Periscope Aircraft Sextant
Prior to the satellite-based Global Positioning System (GPS) of 1973, aircraft navigators used celestial objects and the horizon to calculate their position and path.
A sextant is an instrument which can measure the angle between any two visible objects, which allows one to determine their own location in relationship to those objects. The periscopic aircraft sextant is an optical sextant which mounts to the roof of the aircraft allowing the navigator to view the sky from within an enclosed cockpit.
This periscopic sextant belonged to Robert O. Harder, a navigator-bombardier on a B-52 Stratofortress during the Vietnam War.