The water surface level in the seas and oceans of our planet periodically changes, fluctuates in certain intervals. These periodic fluctuations are ebb and flow of the sea.
The picture of the ebb and flow of the sea
To visualize picture of the ebb and flow of the sea, imagine that you are standing on a sloping ocean shore, 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. The iron hull of a small boat, which has fallen on its side, lies not far away. The bottom of his body in the bow is heavily weathered. Obviously, once this ship, being not far from the coast, flew into the anchor. This accident happened, in all likelihood, during low tide, and, apparently, the ship has been lying in this place for more than one year, since almost all of its hull has managed to be covered with brown rust. You are inclined to consider the unwary captain to be the culprit of the ship accident. Apparently, the anchor was the sharp weapon that the ship fell on its side hit. You are looking for this anchor and cannot find it. Where could he go? Then you notice that the water is already approaching a pile of white stones, and then you guess that the anchor you saw has long been flooded by a tidal wave. The water "comes" on the shore, it continues to rise further and further upward. Now the heap of white stones was almost all hidden under the water.The phenomena of the ebb and flow of the sea
The phenomena of the ebb and flow of the sea 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 gravitation discovered by him. The reason for these phenomena is the action of the attraction of the Moon, exerted on the water shell of the Earth. Another famous Galileo Galilei connected the ebb and flow with the rotation of the Earth and saw in this one of the most reasonable and reliable evidence of the validity of the teachings of Nicolaus Copernicus, (more:). The Paris Academy of Sciences in 1738 announced a prize to the one who gives the most substantiated presentation of the theory of tides. The award then received Euler, Maclaurin, D. Bernoulli and Cavalieri... The first three took Newton's law of gravity as the basis of their work, and the Jesuit Cavalieri explained the tides on the basis of Descartes's vortex hypothesis. However, the most outstanding works in this area belong 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 by a water shell, and look at the globe from one of its poles, then the picture of sea tides 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 attraction 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. Simultaneously with this, on the opposite side of the Earth, the least subjected to the attraction 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 one straight line that passes through the centers of the Earth and the Moon, two large humps are formed, two huge water blisters... At the same time, the two other sides of our planet, located at an angle of ninety degrees from the points of maximum tide indicated above, are experiencing the greatest ebb tides. Here water falls more than anywhere else on the surface of the globe. The line connecting these points at low tide is somewhat reduced, and thus the impression of an increase in the elongation of the Earth in the direction of the maximum tidal points is created. Due to the lunar attraction, these points of maximum tide constantly retain their position relative to the Moon, but since the Earth rotates around its axis, during the time of the day they seem to move across the entire surface of the globe. That's why in each area during the day there are two high tides and two low tides.Solar ebb and flow
The sun, just like the moon, by the force of its attraction produces the ebb and flow. 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 smaller than the lunar ones. That's why solar tides, are not observed separately, but only their effect on the magnitude of the lunar tides is considered. For example, the greatest ebb and flow of the sea occurs during full moons and new moons, since at this time the Earth, the Moon and the Sun are on the same straight line, and our daylight by its attraction enhances the attraction of the Moon. Conversely, when we observe the moon in the first or in the last quarter (phase), there are least sea ebb and flow... This is because in this case the lunar tide coincides with sunshine... The action of the lunar attraction decreases by the amount of the Sun's attraction.Tidal friction
« Tidal friction»Existing in our planet, in turn, has an effect on the lunar orbit, since the tidal wave caused by the lunar gravity has the opposite effect on the moon, creating a tendency to accelerate its motion. As a result, the Moon is gradually moving away from the Earth, the period of its revolution increases, and it, in all likelihood, lags a little behind in its motion.The magnitude of sea tides
In addition to the relative position in space of the Sun, Earth and Moon, on tide magnitude in each particular area, the shape of the seabed and the nature of the coastline are influenced. It is also known that in the closed seas, as, for example, in the Aral, Caspian, Azov and Black, ebb and flow are almost not observed. They can hardly be found in the open oceans; here the tides barely reach one meter, the water level rises very slightly. But on the other hand, 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.
