude of tidal fluctuations is partly due to the funnel-shaped shape of the Bay of Fundy, where the depths decrease and the shores move closer together towards top of the bay. The causes of tides, which have been the subject of constant study for many centuries, are among those problems that have given rise to many controversial theories even in relatively recent times

Charles Darwin wrote in 1911: “There is no need to look for ancient literature for the sake of grotesque theories of tides.” However, sailors manage to measure their height and take advantage of the tides without having any idea of the actual causes of their occurrence.

I think that we don’t have to worry too much about the causes of the tides. Based on long-term observations, special tables are calculated for any point in the earth’s waters, which indicate the times of high and low water for each day. I’m planning my trip, for example, to Egypt, which is famous for its shallow lagoons, but try to plan in advance so that the full water occurs in the first half of the day, which will allow you to fully ride most of the daylight hours.
Another question related to tides that is interesting for kiters is the relationship between wind and water level fluctuations.

A folk superstition states that at high tide the wind intensifies, but at low tide it turns sour.
The influence of wind on tidal phenomena is more understandable. The wind from the sea pushes the water towards the coast, the height of the tide increases above normal, and at low tide the water level also exceeds the average. On the contrary, when the wind blows from land, water is driven away from the coast, and sea level drops.

The second mechanism operates by increasing 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 mmHg. Art., the water level drops by approximately 33 cm. A high pressure zone or anticyclone is usually called good weather, but not for kiters. There is calm in the center of the anticyclone. A decrease in atmospheric pressure causes a corresponding increase in water levels. Consequently, a sharp drop in atmospheric pressure combined with hurricane-force winds can cause a noticeable rise in water levels. Such waves, although called tidal, 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 influence 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

The water surface level in the seas and oceans of our planet changes periodically and fluctuates in certain intervals. These periodic oscillations are sea tides.

Picture of sea tides

To visualize picture of sea ebbs and flows, imagine that you are standing on the sloping shore of the ocean, in some bay, 200–300 meters from the water. There are many different objects on the sand - an old anchor, a little closer a large pile of white stone. Now, not far away, lies the iron hull of a small boat, fallen on its side. The bottom of its hull in the bow is badly damaged. Obviously, once this ship, being not far from the shore, hit an anchor. This accident occurred, in all likelihood, during low tide, and, apparently, the ship had been lying in this place for many years, since almost its entire hull had become covered with brown rust. You are inclined to consider the careless captain to be the culprit of the ship's accident. Apparently, the anchor was the sharp weapon that the ship that had fallen on its side struck. You are looking for this anchor and cannot find it. Where could he have gone? Then you notice that the water is already approaching a pile of white stones, and then you realize that the anchor you saw has long been flooded by a tidal wave. The water “steps” onto the shore, it continues to rise further and further upward. Now the pile of white stones turned out to be almost all hidden under water.

Phenomena of sea tides

Phenomena of sea tides people have long been associated with the movement of the Moon, but this connection remained a mystery until the brilliant mathematician Isaac Newton did not explain on the basis of the law of gravity he discovered. The cause of these phenomena is the effect of the Moon’s gravity on the Earth’s water shell. Still famous Galileo Galilei connected the ebb and flow of the tides with the rotation of the Earth and saw in this one of the most substantiated and true proofs of the validity of the teachings of Nicolaus Copernicus (more details:). The Paris Academy of Sciences in 1738 announced a prize to the one who would give the most substantiated presentation of the theory of tides. The award was then received Euler, Maclaurin, D. Bernoulli and Cavalieri. The first three took Newton's law of gravitation as the basis for their work, and the Jesuit Cavalieri explained tides based on Descartes' vortex hypothesis. However, the most outstanding works in this area belong to Newton and Laplace, and all subsequent research is based on the findings of these great scientists.

How to explain the phenomenon of ebb and flow

How most clearly explain the phenomenon of ebb and flow. If, for simplicity, we assume that the earth’s surface is completely covered with water, and we look at the globe from one of its poles, then the picture of sea ebbs and flows can be presented as follows.

