Triquetrum (astronomy)

The triquetrum (derived from the Latin tri- ["three"] and quetrum ["cornered"]) was the medieval name for an ancient astronomical instrument first described by Ptolemy (c. 90 – c. 168) in the Almagest (V. 12).

The triquetrum performed the same function as the quadrant and was devised to overcome the difficulty of graduating arcs and circles.

By reading the position of the lower rod, in combination with the vertical length, the zenith distance (or, alternatively, the altitude) of a celestial object could be calculated.

[1][3] 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.

[2] Copernicus describes its use in the fourth book of the De revolutionibus orbium coelestium (1543) under the heading "Instrumenti parallactici constructio.