Item History: The cross-staff followed the mariner’s astrolabe in the evolution of celestial navigation instruments. The instrument made its appearance in Europe in the 1300s and by the mid-seventeenth century it had been fully developed for use at sea. Like the astrolabe, it was used to measure the altitude of the sun or a star above the horizon to help determine latitude. At the pinnacle of its development it consisted of a graduated staff with a set of interchangeable vanes that enabled the navigator to measure angles between 10º and 90º.
To use the cross-staff the navigator selected the vane most appropriate for the angle that needed to be measured and slipped it over the staff. The staff was then held up to the eye with one hand while the vane, also called a transom, was held up with the other. The vane was moved along the staff until the sun or star was sighted along the top of the vane and the bottom appeared to rest upon the horizon. The vane would then be clamped into place and the altitude would be read off the scale that corresponded to the vane used to take the sight.
While the cross-staff was an improvement over earlier instruments, it had a few shortcomings of its own. Using the cross-staff required the observer to look in two places at once, while this was not difficult for small angles the technique proved to be highly inaccurate when measuring large ones. Of course the biggest problem was that taking a sight during the day required the navigator to look directly at the sun—a potentially blinding experience! To solve this problem the observer could hold a piece of smoked glass at the top of the vane but this proved awkward at best. For these reasons, navigators used the cross-staff to measure small angles and the astrolabe to measure large angles and to take sun sights.
This early nineteenth century ebony cross-staff was made by Hendrik Noordyk of Amsterdam in 1804. It has reproduction pearwood vanes, ivory horizon vane, and brass aperture disc. The quality of the staff is fair and clearly worn through use, especially at the ends. The side numbers have been erased, but the instrument number, 78, at the eye-end, is clearly visible, as is the entire graduation on all four sides.