The Chinese calendar: lunar months, solar terms and the sexagenary cycle
The Chinese lunisolar calendar: numbered months, the leap month, the twenty-four solar terms, the Taichu and Shoushi reforms and the cycle of sixty.
The traditional Chinese calendar, called nongli, the agricultural calendar, is a refined lunisolar system. Its months begin with the new Moon, but the year is regulated by twenty-four points along the Sun's course that mark the work of the fields. For more than two thousand years its preparation was an affair of state: each dynasty promulgated its own calendar, and a wrongly predicted eclipse could be read as a sign that Heaven had withdrawn its favour.
Lunar months and the leap month
Chinese months have no proper names but numbers: first month, second month and so on up to the twelfth. Each month begins on the day of the conjunction of Moon and Sun, calculated rather than observed, and lasts 29 or 30 days. The fifteenth day therefore falls close to the full moon, which is why many traditional festivals, such as the Lantern Festival and the Mid-Autumn Festival, are celebrated on precisely that date.
To realign months and seasons, a leap month called runyue is inserted roughly every two or three years. The modern rule is elegant: the solar year is divided into twelve principal terms, and a lunar month that contains none of them becomes the leap month and takes the number of the preceding month. In this way the winter solstice always falls in the eleventh month, which serves as the fixed point of the whole system.
The twenty-four solar terms
The solar terms, jieqi, divide the Sun's annual path into twenty-four equal parts of fifteen degrees. Their names describe the climate and farming life of the Yellow River plain: Beginning of Spring, Rain Water, Awakening of Insects, Grain in Ear, White Dew, Frost's Descent, Major Cold. They are the solar part of the calendar, the part farmers used to decide when to sow and when to harvest.
In 2016 UNESCO inscribed the twenty-four solar terms on the list of the Intangible Cultural Heritage of Humanity. Until the seventeenth century mean terms were used, at equal intervals of time; with the reform of 1645, prepared with the contribution of the German Jesuit Johann Adam Schall von Bell, the calendar switched to true terms, calculated on the Sun's actual position, which appears to move faster in winter.
Reforms and court astronomers
The history of the Chinese calendar is a succession of reforms. In 104 BC, under Emperor Wu of the Han dynasty, the Taichu calendar was promulgated, fixing the start of the year in the month in which the New Year still falls today. In 1281, under the Yuan dynasty, the Shoushi calendar came into force, prepared by the astronomer Guo Shoujing on the basis of new measurements and regarded as one of the most accurate of its time.
Older still is the sexagenary cycle, obtained by combining ten heavenly stems and twelve earthly branches. The oracle inscriptions on bones and turtle shells of the Shang dynasty, towards the end of the second millennium BC, already use this cycle to count days. Later it was applied to years as well, and the twelve branches were associated with the twelve animals of the popular zodiac.
In 1912 the Republic of China officially adopted the Gregorian calendar, and the People's Republic has kept the same choice for civil purposes. The lunisolar calendar has nevertheless remained alive for festivals, anniversaries and family life, and it is still officially calculated according to precise astronomical rules. Alongside the civil calendar, hundreds of millions of people continue to count time by the Moon as well.