View source for Astronomy:Triquetrum

{{Short description|Ancient astronomical instrument}}
[[Image:Dreistab 00.jpg|thumb|Triquetrum of Copernicus]]

The '''triquetrum''' (derived from the Latin ''tri-'' ["three"] and ''quetrum'' ["cornered"]) was the medieval name for an ancient astronomical instrument first described by [[Biography:Ptolemy|Ptolemy]] ({{Circa|90|168}}) in the ''[[Astronomy:Almagest|Almagest]]'' (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 and [[Astronomy:Parallax|parallax]] of the [[Astronomy:Moon|Moon]].<ref name="kelley1">Kelley, D., Milone, E., (2005), ''Exploring Ancient Skies: An Encyclopedic Survey of Archaeoastronomy'', pages 77-79. Birkhäuser.</ref><ref name="gassendi1">Gassendi, P., Thill, O., (2002) ''The Life of Copernicus (1473-1543)'', pages 118-120. Xulon Press.</ref>{{Self-published-inline|date=April 2012}}

==Design==
The triquetrum performed the same function as the [[Engineering:Quadrant (instrument)|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|altitude]]) of a celestial object could be calculated.<ref name="kelley1"/><ref>Krebs, R., (2004), ''Groundbreaking Scientific Experiments, Inventions, and Discoveries of the Middle Ages and the Renaissance'', page 225. Greenwood Publishing Group.</ref>

==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 [[Astronomy:Astrolabe|astrolabe]].<ref name="gassendi1"/> Copernicus describes its use in the fourth book of the ''[[Astronomy:De revolutionibus orbium coelestium|De revolutionibus orbium coelestium]]'' (1543) under the heading "''Instrumenti parallactici constructio''."<ref name="gassendi1"/> The instrument was also used by [[Biography:Tycho Brahe|Tycho Brahe]]<ref>Christianson, J., (2000), ''On Tycho's Island: Tycho Brahe and His Assistants, 1570-1601'', pages 73-75. Cambridge University Press.</ref> in the same century.

==See also==
*[[Astronomy:List of astronomical instruments|List of astronomical instruments]]

==Notes==
{{Reflist}}

==External links==

{{Greek astronomy}}
{{Astronomy in medieval Islam}}

[[Category:Ancient Greek astronomy]]
[[Category:Astrometry]]
[[Category:Astronomical instruments]]
[[Category:Historical scientific instruments]]

{{Sourceattribution|Triquetrum (astronomy)|1}}