Before any compass can be trusted, two layers of correction stand between what it reads and where true north lies: the Earth's field is not aligned with the geographic poles, and the aircraft wraps its own magnetism around the sensor. This lesson works through both layers, ending with the two small calculations the exam asks again and again: applying variation and applying deviation.
The Earth's magnetic field
The Earth behaves as if a huge bar magnet lay buried in its core, tilted about eleven degrees from the spin axis and slowly wandering. Its field lines leave the surface in the southern hemisphere, arc through space, and re-enter in the north: near the magnetic equator they run parallel to the surface, and toward the magnetic poles they plunge increasingly steeply into the ground. A magnet's standing properties (the reason any of this is usable) are worth restating:
Two poles — every magnet has them; like poles repel and unlike attract.
Closed field lines — the field runs in closed lines from pole to pole.
Free alignment — a freely suspended magnet aligns itself with the field it sits in, which is all a compass needle is.
Variation and dip
Two angles summarise the field at any place:
Magnetic variation — the horizontal angle between true north and magnetic north, named east or west for the side the magnetic direction lies on; it differs from place to place and drifts slowly with the years, which is why charts print it with a date.
Magnetic dip (inclination) — the vertical angle between the field and the local horizontal: zero at the magnetic equator, ninety degrees at the magnetic poles.
A compass needle does not politely ignore the vertical part: left free, it aligns with the total field, dip included. Near the poles the horizontal component that carries the heading information shrinks toward nothing while the needle tries to point into the ground, and the instrument stops functioning in the vicinity of the magnetic poles: a limitation designed around, never repaired.