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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 appears 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 formulation
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. In 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, i.e. 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.
see also
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
Let's continue the conversation about the forces acting on celestial bodies and the effects caused by this. Today I will talk about tides and non-gravitational disturbances.
What does this mean – “non-gravitational disturbances”? Perturbations are usually called small corrections to a large, main force. That is, we will talk about some forces, the influence of which on an object is much less than gravitational ones
What other forces exist in nature besides gravity? Let us leave aside strong and weak nuclear interactions; they are local in nature (act at extremely short distances). But electromagnetism, as we know, is much stronger than gravity and extends just as far - infinitely. But since electric charges of opposite signs are usually balanced, and the gravitational “charge” (the role of which is played by mass) is always of the same sign, then with sufficiently large masses, of course, gravity comes to the fore. So in reality we will be talking about disturbances in the movement of celestial bodies under the influence of an electromagnetic field. There are no more options, although there is still dark energy, but we will talk about it later, when we talk about cosmology.
As I explained on , Newton's simple law of gravity F = G∙M∙m/R² is very convenient to use in astronomy, because most bodies have a close to spherical shape and are sufficiently distant from each other, so that when calculating they can be replaced by points - point objects containing their entire mass. But a body of finite size, comparable to the distance between neighboring bodies, nevertheless experiences different force influences in its different parts, because these parts are located differently from the sources of gravity, and this must be taken into account.
Attraction crushes and tears apart
To feel the tidal effect, let's do a thought experiment popular among physicists: imagine ourselves in a freely falling elevator. We cut off the rope holding the cabin and begin to fall. Before we fall, we can watch what is happening around us. We hang free masses and observe how they behave. At first they fall synchronously, and we say this is weightlessness, because all the objects in this cabin and it itself feel approximately the same acceleration of free fall.
But over time, our material points will begin to change their configuration. Why? Because the lower one at the beginning was a little closer to the center of attraction than the upper one, so the lower one, being attracted stronger, begins to outstrip the upper one. And the side points always remain at the same distance from the center of gravity, but as they approach it they begin to approach each other, because accelerations of equal magnitude are not parallel. As a result, the system of unconnected objects is deformed. This is called the tidal effect.
From the point of view of an observer who has scattered grains around him and watches how individual grains move while the entire system falls onto a massive object, one can introduce such a concept as a field of tidal forces. Let us define these forces at each point as the vector difference between the gravitational acceleration at this point and the acceleration of the observer or the center of mass, and if we take only the first term of the expansion in the Taylor series for relative distance, we will get a symmetrical picture: the nearest grains will be ahead of the observer, the distant ones will lag behind him, i.e. the system will stretch along the axis directed towards the gravitating object, and along directions perpendicular to it the particles will be pressed towards the observer.
What do you think will happen when a planet is pulled into a black hole? Those who have not listened to lectures on astronomy usually think that a black hole will tear off matter only from the surface facing itself. They do not know that an almost equally strong effect occurs on the other side of a freely falling body. Those. it is torn in two diametrically opposite directions, not in one at all.
The Dangers of Outer Space
To show how important it is to take into account the tidal effect, let's take the International Space Station. It, like all Earth satellites, falls freely in a gravitational field (if the engines are not turned on). And the field of tidal forces around it is a quite tangible thing, so the astronaut, when working on the outside of the station, must tie himself to it, and, as a rule, with two cables - just in case, you never know what might happen. And if he finds himself untethered in those conditions where tidal forces pull him away from the center of the station, he can easily lose contact with it. This often happens with tools, because you can’t link them all. If something falls out of an astronaut’s hands, then this object goes into the distance and becomes an independent satellite of the Earth.
The work plan for the ISS includes tests in outer space of a personal jetpack. And when his engine fails, tidal forces carry the astronaut away, and we lose him. The names of the missing are classified.
This is, of course, a joke: fortunately, such an incident has not happened yet. But this could very well happen! And maybe someday it will happen.
