What is precession?

Precession is the slow conical motion of the earth's axis around the axis passing through the center of the earth and perpendicular to the ecliptic plane. The period is 26,000 years, which is caused by the attraction of planets such as the sun and the moon to the equatorial uplift of the earth. As a result, the vernal equinox gradually moved westward. The equinox (precession) is the year, and precession is the year difference, which is the time difference between the sidereal year and the tropic year. Generally speaking, as we all know, the earth is similar to a top and rotates around its axis. The gyro does not necessarily rotate completely parallel to the ground when it rotates. So the gravity on both sides of the gyro is unbalanced, and the jitter even stops. The same is true of the earth, resulting in "precession." Precession is the phenomenon of vernal equinox displacement caused by precession of the earth's rotation axis. Under the gravity of the sun and the moon, the spatial direction of the earth's rotation axis is not fixed, but moves slowly and continuously around an axis passing through the center of the earth and perpendicular to the ecliptic plane, and rotates clockwise (as seen from the northern hemisphere) for about 25,800 years, depicting a conical surface. The vertex angle of this cone is equal to the intersection angle of yellow and red (23.5 26' 21"). So the celestial pole describes a small circle with a radius of 23.5 around the yellow pole on the celestial sphere, which also makes the vernal equinox rotate once every 25,800 years along the ecliptic in the direction opposite to the annual apparent motion of the sun, and move about 50.3 "to the west every year. This long-term precession of the earth's axis caused by the sun and the moon is called the precession of the sun and the moon. In addition, under the action of planetary gravity, the earth's orbital plane constantly changes its position, which not only changes the ecliptic angle, but also causes the vernal equinox to move slightly along the equator, which is opposite to the precession direction of the sun and the moon. This effect is called planetary precession. The age difference of the planets makes vernal equinox move eastward about 0.13 "every year along the equator. The joint action of the precession of the sun and the moon and the precession of the planets changes the coordinates of celestial bodies, such as right ascension and declination. The change within one year is called precession. In addition, according to the general theory of relativity, the rotating shaft of a rotating object will produce relativistic precession in space, which is called geodesic precession. The precession of the earth is1.98 "/century, and the direction is counterclockwise. The equator of the earth does not coincide with its orbit "ecliptic", and the intersection of their planes is vernal equinox and vernal equinox. The time for the sun to return to vernal equinox from vernal equinox is called tropic year, and the time required for the earth to rotate once is called sidereal year. The tropic year is about 20 minutes shorter than the sidereal year, which is caused by the periodic change of the earth axis due to the gravity of other celestial bodies when the earth moves. Due to precession, with the passage of time, the position of the north celestial pole is constantly changing among the stars. Now the North Celestial Pole is the Polaris, and according to the ancient records of China, the Polaris 4600 years ago was the right pivot star of Tianlong. 14000 years later, Vega will be the "North Star" at that time. Because of precession, the position of the same star changes with time. Around 150 BC, hipparchus, an ancient Greek astronomer and founder of western ancient astronomy, discovered precession by comparing his catalog with that of his predecessors. In 330 AD, ancient astronomers in China independently discovered precession and measured it accurately. [ 1]