Who wouldn't want to take a walk to the bottom of the sea? "It's impossible! you exclaim. - For this you need at least a caisson! " But don't you know that large areas of the seabed open up for observation twice a day? True, woe to those who decide to stay at this "exhibition" beyond the established time! The seabed opens at low tide. - this is a change of high and low water.
This is one of the mysteries of nature. Many naturalists tried to solve it: Kepler who discovered the law of planetary motion, Newton who established the basic laws of motion, the French scientist Laplace who studied the emergence celestial bodies. They all wanted to penetrate the secrets of the life of the oceans.
The wind creates waves on the sea. But the wind is too weak to control the ebb and flow. Even a storm can only help with the tide. What gigantic forces are doing such hard work?
The influence of the moon on ebb and flow
Three giants are fighting for the world's oceans: The sun, the moon and the earth itself... The sun is stronger than all, but it is too far from us to be the winner. The movement of water masses on Earth is mainly controlled by the Moon. Located at a distance of 384,000 kilometers from Earth, it regulates the "pulse" of the oceans. Like a huge magnet, the Moon pulls masses of water up several meters while the Earth rotates on its axis.
Although the difference between the heights of ebb and flow is on average no more than 4 meters, the work that the moon is doing is enormous. It is equal to 11 trillion horsepower. If this number is written in numbers alone, then it will have 18 zeros and look like this: 11,000,000,000,000,000,000 the globe.
Ebb and flow - sources of energy
After the sun ebb and flow- The biggest energy sources... They could give electricity all over the world. Since time immemorial, man has tried to make the moon serve him. In China and elsewhere, tidal waters have been driving millstones for a long time.
In 1913, the first "lunar" power station was put into operation in the North Sea near Husum. In England, France, the USA, and especially in Argentina, feeling a lack of fuel, many bold projects for the construction of tidal stations have been created. However, the farthest went Soviet engineers, who created a project for the construction of a dam 100 kilometers long and 15 meters high in the Mezen Bay Of the White Sea.
At high tide, a reservoir with a capacity of 2 thousand square kilometers... Two thousand turbine generators will provide 36 billion kilowatt-hours. This amount of energy was produced in 1929 by France, Italy and Switzerland combined. A kilowatt hour of this energy will cost about a penny. Unfortunately, the "pulse" ebb and flow of the sea beats with unequal strength, like the pulse of a person. The tides do not provide a constant, uniform flow of water, and this makes the project difficult.
The tide is strongest when the Sun and Moon pull masses of water in the same direction. High tides when the water level rises up to 20 meters, are at full and young moon... They are called "syzygy". In the first and last quarter of the month when the moon is at right angles to the sun, tides are lowest and are called "quadrature".
The ebb and flow of the sea is very great importance for navigation, and therefore their offensive calculated in advance... This calculation is so difficult that it takes many weeks to compile the annual tide calendar. But the ingenious mind of man has created a calculating machine, the "electronic brain" of which makes forecasts of tides in two days. The tide calendar shows that tidal waves travel across the globe at regular intervals. From the sea shores, they rise into rivers.
The surface level of the oceans and seas periodically, approximately twice a day, changes. 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 bends towards it. The moon moves around the Earth, and "runs" after it across the ocean tidal wave... 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 one, 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 vary 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 tide 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 value 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 in the entire globe.
In the north, in the Penzhinskaya Bay, the tide height reached 12-14 m. This is the highest 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.
The content of the article
Ebb and flow, periodic fluctuations in water level (rises and falls) in water areas on Earth, which are caused by the gravitational attraction of the Moon and the Sun, acting on the rotating Earth. All large water areas, including oceans, seas and lakes, are more or less prone to ebb and flow, although they are small on lakes.