Planet-ocean
Let's return to Earth. This is the most interesting object for us, and the tidal forces acting on it are felt quite noticeably. From which celestial bodies do they act? The main one is the Moon, because it is close. The next largest impact is the Sun, because it is massive. The other planets also have some influence on the Earth, but it is barely noticeable.
To analyze external gravitational influences on the Earth, it is usually represented as a solid ball covered with a liquid shell. This is a good model, since our planet actually has a mobile shell in the form of ocean and atmosphere, and everything else is quite solid. Although the Earth's crust and inner layers have limited rigidity and are slightly susceptible to tidal influence, their elastic deformation can be neglected when calculating the effect on the ocean.
If we draw tidal force vectors in the Earth’s center of mass system, we get the following picture: the field of tidal forces pulls the ocean along the Earth-Moon axis, and in a plane perpendicular to it presses it to the center of the Earth. Thus, the planet (at least its moving shell) tends to take the shape of an ellipsoid. In this case, two bulges appear (they are called tidal humps) on opposite sides of the globe: one faces the Moon, the other faces away from the Moon, and in the strip between them, a corresponding “bulge” appears (more precisely, the surface of the ocean there has less curvature).
A more interesting thing happens in the gap - where the tidal force vector tries to move the liquid shell along the earth's surface. And this is natural: if you want to raise the sea in one place, and lower it in another place, then you need to move the water from there to here. And between them, tidal forces drive water to the “sublunar point” and to the “anti-lunar point.”
Quantifying the tidal effect is very simple. The Earth's gravity tries to make the ocean spherical, and the tidal part of the lunar and solar influence tries to stretch it along its axis. If we left the Earth alone and allowed it to fall freely onto the Moon, the height of the bulge would reach about half a meter, i.e. The ocean rises only 50 cm above its average level. If you are sailing on a ship on the open sea or ocean, half a meter is not noticeable. This is called static tide.
In almost every exam I come across a student who confidently claims that the tide occurs only on one side of the Earth - the one facing the Moon. As a rule, this is what a girl says. But it happens, although less often, that young men are mistaken in this matter. At the same time, in general, girls have a deeper knowledge of astronomy. It would be interesting to find out the reason for this “tidal-gender” asymmetry.But in order to create a half-meter bulge at the sublunar point, you need to distill a large amount of water here. But the surface of the Earth does not remain motionless, it rotates quickly in relation to the direction of the Moon and the Sun, making a full revolution in a day (and the Moon moves slowly in orbit - one revolution around the Earth in almost a month). Therefore, the tidal hump constantly runs along the surface of the ocean, so that the solid surface of the Earth is under the tidal hump 2 times per day and 2 times under the tidal drop in ocean level. Let's estimate: 40 thousand kilometers (the length of the earth's equator) per day, that's 463 meters per second. This means that this half-meter wave, like a mini-tsunami, hits the eastern coasts of the continents in the equator region at supersonic speed. At our latitudes, the speed reaches 250-300 m/s - also quite a lot: although the wave is not very high, due to inertia it can create a great effect.
The second object in terms of influence on the Earth is the Sun. It is 400 times farther from us than the Moon, but 27 million times more massive. Therefore, the effects from the Moon and from the Sun are comparable in magnitude, although the Moon still acts a little stronger: the gravitational tidal effect from the Sun is about half as weak as from the Moon. Sometimes their influence is combined: this happens on a new moon, when the Moon passes against the background of the Sun, and on a full moon, when the Moon is on the opposite side from the Sun. On these days - when the Earth, Moon and Sun line up, and this happens every two weeks - the total tidal effect is one and a half times greater than from the Moon alone. And after a week, the Moon passes a quarter of its orbit and finds itself in quadrature with the Sun (a right angle between the directions on them), and then their influence weakens each other. On average, the height of tides in the open sea varies from a quarter of a meter to 75 centimeters.
Sailors have known tides for a long time. What does the captain do when the ship runs aground? If you have read sea adventure novels, then you know that he immediately looks at what phase the Moon is in and waits for the next full moon or new moon. Then the maximum tide can lift the ship and refloat it.
