What day is as long as night?

A: The vernal equinox and the autumnal equinox are March 2 1 and September 23, respectively.

At the vernal equinox and autumnal equinox, that is, March 2 1 and September 23, day and night are equal in length. At the vernal equinox, the sun almost shines directly at the equator of the earth, while at the autumnal equinox, the sun reaches the longitude 180 (autumnal equinox), so that the length of day and night is almost equal all over the world, each with 12 hours.

The vernal equinox and autumnal equinox are the 90-day midpoint of the Spring and Autumn Period, hence the name. The vernal equinox is an important solar term, which not only has astronomical significance: the northern and southern hemispheres are equally divided day and night, but also has obvious characteristics in climate. In ancient times, it was also called "Japan-China", "Day and Night" and "Mid-spring Moon".