Tide is the fluctuation phenomenon of the ocean surface on the earth caused by the tidal force action of the sun and the moon. It refers to the periodic movement of seawater under the tidal force of celestial bodies (mainly the moon and the sun). In ancient times, the water gushing from rivers and seas during the day was called "tide" and "sunseeker" at night, which were collectively called "tide". Traditionally, the vertical fluctuation of sea surface is called tide, and the horizontal flow of sea water is called tidal current.

Tides are caused by the tidal force of the sun and the moon, which makes the sea water on the earth fluctuate periodically. It can not only generate electricity, fish, make salt, develop shipping and marine biological culture, but also have an important impact on many military operations. In history, there are many successful cases of using tidal laws to win.

The seawater fluctuates periodically: at a certain moment, the seawater adds fuel to the flames, rises rapidly and reaches a climax; After a period of time, the rising sea water receded on its own, leaving a beach and low tide. This cycle will never stop. This movement phenomenon of seawater is tide.

Tide is the periodic fluctuation of water level, crust and atmosphere caused by the different attraction of the moon and the sun to various parts of the earth. The periodic fluctuation of ocean water surface is called tide, the corresponding phenomenon in the crust is called land tide (also called solid tide), and the atmosphere is called gas tide. Among the above three tides, the ocean tide is the most obvious. /kloc-in the 7th century, Newton scientifically explained that tides are caused by the attraction of the moon and the sun to seawater with the law of universal gravitation. As for the land tide and the air tide, they are quite small and must be measured with precision instruments.