It can be used as a sun dial to tell time, if the approximate latitude and season is known, or to tell latitude, if the time is known or observed (at solar noon). It may be considered to be a simplified, portable armillary sphere, or a more complex form of astrolabe.
Parts of the instrument go back to instruments made and used by ancient Greek astronomers. Gemma Frisius combined several of the instruments into a small, portable, astronomical-ring instrument. He first published the design in 1534, and in Petrus Apianus's Cosmographia in 1539. These ring instruments combined terrestrial and celestial calculations.
The dial is suspended from a cord or chain; the suspension point on the vertical meridian ring can be changed to match the local latitude. The time is read off on the equatorial ring; in the example below, the center bar is twisted until a sunray passes through a small hole and falls on the horizontal equatorial ring.
Traveller's sundial or universal ring dial, a portable version of astronomical rings, closed for carrying.
Traveller's sundial, open, ready for use. See Commons annotations for labels of parts of the traveller's sundial.
Traveller's sundial, in use, with position of light beam (inside highlighting square) being read off to tell the time. See annotations for how to read the sundial.