True north
Encyclopedia
True 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 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.
North Pole
The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is, subject to the caveats explained below, defined as the point in the northern hemisphere where the Earth's axis of rotation meets its surface...
.
True geodetic north usually differs from magnetic north (the direction a compass points toward the magnetic north pole), and from grid north
Grid north
Grid north is a navigational term referring to the direction northwards along the grid lines of a map projection. It is contrasted with true north and magnetic north...
(the direction northwards along the grid lines of a map projection
Map projection
A map projection is any method of representing the surface of a sphere or other three-dimensional body on a plane. Map projections are necessary for creating maps. All map projections distort the surface in some fashion...
). 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
Polaris
Polaris |Alpha]] Ursae Minoris, commonly North Star or Pole Star, also Lodestar) is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole, making it the current northern pole star....
, so that the star appears to trace a tiny circle in the sky each day. Due to the precession
Precession
Precession is a change in the orientation of the rotation axis of a rotating body. It can be defined as a change in direction of the rotation axis in which the second Euler angle is constant...
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 star to the celestial north pole was Thuban
Thuban
Thuban also known by its Bayer designation Alpha Draconis is a star in the constellation of Draco. A relatively inconspicuous star in the night sky of the Northern Hemisphere, it is historically significant as having been the north pole star in ancient times...
.
On maps published by the United States Geological Survey
United States Geological Survey
The United States Geological Survey is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, and the natural hazards that threaten it. The organization has four major science disciplines, concerning biology,...
and the United States Armed Forces
United States armed forces
The United States Armed Forces are the military forces of the United States. They consist of the Army, Navy, Marine Corps, Air Force, and Coast Guard.The United States has a strong tradition of civilian control of the military...
, 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
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
Ordnance Survey
Ordnance Survey
Ordnance Survey , an executive agency and non-ministerial government department of the Government of the United Kingdom, is the national mapping agency for Great Britain, producing maps of Great Britain , and one of the world's largest producers of maps.The name reflects its creation together with...
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.