Solar terms with equal length of day and night

The solar term of equal length between day and night is the vernal equinox. The vernal equinox is the fourth of the 24 solar terms and an important season for farming. The sun shines directly on the equator of the earth, and day and night are almost equal. In ancient times, it was spring from beginning of spring to long summer, and the vernal equinox was in the middle of three months in spring.

In ancient times, the vernal equinox was also called "Japan-China", "Day and Night Equinox" and "Midspring Moon". "Twenty-four solar terms" also records: "In mid-February, it is divided into 90 days and a half, so it is called minutes." Another article, Spring and Autumn Stories, Yin and Yang in and out, said: "At the vernal equinox, Yin and Yang are also half-phase, so day and night are cold and summer." In addition, Li Mingshi Yi said: "The dividing line is the point where Huang Chi meets, and the sun is here, and the day and night are equally divided." Therefore, the meaning of the vernal equinox, first, refers to the day when the day and night are equally divided; Second, in ancient times, it was spring from beginning of spring to long summer, and the vernal equinox happened to be in the three months of spring, and spring was equally divided.

Every year around March 2 1 in Gregorian calendar, the sun reaches 0 degrees of the Yellow Meridian, which is the vernal equinox of the 24 solar terms. At the vernal equinox, the direct point of the sun is on the equator, and then the direct point of the sun continues to move northward, so the vernal equinox is also called the ascending equinox. At the vernal equinox, theoretically, the world is as long as day and night (see the dawn). After the vernal equinox, the days in the northern hemisphere are getting longer and shorter, and the nights in the southern hemisphere are getting shorter and shorter. At the vernal equinox, there are no extreme days and nights in the world. After the vernal equinox, it began to be extremely daytime near the North Pole, and its scope gradually expanded. Near the Antarctic, the extreme day ends and the extreme night begins, and the scope gradually expands.

The vernal equinox refers to the position of the sun between 0 degrees and 15 degrees of the yellow meridian during the period from March 20th to April 5th. But in fact, it usually refers to the day when the sun is really at 0 degrees of the yellow meridian: March 20 or March 2 1.