North has always served as the fundamental reference for navigation. The “North” used for mapping, however, is fundamentally different from the one a compass points toward. While both are near the top of the globe, True North and Magnetic North are distinct, non-aligned locations. Understanding this difference is essential for accurate navigation, whether using maps or modern technology.
True North: The Fixed Axis
True North is the direction that points directly toward the geographic North Pole, the fixed point where the Earth’s rotational axis intersects its surface. This pole is static and serves as the unmoving geographic reference for the entire planet. All lines of longitude, known as meridians, converge at this precise pole. Because True North is determined by the Earth’s spin, it provides a consistent, unchanging frame of reference for mapping and coordinate systems. Maps, including topographic charts and GPS systems, are universally oriented to this fixed direction.
Magnetic North: The Dynamic Pole
Magnetic North is the direction toward which a magnetic compass needle aligns itself. This point is the location on the Earth’s surface where the planet’s magnetic field lines point vertically downward. A compass needle responds to the horizontal component of the local geomagnetic field, which is generated deep within the Earth. The Magnetic North Pole is constantly moving and is currently located in the Arctic Ocean, often shifting tens of kilometers each year. This dynamic pole is rarely in the same physical location as the geographic North Pole. A compass aligns with the local field direction, which can vary widely across the globe.
Magnetic Declination: Measuring the Difference
The angular difference between True North and Magnetic North at any specific location is known as magnetic declination, or sometimes magnetic variation. This angle is a necessary correction factor for anyone using a magnetic compass with a map. If the magnetic direction is east of True North, the declination is considered positive or “East,” and if it is west, it is negative or “West.”
The declination value is not uniform across the planet; it changes depending on the geographical location. For example, along the east coast of the United States, declination can change from 16 degrees West in Maine to 4 degrees East in Texas. To use a compass bearing on a map, a navigator must apply this correction by adding or subtracting the declination angle from the compass reading. Ignoring a declination of even a few degrees can lead to significant navigational errors over long distances.
Maps often include a declination diagram that provides the angle for a specific year. Due to the pole’s movement, this value must be periodically updated. Lines of equal declination, called isogonic lines, are charted globally to help determine the precise local correction. The imaginary line where magnetic declination is zero is called the agonic line, where a compass and a map align perfectly.
The Shifting Earth: Why Magnetic North Wanders
The movement of the Magnetic North Pole is a direct consequence of processes occurring deep within the Earth’s interior. The planet’s magnetic field is generated by the geodynamo, a complex mechanism operating in the outer core. This outer core is composed of molten iron and nickel, which is electrically conductive and in constant motion.
Convection currents within the molten metal, driven by heat escaping from the core, create a self-sustaining cycle of electric currents. The movement of these charged metals through existing magnetic fields generates new fields in a process known as the dynamo effect. This dynamic fluid motion causes the magnetic field to constantly change, which is reflected on the surface as the wandering of the magnetic poles. This slow, continuous change in the field is referred to as secular variation.