Answer: 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. |
