Xu, the Emptiness of the Black Tortoise

Xu, the Emptiness, is the eleventh of the twenty-eight xiu and the fourth mansion of the northern palace, the Black Tortoise, the central mansion of the winter quarter. Its name means void or emptiness, identified with stars in Aquarius and Equuleus, and it was anciently associated with the winter solstice and with death, ruins, and mourning. In the later astrological tradition its animal totem is the Rat. As one of the twenty-eight lunar mansions it marks a portion of the Moon's path. These are the conventions of the Chinese astronomical and calendrical tradition, in which the twenty-eight xiu divide the sky into four palaces of seven mansions, the Black Tortoise spanning the northern heavens the almanac ties to winter and the deep cold.

Xu's old association with the winter solstice, death and mourning places it at a structural low point comparable to how the Arabic Al Baldah and the Vedic Uttara Ashadha region mark their own respective quiet or empty stretches of the ecliptic, though the three traditions arrive at broadly similar "quiet point" readings by entirely different routes, in this case the calendrical logic of the solstice as the year's dead centre rather than the sparseness of visible stars, which the Arabic mansion's empty patch depends on instead.