Precession refers to the phenomenon that the long-term precession of the earth's rotation axis causes the vernal equinox to move westward along the ecliptic, resulting in the tropical year being shorter than the sidereal year. The reason is that the earth is an ellipsoid and the equator is locally uplifted; The equatorial plane does not coincide with the ecliptic plane. The sun, the moon and other planets exert additional gravity on the equatorial uplift, which causes the change of the inclination angle of the equatorial plane, that is, the precession of the earth axis.

Therefore, vernal equinox moves about 50 ".2" westward along the ecliptic every year, once every 26,000 years. The tropical year is slightly shorter than the sidereal year, which was called "precession" in ancient China. The westward movement of vernal equinox caused by the movement of the sun and the moon does not change the angle of intersection between the yellow and the red, which is called "precession of the sun and the moon"; Affected by other planets, the ecliptic angle changes at the same time, which is called "planetary precession". The sum of the two is called "total precession".

Extended data

Influence: The influence of precession is the frequent change of Polaris. At present, the hook instrument is very suitable for indicating the north pole of the celestial sphere, because the brightness (2.0, etc. ) is enough to undertake this important task, and the deviation from the real North Pole is less than half a degree. On the other hand, the luminosity of the right axis of Draco (α star of Draco) and Polaris in 3000 BC and 3.67 BC is much inferior.

The bright Vega in Lyra used to be the North Star (BC 12000, which will be re-elected in BC 14000), but she never reached the North Pole within 5.

Reference source: Baidu Encyclopedia-precession