*also called ‘geo-detic north’ / ‘geo-graphic north’*
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*jee o DET ic*
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*’true north’ is the direction along earth’s surface towards the ‘geo-graphic (true) north pole’*
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‘geo-detic north’ differs from…
‘magnetic north’
(the direction a compass points toward the ‘magnetic north pole’)
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and from ‘grid north’
(the direction northwards along the ‘grid lines’ of a ‘map projection’)
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(‘geo-detic 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 ‘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 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.[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.[5]
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
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References
^ Meeus 1997, p. 305.
^ McClure 2013.
^ Smiley & Khan 1959, p. 250.
^ Ridpath 2004, p. 5: “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.”
^ “What do the different north arrows on a USGS topographic map mean?”. United States Geological Survey.
Sources[edit]
McClure, Bruce (2013). “Polaris is the North Star”. Astronomy Essentials. EarthSky. Archived from the original on 2014-03-09. Retrieved 2014-04-28.
Meeus, Jean (1997). Mathematical Astronomy Morsels. Richmond, VA: Willmann-Bell. ISBN 0-943396-51-4.
Ridpath, Ian, ed. (2004). Norton’s Star Atlas. New York: Pearson Education. ISBN 0-13-145164-2.
Smiley, C. H.; Khan, A. Majid (1959). “Polaris and Precession”. Journal of the Royal Astronomical Society of Canada. 53: 249. Bibcode:1959JRASC..53..249S.
en.wikipedia.org /wiki/True_north
True north
Contributors to Wikimedia projects3-4 minutes 1/9/2006
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