Triquetrum (astronomy)

The three-rod, also called triquetrum or parallaktisches ruler, is an ancient astronomical goniometer for measuring zenith distances of the stars and the parallax of the moon.

Description

From the set of the similarity of the triangles that have two sides and an angle in common, the obvious idea derives, measuring devices, whose function is based on this geometric law to construct three rulers (rules). The triquetrum was already described by Claudius Ptolemy in the fifth book of the Almagest.

It was one of the most popular astronomical instruments until the invention of the telescope. It was able to measure angles with a greater accuracy than the astrolabe. Numerous scholars have addressed according to Ptolemy with the Triquetum, such as the Arab al - Battani and Jabir ibn Aflah. Also Regiomontanus grappled with the issue of possible use. He dedicated the Hungarian king Matthias Corvinus a small script in which he, however, is limited to the amount of field measurements. In the years 1515-1525 the Triquetum Nicolaus Copernicus for the determination of zenith distances of the moon and the fixed star Spica served. He describes its use in the fourth book of De revolutionibus orbium Coelestium under the heading Instrumenti parallactici constructio. The instrument was also described by Tycho Brahe, reproduced and used.

The left picture shows the instrument of Copernicus. It consists of three rulers, which form an isosceles triangle. The same one of the legs AB is vertical, the other AC is rotatable about the upper end point of the first, provided with visors and is aligned with the star to be observed; on the third, provided with a division BD ruler the length of the unequal side BC is measured and determined by the angle at A, and so the zenith distance of the star.