Why is the spring tide of the third day eighteen floods? What's going on here?

The so-called proverb "the third day, the 18th National Congress" refers to the phenomenon that on the third day and 18th day of the lunar calendar, the positions of the earth, the moon and the sun are almost in a straight line, and the tidal force of the moon and the sun are the same, and the two forces are superimposed to make the sea spring. The actual tide will also be restricted by many factors such as geographical environment, coastal location, ocean current movement and so on.

Tide of the maximum period (that is, the new moon and the moon) when the sun and the moon coincide. Due to the lag of the ocean, the astronomical tides of ocean tides are generally about one and a half days after the new moon and the full moon, that is, the second, third, seventeenth and eighteenth days of the lunar calendar. The biggest astronomical wonder in the world is the Qiantang River tide in Zhejiang.

Calculation method of high tide:

Because the fluctuation of seawater is a geographical phenomenon influenced by the gravity of the moon, the fluctuation of seawater is regular, with a period of 15 days. Therefore, the climax of the next day was delayed by 0.8 hours (48 minutes) compared with the previous day.

The daily high tide time can be calculated according to the date of the lunar calendar. The specific calculation method is: according to the lunar calendar, if the number of days is less than 15, directly multiply the number by 0.8, and the obtained number is the highest high tide time of the day; If the number of days is greater than 15. You must subtract the number of days from 15 and multiply it by 0.8 to get the highest climax time of the day.

At the same time, the interval between high tide and low tide is generally 6 hours, which means that high tide and low tide are twice a day. The high tide generally lasts for more than an hour before the low tide begins, and the low tide time is at least between the two high tides.