The torquetum (or turketum): Was it an observing instrument?

The torquetum was a complex astronomical instrument whose construction is known thanks to certain descriptions, iconography and few extant artefacts. It was used in pre-telescopic astronomy from at least the 13th century. However, the usefulness of the torquetum as an observing instrument remains unknown. It is my intention to introduce a preliminary analysis of the merits and limitations of the torquetum in determining the coordinates of celestial bodies. For this purpose I shall refer to (1) written sources that contain descriptions of the construction of the instrument and its use; (2) the results of an examination of the torquetum constructed by Hans Dorn of Vienna (c.1487) and conserved in the Jagiellonian University Museum in Cracow; (3) elements of a theory of the instrument, which allow us to trace down instrumental errors, both systematic and accidental; (4) the only substantial and extant set of observations made with the torquetum, that is, a catalogue of 58 stars, compiled in Kassel in the years 1560–63.