What is the precession period? What is the ecliptic cycle?

Under the action of external force, the direction of the earth's rotation axis in space does not keep a fixed direction, but changes constantly. Among them, the long-term movement of the earth's axis is called precession, while the periodic movement is called nutation. Precession and nutation cause changes in the positions of the celestial pole and vernal equinox relative to the stars. In the second century BC, the ancient Greek astronomer Hippocrates first discovered precession when compiling a catalog containing 1022 stars. Astronomers in the Jin Dynasty in China independently discovered the precession phenomenon based on the observation of the transit of stars on the winter solstice. According to the "History of Song Dynasty", "Yu:' Yao had a short star in winter, but now more than 2,700 years ago, on the East Wall, it got worse every year, so we can know where it is'". This is the origin of the term precession.

Newton was the first to point out that the cause of precession was the attraction of the sun and the moon to the equatorial uplift of the earth. Under the gravity of the sun and the moon, the earth's rotation axis draws a cone around the yellow pole in space, which takes about 26,000 years, and the radius of the cone is about 23. This long-term movement of the earth's axis caused by the sun and the moon is called precession of the sun and the moon. In addition to the gravity of the sun and the moon, the earth is also attracted by other planets in the solar system, which causes the position of the orbital plane of the earth's movement, that is, the ecliptic plane, to change constantly, thus making vernal equinox have a small displacement along the equator, which is called planetary precession. The age difference of the planets makes the vernal equinox advance eastward along the equator by about 0. 13 arc seconds every year.

While the earth's rotation axis moves precession around the yellow pole in space, it is accompanied by many short-period changes. British astronomer Bradley discovered nutation after analyzing the observation data of 1748 for 20 years. The change of the position of the lunar orbital plane (white plane) is the main reason for nutation. At present, astronomers have analyzed that there are as many as 263 nutation periods, among which the main nutation period, namely 18.6, has the largest nutation term amplitude, which is mainly caused by the movement of the white road, which causes the intersection point of the rising of the white road to move westward along the ecliptic, and it takes about 18.6 years to make a circle. Therefore, the gravitational effect of the moon on the earth also has the same periodic change. On the celestial sphere, it shows that the celestial pole moves around the precession of the yellow pole and around its average position, with a period of 18.6 years. Similarly, the gravitational effect of the sun on the earth changes periodically, causing nutation in the corresponding period.

Zodiac period:

The great circle where the earth's orbital plane around the sun intersects the celestial sphere. Seen from the earth, the ecliptic is very close to the annual apparent motion of the sun on the celestial sphere. Because the earth's revolution is perturbed by the moon and other planets, the orbit of the earth's revolution is not a strict plane, that is, it produces irregular continuous changes in space, which includes many short-term and a slow long-term movements. Short-period motion can be averagely eliminated in a certain time, and the orbital plane with periodic motion eliminated is called instantaneous average orbital plane. The strict definition of the ecliptic is: the great circle where the instantaneous average orbital plane of the center of mass of the Earth-Moon system revolves around the sun and intersects the celestial sphere. The ecliptic is the base circle of the ecliptic coordinate system on the celestial sphere.