True north (geodetic north) is the direction along the earth's surface towards the geographic North Pole.
True geodetic north usually differs from magnetic north (the direction a compass points toward the magnetic north pole), and from grid north (the direction northwards along the grid lines of a map projection). Geodetic true north also differs very slightly from Astronomical true north (typically by a few arc seconds) because the local gravity may not point at the exact rotational axis of the earth.
The direction of astronomical true north is marked in the skies by the north celestial pole. This is within about 1 degree of the position of Polaris, so that the star appears to trace a tiny circle in the sky each day. Due to the precession of the Earth's axis, true north rotates in an arc with respect to the stars that takes approximately 25,000 years to complete. In 2102 Polaris will make its closest approach to the celestial north pole. 5,000 years ago, the closest (visible to the eye) star to the celestial north pole was Thuban.
On maps published by the United States Geological Survey and the United States Armed Forces, true north is marked with a line terminating in a five-pointed star. The east and west edges of the USGS topographic quadrangle maps of the United States are meridians of longitude, thus indicating true north (so they're not exactly parallel). Maps issued by the United Kingdom Ordnance Survey contain a diagram showing the difference between true north, grid north and magnetic north at a point on the sheet; the edges of the map are likely to follow grid directions rather than true, and the map will thus be truly rectangular/square.
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