The sun's revolution in the twenty-four solar terms

Because the direct range of the sun moves back and forth between the tropic of cancer. On the day of the vernal equinox, the sun shines almost directly on the equator of the earth, and the whole world is almost as long as day and night (regardless of the refraction of the sun's light by the atmosphere and the shadow in the morning).

After the vernal equinox, the direct point of the sun continues to move from the equator to the northern hemisphere, where the days are long and the nights are short, that is, the days are longer than the nights in a day; In the southern hemisphere, the days are shorter and the nights are longer, that is, the days are shorter than the nights in a day. Therefore, the vernal equinox is also called the ascending equinox.

Extended data:

In winter solstice, the sun shines directly on the tropic of Capricorn, and the daytime is the shortest in all parts of the northern hemisphere. Then the direct point of the sun began to move northward, and the days in all parts of the northern hemisphere became longer and longer. After the vernal equinox, the direct point of the sun moved to the northern hemisphere, and the phenomenon that the day is longer than the night began to appear in the northern hemisphere. From summer solstice, the sun shines directly on the Tropic of Cancer, and the days are the longest in all parts of the northern hemisphere. Then the direct point of the sun began to move south, and the days in all parts of the northern hemisphere became shorter and shorter. After the autumnal equinox, the direct point of the sun moved to the southern hemisphere, and the phenomenon that the day was shorter than the night began to appear in the northern hemisphere.