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The magnetic declination at a certain place, at a certain time, is the difference in angle between the direction of the magnetic north pole and the (true) north pole.

The magnetic north pole is not the same as the north pole, though it is close. It moves in a diurnal cycle, ellipse-shaped, returning roughly to the same place as it was 24 hours before. The size of the ellipse changes, and the position of the ellipse changes over longer periods of time. So the magnetic declination varies with both position and time.

There is always someplace on the Earth where the magnetic declination is zero. When the magnetic north pole is at the true north pole, this is true everywhere on Earth. But even when it's not, the magnetic pole is on some line of longitude - and all places on the same line of longitude, or 180 degrees away from it, have a zero magnetic declination (possibly excluding places too close to the poles, where compasses don't work).

Source: http://www.geolab.nrcan.gc.ca/geomag/eng_main.html

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