Solar term

A solar term (or jiéqì, traditional Chinese: 節氣; simplified Chinese: 节气) is any of twenty-four moments in traditional Chinese lunisolar calendars that matches a particular astronomical event and signifies some natural phenomenon. The points are spaced 15° apart along the ecliptic and are used by Chinese lunisolar calendar to stay synchronized with the seasons, which is crucial for agrarian societies. The solar terms are also used to calculate intercalary months; which month is repeated depends on the position of the sun at the time. According to the Book of Documents (書經; Shūjīng), the first determined term was Dongzhi (Winter Solstice) by Dan, the Duke of Zhou (周公旦; Zhōu Gōng Dàn), while he was trying to locate the geological center of the Western Zhou dynasty, by measuring the length of the sun's shadow on an ancient type of sundial called tǔguī (土圭). Then four terms of seasons were set, which were soon evolved as eight terms; not until the Taichu Calendar of 104 BC were all twenty-four solar terms officially included in the Chinese calendar. Because the Sun's speed along the ecliptic varies depending on the Earth-Sun distance, the number of days that it takes the Sun to travel between each pair of solar terms varies slightly throughout the year, but it is always between 15 and 16 days. Each solar term is divided into three pentads (候; hòu), so there are 72 pentads in a year, consisting of five, rarely six, days. Most of them are named after phenological (biological or botanical) phenomena corresponding to the pentad. Solar terms originated in China, then spread to Korea, Vietnam, and Japan, countries in the East Asian cultural sphere. Although each term was named based on the seasonal changes of climate in the North China Plain, peoples living in the different climates still...