crossstaff

The cross-staff is a historical navigational instrument used to measure the altitude of celestial bodies in order to determine latitude at sea. It consists of a straight staff with a perpendicular cross-piece that slides along or is fixed at a right angle to the staff. The navigator sights along the edge of the cross-piece toward the horizon and aligns the end of the cross-piece with the Sun or a star. The angle between the staff and cross-piece is read from a scale, giving the altitude of the body above the horizon.

The instrument developed in the late medieval to early modern period and was widely used by mariners in the 16th through 18th centuries. It offered a portable, relatively simple means of taking celestial measurements without the need for more complex devices. In practice, it required careful sighting and introduced potential errors from glare, horizon clarity, and the observer’s eye height above the water.

Design variants existed, with different cross-piece lengths and scales, but the basic principle remained the same: a right-angled cross-piece provided a means to transfer the altitude of a celestial body to a readable angle. While useful, the cross-staff had limitations, including susceptibility to solar glare and reduced accuracy in rough seas.

By the late 18th and 19th centuries, the cross-staff was largely supplanted by more accurate instruments such as the backstaff and, later, the sextant. Today it is primarily of historical interest, illustrating early methods of celestial navigation and the evolution of navigational technology.