Compasses

A compass is a navigational instrument that determines direction relative to the Earth's magnetic poles. Its primary component is a magnetized needle or card that aligns with the planet’s magnetic field, allowing the user to identify cardinal points—north, east, south, and west. Early magnetic compasses appeared in China during the Han dynasty and were later adopted for maritime navigation in the Arab world and Europe, dramatically improving long‑distance travel and trade.

The basic operating principle relies on ferromagnetic materials being attracted to magnetic fields. In a typical magnetic compass, a lightweight, magnetized needle is balanced on a low‑friction pivot within a housing marked with degree graduations. The needle settles pointing toward magnetic north, which differs from true geographic north by a location‑dependent variation called declination. Modern compasses often include a declination scale so users can correct readings for more accurate navigation.

Beyond the traditional magnetic design, other compass types serve specialized purposes. Gyrocompasses use a rapidly spinning rotor and the principles of angular momentum to find true north without reliance on magnetism, making them essential on steel‑hull ships where magnetic interference is problematic. Fluxgate and solid‑state compasses employ electronic sensors to detect magnetic fields, providing digital outputs for integration into smartphones, aircraft, and autonomous vehicles.

Compasses are employed in a wide range of activities, including hiking, surveying, marine navigation, aviation, and military operations. Limitations arise from magnetic anomalies, nearby ferrous objects, and electromagnetic interference, which can cause deviation or erratic behavior. Users mitigate these effects by performing regular calibrations, maintaining a clear view of the horizon, and referencing supplementary navigation aids such as GPS or celestial observations.