How are tides formed?

Tide

A general term for the periodic movements and changes in the earth's lithosphere, hydrosphere and atmosphere caused by the tidal forces of the sun and moon. The elastic-plastic deformation of the solid earth caused by the tidal force of the sun and the moon is called solid tide, referred to as the solid tide or ground tide; the periodic rise and fall, rise and fall, advance and retreat of the sea surface caused by the tidal force of the sun and the moon by sea water, It is called ocean tide, or ocean tide for short; the periodic changes (such as 8, 12, 24 hours) caused by the tidal force of various elements of the atmosphere (such as air pressure field, atmospheric wind field, earth magnetic field, etc.) are called atmospheric tide, or air tide for short. . Among them, the atmospheric tides caused by the sun are called solar tides, and those caused by the moon are called lunar tides. Because the moon is closer to the earth than the sun, the ratio of the tidal forces of the moon and the sun is 11:5. For the ocean, the lunar tide is more significant than the solar tide. The driving forces of earth tides, sea tides and air tides are all caused by the different gravitational forces of the sun and the moon on various parts of the earth, and the three influence each other. The elastic-plastic tidal deformation of the crust at the bottom of the ocean will cause corresponding ocean tides, that is, there is an impact of the ground tide effect on ocean tides; and the migration of seawater mass caused by ocean tides changes the load on the earth's crust, causing the earth's crust to undergo sustainable changes. Complex changes. The air tide is above the ocean tide, and it acts on the sea surface to cause additional vibrations, making the changes in the ocean tide more complicated. As a complete tidal science, its research objects should be ground tides, ocean tides and air tides as a unified whole. However, because the phenomenon of ocean tides is very obvious and closely related to people's lives, economic activities, transportation, etc., it is customary to treat tides as The word (tide) is understood in a narrow sense as ocean tide.

Tidal energy is ocean energy in the form of potential energy, which refers to the potential energy of water formed by the rise and fall of seawater. The tidal phenomenon of rising and falling sea water is caused by the movement of the earth and celestial bodies and the interaction between them. In the oceans, the Moon's gravity raises water levels on the lunar and farside sides of the Earth. Due to the rotation of the Earth, this rise in water level propagates from east to west in the form of deep-sea waves with a period of 12 hours and 25 minutes and an amplitude of less than 1m. The sun's gravity acts similarly, but with a smaller force and a period of 12 hours. When the sun, moon and earth are in a straight line, spring tides occur; when they are at right angles, neap tides occur. In addition to changes in semidiurnal and monthly tides, the rotational motion of the Earth and Moon also produces many other periodic cycles, whose periods can range from days to years. At the same time, the sea water on the surface is affected by the centrifugal force of the earth's motion. The combined force of the moon's gravity and centrifugal force is the tidal force that causes the sea water to rise and fall. In addition to the moon and the sun, other celestial bodies also exert tidal forces on the earth. Although the mass of the sun is much greater than that of the moon, the distance between the sun and the earth is also much greater than the distance between the moon and the earth, so its tidal force is less than half of the moon's tidal force. The tidal force generated by other celestial bodies is negligible either because they are far away from the earth or because their mass is too small. According to the equilibrium tide theory, if the earth is completely covered by water of constant depth, calculated using universal gravity, the maximum tidal force generated by the moon can raise the sea level by 0.563m, and the effect of the tidal force of the sun is O. 246m. The observed tidal range in oceans such as Hawaii is about 1m, which is relatively close to the equilibrium tide theory. However, the actual tidal range in offshore waters is much larger than the above calculated value. For example, the maximum tidal range in Hangzhou Bay in my country reaches 8.93m, and the maximum tidal range in the Bay of Fundy in Canada, North America, reaches 19.6m. There is currently no clear explanation for this actual versus calculated difference. It is generally believed that when ocean tidal waves impact the continental shelf and coastline, they increase the tidal range through movements such as rising, converging, and oscillation. The energy of tidal energy is directly proportional to the tidal volume and tidal range. In other words, it is proportional to the square of the tidal range and the area of ??the reservoir. Compared with hydropower, the energy density of tidal energy is very low, equivalent to the level of micro-hydropower generation. The maximum value of tidal range in the world is about 13~15m, but generally speaking, an average tidal range of more than 3m has practical application value.

Tides vary from place to place, and different areas often have different tidal systems. They all obtain energy from deep-sea tidal waves, but have their own unique characteristics. Despite their complexity, tides can be accurately predicted anywhere. Ocean tides gain energy from the Earth's rotation and slow the Earth's rotation in the process of absorbing the energy. However, this slowdown in the earth's rotation is almost imperceptible in a person's lifetime, and it will not be accelerated by the development and utilization of tidal energy. This energy is dissipated at a rate of 1.7TW through friction between shallow sea areas and coastal areas. Extracting energy from tides is only possible when there are large tides and the energy is concentrated, and in places where the geographical conditions are suitable for building tidal power stations. Although such sites are not available everywhere, countries around the world have selected a considerable number of sites suitable for developing tidal energy. According to the latest estimates, the tidal energy with development potential is about 200TW·h per year.