Reversible waterfall
(reversing direction) is another phenomenon associated with river tides. Typical example- a waterfall on the St. John River (New Brunswick, Canada). Here, along a narrow gorge, at high tide, water penetrates into a basin located above the low water level, but somewhat below the high water level in the same gorge. Thus, a barrier arises, flowing through which water forms a waterfall. At low tide, the water flow rushes downstream through the narrowed passage and, overcoming the underwater ledge, forms an ordinary waterfall. At high tide, a steep wave penetrating into the gorge falls like a waterfall into the overlying basin. The reverse flow continues until the water levels on both sides of the threshold are equal and the tide begins. Then the downstream waterfall is restored again. The average water level drop in the gorge is approx. 2.7 m, however, at the highest tides, the height of the direct waterfall can exceed 4.8 m, and the reversible one - 3.7 m.
The largest amplitudes of tides.
The world's highest tide is generated by strong currents in Minas Bay in the Bay of Fundy. Tidal fluctuations here are characterized by a normal course with a semidiurnal period. The water level during high tide often rises by more than 12 m in six hours, and then decreases by the same amount over the next six hours. When the effect of syzygy tide, the position of the Moon at perigee and the maximum declination of the Moon fall on one day, the tide level can reach 15 m. the top of the bay.
Wind and weather.
The wind has significant influence on tidal phenomena. The wind from the sea drives the water towards the coast, the tide height increases above normal, and at low tide the water level also exceeds the average. On the contrary, when the wind blows from the land, the water is driven away from the coast, and the sea level drops.
By increasing atmospheric pressure over a wide area, the water level decreases, as the superimposed weight of the atmosphere is added. When atmospheric pressure rises by 25 mm Hg. Art., the water level drops by about 33 cm. A decrease in atmospheric pressure causes a corresponding increase in the water level. Consequently, a sharp drop in atmospheric pressure, combined with hurricane force winds, can cause a noticeable rise in the water level. Such waves, although they are called tidal waves, are not actually associated with the influence of tidal forces and do not have the periodicity characteristic of tidal phenomena. The formation of these waves can be associated either with hurricane force winds or with underwater earthquakes (in the latter case, they are called seismic by sea waves, or tsunami).
Using the energy of the tides.
Four methods have been developed to harness tidal energy, but the most practical is the creation of a tidal basin system. At the same time, fluctuations in the water level associated with tidal phenomena are used in the sluice system so that the level difference is constantly maintained, which makes it possible to obtain energy. The capacity of tidal power plants is directly dependent on the area of the trapped basins and the potential level difference. The latter factor, in turn, is a function of the tidal amplitude. The achievable level difference is by far the most important for the generation of electricity, although the cost of the facilities depends on the area of the pools. Currently, large tidal power plants operate in Russia on the Kola Peninsula and in Primorye, in France in the estuary of the Rance River, in China near Shanghai, and also in other regions of the world.
| INFORMATION ABOUT TIDES IN SOME PORTS OF THE WORLD | ||||
| Port | Interval between tides | Average tide height, m | Syzygy tide height, m | |
| h | min | |||
| M. Morris Jesep, Greenland, Denmark | 10 | 49 | 0,12 | 0,18 |
| Reykjavik, Iceland | 4 | 50 | 2,77 | 3,66 |
| R. Coxoac, Hudson Strait, Canada | 8 | 56 | 7,65 | 10,19 |
| St. John's, Newfoundland, Canada | 7 | 12 | 0,76 | 1,04 |
| Barntko, Bay of Fundy, Canada | 0 | 09 | 12,02 | 13,51 |
