Known in China as early as the 4th century, the magnetic compass was introduced in the West by the Arabs in the 12th century. Together with the sea chart, it is the major instrument of modern navigation. The compass consists of a magnetized needle rotating freely in the horizontal plane. In the Earth’s magnetic field, the compass needle always points to the magnetic North Pole, which diverges by several degrees from the geographic North Pole. In some models, the entire compass dial rotates on the central pin, and the dial is engraved with the cardinal points and the windrose. In such versions, the direction is shown by a reference mark on the compass casing.
The windrose surrounding the magnetic needle was the geometric reference that allowed navigators to plot their routes on sea charts. In addition to the four cardinal directions, identified with the north, east, south, and west winds, there were four intermediate directions, associated with the north-east, south-east, south-west, and north-west winds. Every 45° interval was divided into four parts, making a total of 32 directions. These were usually drawn on sea charts to form the thick mesh of rhumb lines that virtually criss-crossed the Earth’s surface.