Coastal problems and features
Tides are especially important for port workers and for sailors who are about to bring their ship into or out of port. As a rule, the problem of shallow water arises near the coast, and to prevent it from interfering with the movement of ships, underwater channels - artificial fairways - are dug to enter the bay. Their depth should take into account the height of the maximum low tide.
If we look at the height of the tides at some point in time and draw lines of equal heights of water on the map, we will get concentric circles with centers at two points (sublunar and anti-lunar), in which the tide is maximum. If the orbital plane of the Moon coincided with the plane of the Earth’s equator, then these points would always move along the equator and would make a full revolution per day (more precisely, in 24ʰ 50ᵐ 28ˢ). However, the Moon does not move in this plane, but near the ecliptic plane, in relation to which the equator is inclined by 23.5 degrees. Therefore, the sublunar point also “walks” along latitude. Thus, in the same port (i.e., at the same latitude), the height of the maximum tide, which repeats every 12.5 hours, changes during the day depending on the orientation of the Moon relative to the Earth's equator.
This “trifle” is important for the theory of tides. Let's look again: the Earth rotates around its axis, and the plane of the lunar orbit is inclined towards it. Therefore, each seaport “runs” around the Earth’s pole during the day, once falling into the region of the highest tide, and after 12.5 hours - again into the region of the tide, but less high. Those. two tides during the day are not equal in height. One is always larger than the other, because the plane of the lunar orbit does not lie in the plane of the earth's equator.
For coastal residents, the tidal effect is vital. For example, in France there is one that is connected to the mainland by an asphalt road laid along the bottom of the strait. There are many people living on the island, but they cannot use this road while the sea level is high. This road can only be driven twice a day. People drive up and wait for low tide, when the water level drops and the road becomes accessible. People travel to and from work on the coast using a special tide table that is published for each coastal settlement. If this phenomenon is not taken into account, water may overwhelm a pedestrian along the way. Tourists simply come there and walk around to look at the bottom of the sea when there is no water. And local residents collect something from the bottom, sometimes even for food, i.e. in essence, this effect feeds people.
Life came out of the ocean thanks to the ebb and flow of the tides. As a result of the low tide, some coastal animals found themselves on the sand and were forced to learn to breathe oxygen directly from the atmosphere. If there were no Moon, then life might not have come out of the ocean so actively, because it is good there in all respects - a thermostatic environment, weightlessness. But if you suddenly found yourself on the shore, you had to somehow survive.
The coast, especially if it is flat, is greatly exposed at low tide. And for some time people lose the opportunity to use their watercraft, lying helplessly like whales on the shore. But there is something useful in this, because the low tide period can be used to repair ships, especially in some bay: the ships sailed, then the water went away, and they can be repaired at this time.
For example, there is the Bay of Fundy on the east coast of Canada, which is said to have the highest tides in the world: the water level drop can reach 16 meters, which is considered a record for a sea tide on Earth. Sailors have adapted to this property: during high tide they bring the ship to the shore, strengthen it, and when the water goes away, the ship hangs, and the bottom can be caulked.
People have long begun to monitor and regularly record the moments and characteristics of high tides in order to learn how to predict this phenomenon. Soon invented tide gauge- a device in which a float moves up and down depending on sea level, and the readings are automatically drawn on paper in the form of a graph. By the way, the means of measurement have hardly changed since the first observations to the present day.
Based on a large number of hydrograph records, mathematicians are trying to create a theory of tides. If you have a long-term record of a periodic process, you can decompose it into elementary harmonics - sinusoids of different amplitudes with multiple periods. And then, having determined the parameters of the harmonics, extend the total curve into the future and make tide tables on this basis. Nowadays such tables are published for every port on Earth, and any captain about to enter a port takes a table for him and sees when there will be sufficient water level for his ship.
