From the Arabic word meaning guide, a navigation instrument of great antiquity. It was used by Arab seamen in the Red Sea and the Indian Ocean for at least six centuries for measuring the altitude of a celestial body. The instrument became known to European navigators through Vasco da Gama after he had rounded the African continent from the Atlantic to the Indian Ocean in 1497. The principle of the kamal, which is the same as that of the cross-staff, depends upon the geometrical properties of similar triangles. The simplest form comprised a rectangular board or tablet of wood, to the centre of which was secured a knotted cord. The tablet was held so that the upper and lower edges coincided respectively with the observed body and the horizon vertically below it. In this position the cord was stretched taut to the observer's eye, and the ratio between the fixed length of the tablet and variable length of the cord between tablet and eye, being a function of the altitude of the observed body, gave the measurement. The positions of the knots on the cord were related to the meridian altitudes of a given star appropriate to the latitudes of headlands and harbours along the route. In a modified form, it can still be seen in use by the Arab navigators of dhows in the Red Sea and off the East African coast.
Observing the altitude with the Kamal
Subjects: Maritime History.