Jacob's staff is a device that has been used for centuries to measure angles and heights. It is also known by other names such as cross staff, Davis quadrant, and backstaff. The term cross staff comes from the appearance of the device, which resembles a cross. The Davis quadrant was named after its inventor, Captain John Davis. The backstaff was later developed as an improved version of the Davis quadrant. Other synonyms for Jacob's staff are nautical quadrant, astronomical quadrant, and mariner's astrolabe. Today, Jacob's staff is used mainly for historical purposes and has been replaced by modern tools such as the sextant.