The most famous story related to predictive calculations took place during the Second World War: in 1944, our allies - the British and Americans - were going to open a second front against Nazi Germany, for this it was necessary to land on the French coast. The northern coast of France is very unpleasant in this regard: the coast is steep, 25-30 meters high, and the ocean bottom is quite shallow, so ships can only approach the coast at times of maximum tide. If they ran aground, they would simply be shot from cannons. To avoid this, a special mechanical (there were no electronic ones yet) computer was created. She performed Fourier analysis of sea-level time series using drums rotating at their own speed, through which a metal cable passed, which summed up all the terms of the Fourier series, and a feather connected to the cable plotted a graph of tide height versus time. This was top secret work that greatly advanced the theory of tides because it was possible to predict with sufficient accuracy the moment of the highest tide, thanks to which heavy military transport ships swam across the English Channel and landed troops ashore. This is how mathematicians and geophysicists saved the lives of many people.
Some mathematicians are trying to generalize the data on a planetary scale, trying to create a unified theory of tides, but comparing records made in different places is difficult because the Earth is so irregular. It is only in the zero approximation that a single ocean covers the entire surface of the planet, but in reality there are continents and several weakly connected oceans, and each ocean has its own frequency of natural oscillations.
Previous discussions about sea level fluctuations under the influence of the Moon and the Sun concerned open ocean spaces, where tidal acceleration varies greatly from one coast to another. And in local bodies of water - for example, lakes - can the tide create a noticeable effect?
It would seem that it should not be, because at all points of the lake the tidal acceleration is approximately the same, the difference is small. For example, in the center of Europe there is Lake Geneva, it is only about 70 km long and is in no way connected with the oceans, but people have long noticed that there are significant daily fluctuations in water there. Why do they arise?
Yes, the tidal force is extremely small. But the main thing is that it is regular, i.e. operates periodically. All physicists know the effect that, when a force is applied periodically, sometimes causes an increased amplitude of oscillations. For example, you take a bowl of soup from the cafeteria and... This means that the frequency of your steps is in resonance with the natural vibrations of the liquid in the plate. Noticing this, we sharply change the pace of walking - and the soup “calms down.” Each body of water has its own basic resonant frequency. And the larger the size of the reservoir, the lower the frequency of natural vibrations of the liquid in it. So, Lake Geneva’s own resonant frequency turned out to be a multiple of the frequency of the tides, and a small tidal influence “looses” Lake Geneva so that the level on its shores changes quite noticeably. These long-period standing waves that occur in closed bodies of water are called seiches.
Tidal Energy
Nowadays, they are trying to connect one of the alternative energy sources with the tidal effect. As I said, the main effect of tides is not that the water rises and falls. The main effect is a tidal current that moves water around the entire planet in a day.
In shallow places this effect is very important. In the New Zealand area, captains do not even risk guiding ships through some straits. Sailboats have never been able to get through there, and even modern ships have difficulty getting through there, because the bottom is shallow and tidal currents have enormous speed.
But since the water is flowing, this kinetic energy can be used. And power plants have already been built, in which turbines rotate back and forth due to tidal currents. They are quite functional. The first tidal power plant (TPP) was made in France, it is still the largest in the world, with a capacity of 240 MW. Compared to a hydroelectric power station, it’s not so great, of course, but it serves the nearest rural areas.
The closer to the pole, the lower the speed of the tidal wave, therefore in Russia there are no coasts that would have very powerful tides. In general, we have few outlets to the sea, and the coast of the Arctic Ocean is not particularly profitable for using tidal energy, also because the tide drives water from east to west. But there are still places suitable for PES, for example, Kislaya Bay.
The fact is that in bays the tide always creates a greater effect: the wave runs up, rushes into the bay, and it narrows, narrows - and the amplitude increases. A similar process occurs as if a whip was cracked: at first the long wave travels slowly along the whip, but then the mass of the part of the whip involved in the movement decreases, so the speed increases (impulse mv is preserved!) and reaches supersonic at the narrow end, as a result of which we hear a click.
