Torquetum

The torquetum or turquet is a medieval astronomical instrument designed to take and convert measurements made in three sets of coordinates: Horizon, equatorial, and ecliptic. It is said to be a combination of Ptolemy's astrolabon and the plane astrolabe. In a sense, the torquetum is an analog computer. The origins of the torquetum are unclear. Its invention has been credited to multiple figures, including Bernard of Verdun, Franco of Poland, and Jabir ibn Aflah. The earliest account of the torquetum appear in the writings of Bernard of Verdun and Franco of Poland. Franco of Poland's work was published in 1284; however, Bernard of Verdun's work does not contain a date. Therefore, it is impossible to know which work was written first. Franco's work was more widely known and is credited with the distribution of knowledge about the torquetum. Some authors have supposed that the first torquetum was built by Jabir ibn Aflah, though there is conflicting evidence that suggests that Jabir simply inspired the invention of the torquetum, and little evidence exists to suggest that it was created by him. The instrument was first created sometime in the 12th or 13th century. However, the only surviving examples of the torquetum are dated from the 16th century. In the middle of the 16th century, the torquetum had numerous structural changes to the original design. The most important change was by instrument-maker, Erasmus Habermel. His alteration allowed for astronomers to make observations to all three of the scales. A torquetum can be seen in the famous portrait The Ambassadors (1533) by Hans Holbein the Younger. It is placed on the right side of the table, next to and above the elbow of the ambassador clad in a long brown coat or robe. The painting shows much of the details of the inscriptions on the disk and half disk, which make up the top of this particular kind of torquetum. A 14th century instrument, the rectangulus, was invented by Richard of Wallingford. This carried out the same task as the torquetum, but was calibrated with linear scales, read by plumb lines.