The reason of dichotomy

Vernal equinox, autumnal equinox, summer solstice and winter solstice are four solar terms in the traditional calendar of the motherland. The traditional calendar in China is a combination of the solar calendar and the yin-yang calendar, which are respectively equipped with the time of the main branch (the 60-base time system of pure mathematics has nothing to do with the other two components), and the solar calendar part is the "twenty-four" solar terms. Strictly speaking, the "twenty-four" solar terms only refer to time. In our traditional calendar, the ecliptic is divided into 24 segments, each segment is 15, so there are 24 nodes on the ecliptic, among which the nodes with longitude of 0, 90, 180 and 270 are vernal equinox, summer solstice, autumnal equinox and winter. So the dichotomy refers to four moments, the dichotomy is the four nodes on the ecliptic, and the dichotomy is the day when the four solar terms are respectively located.