Main field and secular variation

The scope of this paper is the observational aspects of the Earth's main field and its secular change as known from historical observations. A few comments are also included in regard to paleomagnetic evidence, and a brief discussion is added on the main fields of other worlds of the Solar System. Such work as has been done in the past four years should eventually lead to a better understanding of the magnetic field of the Earth as well as planetary interiors. If the health of a discipline can be measured by the amount of controversy, this one must be extremely robust.