Dichotomy introduces dichotomy.

The vernal equinox, autumnal equinox, summer solstice and winter solstice are four solar terms in the traditional calendar of China. China's traditional calendar is a solar calendar and a lunar calendar (that is, the lunar calendar, which is marked by the periodic ups and downs of the moon), in which the solar calendar part is "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, autumn equinox and winter solstice respectively. 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. The relationship between the above three concepts can be expressed as follows: On the vernal equinox, the sun is located in vernal equinox on the ecliptic.