How did the ancients determine the date of solar terms?

Lunar calendar is a fixed calendar, with the characteristics of astronomical calendar, which can well correspond to various astronomical phenomena, such as its solar terms strictly correspond to the height of the sun, calendar days strictly correspond to the moon phase, the frequency of non-appearance and appearance of leap months corresponds to the perihelion and farhelion of the earth, and other astronomical phenomena, such as sunrise, sunset, five-star orientation, solar eclipse, tides and so on. Even calendar months roughly correspond to the height of the sun. Lunar New Moon is the first day of the month-the first day of the month. Astronomically speaking, the so-called "new moon" has a definite moment, that is, the ecliptic of the moon is the same as that of the sun.

The lunar calendar is determined according to the trajectory of the sun, which is the ecliptic.

The positioning method of the ecliptic is the longitude of the ecliptic, which is divided into 360 degrees. In this system, the celestial sphere is divided into two hemispheres by the ecliptic plane. At the vernal equinox, the sun moves to the longitude of the ecliptic at 0 degrees, at the summer solstice at 90 degrees, at the autumnal equinox at 180 degrees and at the winter solstice at 270 degrees.

Solar terms are determined according to the position of the sun on the ecliptic, so the annual solar terms can be determined in advance.