A planispheric astrolabe is an analogue computer for the sky, cast in brass because wood warps. A raised-rim mater holds a stack of tympans — one engraved per latitude — under a pierced rete whose filed star-pointers and off-centre ecliptic ring turn over them.

The surviving instruments sit at a dark antique-gold patina, bright only where pointers and cut scales catch light. Star names run in Kufic beneath the pointers; the back carries the shadow square, the calendar and the trigonometric scales. Nothing on the face is decoration — every line is a working scale.