What is the ecliptic?

The ecliptic is the great circle of the sun's obvious path through the sky; It can also be said that the earth's orbit around the sun is projected on the celestial sphere.

The zodiac signs are arranged along the ecliptic, and the intersection angle between the ecliptic and the celestial equator is 23.4; The two intersections of the ecliptic and the celestial equator are the vernal equinox and the autumnal equinox. In the ecliptic coordinate system, the longitude of celestial bodies is measured eastward along the ecliptic from the vernal equinox, with positive north latitude and negative south latitude. The distance between the south and north poles and the corresponding celestial pole is 23.4.

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.