| Portland, pcs. Maine, USA | 11 | 10 | 2,71 | 3,11 |
| Boston, pcs. Massachusetts, USA | 11 | 16 | 2,90 | 3,35 |
| New York, pcs. New York, USA | 8 | 15 | 1,34 | 1,62 |
| Baltimore, pcs. Maryland, USA | 6 | 29 | 0,33 | 0,40 |
| Miami Beach, pcs. Florida, USA | 7 | 37 | 0,76 | 0,91 |
| Galveston, pcs. Texas, USA | 5 | 07 | 0,30 | 0,43* |
| O. Maraca, Brazil | 6 | 00 | 6,98 | 9,15 |
| Rio de Janeiro, Brazil | 2 | 23 | 0,76 | 1,07 |
| Callao, Peru | 5 | 36 | 0,55 | 0,73 |
| Balboa, Panama | 3 | 05 | 3,84 | 5,00 |
| San Francisco, pcs. California, USA | 11 | 40 | 1,19 | 1,74* |
| Seattle, Washington, USA | 4 | 29 | 2,32 | 3,45* |
| Nanaimo, British Columbia, Canada | 5 | 00 | ... | 3,42* |
| Sitka, Alaska, USA | 0 | 07 | 2,35 | 3,02* |
| Sunrise, Cook Bay Alaska, USA | 6 | 15 | 9,24 | 10,16 |
| Honolulu, pcs. Hawaii, USA | 3 | 41 | 0,37 | 0,58* |
| Papeete, about. Tahiti, French Polynesia | ... | ... | 0,24 | 0,33 |
| Darwin, Australia | 5 | 00 | 4,39 | 6,19 |
| Melbourne, Australia | 2 | 10 | 0,52 | 0,58 |
| Rangoon, Myanmar | 4 | 26 | 3,90 | 4,97 |
| Zanzibar, Tanzania | 3 | 28 | 2,47 | 3,63 |
| Cape Town, South Africa | 2 | 55 | 0,98 | 1,31 |
| Gibraltar, Vlad. Great Britain | 1 | 27 | 0,70 | 0,94 |
| Granville, France | 5 | 45 | 8,69 | 12,26 |
| Lit, UK | 2 | 08 | 3,72 | 4,91 |
| London, Great Britain | 1 | 18 | 5,67 | 6,56 |
| Dover, UK | 11 | 06 | 4,42 | 5,67 |
| Avonmouth, UK | 6 | 39 | 9,48 | 12,32 |
| Ramsey, oh. Maine, UK | 10 | 55 | 5,25 | 7,17 |
| Oslo, Norway | 5 | 26 | 0,30 | 0,33 |
| Hamburg, Germany | 4 | 40 | 2,23 | 2,38 |
| * Daily amplitude of the tide. | ||||
Literature:
V. V. Shuleikin Physics of the sea. M., 1968
Harvey J. Atmosphere and ocean. M., 1982
Drake C., Imbrie J., Knaus J., Turekian K. The ocean itself and for us. M., 1982
© Vladimir Kalanov,
"Knowledge is power".
The phenomenon of tides at sea has been noticed since ancient times. Herodotus wrote about tides as early as the 5th century BC. For a long time people could not understand the nature of the tides. Various fantastic assumptions have been made, such as that the Earth breathes. Even the famous scientist (1571-1630), who discovered the laws of planetary motion, considered the ebb and flow as a result ... of the breathing of the planet Earth.
The French mathematician and philosopher (1596-1650) was the first among European scientists to point out the connection between tides and, but did not understand what this connection was. Therefore, he gave such a far from the truth explanation of the phenomenon of tide: the Moon, revolving around the Earth, presses on the water, forcing it to go down.
Gradually, scientists figured out this, I must say, difficult problem, and it was found that tides are a consequence of the influence of the gravitational forces of the Moon and (in lesser degree) The sun on the surface of the ocean.
In oceanology, the following definition is given: rhythmic rises and falls of waters, as well as their accompanying currents, are called ebb and flow.
High and low tides occur not only in the ocean, but also in the atmosphere and the earth's crust. Raising crust are very insignificant, so they can only be determined with special instruments. Another thing is the water surface. Particles of water move, and, receiving acceleration from the moon, approach it incomparably more than the earth's solid. Therefore, on the side facing the moon, the water rises upward, forming a bend, a kind of water bump on the surface of the ocean. As the Earth rotates on its axis, this water bump moves along the surface of the ocean following.
In theory, even distant stars are involved in the formation of tides. But this remains a purely theoretical premise, since the effect of the stars is negligible and can be neglected. More precisely, even it cannot be neglected, since there is nothing to neglect. The influence of the Sun on the surface of the ocean is 3-4 times weaker than the influence of the Moon due to the great distance of the star. Powerful lunar tides mask the Sun's gravitational pull, so there is no solar tide as such.
