The 28 Lunar Mansions divide the sky into four celestial beasts, marking the moon's progress.
The Twenty-Eight Mansions divide the band of sky the moon crosses into twenty-eight "lodges," each a segment marked by a constellation, grouped under the four celestial animals of the directions. As the moon moves a lodge a night, the heavens become a partitioned, addressable map.
The Mansions unlock the sky as a reference grid — a coordinate system for tracking position, season, and ritual time against fixed stars. Long before instruments, this gave astronomers a shared address book of the heavens, turning the overwhelming dome of night into twenty-eight named, ordered, navigable rooms.