By creating the experimental Kislogubskaya TPP of low power, power engineers tried to understand how effectively tides at circumpolar latitudes can be used to produce electricity. It doesn't make much economic sense. However, now there is a project for a very powerful Russian TPP (Mezenskaya) – for 8 gigawatts. In order to achieve this colossal power, it is necessary to block off a large bay, separating the White Sea from the Barents Sea with a dam. True, it is highly doubtful that this will be done as long as we have oil and gas.
The past and future of tides
By the way, where does tidal energy come from? The turbine spins, electricity is generated, and what object loses energy?
Since the source of tidal energy is the rotation of the Earth, if we draw from it, it means that the rotation must slow down. It would seem that the Earth has internal sources of energy (heat from the depths comes from geochemical processes and the decay of radioactive elements), and there is something to compensate for the loss of kinetic energy. This is true, but the energy flow, spreading on average almost evenly in all directions, can hardly significantly affect the angular momentum and change the rotation.
If the Earth did not rotate, the tidal humps would point exactly in the direction of the Moon and the opposite direction. But, as it rotates, the Earth’s body carries them forward in the direction of its rotation - and a constant divergence of the tidal peak and the sublunar point of 3-4 degrees arises. What does this lead to? The hump that is closer to the Moon is attracted to it more strongly. This gravitational force tends to slow down the Earth's rotation. And the opposite hump is further from the Moon, it tries to speed up the rotation, but is attracted weaker, so the resultant moment of force has a braking effect on the rotation of the Earth.
So, our planet is constantly decreasing its rotation speed (though not quite regularly, in jumps, which is due to the peculiarities of mass transfer in the oceans and atmosphere). What effect do Earth's tides have on the Moon? The near tidal bulge pulls the Moon along with it, while the distant one, on the contrary, slows it down. The first force is greater, as a result the Moon accelerates. Now remember from the previous lecture, what happens to a satellite that is forcibly pulled forward in motion? As its energy increases, it moves away from the planet and its angular velocity decreases because the orbital radius increases. By the way, an increase in the period of revolution of the Moon around the Earth was noticed back in the time of Newton.
Speaking in numbers, the Moon moves away from us by about 3.5 cm per year, and the length of the Earth’s day increases by a hundredth of a second every hundred years. It seems like nonsense, but remember that the Earth has existed for billions of years. It is easy to calculate that in the time of dinosaurs there were about 18 hours in a day (the current hours, of course).
As the Moon moves away, tidal forces become smaller. But it was always moving away, and if we look into the past, we will see that before the Moon was closer to the Earth, which means the tides were higher. You can appreciate, for example, that in the Archean era, 3 billion years ago, the tides were kilometer high.
Tidal phenomena on other planets
Of course, the same phenomena occur in the systems of other planets with satellites. Jupiter, for example, is a very massive planet with a large number of satellites. Its four largest satellites (they are called Galilean because Galileo discovered them) are quite significantly influenced by Jupiter. The nearest of them, Io, is entirely covered with volcanoes, among which there are more than fifty active ones, and they emit “extra” matter 250-300 km upward. This discovery was quite unexpected: there are no such powerful volcanoes on Earth, but here is a small body the size of the Moon, which should have cooled down long ago, but instead it is bursting with heat in all directions. Where is the source of this energy?
Io's volcanic activity was not a surprise to everyone: six months before the first probe approached Jupiter, two American geophysicists published a paper in which they calculated Jupiter's tidal influence on this moon. It turned out to be so large that it could deform the satellite’s body. And during deformation, heat is always released. When we take a piece of cold plasticine and begin to knead it in our hands, after several compressions it becomes soft and pliable. This happens not because the hand heated it with its heat (the same thing will happen if you squish it in a cold vice), but because the deformation put mechanical energy into it, which was converted into thermal energy.