Ebb and flow in the air and solid shells of the Earth
Ebb and flow also occur in the air and solid shells of the Earth... We hardly notice these phenomena in the lower layers of the atmosphere. For comparison, let us point out that ebb and flow are not observed at the bottom of the oceans either. This circumstance is explained by the fact that mainly the upper layers of the water envelope are involved in tidal processes. The ebb and flow in an air envelope can be detected only with a very long-term observation of changes in atmospheric pressure. As for the earth's crust, each part of it, due to the tidal and ebbling action of the moon, rises twice and falls twice by about a few decimeters during the day. In other words, the fluctuations of the solid shell of our planet are approximately three times less than the fluctuations in the level of the surface 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, sometimes rises and then falls. These processes occurring in the solid shell of the Earth can be detected only with the help of instruments used to register earthquakes. It should be noted that ebb and flow occur on other world bodies and have a tremendous impact on their development. If the Moon were stationary in relation to the Earth, then in the absence of other factors affecting the delay of the tidal wave, anywhere in the world every 6 hours there would be two high tides and two low tides per day. But since the Moon continuously revolves around the Earth and, moreover, in the same direction in which our planet rotates around its axis, a certain delay is obtained: the Earth manages to turn to the Moon with each of its parts not within a day, but at about 24 hours and 50 minutes. Therefore, in each locality, the ebb or flow does not last exactly 6 hours, but about 6 hours and 12.5 minutes.Alternating ebb and flow
In addition, it should be noted that the correctness alternating ebb and flow 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 alternation irregularities sometimes reach several hours. Thus, the most "high" water occurs not at the moment of the Moon's climax, as it follows according to the theory, but several hours later than the Moon's passage through the meridian; this delay is called the applied hour of the port and sometimes reaches 12 hours. Previously, it was widely believed that the ebb and flow of the sea were associated with sea currents. Now everyone knows that these are phenomena of a different order. A tide is a kind of wave motion, similar to that which occurs due to the 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 it is not carried away by it like a current. The tidal wave period 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 hot flashes is many times less than the force of gravity... While the force of attraction is inversely proportional to the square of the distance between the attracted bodies, the force that causes tides is approximately is inversely proportional to the cube of this distance rather than squaring it.Let's continue talking about the forces acting on celestial bodies and the resulting effects. 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 the object is much less than gravitational
What other forces are there in nature besides gravity? We leave aside strong and weak nuclear interactions, they are local in nature (they act at extremely small distances). But electromagnetism, as you know, is much stronger than gravity and spreads 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 talk about disturbances in the motion of celestial bodies under the influence of an electromagnetic field. There are no more options, although there is still dark energy, but about it - later, when it comes to cosmology.
As I told you on, Newton's simple law of gravitation F = G∙M∙m/R² is very convenient to use in astronomy, because most bodies are close to spherical 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 a different force effect in its different parts, because these parts are differently removed from the sources of gravity, and this must be taken into account.
Attraction flattens and tears
To experience the tidal effect, let's do a thought experiment popular with physicists: imagine ourselves in a freely falling elevator. Cut off the rope holding the booth and begin to fall. Until we fall, we can watch what is happening around us. We suspend the free masses and watch how they behave. First, they fall synchronously, and we say - this is weightlessness, because all objects in this cabin and it itself feel about the same acceleration of gravity.
But over time, our material points will begin to change their configuration. Why? Because the lower one at the beginning was slightly closer to the center of gravity than the upper one, so the lower one, attracting more strongly, begins to outstrip the upper one. And the lateral 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 unrelated objects is deformed. This is called the tidal effect.
From the point of view of an observer who has scattered cereals around him and watches how individual grains move while this whole system falls on 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 in terms of relative distance, we get a symmetric picture: the near grains will be ahead of the observer, the far ones will lag behind him, i.e. the system will stretch along the axis directed to the gravitating object, and along the directions perpendicular to it, the particles will be pressed against 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. Unbeknownst to them, the effect is almost as strong on the reverse side of a freely falling body. Those. it breaks in two diametrically opposite directions, by no means in one.
