Treatise on geophisics. Geomagnetism. Volume 5/ Трактат о геофизике. Геомагнетизм. Том 5.

The Earth has its own magnetic field (the geomagnetic field), which is confined by the action of the solar wind into a volume called the magnetosphere (see Chapter 5.03). This field is not steady, but varies with time due partly to the interaction with the solar wind, but more importantly by its own physical processes. Direct observation of such changes has been carried out only in the last few centuries, but with indirect measurements we can understand the field behavior millions of years back in time. In this extended time frame, there is evidence that the polarity of the magnetic field reversed frequently, and that the magnetic dipole axis in very ancient times was significantly displaced from the present rotational axis (the North and South geographic Poles). It is of considerable interest how such knowledge was acquired over several centuries. We will take a brief tour of the historical events that provided important steps in formulating our understanding of the geomagnetic field. In doing so, we have to rely solely on the written records, which is the reason why only the European and Chinese histories are referred. There are many works on this topic; among them, the important ones are Mitchell (1932–46), Harradon (1943–45), Needham (1962), and Yamamoto (2003). The English translations of Chinese literature below were taken from Needham (1962). Chinese sentences given together with English were taken from the Japanese translation of this book (Hashimoto et al., 1977). When we talk about the earliest recognition of the magnetism of the Earth, we should be careful to discriminate two separate issues; that is, the attractive force exerted by a magnet on iron, and the north- (or south-) seeking property of the magnet. The former can be taken as the forerunner to the science of magnetism, while the latter is the basis for appreciation of the magnetic field associated with the Earth. Our main interest is in the geomagnetic field, but it is necessary to look into magnets first.