The cross-staff consisted of a long staff with a perpendicular vane which slides back and forth upon it. The staff is marked with graduated measurements - calculated by trigonometry. The angles can then be measured by holding it so the ends of the vane are level with the points to be measured. Early in the sixteenth century it was already in use as a seaman's navigational instrument. 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.