cross-staff

cross-staff, or forestaff, an early navigation instrument for measuring the meridian altitude of the sun or a star to establish latitude at sea. Its precise origins are obscure but the principle is clearly the same as that of the Jacob's staff (with which the cross-staff is often confused), a medieval instrument first referred to in 1342 in a treatise by the Catalan Jew Levi ben Gerson, and used principally by surveyors and for military purposes for distance measurement. The cross-staff measured with precision angles in degrees of arc and its use at sea seems to have been first proposed, somewhat earlier than contemporary technology warranted, by the astronomer Johannes Werner who had suggested the use of lunar distances to find longitude at sea. However, the earliest record of its employment in navigation, with proper instructions as to its use, seems to have been in John of Lisbon's Livro de marinharia written in about 1515. By the middle of that century the Portuguese in their southward exploration by sea of the Atlantic Ocean were using the instrument, which was eventually to displace both the seaman's quadrant and astrolabe.

The cross-staff comprises a square-cut wooden staff about 76 centimetres (30 in.) in length with, at right angles to the shaft, a cross-piece known as the transversal that could slide up and down the staff. The instrument was aimed at the body being observed, much as a crossbow might be aimed, with one end on the observer's cheekbone. The staff was graduated to give the observer the angle of elevation, that is, the altitude of the body. To avoid altering the length of the staff to increase the range of angles, later instruments were fitted with three or four transversals. The back-staff or Davis's quadrant supplemented it from the 1590s and among English seamen supplanted it almost entirely in the 17th century.

Mike Richey