Torquetum

The torquetum, also called the turquet, was a medieval astronomical instrument that let an observer measure and convert readings among the horizon, equatorial and ecliptic coordinate systems without separate calculation, combining the functions of Ptolemy's astrolabon and the plane astrolabe into a single mechanical analog computer. Its origin is disputed among Jabir ibn Aflah, Bernard of Verdun and Franco of Poland, with the earliest known European references appearing in Latin texts of the 1280s and a simplified version reported at the Yuan dynasty's imperial Chinese observatory between 1276 and 1279; in the sixteenth century instrument maker Erasmus Habermel modified the design so astronomers could observe against all three of its scales at once. Peter of Limoges used a torquetum to observe Halleys Comet around 1300, and Peter Apian's 1540 book Astronomicum Caesareum gave the instrument its most detailed published description, cementing its place as a tool of observational astronomy.