Qiūfēn

Qiufen, the Autumn Equinox, is the sixteenth solar term, at the sun's longitude of 180 degrees, around the twenty-third of September. It is a principal (zhongqi) term and the second equinox, when day and night are again equal and the sun crosses the celestial equator going south; thereafter night grows longer than day. In Chinese cosmology it marks the balance point where yin begins to prevail. The farming almanac treats it as a central time of the autumn harvest, and it has in recent times been adopted as a national farmers' festival. Being anchored to the equinox itself, it falls near the same Gregorian date each year. These are the observances of the East Asian agricultural tradition, resting on the astronomical division of the tropical year into twenty-four solar terms.

Qiufen, the Autumn Equinox, falls at 180 degrees of ecliptic longitude, when the sun again crosses the celestial equator heading south, around 23 September, and is a qi, or major, term, one of the four cardinal terms marking an exact astronomical event of day-night equality, mirroring Chunfen's spring equinox six months earlier. As at the spring equinox, day and night are of approximately equal length worldwide at this moment, not only in China. Since 2018, 23 September, at or near Qiufen, has been formally observed in the People's Republic of China as Farmers' Harvest Festival, a modern state-designated holiday explicitly tied to this solar term rather than to a fixed Gregorian or lunar date, honouring the term's older agricultural association with the main autumn harvest.

The twenty-four solar terms divide the ecliptic into equal arcs of fifteen degrees, and this one falls at 180 degrees of the sun's apparent longitude. It is a major term, a zhongqi, one of the twelve that fall in the even positions of the twenty-four. The major terms are the ones that anchor the lunar months, and they alternate strictly with the sectional terms, so a zhong is always followed by a jie fifteen degrees later.

The twelve major terms are what make the Chinese calendar work as a lunisolar one. Each ordinary lunar month is expected to contain exactly one major term, and the month takes its number from the term it contains, so the eleventh month is by definition the one holding the winter solstice. When a lunar month falls entirely between two major terms and contains none, it is declared intercalary and repeats the number of the month before it, which is how a thirteenth month is inserted without any fixed rule about when.

Qiufen is the autumn equinox, the sun at 180 degrees, and day and night are of equal length. Since 2018 the day has been observed in China as the Farmers' Harvest Festival, a national observance established by the State Council, which is a recent addition rather than a traditional one and is recorded here as such. The equinoxes and solstices are the four terms an observer can determine directly, and the other twenty are interpolated between them.