The extreme position of the water level at the end of the tide is called full of water, and at the end of low tide - low water.
Two photographs taken from the same point at moments of low and high water,
give an idea of tidal level fluctuations.
If we start observing the tide at the time of high water, then we will see that after 6 hours the lowest water level will come. After that, the tide will begin again, which will also continue for 6 hours until the highest level is reached. The next high tide will come 24 hours after the start of our observation.
But this will only happen in the case of ideal, theoretical conditions. In reality, during the day there is one full and one low water - and then the tide is called daily. Or it can happen in two tidal cycles. In this case, we are talking about a semi-daily tide.
The period of daily tide lasts not 24 hours, but 50 minutes longer. Accordingly, the semi-daily tide lasts 12 hours and 25 minutes.
The world's oceans are dominated by semi-diurnal tides. This is declared by the rotation of the Earth around its axis. The tide, like a huge gentle wave with a length of many hundreds of kilometers, spreads over the entire surface of the World Ocean. The period of occurrence of such a wave fluctuates in each place of the ocean from half a day to a day. On the basis of the frequency of the onset of tides, they are distinguished as daily and semi-daily.
During full turnover The Moon moves about 13 degrees around the Earth's axis. To "catch up" with the Moon, the tidal wave just takes 50 minutes. This means that the time of arrival of full water in the same place in the ocean is constantly shifting relative to the time of day. So, if today there was full water at noon, then tomorrow it will be at 12 hours 50 minutes, and the day after tomorrow - at 13 hours 40 minutes.
In the open ocean, where the tidal wave does not meet resistance from the continents, islands, uneven bottom and coastline, there are mostly regular semi-diurnal tides. Tidal waves in the open ocean are invisible, where their height does not exceed one meter.
In full force, the tide manifests itself on the open coast of the ocean, where for tens and hundreds of miles, no islands or sharp bends of the coastline are visible.
When the Sun and the Moon are located on the same line on one side of the Earth, the force of attraction of both luminaries adds up, as it were. This happens twice during a lunar month - on a new moon or a full moon. This position of the luminaries is called syzygy, and the tide that comes on these days is called. Syzygy tides are the highest and most powerful tides. In contrast, the lowest tides are called.
It should be noted that the level of syzygy tides in the same place is not always the same. The reason is the same: the movement of the Moon around - the Earth and the Earth - around the Sun. Let's not forget that the orbit of the Moon around the Earth is not a circle, but an ellipse, which creates a rather tangible difference between the perigee and apogee of the Moon - 42 thousand km. If during syzygy the Moon is at perigee, that is, at the smallest distance from the Earth, then this will cause a high tidal wave. Well, if during the same period the Earth, moving in its elliptical orbit around the Sun, is at the smallest distance from it (and also coincidences occasionally occur), then the ebb and flow will reach the maximum value.
Here are some examples showing the maximum height that ocean tides reach at selected locations the globe (in meters):
Name |
Location |
High tide (m) |
Mezen Bay of the White Sea | ||
Colorado River Mouth | ||
Penzhinskaya Bay of the Sea of Okhotsk | ||
Seoul River Mouth | South Korea | |
Mouth of the Fitzroy River | Australia | |
Grenville | ||
Mouth of the Coxoak River | ||
Port of Gallegas | Argentina | |
Bay of Fundy |
The water rises at high tide from different speed... The nature of the tide is highly dependent on the angle of inclination of the seabed. At steep banks, the water rises slowly at first - 8-10 millimeters per minute. Then the tide speed increases, becoming the highest towards the half-water position. Then it slows down to the position of the upper tide limit. The dynamics of the ebb tide is similar to the dynamics of the tide. But the tide looks quite different on wide beaches. Here the water level rises very quickly and is sometimes accompanied by a high tidal wave that rushes rapidly along the shallows. Swimming amateurs who gape on such beaches cannot expect anything good in these cases. The sea element does not know how to joke.