But why on earth does the shape of the satellite change under the influence of tides from Jupiter? It would seem that, moving in a circular orbit and rotating synchronously, like our Moon, it once became an ellipsoid - and there is no reason for subsequent distortions of the shape? However, there are also other satellites near Io; all of them cause its (Io) orbit to shift slightly back and forth: it either approaches Jupiter or moves away. This means that the tidal influence either weakens or intensifies, and the shape of the body changes all the time. By the way, I have not yet talked about tides in the solid body of the Earth: of course, they also exist, they are not so high, on the order of a decimeter. If you sit in your place for six hours, then, thanks to the tides, you will “walk” about twenty centimeters relative to the center of the Earth. This vibration is imperceptible to humans, of course, but geophysical instruments register it.
Unlike the solid earth, the surface of Io fluctuates with an amplitude of many kilometers during each orbital period. A large amount of deformation energy is dissipated as heat and heats the subsurface. By the way, meteorite craters are not visible on it, because volcanoes constantly bombard the entire surface with fresh matter. As soon as an impact crater is formed, a hundred years later it is covered with products of eruptions of neighboring volcanoes. They work continuously and very powerfully, and to this are added fractures in the planet’s crust, through which a melt of various minerals, mainly sulfur, flows from the depths. At high temperatures it darkens, so the stream from the crater looks black. And the light rim of the volcano is the cooled substance that falls around the volcano. On our planet, matter ejected from a volcano is usually decelerated by air and falls close to the vent, forming a cone, but on Io there is no atmosphere, and it flies along a ballistic trajectory far in all directions. Perhaps this is an example of the most powerful tidal effect in the solar system.
The second satellite of Jupiter, Europa, all looks like our Antarctica, it is covered with a continuous ice crust, cracked in some places, because something is constantly deforming it too. Since this satellite is further away from Jupiter, the tidal effect here is not so strong, but still quite noticeable. Beneath this icy crust is a liquid ocean: the photographs show fountains gushing out of some of the cracks that have opened up. Under the influence of tidal forces, the ocean rages, and ice fields float and collide on its surface, much like we have in the Arctic Ocean and off the coast of Antarctica. The measured electrical conductivity of Europa's ocean fluid indicates that it is salt water. Why shouldn't there be life there? It would be tempting to lower a device into one of the cracks and see who lives there.
In fact, not all planets meet ends meet. For example, Enceladus, a moon of Saturn, also has an icy crust and an ocean underneath. But calculations show that tidal energy is not enough to maintain the subglacial ocean in a liquid state. Of course, in addition to tides, any celestial body has other sources of energy - for example, decaying radioactive elements (uranium, thorium, potassium), but on small planets they can hardly play a significant role. This means there is something we don’t understand yet.
The tidal effect is extremely important for stars. Why - more on this in the next lecture.
Our planet is constantly in the gravitational field created by the Moon and the Sun. This causes a unique phenomenon expressed in the ebb and flow of tides on Earth. Let's try to figure out whether these processes affect the environment and human life.
The mechanism of the phenomenon of "ebb and flow"
The nature of the formation of ebbs and flows has already been sufficiently studied. Over the years, scientists have studied the causes and results of this phenomenon.
Similar fluctuations in the level of earthly waters can be shown in the following system:
- The water level gradually rises, reaching its highest point. This phenomenon is called full water.
- After a certain period of time, the water begins to subside. Scientists gave this process the definition of “ebb.”
- For about six hours, the water continues to drain to its minimum point. This change was named in the form of the term “low water”.
You can notice a certain pattern if you observe the tide process in the same place for a month. The results of the analysis are interesting: every day low and high water changes its location. With such a natural factor as the formation of a new moon and a full moon, the levels of the objects being studied move away from each other.
Consequently, this makes the tide amplitude twice a month at its maximum. The occurrence of the smallest amplitude also occurs periodically, when, after the characteristic influence of the Moon, the levels of low and high waters gradually approach each other.
Causes of ebbs and flows on Earth
There are two factors that influence the formation of ebbs and flows. You should carefully consider both objects that affect changes in the Earth's water space.
The effect of lunar energy on the ebb and flow of tides
Although the influence of the Sun on the cause of tides is undeniable, the greatest importance in this matter belongs to the influence of lunar activity. In order to feel the significant impact of the satellite's gravity on our planet, it is necessary to monitor the difference in the gravity of the Moon in different regions of the Earth.
