Tympan

Tympan means skin, and is used in a variety of technical meanings.

In an astrolabe, a tympan is a metal plate on which the coordinates of the celestial sphere (azimuth and altitude) are engraved in a stereographic projection. A tympan is specific to a particular latitude, so most astrolabes come with a set of interchangeable tympans suitable for use at different latitudes, usually those of particular cities of importance (Cairo, Mecca, Medina, Jerusalem...).