Where can the tide come from?

The periodic movements and changes of the earth's lithosphere, hydrosphere and atmosphere are caused by the tidal force of the sun and the moon. 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.