Jacob's Staff

A Jacob's Staff, also known as a cross staff, is a historical optical instrument used by sailors and navigators to measure the angle between the horizon and a celestial object, like the sun or stars. It consists of a straight rod with a sliding crosspiece (runner) that moves along it. By aligning the crosspiece with the horizon and the target in your eye, you can determine the angle between them, which helps in calculating your latitude and improving navigation accuracy at sea. Its design allows quick, practical measurements before more advanced tools were developed.