In the inland seas, fenced off from the rest of the ocean by narrow and shallow winding straits or clusters of small islands, tides come with barely noticeable amplitudes. We see this in the example of the Baltic Sea, which is reliably closed from the tides by shallow Danish straits. The theoretical height of the tide in the Baltic Sea is 10 centimeters. But these tides are invisible to the eye, they are hidden by fluctuations in the water level from the wind or from changes in atmospheric pressure.
It is known that in St. Petersburg there are frequent floods, sometimes very strong. Let us remember how brightly and truthfully the great Russian poet A.S. Pushkin. Fortunately, floods of such force in St. Petersburg have nothing to do with the tides. These floods are caused by the winds of cyclones, which significantly raise the water level by 4–5 meters in the eastern part of the Gulf of Finland and in the Neva.
The ocean tides are even less affected by the inland seas of the Black and Azov, as well as the Aegean and Mediterranean. In the Sea of Azov, connected to the Black Sea by the narrow Kerch Strait, the amplitude of the tides is close to zero. In the Black Sea, fluctuations in water level under the influence of tides do not even reach 10 centimeters.
Conversely, in bays and narrow bays that have free communication with the ocean, the tides reach significant levels. Freely entering the bay, the tidal masses rush forward, and, finding no way out among the narrowing shores, rise up and flood the land over a large area.
During ocean tides, a dangerous phenomenon is observed in the estuaries of some rivers, called boron... Flow sea water entering the riverbed and meeting with the river flow, it forms a powerful foamy wall, rising up as a wall and rapidly moving against the river flow. On its way, the forest erodes the banks and can destroy and sink any ship if it is in the channel of the river.
On the greatest river South America In the Amazon, a powerful tidal wave 5-6 meters high travels at a speed of 40-45 km / h for a distance of up to one and a half thousand kilometers from the mouth.
Sometimes tidal waves stop the flow of rivers and even turn it in the opposite direction.
On the territory of Russia, small-in-height boron is experienced by rivers flowing into the Mezen Bay of the White Sea.
In order to use the energy of tides in some countries, including Russia, tidal power plants have been built. The first tidal power plant, built in the Kislogubskaya Bay of the White Sea, had a capacity of only 800 kilowatts. Subsequently, TPPs were designed with a capacity of tens and hundreds of thousands of kilowatts. This means that hot flashes begin to work for the benefit of the person.
And last, but globally important, about tides. Tidal currents meet resistance from continents, islands and the seabed. Some scientists believe that as a result of the friction of water masses against these obstacles, the rotation of the Earth around its axis slows down. At first glance, this slowdown is quite insignificant. Calculations showed that over the entire time of our era, that is, over 2000 years, the day on Earth became 0.035 seconds longer. But what was the calculation based on?
It turns out that there is evidence, albeit indirect, that the rotation of our planet is slowing down. Studying the extinct corals of the Devonian period, the English scientist D. Wells found that the number of diurnal growth rings is 400 times more than the annual ones. In astronomy, the theory of stability of planetary movements is recognized, according to which the length of the year remains practically unchanged.
It turns out that in the Devonian period, that is, 380 million years ago, a year consisted of 400 days. Consequently, the day then had a duration of 21 hours 42 minutes.
If D. Wells was not mistaken when calculating the daily rings of ancient corals, and if the rest of the calculations are correct, then everything goes to the fact that some 12-13 billion years will not pass, as the earth's day will become equal in duration lunar month... And then what? Then our Earth will be constantly facing one side of the Moon, as is currently the case with the Moon in relation to the Earth. The rise in water will stabilize on one side of the Earth, the tides will cease to exist, and the solar tides are too weak to be felt.
We provide an opportunity for our readers to independently assess this rather exotic hypothesis.
© Vladimir Kalanov,
"Knowledge is power"
British photographer Michael Marten has created a series of original shots that capture the coast of Britain from the same angles, but in different time... One shot at high tide and the other at low tide.
It turned out very unusual, but positive reviews about the project, literally forced the author to start publishing the book. The book, titled "Sea Change", was released in August this year and was released in two languages. It took Michael Marten about eight years to create his impressive series of images. The time between high and low water is on average a little over six hours. Therefore, Michael has to linger at each location longer than just a few clicks of the shutter.