The dangers of outer space
To show how important it is to take into account the tidal effect, take the International Space Station. She, like all satellites of the Earth, freely falls in the gravitational field (if the engines are not turned on). And the field of tidal forces around it is quite a tangible thing, so when an astronaut works on the outer side of the station, he must tie himself to it, and, as a rule, with two cables - just in case, you never know what might happen. And if he turns out to be unattached in those conditions where tidal forces are pulling him away from the center of the station, he can easily lose contact with it. This often happens with tools, because you can't tie all of them. If something falls out of the hands of an astronaut, then this object goes into the distance and becomes an independent satellite of the Earth.
The plan of work on the ISS includes tests in outer space of an individual jetpack. And when his engine fails, the tidal forces carry the astronaut away, and we lose him. The names of the missing are classified.
This, of course, is a joke: fortunately, there has not been such an incident yet. But this could very well have happened! And it might happen someday.
Ocean planet
Let's go back 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 rest of the planets also have some influence on the Earth, but it is barely perceptible.
To analyze the external gravitational influence on the Earth, it is usually represented as a solid sphere covered with a liquid shell. This is not a bad model, since our planet does have a movable shell in the form of an ocean and an atmosphere, and everything else is pretty solid. Although the earth's crust and inner layers have limited rigidity and are slightly tidal, their elastic deformation can be neglected when calculating the effect produced on the ocean.
If we draw the vectors of tidal forces in the Earth's center of mass system, we get the following picture: the tidal force field pulls the ocean along the "Earth-Moon" axis, and in the plane perpendicular to it pushes it to the center of the Earth. Thus, the planet (in any case, its movable 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 from the Moon, and a “bulge” appears in the strip between them (more precisely, the ocean surface has a lower curvature there).
A more interesting thing happens in the gap - where the tidal force vector tries to displace the liquid shell along the earth's surface. And this is natural: if in one place you want to raise the sea, and in another place - to lower it, 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".
It is very easy to quantify the tidal effect. The gravity of the Earth tries to make the ocean spherical, and the tidal part of the lunar and solar influence - to stretch it along the axis. If we left the Earth alone and allowed it to freely fall on the Moon, then 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 steamer on the open sea or ocean, half a meter is not perceptible. 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 that faces 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, the knowledge of astronomy is deeper among girls. 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 sublunary point, you need to distill a large amount of water here. But the surface of the Earth does not remain stationary, it rotates rapidly in relation to the direction to the Moon and the Sun, making a complete revolution in a day (and the Moon slowly goes 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 bulge 2 times per day and 2 times under the ebb and flow of the 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, such as a mini-tsunami, runs on the eastern coasts of the continents in the equatorial 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 the scale 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 weaker than from the Moon. Sometimes their influence adds up: 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. 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 is in square with the Sun (a right angle between the directions on them), and then their influence weakens each other. On average, the height of tides on the high seas varies from a quarter of a meter to 75 centimeters.
Tides have been known to sailors 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 drive it aground.
Coastal issues and features
The tides are especially important for port workers and for seafarers who intend to bring their ship into or out of port. As a rule, the problem of shallow water arises near the coast, and so that it does not interfere with the movement of ships, to enter the bay, they break through underwater channels - artificial fairways. 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 water height on the map, we get concentric circles with centers at two points (in the sublunar and anti-lunar), at 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 in a day (more precisely, in 24ʰ 50ᵐ 28ˢ) would make a complete revolution. However, the Moon walks not in this plane, but near the plane of the ecliptic, with respect to which the equator is tilted by 23.5 degrees. Therefore, the sublunary point "walks" also in 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 "little thing" is important for the theory of tides. Let's look again: the Earth rotates on its axis, and the plane of the lunar orbit is inclined to it. Therefore, each seaport "runs" around the Earth's pole during the day, once falling into the area of the highest tide, and after 12.5 hours - again into the area 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, which 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 as long as the sea level is high. This road can only be traveled twice a day. People drive up and wait for the low tide when the water level drops and the road becomes accessible. People travel to the coast to and from work, using a special tide table, which is published for each settlement on the coast. If this phenomenon is not taken into account, water along the way can overwhelm a pedestrian. Tourists just come there and walk 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 fact, this effect feeds people.
