Shuāngjiàng

Shuangjiang, Frost Descent, is the eighteenth and last solar term of autumn, at the sun's longitude of 210 degrees, around the twenty-fourth of October. It is a principal (zhongqi) term. Its name announces the first frosts settling on the ground as temperatures fall toward winter. In the farming almanac it is the final push to complete the autumn harvest and to store the crops before the cold, and it is associated in tradition with tonic foods eaten to fortify the body against the coming winter and with the eating of persimmons in some regions. Being a solar marker, it falls near the same Gregorian date each year. These are the observances of the East Asian agricultural tradition, resting on the division of the tropical year into twenty-four solar terms.

Shuangjiang, Frost's Descent, falls at 210 degrees, around 23 October, and is a qi, or major, term, the last of the six terms traditionally assigned to autumn before Lidong opens winter. Its name signals the point in the traditional agricultural calendar when the first frost is expected across much of the North China Plain, later than the actual first frost in China's colder northern regions and earlier than in its warmer south, a mismatch that recurs across the twenty-four-term system generally, since the terms were fixed to a single reference climate rather than adjusted regionally. Some traditional almanacs associate Shuangjiang with the persimmon harvest, the fruit's ripening treated as a seasonal marker coinciding with the term.

The twenty-four solar terms divide the ecliptic into equal arcs of fifteen degrees, and this one falls at 210 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 system travelled with the Chinese calendar. Japan, Korea and Vietnam all adopted the twenty-four terms and kept their own readings of the same names, and the terms remain in use in each. In 2016 the twenty-four solar terms were inscribed on the UNESCO Representative List of the Intangible Cultural Heritage of Humanity, on the ground that they remain a living framework for agricultural and everyday practice rather than an obsolete scheme.

Shuangjiang, the descent of frost, closes the autumn group and completes the sequence that begins with the white dew and passes through the cold dew. It is the last of the terms before winter opens at Lidong, and its name is the point at which the graded progression of the autumn terms reaches something that will damage a crop rather than merely mark the season.