What is precession?

The year is the year, and the precession is the year difference, which is the time difference between the tropic year and the sidereal year.

The tropic year is a Gregorian calendar year, which is called a natural timer according to the time on the crust surface. Man-made sidereal year timer.

If there is no solid tide, the crust and mantle rotate synchronously, and the time for the earth to rotate once is 6: 9: 0 on 365. That is, the crust and mantle have the same time.

With the earth tide, the crust rotates behind the mantle, shortening the tropical year and leading to precession.

See the figure below; Crustal time is shorter than mantle time.