Beginning of autumn is the first 13 solar term among the 24 solar terms. bucket handle of the Big Dipper points to the southwest, and the sun reaches135, which is the annual festival on August 7-9 in the Gregorian calendar. Beginning of autumn is the first solar term and the starting point of autumn. Autumn begins in beginning of autumn and ends in beginning of winter, which is the result of celestial bodies' movement.
From the perspective of solar terms:
After beginning of autumn, it really entered autumn, and the arrival of beginning of autumn also marked the beginning of autumn.
But because beginning of autumn is still in the dog days, it is not autumn.
From the point of view of temperature:
After entering beginning of autumn, in order to distinguish the heat in autumn from that in summer, meteorologists now add an autumn condition: "The average temperature has stabilized below 22 degrees for five consecutive days after beginning of autumn," which counts as officially entering autumn, and the autumn ends when it is below 10 degrees.
Therefore, it is not really autumn after beginning of autumn.