Zi, the Turtle Beak of the White Tiger

Zi, the Turtle Beak, is the twentieth of the twenty-eight xiu and the sixth mansion of the western palace, the White Tiger. It is the smallest of the twenty-eight mansions, identified with the faint group of stars at the head of Orion. In the later astrological tradition its animal totem is the Monkey. As one of the twenty-eight lunar mansions it tracks a narrow segment of the Moon's path. These are the conventions of the Chinese astronomical and calendrical tradition, in which the twenty-eight xiu are of very unequal width, some, like the Turtle Beak, spanning only a degree or two, others far wider, because the mansions were defined by named asterisms and their determinative stars rather than by equal division of the sky.

Being the smallest of the twenty-eight mansions by angular width, Zi illustrates most clearly why the xiu are not, unlike the Vedic nakshatras, built on strictly equal arcs: because each mansion is anchored to a determinative star rather than to a fixed degree count, a stretch of sky with two named asterisms close together, as here at the boundary with Shen, produces one very narrow mansion and one correspondingly wider neighbour rather than the even thirteen-degree-twenty-minute segments the nakshatra scheme imposes regardless of what stars happen to fall where.