The Chinese Calendar: New Moons, Solar Terms and the Zodiac

Every year, more than a billion people celebrate a New Year that falls on a different date: sometimes in late January, sometimes in mid February. The reason is the traditional Chinese calendar, one of the oldest calendars still in daily use. It is a lunisolar calendar, which means it follows the Moon for its months and the Sun for its seasons, and it has been refined by astronomers for more than two thousand years.

Months that begin at the new Moon

In the Chinese calendar each month begins on the day of the new Moon, when the Moon is between the Earth and the Sun and invisible. This differs from the Babylonian and Islamic practice of starting a month at the first visible crescent. Because the new Moon can be calculated, Chinese astronomers could predict months in advance rather than waiting to see the sky.

The Moon's cycle is about 29.5 days, so months are either 29 or 30 days long, and the fifteenth day of each month usually falls near the full Moon. That is why the Mid-Autumn Festival, held on the fifteenth day of the eighth month, is celebrated under a bright full Moon.

The 24 solar terms

Twelve lunar months make about 354 days, roughly 11 days short of the solar year. To keep the calendar tied to the seasons, Chinese astronomers divided the Sun's yearly path into 24 solar terms, called jieqi, each about fifteen days long. They have names that describe the season, such as:

• Beginning of Spring

• Rain Water

• Awakening of Insects

• Grain Rain

• Summer Solstice

• Frost Descent

• Winter Solstice

Farmers used the solar terms to decide when to plant, irrigate and harvest. In 2016 UNESCO recognized the 24 solar terms as part of the world's intangible cultural heritage.

The leap month

Every two or three years a leap month is added. Under the rule used since 1645, the leap month is inserted where a lunar month contains no "principal" solar term, one of the twelve terms that mark the middle of each solar month. This keeps the lunar months aligned with the Sun, so that New Year always falls in late winter. In practice it usually lands on the second new Moon after the winter solstice, between 21 January and 20 February.

A 60-year cycle and twelve animals

Years are not only numbered but named. The Chinese calendar uses a sexagenary cycle that combines ten Heavenly Stems with twelve Earthly Branches. Together they create 60 unique pairs, after which the cycle starts again. A person's sixtieth birthday, completing one full cycle, is traditionally a special celebration.

The twelve Earthly Branches are linked to the familiar animals of the zodiac: Rat, Ox, Tiger, Rabbit, Dragon, Snake, Horse, Goat, Monkey, Rooster, Dog and Pig. The Heavenly Stems connect with five elements: wood, fire, earth, metal and water. The year beginning on 6 February 2027, for example, is the Year of the Fire Goat.

Astronomy and the emperor

In imperial China the calendar was a matter of state. The emperor was seen as the link between heaven and earth, and an accurate calendar showed that he ruled with heaven's approval. An official bureau of astronomers watched the sky, predicted eclipses and issued the calendar each year. A failed eclipse prediction could be an embarrassment for the whole court, and the calendar was revised dozens of times over the centuries as methods improved.

The cover of this post shows part of the Dunhuang star map, a chart of the Chinese sky made in the Tang dynasty, around the seventh century AD. Found among thousands of manuscripts in a cave near Dunhuang on the Silk Road, it is the oldest complete star atlas known from any civilization and is now kept at the British Library.

In the seventeenth century the Jesuit astronomer Johann Adam Schall von Bell helped reform the calendar for the new Qing dynasty, introducing the system still used for festivals today.

Two calendars side by side

China officially adopted the Gregorian calendar in 1912. Today daily life, business and government use the same dates as the rest of the world. The traditional calendar has not disappeared, though. It sets the dates of Lunar New Year, the Lantern Festival, the Dragon Boat Festival and Mid-Autumn Festival, and similar lunisolar calendars are used in Korea and Vietnam.

Many printed calendars in East Asia show both systems on the same page, with the Gregorian date in large numbers and the lunar date written small beneath it. It is a neat picture of how old and new ways of counting time can live together, and a reminder that a calendar is as much about culture and celebration as it is about counting days.

Cover image: Dunhuang star map, Public domain, via Wikimedia Commons.