The evolution of celestial navigation witnessed a significant leap with the introduction of the Cross-Staff, an instrument originally described by the scholar Levi ben Gerson in the fourteenth century. Originally intended for astronomical observations on land, the device was adapted by mariners seeking a reliable method to quantify the altitude of celestial bodies above the horizon. By the early sixteenth century, it had become a standard implement aboard vessels traversing the open ocean, serving as a primary tool for European explorers.
The apparatus consisted of a graduated main staff and a sliding perpendicular cross-piece, known as the Transversal. To obtain a reading, the navigator positioned one end of the staff against the cheekbone directly below the eye. The cross-piece was then adjusted along the staff until its upper tip aligned with the target star and the lower tip coincided with the horizon. This geometric arrangement allowed for the calculation of the angle of elevation through the principles of similar triangles, providing the essential data required to determine Latitude.
Despite its widespread adoption, the cross-staff presented distinct challenges, particularly during solar observations. Direct usage required the navigator to gaze towards the sun, resulting in a condition historically referred to as “burning the eye.” While the addition of smoked glass offered limited protection, the inherent physical risks and the difficulty of maintaining stability on a rolling deck eventually necessitated the development of the Backstaff. This subsequent innovation allowed mariners to measure solar altitude with their backs turned to the sun, marking the eventual obsolescence of the cross-staff in favor of safer, more precise optical instruments.
