What does the vernal equinox mean? The origin of this solar term?

The vernal equinox, autumnal equinox, winter solstice and summer solstice, also known as "two points to two days", are the four most important symbolic solar terms among the 24 solar terms. The twenty-four solar terms say, "Mid-February. The division is half, and this is half of ninety days, so it is called division. " At the vernal equinox, the sun reaches the meridian zero, which happens to be in the middle of February. Because the vernal equinox is divided into two halves, it is called the vernal equinox.

At the vernal equinox, the change of nature is characterized by the same length of day and night. The story of Spring and Autumn Annals says, "The vernal equinox is characterized by yin and yang, so the day and night are flat, and the cold and summer are flat." There are two such "semi-cloudy and semi-sunny" days every year, one is the vernal equinox and the other is the autumnal equinox, so there is a proverb that "the vernal equinox and autumnal equinox are equally divided day and night".

"Historical Records Yao Dian" records the length of the day, the length of the night and the shortness of the day, which describes the changes of the sun in four seasons: spring, summer, autumn and winter. It can be known that the vernal equinox and autumnal equinox were observed after the earliest observation of the two landmark points: the longest solstice in winter and the shortest solstice in summer.

The ancient vernal equinox was the day to proofread weights and measures. "The Book of Rites and the Moon Order": "(Mid-spring Moon) is divided into day and night, so it is the same amount. It is the right to level the stone and fight."