Solar terms are widely used. What is the main basis for the formulation of solar terms?

1543, Copernicus, a famous Polish astronomer, first put forward the concepts of the earth's rotation and revolution in his book "On the Operation of Celestial Bodies". The earth's orbit is elliptical, its half-length diameter is 149597870 km, its eccentricity is 0.0 167, and its average orbit speed is 29.79 km per second. The angle between the orbital plane of revolution (ecliptic plane) and the equatorial plane of the earth is 23 27', which is called the ecliptic angle. The rotation of the earth produces the diurnal variation on the earth, and the revolution of the earth and the existence of the angle of intersection between the yellow and the red cause the alternation of the four seasons.

Seen from the earth, the sun moves counterclockwise along the ecliptic, and the ecliptic and the equator have two intersections on the celestial sphere, which are 180 apart. Among them, the point where the sun passes through the celestial equator from south to north along the ecliptic is called vernal equinox, and the other point which is away from vernal equinox 180 is called the autumnal equinox. For people living in the northern hemisphere, when the sun passes through the vernal equinox and the autumnal equinox respectively, it means that it is already spring or autumn. The point at which the sun reaches the northernmost point through the vernal equinox is called the summer solstice, and the other point with a difference of 180 is called the winter solstice. Every year around June 22nd and February 22nd, the sun passes through summer solstice and winter solstice. Similarly, for people living in the northern hemisphere, when the sun approaches the summer solstice and the winter solstice, it enters the astronomical Xia Dong season. The above situation is just the opposite for people living in the southern hemisphere.