An Etymological Dictionary of Astronomy and Astrophysics

An ancient astronomical instrument first described by Ptolemy in the Almagest, used in measuring the → altitude of a celestial body. It consisted of three long arms of wood. The first is perpendicular to the → horizon; the second is connected at the head of the first with an axis. The third had a graduated scale. An object was sighted along one arm and its → zenith distance was read on the graduated scale.
It performed the same function as the → quadrant. Also called parallactic rulers, Ptolemy’s rulers.

Etymology (EN): L. neuter of triquetrus “three corned,” from tri-→ three + -quetrus “corned.”

Etymology (PE): Se-šâx, literally “three corned,” from se,
→ three, + šâx “→ branch, horn.”