One is the rotation of the earth itself, and the other is the gravitational interference of external celestial bodies (tidal force of the sun and the moon).

Simply put, tidal force is an object. Because of its own volume, the distance between each point on the object and the gravity source is different, which leads to different gravity at each point and inconsistent gravity direction, so the object will be deformed.

For the earth, the seawater covered on its surface is most obviously affected by tidal force, and then coupled with the rotation of the earth itself, it produces the phenomenon of daily high tide and low tide.

High tide and low tide time

The regular semidiurnal tide fluctuates once every 12 hour.

The simple calculation method of tidal time is: maximum tidal time = lunar calendar ×0.8, minimum tidal time = maximum tidal time +/-6.

Mastering this rule, it should not be difficult to remember this formula. Here are some practical examples to illustrate this formula:

Suppose: today is 15 of the lunar calendar, and the highest climax time is = 15× 0.8 = 12, that is,12 at noon and midnight; The lowest ebb time is 6 am and 6 pm. Close observation time is from 4: 00 am to 8: 00 am and from 4: 00 pm to 8: 00 pm.

Another example is:

1, 2000/ 12/3 1: The lunar calendar date is the sixth day, so the high tide time is 6*0.8=4.8, which means that the high tide time is 4: 48 in the morning and evening.

2.200110/28: The lunar calendar date is 12, so the high tide time is 12*0.8=9.6, which means the high tide time is 9: 36 in the morning and evening.

3, 2002/11,the eighteenth day of the lunar calendar, the high tide time is (18- 15) * 0.8 = 2.4, that is to say, the high tide time is at 2: 24 in the morning and afternoon.