Editing Magnetic declination (section)

=== Field measurement ===
[[Image:Boussole de déclinaison Lenoir.png|thumb|left|200px|Antique declinometer]]

The magnetic declination at any particular place can be measured directly by reference to the [[celestial pole]]s—the points in the heavens around which the stars appear to revolve, which mark the direction of true north and true south. The [[Measuring instrument|instrument]] used to perform this measurement is known as a ''declinometer''.

The approximate position of the north celestial pole is indicated by [[Polaris]] (the North Star). In the [[Northern Hemisphere|northern hemisphere]], declination can therefore be approximately determined as the difference between the magnetic bearing and a visual bearing on Polaris.  Polaris currently traces a circle 0.73° in radius around the north celestial pole, so this technique is accurate to within a degree.  At high latitudes a [[plumb-bob]] is helpful to sight Polaris against a reference object close to the horizon, from which its bearing can be taken.<ref>{{citation|url=http://earthsci.org/education/fieldsk/declin.htm|title=Magnetic declination, what it is, how to compensate.|access-date=2010-03-03|archive-url=https://web.archive.org/web/20100107205605/http://earthsci.org/education/fieldsk/declin.htm|archive-date=2010-01-07|url-status=dead}}</ref>
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