The Method of Using a Cross Staff
The cross staff measures the angle of altitude of the sun or moon or a star by sighting from one end of a staff, past ends of a cross piece, towards the horizon and the celestial object. The cross arm is moved up or down a graduated staff so that when sighting along the staff the user sees the sun or star and horizon at the ends of the cross arm. The altitude is then read off the staff. The altitude is measured on a scale along the staff.
This illustration comes from Medina's Arte De Navegar, 1545.