1. The idea of creating a series of such works was hatched by the author for a long time. He was looking for how to implement changes in nature on film, without human influence. And I found it by chance, in one of the seaside Scottish villages, where I spent the whole day and found the time of ebb and flow.
3. Periodic fluctuations in water level (rises and falls) in water areas on Earth are called ebb and flow.
The highest water level observed in a day or half a day during high tide is called full water, the lowest level at low tide is called low water, and the moment these limit levels are reached is the standing (or stage), respectively, of high tide or low tide. Average level sea - a conventional value, above which the level marks are located during high tides, and below - during low tides. This is the result of averaging large series of urgent observations.
Vertical fluctuations in the water level during ebb and flow are associated with horizontal movements of water masses in relation to the coast. These processes are complicated by wind surge, river runoff and other factors. Horizontal movements of water masses in the coastal zone are called tidal (or tidal) currents, while vertical fluctuations in the water level are called ebb and flow. All phenomena associated with ebb and flow are characterized by periodicity. Tidal currents periodically change direction to the opposite, in contrast to them oceanic currents, moving continuously and unidirectionally, are caused by the general circulation of the atmosphere and cover large areas of the open ocean.
4. The ebb and flow of the tide cyclically alternate in accordance with changing astronomical, hydrological and meteorological conditions. The sequence of the ebb and flow phases is determined by two highs and two lows in the diurnal cycle.
5. Although the Sun plays an essential role in tidal processes, the decisive factor in their development is the force of the Moon's gravitational attraction. The degree of influence of tidal forces on each particle of water, regardless of its location on the earth's surface, is determined by the law universal gravitation Newton.
This law states that two material particles are attracted to each other with a force directly proportional to the product of the masses of both particles and inversely proportional to the square of the distance between them. This implies that the more the mass of the bodies, the greater the force of mutual attraction arising between them (at the same density, a smaller body will create a smaller attraction than a larger one).
6. The law also means that the greater the distance between two bodies, the less attraction between them. Since this force is inversely proportional to the square of the distance between two bodies, the distance factor plays a much greater role in determining the magnitude of the tidal force than the masses of the bodies.
The gravitational attraction of the Earth, acting on the Moon and keeping it in near-Earth orbit, is opposite to the Earth's gravity by the Moon, which seeks to displace the Earth towards the Moon and "lifts" all objects on Earth in the direction of the Moon.
The point on the earth's surface located directly under the Moon is only 6400 km away from the center of the Earth and, on average, 386 063 km from the center of the Moon. In addition, the mass of the Earth is 81.3 times that of the Moon. Thus, at this point on the earth's surface, the Earth's gravity acting on any object is approximately 300 thousand times greater than the Moon's.
7. It is widely believed that water on Earth, located directly under the Moon, rises in the direction of the Moon, which leads to an outflow of water from other places on the earth's surface, however, since the attraction of the Moon is so small compared to that of the Earth, it would not be enough to lift such a huge weight.
Still oceans, seas and large lakes on earth, being large liquid bodies are free to move under the force of lateral displacement, and any slight tendency to lateral displacement sets them in motion. All waters that are not directly under the Moon are subject to the action of the component of the force of attraction of the Moon, directed tangentially (tangentially) to the earth's surface, as well as its component directed outward, and are subject to horizontal displacement relative to the solid earth's crust.
As a result, there is a flow of water from the adjacent areas of the earth's surface towards a place under the moon. The resulting accumulation of water at a point under the Moon creates a tide there. The actual tidal wave in the open ocean has a height of only 30–60 cm, but it increases significantly as it approaches the shores of continents or islands.
Due to the movement of water from neighboring regions towards a point under the Moon, the corresponding ebb tides of water occur at two other points located at a distance equal to a quarter of the Earth's circumference. It is interesting to note that a drop in sea level at these two points is accompanied by a rise in sea level not only on the side of the Earth facing the Moon, but also on the opposite side.
