How are tides formed? Why twice a day?

The formation of tides: The tidal force of the moon and the sun on the ocean is the cause of the rise and fall of sea water. In a non-inertial system, the tidal force is the combined force of the moon's gravitational force and its corresponding inertia force, and the sun's gravitational force and its corresponding inertia force.

The earth and the moon are regarded as particles, with the center of the earth as the non-inertial reference object. The gravitational force exerted by the earth particles on the moon particles is exactly the centripetal force of the earth particles moving in a circular motion around the center of mass, and this centripetal force corresponds to The inertial force is equal to and opposite to the centripetal force. Therefore, the gravitational force exerted by the earth particle on the moon particle and this inertial force cancel each other out.

In fact, the size of the earth is very large. For objects on the ground closest to the moon, the orbital radius of the circular motion around the center of mass of the earth and the moon is obviously smaller than the orbital radius of the earth's particle. The gravitational pull of the moon will be greater than the corresponding inertial force. These two forces can no longer cancel each other out. Their combined force is in the opposite direction to the gravitational pull of the earth on the object, making the gravity of the object significantly smaller.

If the "object" in question is sea water, then a high tide will occur. High tides also occur in the part of the sea farthest from the moon. The high tide that occurs in the morning is called a tide, and the high tide that occurs in the evening is called a tide, so there are tides.

The reason there are two tides: For the same seawater, if a person faces the moon, the tide is high, and when the moon is turned away, the tide is also high. In this way, people can see two high tides every day.

Extended information:

Tidal cycle:

1. Semi-diurnal tide type: two high tides and two low tides in one lunar day, the previous high tide and low tide The tidal range is roughly the same as the tidal range between high tide and low tide, and the time of high tide and low tide is almost equal (6 hours and 12.5 minutes).

2. Diurnal tide type: There is only one high tide and one low tide in a lunar day. The Beibu Gulf of the South China Sea is a typical all-day tidal area in the world.

3. Mixed tide type: There are two high tides and two low tides on some days in a month, but the tidal difference between the two high tides and the low tides is quite different, and the time of the high tide process and the low tide process are also different. ; while other days have one high and one low.

Regardless of the type of tide, a spring tide with the largest tidal range will occur within two or three days after the first and fifteenth day of each month on the lunar calendar. At that time, the tide will rise the highest and fall the lowest. Within two or three days after the eighth and twenty-third day of each month in the lunar calendar, there will be a neap tide with the smallest tidal range. At that time, the tide will not rise too high and fall not too low.

Baidu Encyclopedia-Tide