Why do models and charts of the geomagnetic field need to be periodically updated?
Models and charts of the magnetic field at the Earth’s surface need to be periodically updated because the field is constantly changing in time. The same fluid motion in the Earth’s core that sustains the main part of the magnetic field also causes the field to slowly change in spatial form, a time-dependence known as ‘secular variation’. This variation can be seen in all vectorial parts of the magnetic field, but it was first noticed in declination several hundred years ago, since it is that quantity that is so important for navigation. In fact, the demands of navigators helped to motivate, centuries ago, some of the original studies of the Earth’s magnetic field. On average the declination at the Earth’s surface changes by about a fifth of a degree per year.
