The twenty-eight lunar mansions

The sky framework

The twenty-eight lunar mansions, ershiba xiu (二十八宿), form a traditional framework for describing the sky. Their names identify star groups and associated regions used to locate the Moon, Sun and planets. They belong to a larger Chinese celestial system that also includes the Three Enclosures and other named star groups. The twelve zodiac animals form a separate classification.

A mansion is a celestial lodging place. The Moon travels around the sky against the stars in approximately 27.3 days, giving a rough association between a night's motion and a mansion. The divisions have unequal widths, however, and their astronomical framework is organised around the celestial equator and north celestial pole. They are not twenty-eight equal sectors.

This journey against the stars differs from the cycle of lunar phases. The interval from one new moon to the next averages about 29.5 days. As Earth moves along its orbit, the Moon must travel farther than one circuit against the stars to regain the same alignment with the Sun. The twenty-eight mansions therefore do not correspond to twenty-eight numbered days in a lunar month.

Four groups of seven form the directional figures of the Azure Dragon in the east, Black Tortoise in the north, White Tiger in the west and Vermilion Bird in the south. The northern figure's Chinese name, Xuanwu, is also translated as Dark Warrior.

The twenty-eight names

The table pairs English names with their Chinese characters. The characters distinguish repeated romanisations: Wei can mean Tail, Rooftop or Stomach, while Bi can mean Wall or Net. English translations vary, so the Chinese name is useful when comparing records.

Shen is the astronomical reading of 参, and Zi is the reading of 觜 in this mansion name. The Dipper mansion, Dou, is distinct from the Northern Dipper, the star group also known in English as the Big Dipper.

Almanac entries

An almanac can assign a mansion name to a date through a recurring calendar sequence. Such an entry does not necessarily give the Moon's position that night. The name can carry a traditional judgement used in date selection, even though no observation of the Moon is involved in assigning it.

The distinction matters when reading a historical record. A statement that the Moon was in the Heart mansion describes a celestial position. Heart printed in a daily almanac's mansion field identifies a calendar classification. The date-assignment rule, rather than the name alone, determines how that field advances.

The list of twenty-eight names is consequently not enough to reconstruct a daily almanac. Its sequence and starting point belong to the particular calendar tradition. Twenty-seven-mansion systems also exist and have their own membership and assignment rules.

History and imagery

The named framework was already established in the Warring States period. A lacquer clothing chest from the tomb of Zeng Hou Yi, Marquis Yi of Zeng, carries the twenty-eight names around a large character Dou representing the Northern Dipper. An eastern dragon and western tiger appear on the lid. The chest brings the names and directional imagery together in an early surviving object.

The mansions also acquired religious identities. Paintings can personify them as deities, including human figures and figures with animal heads. Their clothing, posture and attributes belong to this religious imagery. Star maps, by contrast, arrange the stellar groups in relation to one another. Both draw on the same named sky, but they represent it for different purposes.