The torquetum, a complex instrument used in pre-telescopic astronomy, was used to measure the position of celestial bodies in the horizontal (azimuth and altitude), ecliptical (longitude and latitude) and equatorial systems (hour angle and declination). It is similar in function to Ptolemy's observational armillary sphere (astrolabon organon). However, it was not primarily used for astronomical observations (although such observations have occasionally survived from the 13th and 14th centuries), but rather as an aid to master the difficult coordinate transformations without calculations, as well as to demonstrate the principles of Ptolemaic astronomy.
The complete description of the instrument as PDF for download