Lunar attraction

That part of the surface of our planet that faces the Moon is closest to it; as a result, it is exposed to greater force lunar gravity, than, for example, the central part of our planet and, therefore, is pulled towards the Moon more than the rest of the Earth. Because of this, a tidal hump is formed on the side facing the Moon. At the same time, on the opposite side of the Earth, which is least subject to the gravity of the Moon, the same tidal hump appears. The Earth therefore takes the form of a figure somewhat elongated along a straight line connecting the centers of our planet and the Moon. Thus, on two opposite sides of the Earth, located on the same straight line, which passes through the centers of the Earth and the Moon, two large humps are formed, two huge water swellings. At the same time, on the other two sides of our planet, located at an angle of ninety degrees from the above points of maximum tide, the greatest low tides occur. Here the water drops more than anywhere else on the surface of the globe. The line connecting these points at low tide shortens somewhat, and thus creates the impression of an increase in the elongation of the Earth in the direction of the maximum high tide points. Due to lunar gravity, these points of maximum tide constantly maintain their position relative to the Moon, but since the Earth rotates around its axis, during the day they seem to move across the entire surface of the globe. That's why in each area there are two high and two low tides during the day.

Solar ebbs and flows

The Sun, like the Moon, produces ebbs and flows by the force of its gravity. But it is located at a much greater distance from our planet compared to the Moon, and the solar tides that occur on Earth are almost two and a half times less than the lunar ones. That's why solar tides, are not observed separately, but only their influence on the magnitude of lunar tides is considered. For example, The highest sea tides occur during full and new moons, since at this time the Earth, Moon and Sun are on the same straight line, and our daylight increases the attraction of the Moon with its attraction. On the contrary, when we observe the Moon in the first or last quarter (phase), there are lowest sea tides. This is explained by the fact that in this case the lunar tide coincides with solar ebb. The effect of lunar gravity is reduced by the amount of gravity of the Sun.

Tidal friction

« Tidal friction", existing on our planet, in turn affects the lunar orbit, since the tidal wave caused by lunar gravity has a reverse effect on the Moon, creating a tendency to accelerate its movement. As a result, the Moon gradually moves away from the Earth, its period of revolution increases, and it, in all likelihood, lags a little behind in its movement.

The magnitude of sea tides

In addition to the relative position in space of the Sun, Earth and Moon, on the magnitude of the sea tides In each individual area, the shape of the seabed and the nature of the shoreline influence. It is also known that in closed seas, such as the Aral, Caspian, Azov and Black seas, ebbs and flows are almost never observed. It is difficult to detect them in the open oceans; here the tides barely reach one meter, the water level rises very little. But in some bays there are tides of such colossal magnitude that the water rises to a height of more than ten meters and in some places floods colossal spaces.

Ebbs and flows in the air and solid shells of the Earth

Ebbs and flows also happen in the air and solid shells of the Earth. We hardly notice these phenomena in the lower layers of the atmosphere. For comparison, we point out that ebbs and flows are not observed at the bottom of the oceans. This circumstance is explained by the fact that mainly the upper layers of the water shell are involved in tidal processes. Ebbs and flows in the air envelope can only be detected by very long-term observation of changes in atmospheric pressure. As for the earth’s crust, each part of it, due to the tidal action of the Moon, rises twice during the day and falls twice by about several decimeters. In other words, fluctuations in the solid shell of our planet are approximately three times smaller in magnitude than fluctuations in the surface level of the oceans. Thus, our planet seems to be breathing all the time, taking deep breaths and exhalations, and its outer shell, like the chest of a great miracle hero, either rises or falls a little. These processes occurring in the solid shell of the Earth can only be detected with the help of instruments used to record earthquakes. It should be noted that ebbs and flows occur on other world bodies and have a huge impact on their development. If the Moon were motionless in relation to the Earth, then in the absence of other factors influencing the delay of the tidal wave, two high tides and two low tides would occur every 6 hours in any place on the globe every 6 hours. But since the Moon continuously revolves around the Earth and, moreover, in the same direction in which our planet rotates around its axis, there is some delay: the Earth manages to turn towards the Moon with each part not within 24 hours, but in approximately 24 hours and 50 minutes. Therefore, in each area, the ebb or flow of the tide does not last exactly 6 hours, but about 6 hours and 12.5 minutes.

Alternating tides

In addition, it should be noted that the correctness alternating tides is violated depending on the nature of the location of the continents on our planet and the continuous friction of water on the surface of the Earth. These irregularities in alternation sometimes reach several hours. Thus, the “highest” water occurs not at the moment of the culmination of the Moon, as it should be according to theory, but several hours later than the passage of the Moon through the meridian; this delay is called the port applied clock and sometimes reaches 12 hours. Previously, it was widely believed that the ebb and flow of sea tides were related to sea currents. Now everyone knows that these are phenomena of a different order. A tide is a type of wave movement, similar to that caused by wind. When a tidal wave approaches, a floating object oscillates, as with a wave arising from the wind - forward and backward, down and up, but is not carried away by it, like a current. The period of a tidal wave is about 12 hours and 25 minutes, and after this period of time the object usually returns to its original position. The force that causes tides is many times less than the force of gravity. While the force of gravity is inversely proportional to the square of the distance between the attracting bodies, the force causing tides is approximately is inversely proportional to the cube of this distance, and not at all its square.

