A quadrant is a mathematical or an astronomical instrument featuring a graduated arc encompassing a quarter of a full circle or 90?. Astronomical quadrants were used mainly for determining the altitude of a celestial body above the horizon. Meridian altitudes of the sun or a bright star could be employed for determining the geographical latitude of an observer; or, at a known latitude, the observer could obtain the time from an altitude measured not too close to the meridian direction.

One of the earliest examples of a quadrant was the 'plinth' described around 150 A.D. by Claudius Ptolemy in his Almagest. Here the shadow of an elevated horizontal peg was projected on a graduated arc of 90? that was set up in the plane of the meridian to determine the sun's noontime altitude.

Several large quadrants were constructed by Islamic astronomers: mural quadrants that were fixed in the meridian plane and altazimuth quadrants that could be rotated to arbitrary compass directions for the simultaneous determination of the altitude and the azimuth of a celestial body. Quadrants of both types were also employed by Renaissance astronomers, especially notable in this respect were the large quadrants (up to 2 meters in radius) completed in the 1580s by the Danish astronomer Tycho Brahe at his observatory Uraniborg on the isle of Hven.

Smaller, hand-held versions of the quadrant known as horary quadrants were very popular in the Middle Ages for determining the time from the Sun's altitude. These too were derived from Islamic prototypes and several examples of these quadrants have been preserved. The earliest and most common type, known as the quadrans vetus or 'old quadrant', could be used for every latitude. This type was designed to convert a measured solar altitude to the hour of the day. This was achieved through a bead sliding on a string with a plumb bob that was first set according to the observer's latitude and the day of the year. The sun was then observed through a pair of sights fixed on one edge of the instrument and the time was read off where the bead intersected the hour lines projected on the face of the instrument.

Other types, designed for a fixed latitude, employed a rigid ruler on which a zodiacal scale was inscribed. By observing the sun in a similar fashion, the time was read off where the appropriate zodiacal sign on the ruler intersected the hour lines.

J. A. Bennett,"Quadrant", in R. Bud & D. J. Warner (eds), Instruments of Science: An Historical Encyclopedia (New York and London, 1998), pp. 501-3; N. L. Hahn,"Medieval Mensuration: Quadrans vetus and Geometrie due sunt partes principales", Transactions of the American Philosophical Society, 72 (1982), part 8; D. A. King,"Rubc", in The Encyclopaedia of Islam, (2nd edition, Leiden, 1994), vol. 8, pp. 574-5; W. R. Knorr,"Sacrobosco's Quadrans: Date and Sources", Journal for the History of Astronomy, 28 (1997), 187-222; H. Raeder, E. Str?mgren and B. Str?mgren, Tycho Brahe's Description of his Instruments and Scientific Work as given in Astronomiae Instauratae Mechanica (Wandesburgi 1598) (Copenhagen, 1946); P. Schmalzl, Zur Geschichte des Quadranten bei den Arabern (Munich, 1929); reprinted in: F. Sezgin (ed.), Arabische Instrumente in orientalistischen Studien (6 vols, Frankfurt am Main, 1991), vol. 6, pp. 189-331.

Silke Ackermann and Robert van Gent
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