Triquetrum (astronomy)
Medieval astronomical instrument for measuring celestial altitudes.
The triquetrum, whose name comes from Latin for "three-cornered," is the medieval term for an ancient tool first recorded by Ptolemy in his work the *Almagest*. Also called Parallactic Rulers, it measured the altitudes of celestial bodies. Ptolemy referred to it as a "parallactic instrument" and likely employed it to find the Moon's zenith distance and parallax.
In design, the triquetrum did the same job as a quadrant but was created to avoid the difficulty of marking arcs and circles. It had a vertical post with a graduated scale and two pivoted arms connected at the top and bottom, with the upper arm fitted with sights. The arms were joined so their ends could slide. When a person sighted along the upper arm, the lower arm changed its angle. By reading the lower rod's position along with the vertical length, the zenith distance—or altitude—of a celestial object could be calculated.
The triquetrum was a widely used astronomical instrument until the telescope appeared, offering better angle measurement accuracy than the astrolabe. Copernicus described its use in the fourth book of *De revolutionibus orbium coelestium* under the heading "Instrumenti parallactici constructio." Tycho Brahe also used the instrument in the same century.
- field
- Astronomy
- known_for
- Measuring altitudes of heavenly bodies; determining zenith distance and parallax of the Moon
- inventor
- Ptolemy (described in Almagest V.12)
- other_users
- Copernicus, Tycho Brahe
Lore & Background
The triquetrum performed the same function as the quadrant and was devised to overcome the difficulty of graduating arcs and circles. It consisted of a vertical post with a graduated scale and two pivoted arms hinged at the top and bottom, the upper arm carrying sights. The two arms were joined so that their ends could slide. As a person sighted along the upper arm, the lower one changed its angle. 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.
The triquetrum was one of the most popular astronomical instruments until the invention of the telescope, and 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 under the heading 'Instrumenti parallactici constructio.' The instrument was also used by Tycho Brahe in the same century.
Reader's Guide
The triquetrum represents a significant step in pre-telescopic astronomical instrumentation, offering improved angular measurement precision over the astrolabe. Its design avoided the need for finely graduated arcs, instead using a combination of linear scales and sliding arms to compute angles. This made it a practical tool for determining the altitude and zenith distance of celestial bodies, particularly the Moon, as Ptolemy originally employed it. The instrument's longevity is attested by its continued use by Copernicus and Tycho Brahe, both of whom described or employed it in their own work. The triquetrum thus bridges ancient and early modern astronomy, demonstrating how geometric principles could be applied to observational challenges before the advent of telescopic optics. Its legacy lies in its role as a reliable, widely adopted instrument that helped refine measurements of celestial positions, contributing to the data that eventually supported heliocentric models.
Did You Know?
- The name 'triquetrum' comes from Latin tri- 'three' and quetrum 'cornered'.
- Ptolemy first described the triquetrum in the Almagest (V.12) and called it a 'parallactic instrument'.
- The triquetrum could measure angles with better precision than the astrolabe.
- Copernicus described the triquetrum in De revolutionibus orbium coelestium under the heading 'Instrumenti parallactici constructio'.
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