Cross-staff

The cross-staff was a pre-sextant altitude instrument used from the early sixteenth century to measure the angular height of a celestial body for latitude. The navigator slid a perpendicular crosspiece, the transom, along a graduated staff until it spanned the angle between horizon and star, reading the value off the scale. Its drawback was that observing the sun forced the user to stare toward it; John Davis’s backstaff of about 1594 solved this by letting the observer face away. The cross-staff bridged the astrolabe and the quadrant.