De revolutionibus orbium coelestium
The triquetrum was one of the most popular astronomical instruments until the invention of the telescope, it could measure angles with a better precision than the astrolabe. Copernicus describes its use in the fourth book of the "De revolutionibus orbium coelestium" (1543) under the heading ""Instrumenti parallactici constructio"." The instrument was also used by Tycho Brahe in the same century.