Why do the tides rise and fall?

Due to the tidal force of the sun and the moon, the lithosphere, hydrosphere and atmosphere of the earth have periodic movements and changes respectively. The elastic-plastic deformation of the solid earth caused by the tidal force of the sun and the moon is called solid tide, which is referred to as solid tide or ground tide for short. The periodic rise and fall, fluctuation and advance and retreat of the sea surface caused by the tidal force of the sun and the moon are called ocean tides, referred to as ocean tides for short; Periodic changes of atmospheric elements (such as pressure field, atmospheric wind field, earth's magnetic field, etc.) (such as 8 12, 24 hours). ) caused by the tide is called the atmospheric tides, referred to as the tide. Among them, the atmospheric tide caused by the sun is called the solar tide, and the lunar tide is called the lunar tide. Because the moon is closer to the earth than the sun, the ratio of tidal force between the moon and the sun is 1 1: 5. For the ocean, the moon tide is more important than the sun tide. The driving forces of earth tides, ocean tides and atmospheric tides are all caused by the different gravitational forces of the sun and the moon on various parts of the earth, and they influence each other. The elastic-plastic tidal deformation of the ocean bottom crust will cause corresponding tides, that is, tides have tidal effects; The migration of seawater quality caused by tidal waves changes the load borne by the crust and makes it bend again. The air tide is above the sea tide, which acts on the sea surface and causes additional vibration, making the change of the sea tide more complicated. As a complete tidal science, its research object should be solid tide, ocean tide and atmospheric tide as a unified whole. However, because the tide phenomenon is very obvious and closely related to people's life, economic activities, transportation and so on, it is customary to interpret the word tide as ocean tide in a narrow sense.

Tidal energy is ocean energy in the form of potential energy, which refers to the potential energy of water formed by seawater fluctuation. The tidal phenomenon of seawater fluctuation is caused by the movement and interaction between the earth and celestial bodies. In the ocean, the gravity of the moon raises the water level of the earth facing the moon and the surface facing the moon. Due to the earth's rotation, this water level rise propagates from east to west in the form of deep-sea waves, with a period of 12 hours and 25 minutes and an amplitude less than1m. The effect of solar gravity is similar, but the force is smaller, and its period is 12 hours. When the sun, the moon and the earth are in a straight line, spring tide); Happen; When they are at right angles, there will be a small tide. In addition to the semidiurnal tide and the monthly periodic tide, the rotation of the earth and the moon also produces many other periodic cycles, which range from a few days to several years. At the same time, the seawater on the surface is affected by the centrifugal force of the earth's motion, and the resultant force of the lunar gravity and centrifugal force is the tidal force that causes the seawater to fluctuate. Besides the moon and the sun, other celestial bodies will also generate tidal force on the earth. Although the mass of the sun is much larger than that of the moon, the distance between the sun and the earth is also much larger than that between the moon and the earth, so its tidal force is less than half that of the moon. Because other celestial bodies are far away from the earth or their mass is too small, the tidal force can be ignored. According to the theory of balanced tides, if the earth is completely covered by seawater with equal depth, the maximum tidal force generated by the moon can raise the sea surface by 0.563 m, and the tidal force generated by the sun is 0.246 m. The tidal range observed in Hawaii and other oceans is about 1m, which is close to the theory of balanced tides, but the actual tidal range near the sea is much larger than the above calculated value. For example, the maximum tidal range of Hangzhou Bay in China is 8.93 m, and the maximum tidal range of Fendi Bay in North America and Canada is19.6m. At present, there is no exact explanation for this difference between reality and calculation. It is generally believed that when ocean tidal waves impact the continental shelf and coastline, the tidal range will increase through the movement of rising, gathering and vibration. The energy of tidal energy is directly proportional to tidal volume and tidal range. In other words, it is proportional to the square of tidal range and the area of reservoir. Compared with hydropower, the energy density of tidal energy is very low, which is equivalent to the level of micro-head power generation. The large tidal range in the world is about 13 ~ 15m, but generally speaking, the average tidal range above 3m has practical application value.

