True north
True north (also called geodetic north) is the direction along Earth's surface towards the geographic 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 arcseconds) because the local gravity may not point at the exact rotational axis 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, so that the star 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 2100–02, Polaris will make its closest approach to the celestial north pole (extrapolated from recent Earth precession).[1][2][3] The closest visible star to the celestial north pole 5,000 years ago was Thuban.[4]
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
Citations
^ Meeus (1997), p. 305.
^ McClure (2013), http://earthsky.org/brightest-stars/polaris-the-present-day-north-star
^ Smiley & Khan, Journal of the Royal Astronomical Society of Canada, Vol. 53, p. 250, http://articles.adsabs.harvard.edu//full/1959JRASC..53..249S/0000251.000.html .
^
Ridpath, Ian (ed.) (2004). Norton's Star Atlas. New York: Pearson Education. p. 5. ISBN 0-13-145164-2.Around 4800 years ago Thuban (α Draconis) lay a mere 0°.1 from the pole. Deneb (α Cygni) will be the brightest star near the pole in about 8000 years' time, at a distance of 7°.5.
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Sources
Meeus, Jean (1997). Mathematical Astronomy Morsels. Richmond, VA: Willmann-Bell. ISBN 0-943396-51-4.
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