Solar term

A solar term is any of 24 points in traditional East Asian lunisolar calendars that matches a particular astronomical event or signifies some natural phenomenon. The points are spaced 15° apart along the ecliptic and are used by lunisolar calendars to stay synchronized with the seasons, which is crucial for agrarian societies. The solar terms are also used to calculate intercalary months in East Asian calendars; which month is repeated depends on the position of the sun at the time.

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. Each solar term is divided into three pentads (候 hou), so there are 72 pentads in a year. Each pentad consists of five, rarely six, days, and are mostly 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 North China Plain, peoples living in the different climates still use it with no changes. This is exhibited by the fact that traditional Chinese, Hanja, and Kanji characters for most of the solar terms are identical.

On December 1, 2016, 24 Solar Terms were listed as UNESCO's Intangible Cultural Heritage.

The "Song of Solar Terms" (Chinese: 節氣歌; pinyin: jiéqìgē) is used to ease the memorization of jiéqì: