Why is the vernal equinox equally divided day and night?

Because on the day of the vernal equinox (the sun reaches longitude 0), the sun just points to the equator. At this time, the terminator line passes through the north and south poles and coincides with the meridian circle of the earth, dividing all latitudes into two parts, half in the day hemisphere and the other half in the night hemisphere, so the day and night in the world are almost equal.

On the day of the autumnal equinox (the sun reaches the longitude 180), the sun points directly at the equator again, and the terminator line also passes through the north and south poles, which coincides with the meridian circle of the earth and divides all latitudes into two parts, so there is a saying that the vernal equinox and autumnal equinox are equally divided day and night.

Extended data:

The difference is that after the vernal equinox in the northern hemisphere, the direct point of the sun gradually moved northward, the solar radiation gradually increased, the weather became warmer day by day, the days became longer and the nights became shorter. After the autumnal equinox, the direct point of the sun gradually moved southward, the solar radiation gradually decreased, the weather became colder day by day, the days became shorter and the nights became longer. The southern hemisphere is just the opposite.