Compass & magnetic declination
Magnetic north is not true north. The angle between them — magnetic declination — varies by location, ranging from near zero to over 20° in some regions. Click the map to place your compass and watch the needle respond to Earth's magnetic field. The magnetic poles wander over time; the north magnetic pole has moved over 1000 km since 1900.
Earth's magnetic field is generated by convection currents in the liquid iron outer core, a process called the geodynamo. The resulting field is approximately dipolar (like a bar magnet), but with significant higher-order terms that create regional variations in declination, inclination, and intensity.
Magnetic declination is the angle between magnetic north (where a compass points) and true north (toward the geographic North Pole). In London, declination is currently about −0.5° (slightly west). In parts of Alaska, it can exceed −20°. Navigators must correct for this: an uncorrected compass bearing in Alaska could be off by enough to miss a destination by kilometers.
The dip angle (or inclination) measures how steeply the field lines plunge into the Earth. At the magnetic equator, field lines are horizontal (dip = 0°). At the magnetic poles, they are vertical (dip = ±90°). A dip circle held vertically reveals this angle.
The magnetic poles wander over time — a phenomenon called secular variation. The north magnetic pole was in the Canadian Arctic for centuries but has been accelerating toward Siberia since the 1990s, currently moving at about 55 km/year. This model uses a simplified version of the International Geomagnetic Reference Field (IGRF) to approximate these changes.
Local anomalies — iron ore deposits, volcanic rock, or large metal structures — can distort the local magnetic field, causing compass deviations beyond what declination charts predict. Mariners call this "deviation" and correct for it separately.