Building a Cross-Staff — The Medieval Navigator's Angle-Measuring Instrument

The cross-staff (also called Jacob's staff or balestilha) is one of the simplest and most effective angle-measuring instruments ever devised — a graduated wooden staff with a sliding crosspiece that measures the angular distance between two objects, most commonly the altitude of the Sun or a star above the horizon. First described by the Provençal Jewish mathematician Levi ben Gershon in 1342, the cross-staff became the standard navigation instrument of the Age of Exploration. Columbus, Vasco da Gama, and Magellan all used cross-staffs to determine their latitude at sea. The principle is elementary trigonometry: the observer holds one end of the main staff to their eye, slides the crosspiece along the staff until one end aligns with the horizon and the other with the celestial object, and reads the angle from the graduated scale on the staff. Despite its simplicity, a well-made cross-staff can measure angles to within half a degree — accurate enough for practical ocean navigation. This blueprint builds a functional cross-staff from hardwood following medieval construction methods.