astrolabe

An astrolabe is a flat brass disc engraved with a stereographic projection of the celestial sphere, overlaid with a rotating star map. By aligning the instrument with a particular star or the Sun, a user could read off the local time, the altitude of celestial bodies, and their own approximate latitude.

It was part sighting instrument, part slide rule, part pocket planetarium — and for nearly a thousand years it was the most sophisticated scientific instrument in existence.

The astrolabe descends from Hellenistic Greek designs and was perfected by Islamic astronomers between roughly the 8th and 13th centuries. Medieval European astronomers inherited it through translations from Arabic, and it remained in widespread use until the sextant displaced it at sea in the eighteenth century.