Colorized drawing of a man siting a pole star with a cross-staff. The cross-staff consisted of a long staff with a perpendicular vane which slides back and forth upon it. Navigational angles are measured by holding it so the ends of the vane are level with the points to be measured. A pole star is a visible star that is aligned with the Earth's axis of rotation and lies directly overhead when viewed from the Earth's North Pole or South Pole. In practice, the term Pole Star usually refers to Polaris, which is the current northern pole star, also known as the North Star. Early in the sixteenth century it was used as a seaman's navigational instrument.