True north (also called geodetic north or geographic north) is the
direction along
Earth's surface towards the geographic
North Pole or True North Pole.
Geodetic north 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
arcsecond
A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn. The na ...
s) because the local
gravitational field
In physics, a gravitational field is a model used to explain the influences that a massive body extends into the space around itself, producing a force on another massive body. Thus, a gravitational field is used to explain gravitational phenome ...
may not point at the exact
rotational axis
Rotation around a fixed axis is a special case of rotational motion. The fixed-axis hypothesis excludes the possibility of an axis changing its orientation and cannot describe such phenomena as wobbling or precession. According to Euler's rota ...
of Earth.
The direction of astronomical true north is marked in the skies by the north
celestial pole. This is within about 1° of the position of
Polaris
Polaris is a star in the northern circumpolar constellation of Ursa Minor. It is designated α Ursae Minoris ( Latinized to ''Alpha Ursae Minoris'') and is commonly called the North Star or Pole Star. With an apparent magnitude that ...
, meaning
the star
''The'' () is a grammatical article in English, denoting persons or things already mentioned, under discussion, implied or otherwise presumed familiar to listeners, readers, or speakers. It is the definite article in English. ''The'' is the m ...
would appear to
trace a tiny circle in the sky each
sidereal day. Due to the
axial precession of Earth, true north rotates in an arc with respect to
the stars that takes approximately 25,000 years to complete. Around 2101–2103, Polaris will make its closest approach to the celestial north pole (extrapolated from recent Earth
precession). The visible star nearest the north celestial pole 5,000 years ago was
Thuban.
On maps published by the
United States Geological Survey (USGS) 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 are 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.
References
Sources
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Geography of the Arctic
Cartography
Chinese discoveries
Navigation
Orientation (geometry)
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