Life came out of the ocean thanks to the ebb and flow. As a result of the low tide, some coastal animals found themselves on the sand and had to learn to breathe oxygen directly from the atmosphere. If it were not for the Moon, then life, perhaps, would not so actively leave the ocean, because it is good there in all respects - a thermostated environment, weightlessness. But if you suddenly hit the shore, you had to somehow survive.
The coast, especially if it is flat, is strongly exposed at low tide. And for some time people lose the opportunity to use their floating craft, helplessly lying 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 left, and they can be repaired at this time.
For example, there is a 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: at high tide they bring the ship to the shore, strengthen it, and when the water leaves, the ship hangs, and it can be caulked up the bottom.
For a long time, people began 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 the 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 measuring instruments have hardly changed from the moment of the first observations to the present day.
Based on a large number of hydrographic records, mathematicians try to create a theory of tides. If you have a long-term record of a periodic process, you can decompose it into elementary harmonics - different amplitudes of a sinusoid with multiple periods. And then, having determined the parameters of the harmonics, extend the total curve into the future and, on this basis, make tide tables. Now such tables are published for every port on Earth, and any captain who is about to enter the port takes a table for him and looks when there will be a sufficient water level for his ship.
The most famous story associated with 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 respect: the coast is steep, 25-30 meters high, and the ocean floor is rather shallow, so that ships can approach the coast only at the moments of maximum tides. If they ran aground, they would simply be shot with cannons. To avoid this, a special mechanical (electronic were not yet available) computing machine was created. She performed a 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 wrote out a graph of the tide height versus time. This was a top secret work that greatly advanced the theory of tides, because it was possible to predict the moment of the highest tide with sufficient accuracy, thanks to which heavy military transport ships sailed across the Channel and landed troops ashore. So mathematicians and geophysicists have saved the lives of many people.
Some mathematicians try to generalize the data on a planetary scale, trying to create a unified theory of tides, but it is difficult to compare records taken in different places, because the Earth is very wrong. It is only in a zero approximation that a single ocean covers the entire surface of the planet, but in fact 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 there 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 has nothing to do with the oceans, but people have long noticed that there are significant daily fluctuations in water. Why do they arise?
Yes, the tidal force is extremely small. But the main thing is that it is regular, i.e. acts periodically. All physicists know the effect that, with periodic action of force, sometimes causes an increased amplitude of oscillations. For example, you take a bowl of soup in the dining room and. This means that the frequency of your steps is in resonance with the natural vibrations of the liquid in the tray. Noticing this, we abruptly 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 oscillations of the liquid in it. So, at Lake Geneva, its own resonant frequency turned out to be a multiple of the frequency of the tides, and a small tidal influence "blurs" Lake Geneva so that the level on its shores changes quite noticeably. These standing waves of a long period, arising in enclosed bodies of water, are called seiches.
Energy of the tides
Nowadays, they are trying to associate one of the alternative energy sources with the tidal effect. As I said before, the main effect of tides is not that the water rises and falls. The main effect is a tidal current, which drives water around the entire planet in a day.
In shallow places, this effect is very important. In the area of New Zealand, captains do not even risk escorting ships through some straits. Sailboats have never been able to pass there, and modern ships can hardly pass, because the bottom is shallow and the tidal currents have tremendous speed.
But once the water is flowing, this kinetic energy can be used. And power plants have already been built, in which the turbines rotate back and forth due to the tidal and ebb flow. They are quite workable. 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 the hydroelectric power station, it is not so hot, 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 with very powerful tides. In general, we have few outlets to the sea, and the coast of the Arctic Ocean for using tidal energy is not particularly profitable because the tide drives water from east to west. Still, there are places suitable for PES, for example, the Kislaya lip.
