Quadrant, Davis

The Davis quadrant, or backstaff, is a sixteenth-century instrument for measuring the altitude of the sun at sea without looking directly at it, invented by the English navigator John Davis around 1594 and described in his Seaman’s Secrets (1595). The observer stood with his back to the sun and aligned the shadow cast by a vane against the horizon through a sight, reading the sum of two graduated arcs. It replaced the cross-staff for solar sights because it spared the eyes and improved latitude accuracy.