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. In a sense, the Torquetum is an analog computer.
The first Torquetums are thought to have been built some time in the 13th century. The only surviving examples date from the 16th century. Though such an instrument was described by Ptolemy in a much earlier era, it is not certain if one was built at the time.
A Torquetum can be seen in the famous portrait The ambassadors (1533) by Hans Holbein the Younger (e.g. [1]]). It is placed on the right side of the table right next to and above the elbow of the ambassador clad in a long brown coat or robe. The original of the painting, or good high resolution copies, 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.
Many Web pages explain how to build and use a Torquetum, as a learning process for the Science of Astronomy.
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