True north is a navigational term referring to the direction of the North Pole relative to the navigator's position. Its concept was first discovered and noted by the Chinese polymath Shen Kuo in the 11th century. True north is compared to magnetic north (the direction of the Magnetic North Pole) and grid north (the direction northwards along the grid lines of a map projection). The direction of true north is marked in the skies by the celestial north pole. For most practical purposes, this is the position of Polaris. However, due to the precession of the Earth's axis, true north rotates in an arc that takes approximately 25,000 years to complete. In 2002, Polaris was at its closest approach to the celestial north pole. 2,000 years ago, the closest star to the celestial north pole was Thuban. On maps issued by the United States Geological Survey, and the U.S. military, true north is marked with a line terminating in a five-pointed star. Maps issued by the Ordnance Survey contain a diagram showing the difference between true north, grid north and magnetic north at a point on the sheet.
