seaman's quadrant

seaman's quadrant, often called the simple quadrant, the earliest instrument used by navigators for measuring the altitude of a heavenly body. It was in the form of a quarter circle of brass or wood with a plumb line suspended, when the instrument was in use, from the centre of the circle of which the quadrant formed part. One radial edge of the instrument was fitted with two pins or sights by means of which a sight of the heavenly body was acquired. This quadrant required two observers, one to bring the observed body into the line of the two pins and the other to note the position where the plumb line crossed the arc of the instrument.

This type of quadrant was in use by the early Portuguese navigators during their exploration by sea along the West African coast during the second half of the 15th century. Its first use appears to have been for measuring the altitude of Polaris, the Pole Star, to find the distance made good south of Lisbon or any other port of departure. At that time the scale on the quadrants was often marked with the names of places where the latitude had been established rather than in degrees. Once tables of the sun's declination had been published—they appeared around 1480 in copies of Regimento do astrolabio e do quadrante, the first navigation manual ever published—the arc of the quadrant was graduated in degrees and minutes so that altitudes could be read in angles.

This simple quadrant could not be used on board ship unless the sea was smooth and the air calm, and even in the most suitable conditions its degree of accuracy was coarse. But it was an instrument which could be easily manufactured by the navigator himself if the need arose and, because the vertical was defined by a plumb line, the quadrant could be used for measuring altitudes when the horizon was obscured by darkness or fog.