How are tides formed?

On the first or fifteenth day of the first lunar month, the earth, the moon and the sun are almost on the same straight line, and the sum of the gravitational forces of the sun and the moon makes the sea water fluctuate greatly, which is the so-called spring tide.

On the 8th and 23rd day of the lunar calendar, the relative positions of the earth, the moon and the sun are almost at right angles, and the gravity of the moon is partially offset by the gravity of the sun, so the fluctuation of the sea water is relatively small, which is called a small tide.

Under the gravity of the sun and the moon, the sea water on the earth fluctuates twice a day and night. When it rises, it is the tide. When the sun, the earth and the moon are in a straight line on the first day and half month of the lunar calendar, and the gravity is very high, the tide will rise, which is called tidal flood.

Cause of formation

Because the moon orbits the earth in January, the tides change on Sunday with the different relative positions of the moon, the sun and the earth, and two large tides and two small tides are formed in January.

On the first day of the new moon and the fifteenth day of the lunar calendar, the moon, the sun and the earth are in a straight line, and the tidal force of the moon and the sun are superimposed on each other. At this time, the sea level rises and falls the most, forming the highest and lowest tides twice a month, which is called the spring tide.

In the first quarter moon (the seventh or eighth day) and the last quarter moon (the 22nd or 23rd day), because the positions of the moon, the sun and the earth are close to right triangles, the tidal force of the moon and the sun to the earth decreases each other. At this time, the sea level rises and falls the least, which is called a small tide.

In fact, due to the influence of natural environment, inertia of seawater movement and seabed friction, the spring tide usually occurs in the new moon and 2-3 days after observation (customarily called late tide), and the small tide usually occurs in the first quarter and 2-3 days after the second quarter. Traditionally, big and small tides are called big and small tides.