Was it beginning of autumn in ancient times or after the autumnal equinox?

"beginning of autumn" here means after the autumnal equinox.

At the autumnal equinox, the sun moves to longitude 180, while the vernal equinox, summer solstice and winter solstice are located at 0, 90 and 270 respectively. The autumnal equinox, like the vernal equinox, is divided equally between day and night. On the contrary, after the autumnal equinox, the days will get shorter and shorter. The ancients were full of awe for such solar terms, and gradually formed a series of autumnal equinox phenomena, such as offering sacrifices to the moon at the autumnal equinox, asking questions after the autumnal equinox, waiting at the autumnal equinox, and lying early and getting up early.

Dong Zhongshu, a Confucian in the Han Dynasty, wrote in the story of Spring and Autumn Annals: "There are four seasons in the sky, and you have four strategies. If the four strategies are four seasons, so is the general class. Heaven and man are similar. Celebrate spring, enjoy summer, punish autumn and punish winter. " Celebration, reward, punishment and punishment are the four ruling behaviors of emperors, which should adapt to the changes of the four seasons. Dong Zhongshu believes that the reward should be carried out in spring and summer, and the execution can only be carried out in autumn and winter, which is later called "asking after autumn."