On the day of the autumnal equinox, the direct sunlight reaches 18 (the autumnal equinox), which is almost direct to the equator of the earth, and the length of day and night is the same all over the world (regardless of the refraction of sunlight by the atmosphere and the shadow of morning twilight).
after the autumnal equinox, the direct point of the sun continues to move from the equator to the southern hemisphere, and the days are shorter and the nights are longer in all parts of the northern hemisphere, that is, the days are shorter than the nights in one day; The southern hemisphere is the opposite. Therefore, the autumnal equinox is also called falling points. At the north and south poles, on the day of the autumnal equinox, the sun is on the horizon all day.
for the same solar term, every year is 6 hours or 5 hours behind the previous year, and in leap year, it is 24 hours ahead of schedule. In this way, the reflection on the date is that the same solar term remains unchanged or lags behind by one day in the normal year and the previous year; In leap year, it remains the same as the previous year or one day earlier.
In this way, the starting point of the same solar term has three different Gregorian calendar dates. Take the "autumnal equinox" as an example, there are three situations: September 22nd, September 23rd and September 24th.