CoachCurrents in the Earth’s molten iron core make the planet a giant magnet: at the surface , and a free needle aligns with the field’s horizontal component — this is the compass. The geomagnetic poles are not the geographic ones; the angle between compass north and true north is the declination, corrected on every bearing.
Examples
Example 15.5 (Correcting a bearing)
Where the declination is west, a ship steering “compass north” for drifts west of its track: to sail true north, steer east of the needle.