Why is there a saying of "vernal equinox and autumn equinox"?

Both the vernal equinox and the autumnal equinox are one of the 24 solar terms. Among the traditional 24 solar terms in China, only the vernal equinox and the autumnal equinox are the only two solar terms with the word "fen". The vernal equinox divides spring equally, and the autumnal equinox divides autumn equally. There are four seasons in a year: spring, summer, autumn and winter. From the temperature point of view, summer and winter are two extremes, one is extremely hot and the other is extremely cold. Spring and autumn are almost the same, and the average temperature is more suitable. However, from the temperature changes, the temperature in spring is from low to high, and the temperature in autumn is from high to low. The vernal equinox is the fourth solar term in spring. The longitude of the sun reaches 0, every year in March of Gregorian calendar 19-22. The vernal equinox is of great significance in astronomy. On this day, the northern and southern hemispheres are divided equally day and night. Since then, the direct position of the sun has continued to shift from the equator to the northern hemisphere. The days in the northern hemisphere are longer than the nights, while the southern hemisphere is the opposite.

The autumnal equinox is the sixteenth of the twenty-four solar terms, and the time is generally from September 22nd to September 24th in the Gregorian calendar every year. On the day of the autumnal equinox, the sun reaches longitude 180 degrees (autumnal equinox), almost reaching the equator of the earth, and the length of day and night is the same all over the world (regardless of the refraction of the sun's light by the atmosphere and the dim shadow in the morning). In ancient times, there was a folk activity of "offering sacrifices to the sun in spring and the moon in autumn". The autumnal equinox used to be the traditional "Moon Festival" (Mid-Autumn Festival), which evolved from offering sacrifices to the moon in autumn.