The theoretical reserve of tidal energy in the world is about 3 × 109kw. my country's coastline is tortuous, with a total length of about 1.8×104km. There are more than 6,000 large and small islands along the coast, forming a 1.4×104km coastline. The long coast contains very rich tidal energy resources. my country's theoretical tidal energy reserves amount to 1.1×108kw, of which Zhejiang and Fujian provinces have the largest reserves, accounting for approximately 80.9% of the country's total. However, these are theoretical estimates, and the actual available tidal energy reserves are far less than the above figures.

Tide is a natural phenomenon in coastal areas. In ancient times, the tide during the day was called "tide", and the tide at night was called "tide". Together, it is called "tide". Its occurrence is related to the sun and the moon. It has something to do with it, and it also corresponds to our country’s traditional lunar calendar. On the first day of each month in the lunar calendar, the sun and moon are on one side of the earth, so they have the greatest tidal force, causing "spring tides". Around the fifteenth or sixteenth day of each month in the lunar calendar, the sun and the moon are on both sides of the earth, the tidal force of the sun and the moon will also cause "spring tides" when you push and pull them; when the moon phase is in the first quarter and last quarter, that is, on the eighth and twenty-third o'clock of the lunar calendar, the tidal force of the sun and the moon will The tidal forces partially offset each other, so a "neap tide" occurs. Therefore, there is a saying in the farmer's proverb that "the tide rises on the 15th day of the lunar month, and there are beaches everywhere on the 8th and 23rd day of the lunar month." In addition, high tides also occur on the first day. Since the moon moves more than 13 degrees eastward on the celestial sphere every day, the total time is about 50 minutes. That is, the moon's zenith time (1 lunar day = 24:50) is delayed by about 50 minutes every day. (Tides also occur in the middle of the day, and there are usually two tides every day.) Therefore, the time of high tide is also delayed by about 50 minutes every day.

However, due to the complexity of the movement of the moon and the sun, the spring tide may sometimes be delayed by a day or several days, and the high tide during a lunar day is often one hour or more behind the moon's superior or inferior celestial phase. For several hours, a tide occurs in some places on a lunar day.

The gravity of the sun and the moon has such a great effect on the water (liquid) on the earth, and it also affects the solid continents of the earth's crust, causing "land tides". "Land tides" may trigger earthquakes, so The moon phase should be taken into consideration when making earthquake predictions;

The gravitational pull of the sun and the moon will also have a great effect on the atmosphere (gas) on the earth, causing "atmospheric tides", causing atmospheric convection and atmospheric movement. changes will cause climate changes. (This is contrary to the traditional view that climate change has nothing to do with the moon.) Therefore, meteorological experts recommend that the moon phase should be considered when making weather forecasts.

According to modern science, it has been discovered that the gravity of the sun and the moon may also have an effect on the human body or liquids in organisms, forming mysterious "biological tides" and "human body tides". Some Japanese scientists are working on this issue Doing research. There is a proverb in ancient my country, "If you escape the first day of the lunar month, you cannot escape the fifteenth day." It is also a vivid portrayal of this mysterious biological tide and human body tide that may cause aggravation of illness or mental changes in people or other creatures.

The working people of our country have summarized their experience over thousands of years and come up with many tide calculation methods (tide timing), such as the eight-point tide calculation method, which is one example: the concise formula is:

High tide = 0.8h × [Lunar calendar date - 1 (or 16)] + high tide gap

The above formula can be used to calculate a high tide in a day. For regular semi-diurnal tidal areas, add or subtract 12 to the value Another climax time can be obtained by adding or subtracting 12 hours and 24 minutes (or adding or subtracting 12 hours and 24 minutes for the convenience of calculation). If you add or subtract 6 hours and 12 minutes from the value, you can get the time when the low tide occurs - the time of the low tide.

1. At any moment, there are two protruding parts of the sea level surrounding the Earth. Under ideal circumstances, they appear at the closest and furthest parts of the Earth's surface to the Moon. If tides are caused by the moon's gravity, why do ocean waters bulge far away from the moon?

2. If tides are a gravitational phenomenon, it seems that they should be directly proportional to mass and inversely proportional to the square of distance. The mass of the sun is 2.7×105 times greater than that of the moon, and the square of the distance from the sun to the earth is only 1.5×105 times greater than the square of the distance from the moon to the earth. Dividing the two, it seems that the gravitational pull of the sun on seawater should be 180 times greater than that of the moon. , why does the moon play a major role in tides?

The first question: Different points on the earth receive different gravitational pulls from the moon. The places facing the moon have the greatest gravitational pull, so they are convex; while the places facing away from the moon have the smallest gravitational pull, that is, The binding force here is relatively weak, and of course it will bulge in the other direction. In other words, the position of the earth's center remains unchanged and is convex in the direction of the moon. It must also be convex in the opposite direction. Otherwise, the position of the earth's center of mass will be like the moon moving - is that okay?

Second question: On the earth, the gravitational pull of the sun is much greater than the gravitational pull of the moon. This is correct. However, the tidal phenomenon is caused by the difference in the gravitational force of celestial bodies on different locations on the earth. Although the gravitational force of the sun is large, because it is very far away, the difference in gravitational force on different locations on the earth is almost zero, so its impact on the earth's tides But very small.