The surface level of oceans and seas changes periodically, approximately twice a day. These fluctuations are called ebb and flow. During high tide, the ocean level gradually rises and reaches its highest position. At low tide the level gradually drops to its lowest level. At high tide, water flows towards the shores, at low tide - away from the shores.

The ebb and flow of the tides are standing. They are formed due to the influence of cosmic bodies such as the Sun. According to the laws of interaction of cosmic bodies, our planet and the Moon mutually attract each other. The lunar gravity is so strong that the surface of the ocean seems to bend towards it. The Moon moves around the Earth, and a tidal wave “runs” behind it across the ocean. When a wave reaches the shore, that’s the tide. A little time will pass, the water will follow the Moon and move away from the shore - that’s the low tide. According to the same universal cosmic laws, ebbs and flows are also formed from the attraction of the Sun. However, the tidal force of the Sun, due to its distance, is significantly less than the lunar one, and if there were no Moon, the tides on Earth would be 2.17 times less. The explanation of tidal forces was first given by Newton.

Tides differ from each other in duration and magnitude. Most often, there are two high tides and two low tides during the day. On the arcs and coasts of Eastern and Central America there is one high tide and one low tide per day.

The magnitude of the tides is even more varied than their period. Theoretically, one lunar tide is equal to 0.53 m, solar - 0.24 m. Thus, the largest tide should have a height of 0.77 m. In the open ocean and near the islands, the tide value is quite close to theoretical: on the Hawaiian Islands - 1 m , on St. Helena Island - 1.1 m; on the islands - 1.7 m. On the continents, the magnitude of the tides ranges from 1.5 to 2 m. In the inland seas, the tides are very insignificant: - 13 cm, - 4.8 cm. It is considered tidalless, but near Venice the tides are up to 1 m. The largest tides are the following, recorded in:

In the Bay of Fundy (), the tide reached a height of 16-17 m. This is the highest tide in the entire globe.

In the north, in Penzhinskaya Bay, the tide height reached 12-14 m. This is the highest tide off the coast of Russia. However, the above tide figures are the exception rather than the rule. At the vast majority of tidal level measurement points, they are small and rarely exceed 2 m.

The importance of tides is very great for maritime navigation and the construction of ports. Each tidal wave carries a huge amount of energy.

Ebb and flow

Tide And low tide- periodic vertical fluctuations in ocean or sea level, resulting from 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 a given relief and manifested in periodic horizontal displacement of water masses. Tides cause changes in sea level height, as well as 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 world ocean. The more closed the body of water, the less the degree of manifestation of tidal phenomena.

The annually repeated tidal cycle remains unchanged due to the precise 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.

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

Low tide at Saint-Malo

Story

Low tides played a significant role in the supply of seafood to coastal populations, allowing edible food to be collected from the exposed seabed.

Terminology

Low Water (Brittany, France)

The maximum surface level of the water at high tide is called full of water, and the minimum during low tide is low water. In the ocean, where the bottom is flat and the land is far away, full water manifests itself as two “swells” 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 Sun move relative to the Earth, water humps also move with them, forming tidal waves And tidal currents. In the open sea, tidal currents have a rotational character, and near the coast and in narrow bays and straits they are reciprocating.

If the entire Earth were covered with water, we would experience two regular high and low tides every day. But since the unimpeded propagation of tidal waves is hampered 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, in which the tidal amplitude is zero. The dominant component of the tide (lunar tide M2) forms about a dozen amphidromic points on the surface of the World Ocean with the wave moving clockwise and about the same number counterclockwise (see map). All this makes it impossible to predict the time of tide only based on the positions of the Moon and Sun relative to the Earth. Instead, they use a "tide yearbook" - a reference guide for calculating the time of the onset of tides and their heights in various points of the globe. Tide tables are also used, with data on the moments and heights of low and high waters, calculated a year in advance for main tidal ports.

