. The Middle Ages have left a variety of instruments intended to measure celestial phenomena. Most were based on Hellenistic predecessors, but some were invented or developed during the medieval period. Ara-bic scholars served as the principal means of transmitting knowledge of these instruments and their design principles to the West.
To tell time by celestial means requires at least a simple sundial, and this was frequently incorporated into the architectural detail of churches. An excellent specimen is the angel holding a sundial found on the south tower of Chartres cathedral (12th c.). Portable sundials for travelers also were fabricated.
The astrolabe was the next most common instrument. By measuring the angle above the horizon of the sun or a selected star, the astrolabe projects onto a celestial map the position of the body and thus allows the observer to read the time of day from a dial or graph. Indeed, the astrolabe dial is the ancestor of the analogue clock face. The most common subtype of astrolabe was a simplified version without the celestial map, called the mariner’s astrolabe. The spread of astrolabes stimulated interest in precise astronomical tables giving the position of the sun, and sometimes other celestial bodies, for each day of the year. One of the earliest was produced at Montpellier by Robert de Montpellier in 1141.
Southern France was the source of several other astronomical instruments as well. Robertus Anglicus, writing at Montpellier ca. 1276, described a form of quadrant (quadrans vetus) whose ultimate origins lie in India. Shortly thereafter, a member of the scholarly Provençal family of translators Yacob ben Machir ibn Tibbon (Prophatius Judaeus; d. ca. 1304), developed the “quadrant of Israel” incorporating features from astrolabes. It later was known as the “new quadrant” (quadrans novus). Another Jewish scholar born in Languedoc, Levi ben Gershom, invented ca. 1342 a simplified form of measuring device, the cross-staff or Jacob’s staff, later favored by mariners. Sightings from the cross-staff or the mariner’s astrolabe were referred to tables like that mentioned above to determine latitude while at sea. These instruments remained in common use into the 17th century.
A novel navigational instrument appeared in the 12th century, the magnetic compass. Although there is still controversy over how the compass reached Europe, its Chinese origin is indisputable. The earliest European description comes from an English scholar resident in Paris, Alexander Neckham, writing in 1187. The earliest complete treatise on the compass was composed by the French soldier Pierre de Maricourt (Petrus Peregrin-us) in 1269.
Medieval adaptations of earlier instruments include the addition of a sighting tube to the armillary sphere. Resembling a modern telescope but lacking lenses, the tube was apparently used to better orient the sphere to the north celestial pole by sighting Polaris in isolation from its surrounding stars. The earliest representation of such a device is in a manuscript by Gerbert of Aurillac (930–1003), later Pope Sylvester II. Perhaps related to the sighting tube was the nocturlabe, or nocturnal, a device with a dial and a sighting hole. One aligned pointers on the device with stars in the circumpolar constellations and read the time from a dial.
Bert S.Hall
Gunther, Robert T. Early Science in Oxford. 15 vols. Oxford: Clarendon, 1920–40, Vol. 2: Astronomy (1927).
North, J.D. “The Astrolabe.” Scientific American 230(1974): 96–106.
Poulle, Emmanuel. “Le quadrant nouveau médiéval.” Journal des savants (1964): 148–67, 182–214.
——. Les instruments astronomiques du moyen âge. Paris: Brieux, 1983.
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