The poles of the Earth's magnetic field are close to the geographic poles, which lie on the axis of the Earth's rotation, but not coincident, so that a compass pointing to magnetic north will not point exactly at the geographic north pole.

The angle between the true meridian and the magnetic meridian, or the difference between the true north and the magnetic north, is called variation and varies between 180^o and zero.

Because the Earth's magnetic pole is slowly moving around, the variation changes over time and therefore the date of any map showing variation is important, because if it is an old map, its variations will be incorrect.

Thus, the pilots should always fly with the most up-to-date chart for flight safety reasons. If the chart is republished at frequent intervals, the variation should be comparatively recent as well.