The results of the experiment will show that the difference in their parameters is quite small. The thing is that the point on the earth's surface closest to the Moon is literally 6% more susceptible to external influence than the point that is most distant. It is safe to say that this disengagement of forces is pushing the Earth apart in the direction of the Moon-Earth trajectory.
Taking into account the fact that our planet constantly rotates around its axis during the day, a double tidal wave passes twice along the perimeter of the created stretch. This is accompanied by the creation of so-called double “valleys”, the height of which, in principle, does not exceed 2 meters in the World Ocean.
On the territory of the earth's land, such fluctuations reach a maximum of 40-43 centimeters, which in most cases goes unnoticed by the inhabitants of our planet.
All this leads to the fact that we do not feel the force of the ebb and flow of the tides either on land or in the water element. You can observe a similar phenomenon on a narrow strip of coastline, because the waters of the ocean or sea sometimes gain impressive heights by inertia.
From all that has been said, we can conclude that the ebb and flow of tides are most closely related to the Moon. This makes research in this area the most interesting and relevant.
The influence of solar activity on the ebb and flow of tides
The significant distance of the main star of the solar system from our planet means that its gravitational influence is less noticeable. As an energy source, the Sun is certainly much more massive than the Moon, but still makes itself felt by the impressive distance between the two celestial objects. The amplitude of solar tides is almost half that of the tidal processes of the Earth's satellite.
A well-known fact is that during the full moon and the waxing of the moon, all three celestial bodies - the Earth, the Moon and the Sun - are located on the same straight line. This leads to the addition of lunar and solar tides.
During the period of direction from our planet to its satellite and the main star of the Solar system, which differs from each other by 90 degrees, there is some influence of the Sun on the process under study. There is an increase in the level of ebb and a decrease in the level of tide of earth's waters.
Everything indicates that solar activity also affects the energy of the tides on the surface of our planet.
Main types of tides
This concept can be classified according to the duration of the tide cycle. The demarcation will be recorded using the following points:
- Semi-diurnal changes in the water surface. Such transformations consist of two full and the same amount of incomplete water. The parameters of alternating amplitudes are almost equal to each other and look like a sinusoidal curve. They are most localized in the waters of the Barents Sea, on the vast coastal strip of the White Sea and on the territory of almost the entire Atlantic Ocean.
- Daily fluctuations in water level. Their process consists of one full and incomplete water for a period calculated within a day. A similar phenomenon is observed in the Pacific Ocean region, and its formation is extremely rare. During the passage of the Earth's satellite through the equatorial zone, the effect of standing water is possible. If the Moon is inclined at its lowest rate, small tides of an equatorial nature occur. At the highest numbers, the process of formation of tropical tides occurs, accompanied by the greatest power of water influx.
- Mixed tides. This concept includes the presence of semidiurnal and diurnal tides of irregular configuration. Semi-diurnal changes in the level of the earth's water shell, which have an irregular configuration, are in many ways similar to semi-diurnal tides. In altered daily tides, one can observe a tendency towards daily fluctuations depending on the degree of declination of the Moon. The waters of the Pacific Ocean are most susceptible to mixed tides.
- Abnormal tides. These rises and falls of water do not fit the description of some of the signs listed above. This anomaly is associated with the concept of “shallow water,” which changes the cycle of rise and fall of water levels. The influence of this process is especially noticeable in river mouths, where high tides are shorter than low tides. A similar cataclysm can be observed in some parts of the English Channel and in the currents of the White Sea.
Earth's tide chart
There is a so-called tide table. It is necessary for people who, by the nature of their activities, depend on changes in the earth’s water level. To have accurate information on this phenomenon, you need to pay attention to:
- Designation of an area where it is important to know tide data. It is worth remembering that even closely located objects will have different characteristics of the phenomenon of interest.
