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Cross-Staff

The cross-staff, originating sometime in the thirteenth or early part of the fourteenth century, was a better instrument for taking readings of the altitude of a celestial body than the quadrant or the astrolabe. The ultimate in simplicity, it was but a long stick with a movable cross-bar called the transversary. The navigator aimed the lower point of the cross-bar at the horizon and moved the cross-bar until its upper tip touched the celestial body; then he read the altitude on the scale inscribed along the length of the staff. To prevent painful damage to the eyes by having to look directly at the sun, a small shield blocked the sun (except its uppermost edge), and the navigator made a correction value to find the true reading. Alternatively, a small piece of smoked glass was used.

In spite of the knowledge of how to determine latitude by means of celestial observations, and the existence of nautical instruments to take these measurements, there was a vast difference between theory and practice. The instruments themselves caused a certain amount of error; with the astrolabe, this could be as great as one whole degree of arc, equal to an error of sixty nautical miles. It was no small feat for the navigator to keep the cross-staff aimed at the horizon, while at the same time moving the cross-bar so as to have its upper tip touch the celestial body, all the while trying to brace himself on the pitching and rolling deck of a ship. Under these conditions it was no easier to maintain a star or the sun in the sighting holes of the alidade of an astrolabe.