A navigational instrument.
During the age of discovery, mariners were armed with few reliable navigational instruments. Of utmost importance was the cross-staff. Taking its name from its shape, the cross-staff consisted of movable wooden crossed pieces, one longer than the other, wherein a sighting of the sun or pole-star was taken to measure the meridian altitude of the sun or a star to determine latitude. Known in principle to both ancient Chaldeans (400 B.C.E.) and to medieval astronomers for determining latitude, the cross-staff was first recommended for use at sea by Johannes Werner in 1514. Undoubtedly, without the cross-staff, the globe could not have been encompassed by the sixteenth century.
The origins of the cross-staff are shrouded in history. However, its genesis and evolution at sea are not. Arab sailors are known to have used a similar instrument, known as the kamal, well before the sixteenth century. Knots in a piece of string, instead of a staff, designated the polestar’s latitude of the intended ports of call. An Arab pilot’s use of a kamal on Vasco da Gama’s 1498 voyage is the first recorded introduction of its use to European sailors. Conceivably, the kamal may have influenced Portuguese mariners during their fifteenth-century voyages down and around the African coast. The cross-staff, however, was a direct descendant of the astronomical cross-staff, though its use at sea may have been prompted by reports of the kamal.
The cross-staff’s manufacture was first published in Martin Cortes’s Arte de Navegar (1551) and translated into English in Richard Eden’s Arte of Navigation (1561). For convenience, the sea cross-staff was shorter than its relative, consisting of a piece of wood approximately thirty-six inches long and eventually with four variably sized crosspieces. Originally, there was only one crosspiece, though by 1581 it was common practice to have three different crosses to differentiate the latitudes 15°, 30°, and 60°. The 10° piece became commonplace sometime during the early seventeenth century. In effect, by aligning the appropriate crosspiece of one’s approximate known latitude along the shaft to the horizon and either the sun or polestar from one’s eye, the current latitude could be determined. The danger of using the sun for observation was noted and it was avoided through the use of smoked glass as described by William Bourne in 1574. (The various editions of Mariner’s Compass Rectified [1633], written by Andrew Wakely and revised by James Atkinson [1694], offer an excellent contemporary account of the cross-staff and how to use it.)
The cross-staff, however, was an awkward instrument because of its size and was unable to provide accurate observations because of its crude construction and operation. It was known that the distance one held it from one’s eye incurred inaccuracies, as it could not account for the discrepancies of the human form. Yet, the simplicity of its use ensured that the cross-staff remained viable even into the early nineteenth century despite more accurate instrumentation.
In 1595 John Davis created the cross-staff’s successor and the first truly nautical instrument, the back staff. Initially consisting of a shaft with a vane secured at the end and a sliding vertical arm, the back staff, so-named since the observer’s back faced the object being viewed, represented a great advance in measuring latitude. The back staff was used like its predecessor, except that its arms were first positioned at preselected places on the shaft and, with fine-tuning made possible, the eyepiece could be moved on the lower arm so as to cast a shadow over the vane. It must be noted, however, that there were two variants of the back staff that became commonplace: the Davis Quadrant, with two sliding arms, and the English Quadrant, with a sliding arm and vane. By refining the cross-staff, John Davis helped create an instrument that was much more accurate as it minimized the irregularities of holding the cross-staff to one’s eye. The back staff continued to be used until the end of the eighteenth century, like the cross-staff, because of its simplicity. However, with the creation of the Hadley Quadrant in 1734, the basis of most marine instruments since, determining meridian altitudes was revolutionized and its subsequent alterations guaranteed its supremacy.
The cross-staff, and its cousin the back staff, allowed mariners from the sixteenth and seventeenth centuries to navigate the globe. This instrument of antiquity, modified for the sea and continually refined, armed mariners with the means to provide merchants, ministers, and monarchs the opportunity to achieve their aspirations by being able to navigate the oceans and seas of the world more safely.
Alistair Maeer
See also: Navigation; Quadrant.
Fisher, Dennis. Latitude Hooks and Asimuth Rings: How to Build and Use 18 Traditional Navigational Tools. Camden, ME: International Marine, 1995.