Xiàzhì

Xiazhi, the Summer Solstice, is the tenth solar term, at the sun's longitude of 90 degrees, around the twenty-first of June. It is a principal (zhongqi) term and the longest day of the year in the northern hemisphere, when the sun stands highest and yang reaches its peak, after which, in Chinese cosmology, yin begins its slow return. In the farming almanac it falls in the height of the growing season, and its customs include the eating of cold noodles and other foods suited to the heat. Being anchored to the solstice 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, of which the two solstices and two equinoxes are the cardinal points.

Xiazhi, the Summer Solstice, falls at 90 degrees of ecliptic longitude, the sun's most northerly point relative to the celestial equator, around 21 June, and is a qi, or major, term, one of the four cardinal terms marking an exact astronomical event rather than an intermediate stage. It is the day of longest daylight in the Northern Hemisphere and, unlike most solar terms, was already recognised as an important calendrical fixed point in China well before the full twenty-four-term system was formalised, since solstices and equinoxes are directly observable through shadow measurement using a simple gnomon, the technique ancient Chinese astronomers used to fix these four cardinal terms with considerable precision centuries before the intervening terms were interpolated between them.

The twenty-four solar terms divide the ecliptic into equal arcs of fifteen degrees, and this one falls at 90 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.

Xiazhi is the summer solstice, the sun at 90 degrees and the longest day. Together with the winter solstice it is the oldest determinable point in the Chinese calendar, measurable with a gnomon by anyone who can mark the shortest noon shadow of the year, and the two solstices are attested in Chinese sources long before the full set of twenty-four was assembled. The sanfu, the three periods of greatest summer heat, are counted from Xiazhi rather than from any term of their own.