8. This fact is also explained by Newton's law. Two or more objects located on different distances from the same source of gravity and, therefore, subject to acceleration of gravity of different magnitude, move relative to each other, since the object closest to the center of gravity is most strongly attracted to it.
Water at the sublunary point experiences a stronger attraction to the Moon than the Earth below it, but the Earth, in turn, is more attracted to the Moon than water on the opposite side of the planet. Thus, a tidal wave arises, which is called forward on the side of the Earth facing the Moon, and backward on the opposite side. The first of them is only 5% higher than the second.
9. Due to the rotation of the Moon in its orbit around the Earth, between two successive high tides or two low tides in a given location, approximately 12 hours and 25 minutes pass. The interval between the culminations of successive ebb and flow is approx. 6 hours 12 minutes The period of 24 hours 50 minutes between two successive tides is called tidal (or lunar) days.
10. Inequalities in the magnitude of the tide. Tidal processes are very complex, so there are many factors that need to be taken into account to understand them. In any case, the main features will be determined:
1) the stage of development of the tide relative to the passage of the moon;
2) the amplitude of the tide and
3) the type of tidal fluctuations, or the shape of the curve of the course of the water level.
Numerous variations in the direction and magnitude of tidal forces create a difference in the magnitude of morning and evening tides in a given port, as well as between the same tides in different ports. These differences are called tide inequalities.
Semi-daily effect. Usually, during the day, due to the main tidal force - the rotation of the Earth around its axis - two complete tidal cycles are formed.
11. Seen from the side North Pole ecliptic, it is obvious that the Moon rotates around the Earth in the same direction in which the Earth rotates around its axis - counterclockwise. With each subsequent revolution, this point on the earth's surface again takes a position directly under the Moon a little later than during the previous revolution. For this reason, the ebb and flow of the ebb and flow every day are delayed by about 50 minutes. This value is called lunar lag.
12. Half-month inequality. This main type of variation is characterized by a periodicity of about 143/4 days, which is associated with the rotation of the Moon around the Earth and its passage through successive phases, in particular syzygies (new moons and full moons), i.e. moments when the Sun, Earth and Moon are located on one straight line.
So far, we have only dealt with the tidal effect of the moon. The sun's gravitational field also acts on the tides, however, although the mass of the sun is much greater than the mass of the moon, the distance from the earth to the sun is so much greater than the distance to the moon that the tidal force of the sun is less than half the tidal force of the moon.
13. However, when the Sun and the Moon are on the same straight line, both on the same side of the Earth, and on different sides (in the new moon or full moon), the forces of their attraction add up, acting along one axis, and the solar tide is superimposed on the lunar.
14. Likewise, the attraction of the sun intensifies the ebb tide caused by the influence of the moon. As a result, the tides become higher and the ebb tides lower than if they were caused only by the attraction of the moon. Such tides are called syzygy.
15. When the gravitational vectors of the Sun and the Moon are mutually perpendicular (during quadratures, ie, when the Moon is in the first or last quarter), their tidal forces oppose, since the tide caused by the attraction of the Sun is superimposed on the ebb caused by the Moon.
16. In such conditions, the tides are not so high, and the ebb tides are not so low, as if they were due only to the force of gravity of the moon. Such intermediate ebb and flow are called quadrature.
17. The range of elevations of high and low waters in this case is reduced by approximately three times in comparison with the syzygy tide.
18. Lunar parallax inequality. The period of fluctuations in the heights of the tides, arising from the lunar parallax, is 271/2 days. The reason for this inequality is the change in the distance of the Moon from the Earth during the rotation of the latter. Due to the elliptical shape of the lunar orbit, the tidal force of the moon at perigee is 40% higher than at apogee.
Daily inequality. The period of this inequality is 24 hours 50 minutes. The reasons for its occurrence are the rotation of the Earth around its axis and the change in the declination of the Moon. When the moon is near celestial equator, two high tides on a given day (as well as two low tides) differ slightly, and the heights of the morning and evening full and low waters are very close. However, as the Moon's north or south declination increases, morning and evening tides of the same type differ in height, and when the Moon reaches its highest north or south declination, this difference is greatest.
