The Torquetum - History
The Torquetum - History
The first known accounts of the torquetum are those of Bernardus de Virduno (Bernard of Verdun) in the Tractus super totum astrologium3 and Franco de Polonia (Franco of Poland). It is not possible to ascribe priority to either since the date of Bernard's account is unknown. The earliest known manuscript of Franco's account is dated at 1284.4 Franco does appear to be the main disseminator of the instrument, since manuscript versions of his account are much more common, and other descriptions, up to the late fifteenth century, are based on Franco's work. A mid-fourteenth century manuscript at the Ashmolean Library of Oxford, based on Franco's tract,5 includes labeled construction diagrams, as well as a diagram and description of the semis.6
Torquetum labeled construction diagrams
Torquetum diagram of semis
A number of observations using the torquetum have been recorded, beginning in the thirteenth century. Peter of Limoges used one to measure the initial position of the comet of 1299 in the eighteenth degree of Taurus.7 Jean de Murs, a Parisian astronomer famous for introducing the Alphonsine tables to Medieval astronomy, and exceptional in his time for his record of astronomical observations made between c. 1318 and 1344, recorded the entry of the Sun into Aries on March 12, 1318 using a torquetum. For this observation he invoked the the authority of Alphonse X, thus becoming one of the earliest advocates of these tables.8
In the late fifteenth and early sixteenth centuries Franco's description was superseded by a number of new accounts, among the best known being those of Regiomontanus and Apian. Johannes Regiomontanus (Johann Müller, 1436-76) was a German astronomer famous for his tables of planetary motion as well as his important celestial observations. In 1472 he made scientific observations of what later became known as Halley's comet.9
Regiomontanus instrument, as depicted in 1472, included a variety of scales (like those found on an astrolabe) allowing the determination of time, the position of the sun in the zodiac on a given day etc., as does the instrument depicted by Gregor Reisch.
Perhaps the best known account, and certainly the most commonly reproduced image of a torquetum, is that of Peter Apian (1495-1552) published in 1532.10 Apian depicts a "back-to-basics" instrument made of wood with just the required observational scales. This instrument is also set for a fixed latitude.
An illustration of comet observation published by Apian in 1532 includes a torquetum, collapsed into the horizon configuration, and a cross-staff.11 In 1540 Apian published his second major work, the Astronomicum Caesarium, including his pioneering observations on comets. This was the first scientific description of comets other than their positions in the sky, describing the appearances of five comets (including Halley's) and the fact that their tails always point away from the sun.12 This delightful illustration is incredibly rich in information. Apian demonstrates diagramatically how the tail of the comet always points directly away from the sun (he was the first to note this phenomena), showing both the comets position in the sky (including its approximate position along the ecliptic) and the sun's position on the ecliptic below the horizon (and thus invisible to the observer!). He also illustrates how a cross-staff is used to determine the angle between a celestial object such as a comet and a bright star (Cauda Leonis).A late observation in which a torquetum was involved was the observation of Spica in 1575 by Landgrave William IV working with Tycho Brahe.13
Comet Observation using a Cross-staff (Jacob's Staff) and Torquetum
from Peter Apian. Ein kurtzer bericht d' Observation unnd urtels / des Jüngst erschinnen
Cometen / im weinmon un wintermon dises XXXII. Jars. Ingolstadt (1532).
Despite this late example of its usage as an observational instrument, most scholars seem to consider its popularity in the sixteenth century to be do to its other uses: 1) as a device to demonstrate the various coordinate systems of Ptolomaic astronomy, 2) as an analog computer to inter-convert measurements between coordinate systems without the use of tedious calculations, and 3) as a demonstration of the owners astronomical expertise and sophistication (the torquetum is, after all a very impressive instrument). The most famous example of this last usage is in the painting The French Ambassadors by Hans Holbien the Younger (1533).14
3 Gillespie, Charles Coulston (ed.). Dictionary of Scientific Biography Charles Scribner's Sons, New York (1970-80) II p 24.
4 Hudson, p 624.
5 Giles Hudson, personal communication.
6 Gunther, R. T. Early Science in Oxford: v. II, Astronomy. Oxford University Press, Oxford (1923) pp 35-7. The images here are from Gunther, apparently redrawn from the original manuscript.
7 Hudson, p 624.
8 Gillespie, VII p 129, 130.
9 Azimov, Issac. Azimov's Biographical Encyclopedia of Science and Technology 2nd ed. Doubleday & Company, Inc. Garden City (1982) p 70.
10 Apian's Astronomicum Caesarium is now available on-line. The RARE BOOK ROOM site from the Bodleian Library at the University of Oxford has the book at http://www.rarebookroom.org/Control/appast/index.html. The torquetum, including construction diagrams, is shown on thumbnails 61-62 (November 2010). The entire book may also be downloaded as a pdf from the Vienna University Observatory rare book collection at: http://www.univie.ac.at/hwastro/ (December 2007).
11 Wolfschmidt, Gudrun. Planeten, Kometen, Finsternisse - Peter Apian als Astronom und Instrumentenbauer in Peter Apian: Astronomie, Kosmographie und Mathematik am Beginn der Neuzeit. Polygon-Verlag Buxheim, Eichstätt (1995) p 101.
12 Gillespie, I p 179.
13 Ibid, II p 404.
14 Images of this painting are avaialble on a variety of sites. One with very high quality images and closeups is the Web Gallery of Art site (December 2007). Click here to see an example of a closeup of the torquetum from this painting.
© R. Paselk 2013, Last modified 27 December 2020