An orrery consisted of a silver ball that was surrounded by twelve rings. Each of the rings represented one of the constellations visible in the night sky. A special lens held within the silver ball emitted a cone of blue light that illuminated the rings surrounding the sphere.
Once calibrated by an astrologer, an orrery would let the user know which constellations were currently visible. By moving the outer rings the astrologer could see which constellations were visible on any date based upon the light shining on them from the lens.
Astrologers used orreries to determine the constellations they would use to "hang" their spells.