Background
Little is known of the life of Bernard of Verdun, except that he was born in Verdun, a town in the French region of Lorraine, in the department of Meuse, and lived in the second half of the thirteenth century.
Little is known of the life of Bernard of Verdun, except that he was born in Verdun, a town in the French region of Lorraine, in the department of Meuse, and lived in the second half of the thirteenth century.
Bernard's place in the history of astronomy is assured through his Tractatus super totam astrologiam (Treatise on all of Astrology), which is thought to have been written at the end of the thirteenth century. The treatise did not attempt to explain astrology (there was no well-defined boundary between astrology and astronomy in Bernard’s epoch, and both notions were often interchanged) but is instead a clearly presented didactic explanation of Ptolemy’s geocentric teaching intended for Bernard’s students.
It begins with a description of the four basic elements in the sense used by Aristotle and Ptolemy, and deals with the nature of the celestial sphere and its circular motion.
Bernard then treats the tables of arcs and chords, and covers the basic astronomical notions. He studies in detail the motions of the Sun and Moon and their related effects (above all eclipses), as well as the motions of the other five planets then known. Some efforts to formulate an alternative View of the movements of celestial bodies to replace Ptolemy’s teaching had already been made in Bernard’s lifetime. One of the most serious such attempts was the De motibus celormn of al-Bitruji (Alpetragius), the twelfth-century Spanish—Islamic astronomer who was active in Cordoba and Seville. His system, however, was not sufficiently developed to challenge Ptolemy’s teachings, and Bernard himself rejected it utterly and conceived his own work as a defense of Ptolemy.
Bernard of Verdun defended Ptolemy's theory of epicycles and eccentrics against al-Bitruji's system of homocentric spheres. Contrary to the criticisms of Averroes and al-Bitruji, Bernard maintained that Ptolemy's theory was consistent with Aristotle's physics. He further pointed out many technical inadequacies of al-Bitruji's model, and considered it to be insufficiently developed to challenge Ptolemy's system.
Bernard also rejected the hypothesis of the ninth-century Baghdad astronomer Thahit ibn Qurra that trepidation (trepidatio as "agitated" movement of the intersections of the equator and the ecliptic) caused the assumed unevenness of precession, even though that explanation was widely accepted until the late sixteenth century, when it was disproved by Tycho Brahe.
On the other hand, Bernard’s work has many similarities to the Theorica planetarum by Campanus de Novara, as well as to the work of al-Battani, Muslim astronomer of the ninth to tenth centuries. Of major importance is the final, tenth part of Bernard’s book, in which he describes the principles of the torquetum and the noctilabium. The torquetum is one of the oldest astronomical instruments, measuring angles in horizontal, equatorial, and ecliptic coordinate systems. The inclination of the first oblique board to the instrument’s horizontal base reproduces the equator’s inclination to the horizon; the second oblique board is adjusted with regard to the first board to the ecliptic’s inclination to the equator. The ecliptic latitudes are measured on the circle, which is perpendicular to the ecliptic, and then on the freely hanging semicircle by means of the plumb line of the height above the horizon. The noctilabium is an instrument that determines the time at night (assuming that the day’s date is known) using the Pole Star and other bright stars in its vicinity.