- Finding the necessary information using Internet resources. For more accurate information, you can visit the port of the region being studied.
- Specification of the time of need for accurate data. This aspect depends on whether the information is needed for a specific day or the research schedule is more flexible.
- Working with the table in the mode of emerging needs. It will display all information about the tides.
- The columns at the top of the table indicate the days and dates of the alleged phenomenon. This point will make it possible to clarify the point at which the time frame of what is being studied is determined.
- Below the temporary accounting line there are numbers placed in two rows. In the format of the day, a decoding of the phases of moonrise and sunrise is placed here.
- Below is a wave-shaped chart. These indicators record the peaks (high tides) and troughs (low tides) of the waters of the study area.
- After calculating the amplitude of the waves, the data of the setting of celestial bodies are located, which affect changes in the water shell of the Earth. This aspect will allow you to observe the activity of the Moon and the Sun.
- On both sides of the table you can see numbers with plus and minus indicators. This analysis is important for determining the level of rise or fall of water, calculated in meters.
All these indicators cannot guarantee one hundred percent information, because nature itself dictates to us the parameters according to which its structural changes occur.
The influence of tides on the environment and humans
There are many factors influencing the ebb and flow of tides on human life and the environment. Among them there are discoveries of a phenomenal nature that require careful study.
Rogue waves: hypotheses and consequences of the phenomenon
This phenomenon causes a lot of controversy among people who trust only unconditional facts. The fact is that traveling waves do not fit into any system for the occurrence of this phenomenon.
The study of this object became possible with the help of radar satellites. These structures made it possible to record a dozen waves of ultra-large amplitude over a period of a couple of weeks. The size of such a rise of a body of water is about 25 meters, which indicates the enormity of the phenomenon being studied.
Rogue waves directly affect human life, because over the past decades, such anomalies have carried huge vessels such as supertankers and container ships into the ocean depths. The nature of the formation of this stunning paradox is unknown: giant waves form instantly and disappear just as quickly.
There are many hypotheses regarding the reason for the formation of such a whim of nature, but the occurrence of whirlpools (single waves due to the collision of two solitons) is possible with the intervention of the activity of the Sun and the Moon. This issue is still becoming a source of debate among scientists specializing in this topic.
The influence of tides on the organisms inhabiting the Earth
The ebb and flow of the ocean and sea especially affects marine life. This phenomenon puts the greatest pressure on residents of coastal waters. Thanks to this change in the level of earth's water, organisms leading a sedentary lifestyle develop.
These include mollusks, which have perfectly adapted to the vibrations of the liquid shell of the Earth. At the highest tides, oysters begin to actively reproduce, which indicates that they respond favorably to such changes in the structure of the water element.
But not all organisms react so favorably to external changes. Many species of living beings suffer from periodic fluctuations in water levels.
Although nature takes its toll and coordinates changes in the overall balance of the planet, biological substances adapt to the conditions presented to them by the activity of the Moon and the Sun.
The impact of ebbs and flows on human life
This phenomenon affects the general condition of a person more than the phases of the moon, to which the human body may be immune. However, the ebb and flow of the tides most influence the production activities of the inhabitants of our planet. It is unrealistic to influence the structure and energy of the tides of the sea, as well as the oceanic sphere, because their nature depends on the gravity of the Sun and Moon.
Basically, this cyclical phenomenon brings only destruction and trouble. Modern technologies make it possible to channel this negative factor into a positive direction.
An example of such innovative solutions would be pools designed to trap such fluctuations in water balance. They must be built taking into account that the project is cost-effective and practical.
To do this, it is necessary to create such pools of quite significant size and volume. Power plants to retain the effect of the tidal force of the Earth's water resources are new, but quite promising.
Watch a video about the ebb and flow of the tides:
The study of the concept of tides on Earth, their influence on the life cycle of the planet, the mystery of the origin of rogue waves - all these remain the main questions for scientists specializing in this field. The solution to these aspects is also interesting to ordinary people who are interested in the problems of the influence of foreign factors on planet Earth.
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.
