What is the Chinese zodiac?

The twelve animals run in a fixed order: rat, ox, tiger, rabbit, dragon, snake, horse, sheep, monkey, rooster, dog and pig. Each one belongs to one of the twelve earthly branches (地支), an ancient set of counting signs.

The branches are paired in order with ten heavenly stems (天干), which produces sixty different pairs before the cycle begins again. In popular terms, that makes each year an animal with one of five elements and a yin or yang quality.

Most people use the zodiac to name a birth year. Calendars differ on when the animal changes. Popular calendars switch at Lunar New Year, and Four Pillars and other astrological practice switch at Lichun, the solar term that opens spring in early February. A birth between those two dates has two possible answers.

The animals can also mark hours and days, and traditional rules describe how the twelve branches combine or clash. Describing a person's character from a birth animal is a more recent habit. The Victoria and Albert Museum notes that ancient Chinese texts do not mention it.

The stems and branches appear in the oracle bone inscriptions of the late Shang state, around 1200 BCE. The sinologist William Boltz points out that they served there only as a counting system for days. He finds no evidence of associations such as the zodiac animals until around the unification of China in 221 BCE, about a thousand years later.

Counting years by the same cycle arrived late as well. The earliest known documents that do it come from Mawangdui tomb 3, sealed in 168 BCE, and the practice dates from about the middle of the third century BCE.

The oldest animal lists found so far come from Qin daybooks, manuscripts for choosing lucky days. A daybook from tomb 11 at Shuihudi in Hubei, a tomb dated to 217 BCE, pairs animals with branches in a section on identifying thieves. A daybook from Fangmatan in Gansu has a similar list. Both differ from the modern set, and parts of them are garbled or unclear. One entry at Shuihudi reads deer, which may be a copying error for horse.

The philosopher Wang Chong (27 to about 97 CE) sets out branch and animal pairs in his Lunheng (論衡), for example tiger with the branch yin, horse with wu and rat with zi. Chapter 14 names eleven animals, and the dragon, paired with chen, appears in chapter 66. Together they are usually cited as the earliest full listing in a text handed down through the centuries.

The earliest case that reference works cite of people being described by their birth animals comes from the Book of Zhou. A letter to Yuwen Hu of the Northern Zhou, sent in his mother's name, recalls her sons, the eldest a rat, the next a rabbit, and Hu himself a snake.

Sui and Tang tombs held the animals in many forms. Figures with human bodies and animal heads became popular in the eighth century, and the Victoria and Albert Museum holds an early eighth-century set in earthenware.

The cycle spread across East and Central Asia and changed along the way. Vietnam has a water buffalo in place of the ox and a cat in place of the rabbit. Turkic peoples of Central Asia substituted a leopard for the tiger, an elephant for the pig and a camel for the rat.

No early source explains why these twelve animals were chosen or why they stand in this order. The familiar tale of a race among the animals is a folk story. The story that a rat tricked the cat out of the race is a folk answer to the cat's absence, and one suggestion is that cats may not yet have reached China when the cycle formed. None of these can be shown.

Where the cycle came from is also unsettled. The Qing scholar Zhao Yi argued it came from northern nomads, Guo Moruo proposed a Babylonian source, and others have pointed to India and to Buddhist texts. Those texts are much later than the Qin slips. The Chinese translation of the Great Collection Sutra, for one, was completed in 414 CE.

Passages in the Classic of Poetry and the Zuo Commentary are sometimes read as early links between single animals and branches, but they give no full list. The start date also depends on what counts as the zodiac. The Victoria and Albert Museum's pages date animal calendar signs to the Warring States period and the pairing of animals with years to at least the Han.