The triquetrum (derived from the Latin ''tri-''
three"and ''quetrum''
cornered" was the medieval name for an ancient
astronomical
Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, galaxies ...
instrument first described by
Ptolemy
Claudius Ptolemy (; grc-gre, Πτολεμαῖος, ; la, Claudius Ptolemaeus; AD) was a mathematician, astronomer, astrologer, geographer, and music theorist, who wrote about a dozen scientific treatises, three of which were of importanc ...
(''c.'' 90–''c.'' 168) in the ''
Almagest
The ''Almagest'' is a 2nd-century Greek-language mathematical and astronomical treatise on the apparent motions of the stars and planetary paths, written by Claudius Ptolemy ( ). One of the most influential scientific texts in history, it canoni ...
'' (V. 12). Also known as ''Parallactic Rulers'', it was used for determining altitudes of heavenly bodies. Ptolemy calls it a "''parallactic instrument''" and seems to have used it to determine the
zenith distance
The zenith (, ) is an imaginary point directly "above" a particular location, on the celestial sphere. "Above" means in the vertical direction (plumb line) opposite to the gravity direction at that location (nadir). The zenith is the "highest" ...
and
parallax
Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between those two lines. Due to foreshortening, nearby objects ...
of the
Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
.
[Kelley, D., Milone, E., (2005), ''Exploring Ancient Skies: An Encyclopedic Survey of Archaeoastronomy'', pages 77-79. Birkhäuser.][Gassendi, P., Thill, O., (2002) ''The Life of Copernicus (1473-1543)'', pages 118-120. Xulon Press.]
Design
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
The zenith (, ) is an imaginary point directly "above" a particular location, on the celestial sphere. "Above" means in the vertical direction (plumb line) opposite to the gravity direction at that location (nadir). The zenith is the "highest" ...
(or, alternatively, the
altitude
Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context ...
) of a
celestial object
An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical object, physical entity, association, or structure that exists in the observable universe. In astronomy, the terms ''object'' and ''bod ...
could be calculated.
Use
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
An astrolabe ( grc, ἀστρολάβος ; ar, ٱلأَسْطُرلاب ; persian, ستارهیاب ) is an ancient astronomical instrument that was a handheld model of the universe. Its various functions also make it an elaborate inclin ...
.
Copernicus
Nicolaus Copernicus (; pl, Mikołaj Kopernik; gml, Niklas Koppernigk, german: Nikolaus Kopernikus; 19 February 1473 – 24 May 1543) was a Renaissance polymath, active as a mathematician, astronomer, and Catholic canon, who formulated ...
describes its use in the fourth book of the ''
De revolutionibus orbium coelestium
''De revolutionibus orbium coelestium'' (English translation: ''On the Revolutions of the Heavenly Spheres'') is the seminal work on the heliocentric theory of the astronomer Nicolaus Copernicus (1473–1543) of the Polish Renaissance. The book, ...
'' (1543) under the heading "''Instrumenti parallactici constructio''."
The instrument was also used by
Tycho Brahe
Tycho Brahe ( ; born Tyge Ottesen Brahe; generally called Tycho (14 December 154624 October 1601) was a Danish astronomer, known for his comprehensive astronomical observations, generally considered to be the most accurate of his time. He was k ...
[Christianson, J., (2000), ''On Tycho's Island: Tycho Brahe and His Assistants, 1570-1601'', pages 73-75. Cambridge University Press.] in the same century.
See also
*
List of astronomical instruments
Astronomical instruments include:
*Alidade
*Armillary sphere
*Astrarium
*Astrolabe
*Astronomical clock
*the Antikythera mechanism, an astronomical clock
*Blink comparator
*Bolometer
*the Canterbury Astrolabe Quadrant
*Celatone
*Celestial sphere
*C ...
Notes
External links
*
{{Portal bar, Astronomy, Stars, Solar System, Greece, Science
Ancient Greek astronomy
Astrometry
Astronomical instruments
Historical scientific instruments