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Tides are caused by the attraction of the moon.

Tides are periodic fluctuations of seawater. Because the day is morning and the night is evening, the fluctuation of sea water during the day is called "tide" and the fluctuation of sea water at night is called "sunseeker". This phenomenon once puzzled the ancients and I don't know what caused it. Later, careful people found that the tide is delayed for a period of time every day, which is the same as the time when the moon is late every day, so the tide is inevitably related to the moon. The relationship between tides and the moon is mentioned in China's ancient geography book Shan Hai Jing. Wang Chong's Lun Heng in the Eastern Han Dynasty clearly pointed out that "Tao rises and falls with the moon". But it was not until Newton discovered the law of universal gravitation that Laplace mathematically proved that tidal phenomena were indeed caused by the gravity of the sun and the moon, mainly the moon.

The law of universal gravitation shows that gravity is directly proportional to the product of the mass of two objects and inversely proportional to the square of the distance between them. The attraction of the sun to the earth is much stronger than that of the moon, but the tidal force of the sun is less than that of the moon 1/2. What's going on here? Although the original tidal force (or tidal force) that causes seawater fluctuation is caused by the gravity of the sun and the moon, it is not the absolute gravity of the sun and the moon, but the difference between the gravity of the attracted object and the gravity. The tidal force is directly proportional to the mass of tidal celestial bodies and inversely proportional to the cube of the distance from celestial bodies to the earth. Because the mass of the sun is 27 10X 104 times that of the moon, and the average distance between the sun and the earth is 389 times that of the moon, the tidal force of the moon is 2. 17 times that of the sun, which proves that tides are really mainly caused by the moon. For example, the highest tidal level in a certain place 10 meter high, almost 7 meters is caused by the moon, the contribution of the sun is only 3 meters, and other planets are less than 0.6 mm.

Although the tidal force of the sun is not too great, it can affect the size of the tide. Sometimes they work together with the moon to complement each other, and sometimes they repel and cancel each other out. At the new moon or full moon, the sun and the moon exert gravity in the same direction or in the opposite direction, resulting in climax; But in the first quarter or the second quarter, the gravity of the moon is opposite to that of the sun, leading to low tide. Its cycle is about half a month. From a year's perspective, there are also two high tides and two low tides. At the vernal equinox and autumnal equinox, if the earth, the moon and the sun are almost on the same plane, the tidal force is greater than that in other months, resulting in two climaxes in spring and autumn. In addition, tides are also related to the distance between the moon and the sun from the earth. The moon's orbit is elliptical, approaching and leaving the earth once every 27.55 days. Near-earth tides are 39% larger than far-earth tides. When the near-earth tide coincides with the high tide, the tidal range is particularly large. If the distant tide and low tide coincide, the tidal range is particularly small. The earth's orbit around the sun is also elliptical. At perihelion, the sun has strong gravity and strong tide, and at perihelion, the gravity is small and the tide is weak.

From a certain day on, due to the rotation of the earth and the revolution of the moon, tidal waves spread from east to west along the direction of Sunday movement. The time of a tidal fluctuation is half a lunar day, that is, 12 hours and 25 minutes. This is the so-called semidiurnal tide. People who live by the sea can see the regular fluctuation of the sea twice a day. Bai Juyi's famous saying that "the drought and tide only come at night, and it flows ten times a Saturday in January" comes from this.

The actual tide will also be restricted by many factors such as geographical environment, coastal location, ocean current movement and so on. Taking the tide of Qiantang River as an example, we know that Hangzhou Bay at the mouth of Qiantang River is flared and narrower and narrower. In addition, the sediment brought in during the high tide is deposited at the bottom of the river to form sandbars, which leads to the surge of the tide.

The tidal force of the moon will not only produce tides on the earth, but also cause atmospheric tides. But the atmospheric tide is far less earth-shattering and magnificent than the ocean tide. And because we are in it, it is difficult to detect. In addition, tidal force will make the earth's body, including the earth's surface (the part below the mainland and the seabed) produce tides, which are called solid tides. The earth's surface fluctuation caused by solid tide is very small, which can only be measured by precision instruments, and may have a slight impact on the earth's gravity field. Part of the earth is liquid, so there will be tides. Some people think that tides on the earth are one of the causes of earthquakes.

Actions are always relative. Where there is action, there is reaction. The moon has tidal force to the earth, and conversely, the earth has tidal force to the moon. It stands to reason that the mass of the earth is more than 80 times larger than that of the moon, and the tidal force of the earth to the moon should be more than 20 times that of the moon to the earth. However, because there is no water on the moon, the tidal force of the earth can't "make waves" on the surface of the moon, but it has a braking effect on the moon's rotation, making the moon a synchronous rotating satellite, so the moon always faces us. The moon also slows down the rotation of the earth through the same tidal friction, making the daily time longer and the distance between the earth and the moon longer.

Tide is a natural wonder, not only an important tourism resource, but also has an important impact on navigation, fisheries, salt industry and so on. At the same time, tides can also be used to generate electricity.

The principle of tidal power generation is similar to hydropower generation, that is, the potential energy of water level difference caused by tidal fluctuation is converted into mechanical energy, and then the mechanical energy is converted into electrical energy. According to calculation, the world's ocean tidal energy reserves are about 2.7 billion kilowatts. If all of them are converted into electric energy, the annual power generation is about 1.2 trillion kwh. Tidal energy is not only pollution-free, but also has its advantages compared with renewable energy such as wave energy, wind energy and solar energy. Tidal energy can generate electricity continuously, while wave energy, wind energy and solar energy are greatly influenced by climate. Therefore, how to develop and utilize the huge energy of tides has become a research topic in many countries. According to media reports, the first commercial underwater tidal power station in Norway was connected to the grid in 2003, and it is estimated that 654.38 million people will use this new energy source in five years.

References:

/magazine/tw ahz/2005/04 _ 5- 17 . htm