The sea water has the phenomenon of flood and ebb. When it floods, the sea water rises and the waves roll, and it is very spectacular; When it ebbs, the sea quietly recedes, revealing a beach. High tide and low tide usually occur twice a day. There is a period between high tide and low tide when the water level does not rise or fall, which is called flat tide. Tides are caused by the attraction of the moon. The moon and the sun effect the gravity on the earth, that is, tidal force.

Sea water is also rotating with the rotation of the earth, and rotating objects are affected by centrifugal force, which makes them leave the center of rotation. At the same time, the sea water is also attracted by the moon, the sun and other celestial bodies. Because the moon is closest to the earth, the moon is more attractive. In this way, the sea water forms the tide generating force under the joint action of these two forces.

The tidal force of the moon does not only produce ocean tides on the earth, but it also causes atmospheric tides. The atmospheric tide is far less magnificent than the sea tide and because we're in it, it's hard to detect. In addition, the tidal force will also make the earth's body, including the earth's surface (the mainland and the parts below the ocean floor), produce tides, which are called solid tides. The fluctuation of the earth's surface caused by solid tide is very small, which can only be measured with precision instruments, which may have a slight impact on the earth's gravitational field. Part of the earth's interior is liquid, so tides will also occur there. Some people believe that tides in the earth's interior are one of the things that cause earthquakes.