cross-staff

The cross-staff was a navigational instrument used at sea primarily from the 16th to the 18th century to measure the altitude of celestial bodies, such as the polestar or the Sun. This measurement helped sailors determine their latitude, which is their north-south position on Earth.12

The instrument consisted of a staff, typically about 3 feet (1 meter) long, with a sliding crosspiece. To use it, a navigator would hold the staff to their eye and adjust the crosspiece until its ends aligned with the horizon and the celestial object. The angle could then be read from a scale marked on the staff.2

A significant drawback of the cross-staff was that it required the user to look directly at the Sun, necessitating the use of colored shades. While improvements were made, such as the backstaff, the cross-staff was eventually superseded by more accurate instruments like the octant and sextant.2