Davis Backstaff

Timeline

1. 1594–1595London · LondonDrawing on his Arctic and Atlantic voyaging experience, Davis criticized the cross-staff for requiring the observer to watch both sun and horizon at once, and for parallax errors from the staff end held against the eye. In his 1585 manual he gave a method of facing away from the sun, aligning the shadow vane's projection with the horizon slit, and he followed with The World's Hydrographical Description (1595) to spread his seafaring knowledge. The early figures still showed a sliding staff with an arc, first covering about 45°, later extended to 90°.
2. 1604–1680England · EnglandIn the early 17th century instrument makers mounted large and small arc segments on a central main staff, achieving reading precision near that of a full-radius quadrant with a smaller radius, and added diagonal braces for stability. Continental navigators called it the English quadrant. Flamsteed later added a lens shadow vane for hazy weather, so weak sunlight could still cast a shadow on the horizon vane. By then the back-observation instrument was standard equipment on English and Dutch ocean-going ships for measuring noon solar altitude.
3. 1620–1731Atlantic sea routes · North AtlanticIn the East India Company and Dutch ocean fleets, the backstaff gradually replaced the cross-staff as the first choice for measuring solar altitude by day, while the mariner's quadrant and astrolabe were relegated to auxiliary roles. It cost less than a precision astrolabe and was easier to handle in a swell than a plumb-line quadrant. Its limitation was dependence on solar shadow, requiring other methods at night or near the sun's zenith passage.
4. 1731–1800London · LondonJohn Hadley's reflecting octant let the observer see the celestial body and the horizon's image simultaneously, surpassing shadow-based methods in accuracy and all-weather capability. In the later 18th century navies and company ships switched to octants and sextants, and the backstaff left front-line service, appearing only as a teaching tool or low-cost spare.