Tide component M2

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

Tide height- the difference between the highest water level at high tide (high water) and its lowest level at low tide (low water). The height of the tide is not a constant value, but its average 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. Special names have historically been developed for such tides:

Quadrature tide- the lowest tide, when the tidal 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 highest tide, when the tidal forces of the Moon and the Sun act along the same direction (this position of the luminaries is called syzygy).

The lower or higher the tide, the lower or higher the ebb.

Highest tides in the world

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 peninsulas of Cornwall (England) and Cotentin (France).

Physics of the tide

Modern wording

In relation to planet Earth, the cause of tides is the presence of the planet in the gravitational field created by the Sun and Moon. Since the effects they create are independent, the impact of these celestial bodies on 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 deep in 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 tidal forces, the source of which is the force of gravity and internal forces that ensure the integrity of the celestial body, in the role of which is the force of its own attraction, hereinafter called self-gravity. The emergence of tidal forces can be most clearly seen in 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 in inverse proportion to the square of the distance from it, and the fictitious centrifugal force of inertia caused by the rotation of the 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. Thanks to the action of internal forces, the Earth rotates around the center of the Sun as a whole with a constant angular velocity for each element of its constituent mass. Therefore, as this element of mass moves away from the center of gravity, the centrifugal force acting on it increases in proportion to the square of the distance. A more detailed distribution of tidal forces in their projection onto a plane perpendicular to the ecliptic plane is shown in Fig. 1.

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

The reproduction of changes in the shape of bodies exposed to them, achieved as a result of the action of tidal forces, can, in accordance with the Newtonian paradigm, be achieved only if these forces are completely compensated by other forces, which may include the force of universal gravity.

Fig. 2 Deformation of the Earth’s water shell as a consequence of the balance of tidal force, self-gravitational force and the force of reaction of water to compression 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 gravitational nature, while the force directed away from the Sun is a consequence of the fictitious force of inertia.

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 gravity). However, they cause a shift in the water particles of the World Ocean (the resistance to shear in water at low speeds is practically zero, while to compression it is extremely high), until the tangent to the surface of the water becomes perpendicular to the resulting force.

As a result, a wave appears on the surface of the world's 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 shores. Thus (ignoring ocean currents), each particle of water undergoes an oscillatory movement up and down twice during the day.

Horizontal movement of water is observed only near the coast as a consequence of a rise in its level. The more shallow the seabed is, the greater the speed of movement.

Tidal potential

(concept of acad. Shuleikina)

Neglecting the size, structure and shape of the Moon, we write down the specific gravitational force of the test body located on Earth. Let be the radius vector directed from the test body towards the Moon, and let 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. Let's place the test body at point . The force of attraction of a test body placed at the center of mass of the Earth will be equal to

Here, and refers to 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 a ball with a spherically symmetrical mass distribution. The force function of attraction of a test body by the Moon is no different from the force function of attraction of a 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. Let's draw the axis from the center of the Earth and direct it towards the Moon. The directions of the other two axes will be left 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. We denote it , we obtain The constant is determined from the normalization condition, 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 . Consequently, we obtain the final formula for the tidal potential in the form (4)

Because the

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

Substituting (5) into (4), we get

Deformation of the planet's surface under the influence of tides

The disturbing influence of the tidal potential deforms the leveled surface of the planet. Let us evaluate this impact, assuming that the Earth is a ball with a spherically symmetrical mass distribution. The unperturbed gravitational potential of the Earth on the surface will be equal to . For point . , located at a distance from the center of the sphere, the gravitational potential of the Earth is equal to . 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 obtain

Considering the smallness of the ratio, the last expressions can be written as follows

We have thus obtained the equation of a biaxial ellipsoid, whose axis of rotation coincides with the axis, that is, with the straight line connecting the gravitating body with the center of the Earth. The semi-axes 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 Earth caused by the attraction of the Moon. The radius of the Earth is equal to 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 Earth's mass to the Moon's mass is 81:1. Obviously, when substituting into the formula, we get a value approximately equal to 36 cm.

Notes

Literature

Frisch S. A. and Timoreva A. V. Course of general physics, Textbook for physics-mathematics and physics-technical faculties of state universities, Volume I. M.: GITTL, 1957
Shchuleykin V.V. Physics of the sea. M.: Publishing house "Science", Department of Earth Sciences of the USSR Academy of Sciences 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 freeware tide table program

Portal for schoolchildren. Self-preparation

What is called ebb and flow. The Mystery of Ocean Tides

Definition.

Picture of sea tides

Phenomena of sea tides