Autumnal Equinox (Qiufen) is the sixteenth of the twenty-four solar terms and the fourth solar term of autumn. It occurs when the Big Dipper points to the terrestrial branch you and the sun reaches the ecliptic longitude of 180°, typically falling on September 22–24 in the Gregorian calendar. On the day of the Autumnal Equinox, the sun shines almost directly over the Earth's equator, and day and night are of equal length across the entire globe.

The term “fen” in Qiufen means “equal division” or “half.” Just like the Spring Equinox, on this day the 24-hour period is evenly split between day and night. A further layer of meaning is that it equally divides the autumn season. According to the ancient Chinese system where the four seasons are delineated by the Beginning of Spring, Beginning of Summer, Beginning of Autumn, and Beginning of Winter, autumn begins at “Beginning of Autumn” and ends at “Frost's Descent.” The Autumnal Equinox falls exactly in the middle of autumn's 90 days, serving as the watershed between early autumn and late autumn. After the Autumnal Equinox, the point of direct solar incidence begins to move into the Southern Hemisphere, and the Northern Hemisphere experiences shorter days and longer nights, an increasing diurnal temperature range, and a daily decline in temperature. The Autumnal Equinox represents a moment of yin-yang equilibrium, perfectly aligning with the balance that Chinese culture has always pursued. At this time, the summer heat has receded while the cold has yet to arrive — it is the most pleasant and comfortable season of the year.

Qiufen is one of the earliest solar terms established by the ancients. The great Han dynasty Confucian scholar Dong Zhongshu, in his Chunqiu Fanlu·Yin Yang Chu Ru Shang Xia Pian (Luxuriant Dew of the Spring and Autumn Annals: Chapter on the Ascent and Descent of Yin and Yang), stated: “At the Autumnal Equinox, yin and yang accompany one another; thus day and night are equal, and cold and heat are balanced.” Qiufen was traditionally the “Moon Sacrifice Festival,” from which the Mid-Autumn Festival — originally “Autumn Evening Moon Sacrifice” — evolved. The ancients divided the fifteen days of the Autumnal Equinox season into three pentads: in the first pentad, thunder begins to cease its sound; in the second pentad, hibernating insects seal their burrows with earth; in the third pentad, water begins to dry up.

Since ancient times, Qiufen has given rise to a rich variety of solar term customs, including egg-standing, sticking sparrow beaks, moon worship, and flying paper kites.

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