Why does a compass point north?
A magnetized compass needle aligns with the local horizontal component of Earth’s magnetic field. This direction usually differs somewhat from geographic north. The angle between them is magnetic declination, which varies with location and time.
Operating principle and essential concepts
1. What turns the needle A magnetized needle acts as a magnetic dipole. An external field exerts a torque that tends to align it with that field. Supported by a low-friction bearing or liquid, the needle can settle along a particular direction. It is misleading to picture its behavior solely as attraction toward one distant point magnet. Earth’s field is a vector field extending through space, and the needle responds to its local direction. Field direction and strength vary between regions, so a compass provides information about its immediate magnetic environment. 2. Geographic and magnetic north Geographic north is associated with the northern end of Earth’s rotation axis. In practical navigation, magnetic north refers to the horizontal direction of the local magnetic field. The difference between these references matters when using a map. A needle does not necessarily aim directly at one magnetic-pole location from every point on Earth; the field’s complex structure determines its local direction. Magnetic-pole positions and field maps change with time, so an old declination value should not automatically be used for a present location. 3. The importance of declination Magnetic declination is the angular difference between true north and the compass direction. Sign conventions and direction labels need to be read according to the source. Different locations within one country may have different values. Map projections also introduce grid north, another reference direction. Interpreting a bearing therefore involves the map date, coordinate system and current regional magnetic information. A correction stated for one city should not be transferred uncritically to another area where the field and map geometry may differ.
Examples, measurement conditions and limitations
4. Why a needle can tilt Earth’s field has a vertical component as well as a horizontal one. The vertical component can make a needle dip, an effect associated with magnetic inclination. Some compasses are balanced for particular geographical zones. Outside their intended zone, tilting and mechanical friction can interfere with readings. Near magnetic-pole regions the horizontal field becomes weak, making direction-finding with an ordinary compass difficult. This does not necessarily mean the compass has failed; it may show the limitations of the measured field and the instrument’s geometry. 5. Why nearby objects interfere Magnets, steel structures, current-carrying cables and some electronic devices alter the local field. A needle in such surroundings is responding to more than Earth’s field. Phone compasses also use magnetic sensors, and software calibration cannot eliminate every external influence. Nearby objects and the instrument instructions therefore matter when assessing a reading. Comparing a bearing with other appropriate navigation information is more reliable than treating one isolated reading as definitive, especially during travel where a navigation error could have serious safety consequences. 6. The source of Earth’s field Motion in Earth’s electrically conducting liquid outer core, together with planetary rotation, contributes to geodynamo processes. This dynamo is the main source of the large-scale field detected by a compass; crustal rocks and space weather can contribute to local measurements too. Earth does not contain a fixed bar magnet. A compass is also not a complete positioning system: it does not independently supply latitude, longitude or travel distance. Under appropriate conditions it provides a directional reference that gains practical meaning when combined with a map and position information.