Magnetic declination

Magnetic declination is the horizontal angle between magnetic north and geographic north at a location. It varies across Earth and changes over time.

Earth's magnetic field: The magnetic field surrounding Earth, generated mainly by motion in its liquid outer core. Its direction at a location defines magnetic north and therefore the local declination.
Magnetic compass: A navigation instrument whose magnetized needle aligns approximately with the local horizontal magnetic field. Its north indication differs from map north by the local declination.
Geographic north: The direction toward the geographic North Pole along a local meridian. It is one of the two directions whose horizontal angle defines declination.
Compass error: The difference between a compass indication and the direction it is intended to show. Declination is one source of compass-direction mismatch, but not the only one.
Magnetic poles: The regions where Earth’s magnetic field is vertical and its field lines enter or leave the planet. Their positions help determine how magnetic north differs from geographic north.
True north: The direction along Earth’s surface toward the geographic North Pole. It is the geographic reference direction compared with magnetic north.
Magnetic north: The horizontal direction indicated by Earth’s magnetic field at a location. It is the other direction in the declination angle.
Compass calibration: The adjustment or characterization of a compass so its readings correspond to known directions. Calibration addresses instrument behavior; declination is a geographic field angle.
Geomagnetic secular variation: The gradual change in Earth’s magnetic field over years to centuries. It makes declination change over time even when a location stays fixed.
Bearing (navigation): A horizontal direction to a destination, expressed as an angle from a stated reference direction. Converting magnetic bearings to true bearings requires accounting for declination.

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