Dōngzhì

Dongzhi, the Winter Solstice, is the twenty-second solar term, at the sun's longitude of 270 degrees, around the twenty-second of December. It is a principal (zhongqi) term and the shortest day of the year in the northern hemisphere, when yin reaches its extreme and, in Chinese cosmology, yang begins its return, the turning point from which the light grows again. It is among the most important of the terms as a festival: families gather, and in the north dumplings and in the south glutinous rice balls are eaten to mark the day, an occasion sometimes ranked with the new year. Being anchored to the solstice itself, it falls near the same Gregorian date each year. These are the observances of a living East Asian tradition, resting on the solar-term division of the tropical year.

Dongzhi, the Winter Solstice, falls at 270 degrees of ecliptic longitude, the sun's most southerly point relative to the celestial equator, around 21 or 22 December, and is a qi, or major, term, the shortest day and longest night of the year in the Northern Hemisphere and, like Xiazhi, one of the four cardinal terms fixed by direct astronomical observation using a gnomon well before the full twenty-four-term system was completed. Dongzhi has historically been treated in China as a major festival in its own right, in some periods and regions rivalling the Lunar New Year in importance, marked by family reunion meals, the eating of dumplings in the north and of glutinous rice balls, tangyuan, in the south, customs that persist widely today.

The twenty-four solar terms divide the ecliptic into equal arcs of fifteen degrees, and this one falls at 270 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.

Dongzhi is the winter solstice and the pivot of the entire calendar, because the eleventh lunar month is defined as the month containing it. Every other month number follows from that anchor, and so does the placement of any intercalary month. The day is also a substantial festival, marked in different regions by tangyuan or by dumplings, and the shujiu, a count of nine periods of nine days through the depth of winter, is reckoned from it.