A planispheric astrolabe uses a stereographic projection of the celestial sphere as a physical calculating device. A rotating star map is compared with a latitude-specific plate; a sighting rule measures an object's angular altitude. Aligning that measurement with the engraved geometry permits time and celestial-configuration calculations.[ref]
A sighting rule used to measure a direction or angle. On a conventional astrolabe, it rotates over the back's angular scale; it is distinct from the front rule used to read the projected sky geometry.

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An astrolabe is an ancient astronomical instrument used for solving problems related to time and the position of celestial bodies. It can be used to measure the altitude of stars and planets above the horizon, determine local latitude, and assist in navigation. The device consists of a flat circular disk, often made of brass or other materials, with a rotatable arm called an alidade that allows the user to point at celestial objects.
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