The fact is that in bays the tide always creates a greater effect: a wave runs in, rushes into the bay, and it narrows, narrows - and the amplitude increases. A similar process occurs as if the whip were clicked: at first the long wave goes 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 persists!) and reaches the supersonic end to the narrow end, as a result of which we hear a click.
Creating an experimental Kislogubskaya TPP of small capacity, power engineers tried to understand how efficiently the tides in the circumpolar latitudes can be used to generate electricity. It has no particular economic meaning. However, now there is a project of a very powerful Russian TPP (Mezenskaya) - 8 gigawatts. In order to achieve this colossal capacity, it is necessary to block off a large bay, separating the White Sea from the Barents Sea by a dam. True, it is highly doubtful that this will be done as long as we have oil and gas.
Past and future of the tides
By the way, where does the energy of the tides come from? The turbine is spinning, electricity is being generated, and which object is losing energy?
Since the source of energy for the tide is the rotation of the Earth, then since we draw from it, then the rotation should slow down. It would seem that the Earth has internal sources of energy (heat comes from the depths due to geochemical processes and the decay of radioactive elements), there is something to compensate for the loss of kinetic energy. This is so, but the energy flux, spreading on average almost uniformly 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 in the opposite direction. But, rotating, the Earth's body carries them forward in the direction of its rotation - and there is a constant discrepancy between the tidal peak and the sublunary point of 3-4 degrees. What does this lead to? The hump, which is closer to the moon, is attracted to it more strongly. This gravity tends to slow down the Earth's rotation. And the opposite hump is farther from the Moon, it tries to accelerate the rotation, but is attracted weaker, therefore the resultant moment of forces has a braking effect on the Earth's rotation.
So, our planet is constantly decreasing its rotation speed (albeit not quite regularly, in jumps, which is associated with the peculiarities of mass transfer in the oceans and the atmosphere). And what is the impact of the earth's tides on the moon? The near tidal bulge pulls the moon with it, the distant one, on the contrary, slows it down. The first force is greater; as a result, the Moon is accelerating. 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 at the same time, because the radius of its orbit increases. By the way, an increase in the period of the Moon's revolution around the Earth was noticed back in the time of Newton.
In terms of numbers, the Moon is moving away from us by about 3.5 cm per year, and the duration of the Earth's day every hundred years increases by a hundredth of a second. It seems to be nonsense, but remember that the Earth has been around for billions of years. It is easy to calculate that in the days of the dinosaurs there were about 18 hours in a day (the current hours, of course).
As the moon recedes, the tidal forces become smaller. But it was always moving away, and if we look back in time, we will see that earlier the Moon was closer to the Earth, which means that the tides were higher. You can estimate, for example, that in the Archean era, 3 billion years ago, the tides were one kilometer high.
Tidal phenomena on other planets
Of course, in the systems of other planets with satellites, the same phenomena occur. Jupiter, for example, is a very massive planet with a large number of satellites. Its four largest moons (they are called Galilean, because Galileo discovered them) are influenced by Jupiter quite tangibly. The nearest of them, Io, is entirely covered with volcanoes, among which there are more than fifty active ones, and they throw out "excess" matter 250-300 km up. 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 for a long time, but instead it glows 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 flew to Jupiter, two American geophysicists published a paper in which they calculated the tidal influence of Jupiter on this moon. It turned out to be so large that it can deform the satellite's body. And with deformation, heat is always released. When we take a piece of cold plasticine and begin to crumple it in our hands, it becomes soft, pliable after several squeezes. This happens not because the hand has heated it with its heat (it will be the same if you flatten it in a cold vice), but because the deformation has put mechanical energy into it, which has been converted into heat.
But why on earth is the shape of the satellite changing under the influence of the tides from Jupiter? It would seem that moving in a circular orbit and rotating synchronously, like our Moon, once became an ellipsoid - and there is no reason for further distortions of the shape? However, there are other satellites near Io; all of them make his (Io) orbit shift a little back and forth: it either approaches Jupiter or recedes. 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 the tides in the solid body of the Earth: they, of course, also exist, they are not so high, of the order of a decimeter. If you sit for about six hours in your places, then thanks to the tides, you will "walk" about twenty centimeters relative to the center of the Earth. This oscillation is imperceptible for a person, of course, but geophysical instruments register it.
