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Triquetrum Facts

Triquetrum Facts
The triquetrum is the medieval name for parallactic rulers, which was an ancient astronomical tool described by Ptolemy in the Almagest (2nd century treatise on star and planet paths). The triquetrum's purpose was to measure the altitude of celestial objects. Ptolemy used it to determine the zenith distance of the moon. The triquetrum was capable of doing the same job as the quadrant, without the difficult arcs and circles associated with the quadrant's design. The triquetrum had a vertical post, graduated scale, and two arms that pivoted and were hinged at the bottom and top. The upper arm of the triquetrum had sights. While sighting the upper arm, the lower arm changed its angle. The arms were capable of sliding because they were joined. The reading of the lower rod's position provided the altitude or zenith distance of a celestial object. The name triquetrum is derived from the Latin words tri (three), and quetrum (cornered).
Interesting Triquetrum Facts:
Until the telescope was invented, the triquetrum was considered to be one of the most popular of the astronomical tools.
The triquetrum could measure angles much more accurately than an astrolabe.
The triquetrum was used by Ptolemy to parallax (difference in position viewed from different lines of sight) of the moon as well as the zenith distance of the moon.
The triquetrum used three wooden rulers. The first ruler was perpendicular to the horizon while the second was connected to the head of the first ruler via an axis. The third ruler had the chord and was nailed to the base.
The triquetrum is mentioned in Copernicus' book De revolutionibus orbium coelestium, written in 1543.
Tycho Brahe, a Danish astrologer, used the triquetrum in the 1500s. His small southern observatory built on an island, contained several instruments of astrology including the triquetrum, a small astrolabe, small quadrants, small sextants, an azimuth circle, and an astronomical ring. In the collection was a triquetrum that belonged to Copernicus.
Ptolemy designed a triquetrum to replace the quadrant, which was more difficult to use.
A Norwich born physician named William Cuningham (1531 - 1586) wrote about Ptolemy's parallactic rulers in The Cosmographical Glasse, which was published in 1559. The book discussed surveying and practical mathematics, and he discussed and promoted the use of Ptolemy's rulers (triquetrum) and the quadrant.
As time passed the triquetrum (parallactic rulers) became less popular in writings than the astrolabe and the quadrant.
The triquetrum was also written about in Commentaria, in 1538, which used the manuscript owned by Regiomontanus.
The parallax of the moon was traditionally measured with the use of a triquetrum.
The triquetrum has also been referred to as the 'three rod'.
An 1873 oil on canvas painting called 'Nicolas Copernicus with Triquetrum' shows the astronomer with a triquetrum. The artist was Jan Matejko
In the 1967 TV series The Prisoner, a triquetrum is used in the episode titled "The Chimes of Big Ben".
The triquetrum bone, located in the wrist, also has its name derived from the Latin words meaning 'three cornered'.


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