The solar terms related to the length of day and night are

The solar terms related to the length of day and night are vernal equinox, summer solstice, autumn equinox and winter solstice.

The sun travels back and forth between the Tropic of Cancer and the Tropic of Cancer every year. At the vernal equinox and the autumnal equinox, the sun shines almost directly on the equator, so the length of day and night in the world is equally divided.

On the solstice of summer, the sun is almost direct to the Tropic of Cancer, with long days and short nights in the northern hemisphere and short nights in the southern hemisphere. This day is also the longest day and the shortest night in all parts of the northern hemisphere.

On the solstice of winter, the sun shines almost directly on the tropic of Capricorn. The days in the northern hemisphere are short and the nights in the southern hemisphere are short. This day is also the shortest day and the longest night in all parts of the northern hemisphere. After the winter solstice, the days get longer and the nights get shorter. When the vernal equinox comes next year, it will be as long as day and night, and this change will last until the summer solstice.

The length of day and night is related to solar terms.

The earth is a non-luminous and opaque sphere, which keeps rotating, so at the same time the sun can only shine on half of the earth's surface, which leads to the phenomenon of alternating day and night everywhere on the surface.

Half the earth illuminated by sunlight is daytime, and half the earth not illuminated by sunlight is night. The dividing line between the solar hemisphere and the night hemisphere is called the terminator line, which divides the latitude into two parts: the solar arc and the night arc. The length of solar arc and night arc determines the change of day and night in a place.

Because the earth's continuous revolution produces seasonal changes, the change of day and night is related to seasonal solar terms.