Since when do the days get shorter and the nights get longer?

The autumnal equinox begins every year. From the autumnal equinox every year, the days in the northern hemisphere will gradually shorten and the nights will gradually lengthen until the vernal equinox of the following year. The autumnal equinox is the fourth solar term in autumn, which falls on September 22nd and 23rd every year. At this time, the sun shines directly on the equator, and the whole world is as long as day and night. After the autumnal equinox, the direct point of the sun gradually moved southward, and the daytime time in the northern hemisphere gradually became shorter and the night time gradually became longer.