'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.