Unlike the earth's solid, Io's surface fluctuates with an amplitude of many kilometers for each orbital period. A large amount of deformation energy is dissipated in the form of heat and heats the bowels. By the way, meteorite craters are not visible on it, because volcanoes constantly throw fresh matter on the entire surface. As soon as an impact crater forms, in a hundred years the products of the eruption of neighboring volcanoes fall asleep. They work continuously and very powerfully, to this are added faults in the planet's crust, through which melt of various minerals flows from the depths, mainly sulfur. At high temperatures, it darkens, so the jet 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 slowed down 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. This is perhaps the most powerful tidal effect in the solar system.
The second moon of Jupiter, Europa looks like our Antarctica, it is covered with a continuous ice crust, cracked here and there, because something is constantly deforming it too. Since this moon is farther from Jupiter, the tidal effect is not so strong here, but it is also quite noticeable. Beneath this ice crust there is a liquid ocean: the pictures show fountains gushing from some of the open cracks. Under the influence of tidal forces, the ocean boils, and ice fields float and collide on its surface, almost like we do in the Arctic Ocean and off the coast of Antarctica. The measured electrical conductivity of the Europa ocean fluid indicates that it is salt water. Why shouldn't there be life? It would be tempting to lower the device into one of the cracks and see who lives there.
In fact, not all planets make ends meet. For example, Enceladus, the moon of Saturn, also has an ice crust and an ocean below it. But calculations show that the energy of the tides is not enough to keep the sub-ice 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 that we do not understand something yet.
The tidal effect is extremely important for stars. Why - more on that in the next lecture.
The surface level of the 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 the lowest. At high tide, water flows to the coast, at low tide - from the coast.
The ebb and flow is standing. They are formed due to the influence of such cosmic bodies as the Sun. According to the laws of interaction of cosmic bodies, our planet and the Moon mutually attract each other. The lunar attraction is so great that the surface of the ocean, as it were, bends towards it. The moon moves around the Earth, and a tidal wave "runs" along the ocean after it. When the wave reaches the shore, the tide is coming. A little time will pass, the water after the Moon will move away from the coast - that is the low tide. According to the same universal cosmic laws, ebb and flow are formed from the attraction of the Sun. However, the tidal force of the Sun, due to its remoteness, is much less than the lunar, and if there was no moon, then the tides on Earth would be 2.17 times less. The explanation of tidal forces was first given by Newton.
Tides differ in duration and magnitude. Most often, there are two high tides and two low tides during the day. On the arcs and coasts of East and Central America, there is one high and one low tide during the day.
The magnitude of the tides is even more varied than their period. Theoretically, one lunar tide is 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 is quite close to the theoretical one: in the Hawaiian Islands - 1 m , on the island of Saint Helena - 1.1 m; on the islands - 1.7 m.On the continents, the tide value ranges from 1.5 to 2 m.In the inland seas, the tides are very insignificant: - 13 cm, - 4.8 cm.It is considered tide-free, but around Venice the tides are up to 1 m. The largest can be noted the following tides recorded in:
In the Bay of Fundy (), the tide has reached a height of 16-17 m. This is the highest tide indicator in the entire globe.
In the north, in the Penzhinskaya Bay, the tide height has reached 12-14 m. This is the largest tide off the coast of Russia. However, the above tide rates are the exception rather than the rule. In the overwhelming majority of tide 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.
In order to exhaust the main questions connected with the existence of its satellite, the Moon, near the Earth, we need to say a few words about the phenomenon of tides. It is also necessary to answer the final question raised in this book: where did the moon come from and what is its future? What is the tide?
During high tides, on the shores of the open seas and oceans, water attacks the shores. Low shores are literally overwhelmed by huge masses of water. Huge spaces are covered with water. The sea, as it were, protrudes from the shores and presses on land. The sea water is clearly rising.
