Which day of the 24 solar terms is equal to day and night?

The vernal equinox and autumnal equinox in the twenty-four solar terms are equal day and night.

After the autumnal equinox, the direct point of the sun continued to move from the equator to the southern hemisphere, and the days in the northern hemisphere began to shorten and the nights became longer, that is, the days in a day began to be shorter than the nights; The southern hemisphere is the opposite. Therefore, the autumnal equinox is also called the landing point. At the North Pole and South Pole, on the day of the autumn equinox, the sun is on the horizon all day.

After that, as the direct point of the sun continued to move southward, a six-month polar night began near the North Pole, which gradually expanded and then narrowed. A six-month polar day began near the South Pole, which gradually expanded and then narrowed.

The solar term of the vernal equinox refers to the position where the sun is located between 0 and15 of the yellow meridian, that is, from the vernal equinox to Tomb-Sweeping Day, and the Gregorian calendar is from March 20th to April 5th.

In ancient times, the vernal equinox was also called "Japan-China", "Day and Night Equinox", "Vernal Equinox" and "Ascending Equinox". The significance of the vernal equinox, first, refers to the equal division of day and night, each of which is 12 hours; Second, the vernal equinox is in the middle of spring (from beginning of spring to Changchun), and the spring is equally divided.

Extended data:

Because the earth is non-luminous and opaque, and the sun can only illuminate half of the earth, the illuminated hemisphere is daytime, the unlighted hemisphere is night, and the earth rotates for one day and night, and the dividing line between day and night is called terminator line.

While the earth is rotating, it is still revolving around the sun obliquely, and the inclination angle remains constant for one year. In this way, the earth's exposure to the sun at different positions in the orbit of revolution (different latitudes of direct sunlight) is not exactly the same, thus forming four seasons: spring, summer, autumn and winter.

The position of direct sunlight continues to move from the equator to the southern hemisphere, and the day in the northern hemisphere is shorter than the night until the shortest day is reached from the winter solstice. The temperature difference between day and night gradually increases. The temperature drops day by day, getting colder every day, and gradually entering the late autumn season. The situation in the southern hemisphere is just the opposite.