Cross-staff (Jacob's staff)

Timeline

1. 1342–1470Provence · ProvenceThe Jewish scholar Levi ben Gerson described in a 1342 text an angle-measuring device of a main staff with a sliding crossbar, originally used for astronomical observation and geodesy; Renaissance mathematicians such as Regiomontanus further promoted its geometric angle-measuring principles.
2. 1480–1490Iberia and the Atlantic · AtlanticIn the 1480s Portuguese sailors derived latitude from the altitudes of the sun and Polaris on the open sea; brass astrolabes were accurate but expensive, and the cross-staff principle, similar to the Arabic kamal, became a cheap and easy-to-make alternative, laying the conceptual groundwork for its 16th-century maritime spread.
3. c. 1515–1650Atlantic and Northern European sea routes · Northern EuropeIn 1514 Johannes Werner recommended using the cross-staff at sea; around 1515 Portuguese sailors began adopting it systematically. In 1571 William Bourne published a sailing manual detailing its use; in 1594 Davis's backstaff solved the inconvenience of looking directly at the sun. By the 18th century, the octant and sextant replaced the cross-staff with superior designs.