Purple Forbidden Enclosure

Traditional Chinese astronomy organised the sky around the one point that does not move. Around the north celestial pole, inside the circle of stars that never dip below the horizon, the Han star catalogues placed the Purple Forbidden Enclosure, the walled palace of the celestial emperor. The choice is deliberate and political: everything else in the sky rises, crosses and sets, but the circumpolar stars only wheel in place, as a sovereign remains enthroned while the court moves around him.

Within the enclosure's two chains of wall stars sit the pole star and the figures of the innermost court, and the whole region functions as the sky's still centre. The scheme descends from the star catalogues of the schools of Shi Shen and Gan De, whose work was gathered and reconciled in the early imperial centuries, and it differs from the Greek inheritance at its root. The Greek sky is a gallery of myths scattered without hierarchy; the Chinese sky is a single organised state, with the palace at the pole, a court of government to its south, and a marketplace beyond that. Watching it was state business. The Han histories record officials whose duty was to note every guest star, comet or vapour that entered the enclosure, because a disturbance in the sky's palace was read as advance word of a disturbance in the earthly one.

When the Ming court built its palace compound in Beijing in the early fifteenth century, it named the complex Zijincheng, the Purple Forbidden City. The purple is not the colour of its walls, which are red; it is the purple of Ziwei, the Purple Forbidden Enclosure around the pole of the sky. The name declares a correspondence: as the celestial emperor dwells at the unmoving centre of the heavens, so the earthly emperor dwells at the centre of the realm, and the palace grounds were closed to ordinary subjects as the circumpolar palace is closed to ordinary stars. The correspondence ran through the city's whole plan, with the palace on a north to south axis and the throne facing south, the direction a sovereign faces because the pole stands at his back.

The habit is much older than the Ming. Chinese political cosmology had long described the ruler by the pole: the Analects already say that one who governs by virtue is like the north star, which keeps its place while the multitude of stars turn about it. The enclosure also lends its name to Zi Wei Dou Shu, the purple star astrology that this atlas records among the divinatory disciplines, which organises a birth chart around the pole star Ziwei as the sky organises itself around the pole. One name, borrowed twice, for a palace, a city and a fate.