During high tides (64), deep-seated ocean-going vessels have the opportunity to freely enter relatively shallow-water harbors and estuaries of rivers flowing into the oceans.
The tidal wave is very high in some places, reaching ten or more meters.
Approximately six hours pass from the beginning of the rise of water, and the tide gives way to the ebb (65), the water begins gradually
subside, the sea near the coast becomes shallow, and significant areas of the coastal strip are freed from water. Recently, steamers sailed in these places, and now residents wander on wet sand and gravel and collect shells, seaweed and other "gifts" of the sea.
What explains these constant ebbs and flows? They are due to the attraction that the moon has on the earth.
Not only does the Earth attract the Moon, but the Moon also attracts the Earth. The gravity of the Earth affects the movement of the Moon, forcing the Moon to move in a curved path. But along with this, the attraction of the Earth somewhat changes the shape of the Moon. The parts facing the Earth are attracted by the Earth more strongly than other parts. Thus, the Moon should have a somewhat elongated shape towards the Earth.
The attraction of the moon also affects the shape of the earth. In the side facing the Moon at the moment, there is some swelling, stretching of the earth's surface (66).
Particles of water, as more mobile and possessing little cohesion, are more amenable to this attraction of the moon than particles of solid land. In this regard, a very noticeable rise in water is created in the oceans.
If the Earth, like the Moon, were always facing the Moon with the same side, its shape would be somewhat elongated in the direction of the Moon and no alternating ebb and flow would exist. But the Earth turns in different directions to all the heavenly bodies, including the Moon (diurnal rotation). In this regard, a tidal wave seems to be running along the Earth, running after the Moon, which soars the water of the oceans higher in the parts of the earth's surface facing it at the moment. The ebb and flow should alternate.
During the day, the Earth will make one turn around its axis. Consequently, exactly one day later, the same parts of the earth's surface should be facing the moon. But we know that the Moon manages to pass some part of its path around the Earth in a day, moving in the same direction in which the Earth rotates. Therefore, the period is lengthened, after which the same parts of the Earth will face the Moon. Therefore the ebb and flow cycle does not occur per day, but at 24 hours 51 minutes. During this period of time, two high tides and two low tides alternate on Earth.
But why two and not one? We find the explanation for this by recalling once again the law of universal gravitation. According to this law, the force of attraction decreases with increasing distance, and, moreover, is inversely proportional to its square: the distance doubles - the attraction decreases fourfold.
On the side of the Earth, directly opposite to the one that faces the Moon, the following happens. Particles close to the Earth's surface are attracted by the Moon weaker than the inner parts of the Earth. They tend to the Moon less than particles closer to it. Therefore, the surface of the seas here, as it were, lags somewhat behind the solid inner parts of the globe, and here it also turns out to be a water hump, a tidal height mn, approximately the same as on the opposite side. Here, too, the tidal wave runs over low shores. Consequently, the shores of the oceans will be tide both when these shores are facing the Moon, and when the Moon is in the opposite direction. Thus, on the Earth there must necessarily be two high tides and two low tides during the period of a complete revolution of the Earth around its axis.
Of course, the tide is also influenced by the sun's gravity. But although the Sun is colossal in size, it is, however, much farther from the Earth than the Moon. Its tidal influence is half that of the Moon (it is only 5/11 or 0.45 of the tidal influence of the Moon).
The magnitude of each tide also depends on the height at which the moon is at a given time. In this case, it is completely indifferent what phase the Moon has at this time and whether it is visible in the sky. The moon can stay at this moment is not visible at all, that is, be in the same direction as the sun, and vice versa. Only in the first case will the tide in general be stronger than usual, since the attraction of the Sun is also added to the attraction of the Moon.
The calculation shows that the tidal force of the Moon is only one nine-millionth part of the force of gravity on Earth, that is, the force with which the Earth is attracted to itself. Of course, this attraction of the Moon is negligible. The rise of water by several meters is also negligible in comparison with the equatorial diameter of the globe, equal to 12,756,776 m. But the tidal wave, even such a small one, is very, as we know, perceptible for the inhabitants of the Earth, located off the shores of the oceans.