19. Tropical tides are also known, so called because the moon is located almost over the Northern or Southern tropics.
Daily inequality does not significantly affect the heights of two successive low tides in Atlantic Ocean, and even its effect on tide heights is small compared to the overall amplitude of fluctuations. However, in Pacific the diurnal unevenness is manifested in the ebb levels three times stronger than in the tide levels.
Semi-annual inequality. It is caused by the revolution of the Earth around the Sun and a corresponding change in the declination of the Sun. Twice a year for several days during the equinoxes, the Sun is near the celestial equator, i.e. its declination is close to 0. The moon is also located near the celestial equator for about a day every half a month. Thus, during the equinoxes, there are periods when the declination of both the Sun and the Moon is approximately 0. The total tidal effect of the attraction of these two bodies at such moments is most noticeably manifested in regions located near the Earth's equator. If at the same time the Moon is in the phase of the new moon or full moon, the so-called. equinox syzygy tides.
20. Solar parallax inequality. The period for this inequality is one year. It is caused by the change in the distance from the Earth to the Sun during the Earth's orbital motion. Once for each revolution around the Earth, the Moon is at the shortest distance from it at perigee. Once a year, around January 2, the Earth, moving in its orbit, also reaches the point of closest approach to the Sun (perihelion). When these two moments of closest approach coincide, causing the greatest total tidal force, more high levels tides and more low levels ebb tides. Similarly, if the passage of the aphelion coincides with the apogee, less high tides and shallow tides.
21. The largest amplitudes of tides. The world's highest tide is generated by strong currents in Minas Bay in the Bay of Fundy. Tidal fluctuations here are characterized by a normal course with a semidiurnal period. The water level during high tide often rises by more than 12 m in six hours, and then decreases by the same amount over the next six hours. When the effect of syzygy tide, the position of the Moon at perigee and the maximum declination of the Moon fall on one day, the tide level can reach 15 m. the top of the bay. The reasons for the tides, being subject constant study for many centuries, are among the problems that have given rise to many conflicting theories even in relatively recent times
22. Charles Darwin wrote in 1911: “There is no need to search antique literature for the sake of grotesque theories of tides ”. However, sailors manage to measure their height and use the possibilities of tides without knowing the real reasons for their occurrence.
I think that we also need not bother especially about the causes of the origin of tides. Based on long-term observations, special tables are calculated for any point in the water area of the earth, which indicate the time of high and low water for each day. I am planning my trip to, for example, Egypt, which is just famous for its not deep lagoons, but try to guess in advance so that full water falls in the first half of the day, which will allow most daylight hours to fully ride.
Another tide-related issue of interest to the kiter is the relationship between wind and water level fluctuations.
23. Folk omen asserts that the wind increases at high tide, and on the contrary, it turns sour at low tide.
The effect of wind on tidal phenomena is more understandable. The wind from the sea drives the water towards the coast, the tide height increases above normal, and at low tide the water level also exceeds the average. On the contrary, when the wind blows from the land, the water is driven away from the coast, and the sea level drops.
24. The second mechanism acts by increasing atmospheric pressure over a vast area, a decrease in water level occurs, as the superimposed weight of the atmosphere is added. When atmospheric pressure rises by 25 mm Hg. Art., the water level drops by about 33 cm. Zone high pressure or anticyclone is usually called good weather, but not for a kiter. Calm in the center of the anticyclone. A decrease in atmospheric pressure causes a corresponding increase in the water level. Consequently, a sharp drop in atmospheric pressure, combined with hurricane force winds, can cause a noticeable rise in the water level. Such waves, although they are called tidal waves, are not actually associated with the influence of tidal forces and do not have the periodicity characteristic of tidal phenomena.
But it is quite possible that low tides can also affect the wind, for example, a decrease in the water level in coastal lagoons, leads to greater heating of the water, and as a result, to a decrease in the temperature difference between the cold sea and the heated land, which weakens the breeze effect.
