Chǔshǔ

Chushu, the End of Heat, literally the Stopping of Heat, is the fourteenth solar term, at the sun's longitude of 150 degrees, around the twenty-third of August. It is a principal (zhongqi) term. Its name announces that the oppressive summer heat is finally breaking and the cooler air of autumn is settling in. In the farming almanac it is a time of harvest and of preparing the autumn crops, as the fields ripen and the days shorten. In some traditions it coincides with observances honouring the dead late in the seventh lunar month. 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 of fifteen degrees each.

Chushu, the End of Heat, falls at 150 degrees, around 23 August, and is a qi, or major, term, whose name signals the retreat of the summer heat that peaked at Dashu roughly a month earlier, even though the term still falls within the traditional sanfu period's tail end in some years. In parts of coastal and southern China, Chushu traditionally marks the point in the fishing calendar when a summer fishing moratorium, intended to allow fish stocks to recover during the breeding season, is lifted, and the term is associated in some regions with seafood-eating customs marking the season's fresh catch, a modern continuation of the term's older role as a marker for the agricultural and fishing year rather than a strictly meteorological one.

The twenty-four solar terms divide the ecliptic into equal arcs of fifteen degrees, and this one falls at 150 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 terms have not always been spaced the same way. Until the seventeenth century they were reckoned by equal intervals of time, dividing the year into twenty-four equal parts, a method called pingqi. The Shixian calendar, prepared with Jesuit assistance and promulgated in 1645, redefined them by equal intervals of the sun's apparent longitude instead, fifteen degrees apart, which is the definition still used. Because the Earth's orbit is elliptical the sun does not move at a uniform apparent rate, so the intervals between terms now vary by about a day between the shortest and the longest.

Chushu, the ending of heat, is the major term of the seventh month in the ordinary numbering and marks the retreat of the summer. Its name is built on the same word for heat that gives Xiaoshu and Dashu, so three of the twenty-four terms share a root, which is a small illustration of how tightly the list is constructed as a set rather than assembled from unrelated observations.