The cross-staff uses two pieces of wood to measure altitude, or the distance of an object above sea level or the horizon.
The navigational cross-staff was made of two wooden pieces: a long rectangular pole (staff) and a second rectangle of wood, set perpendicular to the staff, with a square hole in the middle (transom). The transom would slide back and forth along the staff. This tool is similar to the Arabic kamal, but replaces the knot rope of a kamal with a wooden staff that was marked with finer measurements. In the 15th century, Jewish mathematician Levi ben Gerson first described the cross-staff as a navigational tool, and it remained popular until the invention of other instruments that were simpler to use.
The navigator would place one end of the cross-staff below his eye and observe the sun, Polaris (North Star) or another bright star/planet across the upper part of the transom while also observing the horizon at the bottom of the transom. In order to align both the star and the horizon with the transom at the same time, the navigator would move the transom closer or farther from their eye along the staff. When both the star and the horizon were lined up with the transom, the navigator could determine the angle measurement by reading the scale written on the staff. After some quick calculations the navigator was able to determine the altitude of the star. This altitude measurement could then be converted to the latitude of the observer.
Bathe, Basil W. The Visual Encyclopedia of Nautical Terms Under Sail. New York, Crown Publishers Inc., 1978.
Goldstein, B. R. “Levi ben Gerson and the Cross-staff Revisited.” Aleph: Historical Studies in Science & Judaism, 2 (2011): 364-383, 414. Retrieved from http://0-search.proquest.com.read.cnu.edu/docview/902579499?accountid=10100.