Why is the length of day and night in the world equal to the vernal equinox and the autumnal equinox?

Meaning 1: Day and night are equal in length. On this day, the sun reached longitude 180 degrees and directly hit the equator of the earth. Therefore, in most parts of the world, 24 hours are divided equally between day and night, each being 12 hours. After the autumnal equinox, the direct point of the sun began to enter the southern hemisphere from the equator, and the northern hemisphere began to have shorter days and longer nights, and the day began to be shorter than the night. Near the North Pole, there are long nights and stars for six months in a year.

Meaning 2: The climate changes from hot to cold. According to the division of seasons in ancient China, which started with beginning of spring, Changxia, beginning of autumn and beginning of winter, the autumnal equinox is divided into 90 days in autumn and divided equally in autumn.

Meaning 3: It means mid-autumn. Because the autumnal equinox solar terms are just in the middle of autumn, and the autumn is equally divided.

According to the work "Twenty-four solar terms", "See the vernal equinox in mid-August". "Spring and Autumn Stories Yin and Yang in and out" said: "The autumn equinox, Yin and Yang are also half-phase, so it is cold and summer day and night." In ancient China, the autumnal equinox was divided into three periods: "When thunder comes, the sound will stop;" Second, wait for scorpion blank households; Wait for the water to dry. "The ancients thought it thundered because the sun be the spirit is strong, and the yin began to fill after the autumnal equinox, so it didn't thunder.