Earth’s magnetic field is acting up
Oddly enough, this has a sizable impact to general aviation. Heading is typically measured using a gyro, but the heading values are calibrated to magnetic north (typically via a magnetometer or manually against a compass).<p>The creep of the magnetic field is noticeable in particular in the numbering of runways - they are numbered according to their heading. Some runways that have existed for a few decades are, accordingly, off by a few degrees. An older runway numbered "35" may have an actual approach heading of 340 (340 degrees from magnetic north). Not a huge problem when flying by eye, but it has the capability of causing some confusion when flying using instruments. Alternately, the runway numbers can even change.<p>As for GPS, this won't solve the problem either, since GPS can give you a "true" flight path using the movement over time, which with wind will not always match the actual heading of the aircraft due to wind.