Tides vary from place to place, and different regions often have different tidal systems. They all get energy from deep-sea tidal waves, but they have their own characteristics. Although the tides are complex, they can be accurately predicted anywhere. Ocean tides get energy from the earth's rotation and slow it down in the process of absorbing energy. However, this deceleration of the earth's rotation is almost imperceptible in a person's life and will not be accelerated by the development and utilization of tidal energy. This energy is dissipated at the rate of 1.7TW through the friction between the shallow sea area and the coastal area. Only where there is a spring tide, energy concentration and geographical conditions are suitable for building tidal power stations can energy be extracted from the tides. Although such places are not everywhere, countries all over the world have chosen a considerable number of sites suitable for developing tidal energy. According to the latest estimate, the tidal energy with development potential is about 200 MWh per year.

The theoretical reserve of tidal energy in the world is about 3 × 109kw. China's coastline is tortuous, with a total length of about 1.8× 104km, and there are more than 6,000 large and small islands along the coast, forming a coastline of 1.4× 104km. The long coastline contains abundant tidal energy resources. The theoretical reserves of tidal energy in China are 1. 1× 108kw, of which Zhejiang and Fujian provinces have the largest reserves, accounting for about 80.9% of the whole country, but these are theoretical estimates, and the actual available ones are far less than the above figures.

Tide is a natural phenomenon in coastal areas. In ancient times, it was called "Tide" during the day and "Sunseeker" at night, both of which were called "Tide". Its occurrence is related to the sun and the moon, and also corresponds to the traditional lunar calendar in China. On the first day of the first lunar month, the sun and the moon are on one side of the earth, so tidal force is the largest, which will cause a "spring tide". Near the 15th or 16th day of the first lunar month, the sun and the moon are on both sides of the earth, and the tidal force of the sun and the moon will also cause "spring tide". When the phases of the current month are the first quarter moon and the second quarter moon, that is, August and 23rd of the lunar calendar, the tidal force of the sun and the tidal force of the moon cancel each other out, so a "small tide" occurs. Therefore, there is a saying in agricultural proverbs that "the spring tide begins on the fifteenth day of the first lunar month, and beaches can be seen everywhere on the 23rd of the eighth lunar month". In addition, there was a climax on the first day. Because the moon moves eastward more than 13 degrees on the celestial sphere every day, which is about 50 minutes in total, that is, the transit time of the moon (1 lunar day =24: 50) is delayed by about 50 minutes every day, (there will be tides in the next transit, usually twice a day), so the time of high tide is also delayed by about 50 minutes every day.

However, due to the complexity of the movements of the moon and the sun, the spring tide may sometimes be delayed by one or several days, and the climax during lunar day is often one hour or several hours later than the transit time on the moon. In some places, the tide will rise as soon as the lunar calendar is over.

The gravity of the sun and the moon plays such a great role in the water (liquid) on the earth, and also in the solid continent of the earth's crust, so "land tide" may lead to earthquakes, so the moon phase should be considered in earthquake prediction;

The gravity of the sun and the moon will also have a great effect on the atmosphere (gas) on the earth, and a "atmospheric tides" will occur, which will cause changes in atmospheric convection and atmospheric movement, as well as climate change. This contradicts the traditional view that climate change has nothing to do with the moon. Therefore, meteorologists suggest that the moon phase should be considered when making weather forecast.

According to modern science, the attraction of the sun and the moon may also act on the liquid in human body or organism, forming mysterious "biological tide" and "human tide". Some Japanese scientists are studying this problem. There is an ancient saying in China that "you can't escape from the first day, but you can't escape from the fifteenth day", which is also a vivid portrayal of the mysterious biological tide and human tide that may lead to the aggravation of human or other biological diseases or mental changes.

Working people in China have summed up thousands of years' experience and put forward many tidal calculation methods (tidal pushing time), such as the eight-point tidal calculation method, for example, the concise formula is:

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

When the above formula can be regarded as one-day high tide, for the normal semidiurnal tide sea area, add or subtract 12: 25 (or add or subtract 12: 24 for calculation convenience) to get another high tide time. If you add or subtract 6: 00 and 12 points, you can get the time of low tide-low tide.