Xiǎoshǔ

Xiaoshu, Minor Heat, is the eleventh solar term, at the sun's longitude of 105 degrees, around the seventh of July. It is a sectional (jie) term. Its name signals the arrival of hot weather that is strong but not yet at its fiercest, the approach of the peak of summer. In the farming almanac it is a time of intensive care for the crops as heat and humidity climb, with attention to irrigation and to the threat of storms and flooding. It falls around the start of the traditional hottest period known as the dog days. Being a solar marker, it comes near the same Gregorian date each year. These are the observances of the East Asian agricultural tradition, resting on the twenty-four-fold division of the tropical year into equal solar arcs.

Xiaoshu, Minor Heat, falls at 105 degrees, around 7 July, and is a jie, or sectional, term, the first of the pair of heat-themed terms, alongside the following Dashu, that mark the hottest stretch of the traditional Chinese year. As its name signals, Xiaoshu is read as the beginning of the year's heat rather than its peak, which traditional reckoning places at Dashu roughly two weeks later, though in many parts of China, particularly the south, the very hottest days by direct temperature measurement in fact fall somewhat later still, during the extended hot period folk custom calls sanfu, the "three fu" periods calculated from a separate traditional formula rather than from the solar term boundaries themselves.

The twenty-four solar terms divide the ecliptic into equal arcs of fifteen degrees, and this one falls at 105 degrees of the sun's apparent longitude. It is a sectional term, a jie, one of the twelve that fall in the odd positions of the twenty-four and open the twelve solar months. The sectional terms and the major terms alternate strictly around the year, so a jie is always followed by a zhong 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.

Xiaoshu, minor heat, is the first of the pair of heat terms that run through the height of summer, with Dashu following it. The pairing of a lesser and a greater term is one of the organising devices of the list, and it allows a single climatic condition to be given two positions fifteen degrees apart rather than one, which is a more useful division for practical purposes than a single named point would be.