A compass has guided explorers and travelers for centuries. Its ability to consistently point north has made it an indispensable navigation tool. Understanding how a compass works reveals the fundamental forces of our planet that cause its magnetic needle to align.
Understanding Basic Magnetism
A magnet produces an invisible magnetic field that interacts with other magnetic fields. Every magnet has two poles, a north and a south, where its force is strongest. Opposite poles attract, while like poles repel. The compass needle is a small, lightweight magnet designed to freely rotate, allowing it to respond to external magnetic forces.
Earth’s Global Magnetic Field
Earth behaves like a giant magnet, generating a magnetic field that extends into space. This field originates deep within the fluid outer core through a process called the geodynamo. Convection currents of molten iron and nickel create electric currents, producing the magnetic field. This magnetic field has poles distinct from the geographic North and South Poles (Earth’s rotational axis). What is commonly referred to as Earth’s magnetic North Pole is actually a magnetic south pole in geophysical terms, because it attracts the north-seeking pole of a compass. This magnetic south pole is located near Earth’s geographic North Pole.
How the Compass Needle Responds
The compass needle interacts with Earth’s magnetic field. Mounted on a low-friction pivot, it rotates freely to align with Earth’s invisible magnetic field lines. The north-seeking pole of the compass needle is attracted to Earth’s magnetic South Pole. Since this magnetic South Pole is near the geographic North Pole, the compass needle’s north end points in that general direction. This alignment positions the needle parallel to the magnetic field lines, similar to how iron filings align around a bar magnet.
The Difference Between Norths
The direction a compass points, magnetic north, is not precisely the same as true north (geographic north). True north is a fixed point representing Earth’s rotational axis, while magnetic north is where Earth’s magnetic field lines dip vertically into the surface. The angle between magnetic north and true north is called magnetic declination. This declination varies by location and changes slowly over time due to Earth’s dynamic molten core. For precise navigation, such as in surveying or aviation, accounting for magnetic declination is essential to accurately determine true north from a compass reading.