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5.01 Geomagnetism in Perspective M. Kono, Okayama University, Misasa, Japan © 2007 Elsevier B.V. All rights reserved. 5.01.1 Early History 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. 5.01.1.1 Attractive Force of the Magnets The earliest observation of the natural magnets (lodestone or loadstone) is attributed to the Greek philosopher Thales of Miletos (624–546 BC). Thales did not leave any writings of his own, but Aristoteles (384–332 BC) wrote about him in De Anima (On the Soul) about two centuries later. According to this, Thales taught that the lodestone has a soul, because it could set another body (iron) in motion. Diogenes Laertius also wrote that Thales admitted that souls exist even in nonliving matters based on the observation that the magnets and ambers can attract things. This suggests that Thales knew not only about the attractive force of magnets, but also that due to the static electricity of ambers, that can be seen when they are rubbed by clothes (Mitchell, 1937). References to the attractive force of magnets appear quite often in Greek manuscripts (e.g., Plato, Aristotle, Democritus, Lucretius), and there is no doubt that this force was well known to the ancient Greeks. This may be because the attractive force appeared to them as a very remarkable phenomenon since it can act on materials which are not in contact. In these, the magnets were referred to mostly as the rock of Magnesia (x15 x13 x12o& m, x17 x11 x1b x13 x11). Magnesia is the name of a place either in Macedonia, Crete, or Asia Minor. The names of magnetism as well as magnetite (Fe3O4) were derived from this Greek word. Ancient Chinese people made similar observations, but the records are somewhat later than the corresponding Greek ones. In Lu Shih Chhun Chhiu (Master Lu’s Spring and Autumn Annals), written in the late third century BC, it is said that “the lodestone calls the iron to itself, or attracts it.” After that, references to magnets appear abundantly in Chinese literature (e.g., Huai Nan Tzu (The Book of Huai Nan) in the first century BC, and Lun He (Discourses Weighed in the Balance) in 83 AD). The attractive force that magnets exert on iron was a wonder in ancient times, and it was often attributed to magical power. Its full understanding had to wait until the nineteenth century when the magnetic force was explained by physical theorems such as Ampère’s and Gauss’s laws in the framework of electromagnetic theory. 5.01.1.2 Early Chinese Compasses The fact that magnets have the property to align in the north–south direction was discovered by ancient Chinese. From about the second century AD, there are many Chinese texts referring to “south-pointing carriage” which, in many instances, were described as guiding soldiers in thick fogs to the right direction to beat the enemies. Many people thought that this was a device that used the property of magnets. However, it is now considered to be some mechanical device made up of gears and axles rather than an instrument similar to a magnetic compass (Needham, 1962). A more interesting sentence appears in the above-mentioned Lun He (AD 83), which means that “when the south-controlling ladle is thrown upon the ground, it comes to rest pointing at the south.” Wang Chen-To (1948) suggested that the first two letters (south-controlling) were changed in the process of hand copying from the original “south-pointing,” the fourth letter (ladle) means a spoon worked out from a lodestone into that shape, and the eighth letter (ground) actually indicates a diviner’s board. Now, a diviner’s board was used in ancient China for the purpose of telling fortunes, and it is inscribed with the constellation of Great Bear in the center, and the names of 24 directions on the circle around it. With these interpretations, the sentence can be taken to describe an instrument for seeking south using a magnet! Note that the Great Bear is the symbol of the pole and the spoon also has a shape reminiscent of its form. Wang (1948) went further to show the credibility of his interpretation by making a model of this instrument, with a bronze earth-plate and a spoon cut from the lodestone (see Figure 1). A photo of the actual instrument is shown in Needham (1962) and reproduced in Merrill et al. (1996). When Needham visited China, he was shown by Wang Chen-To himself the experiment in which the lodestone spoon gradually rotated to the southward direction and settled there. Although this effort is very impressive, it is rather doubtful if Chinese at this early age really used an instrument which can be identified as the ancestor of the magnetic compass. The interpretation, as suggested by Wang, is not completely convincing. Moreover, there is a conspicuous absence of references to compass-like instruments for about a thousand years afterwords. A well-known early record about the magnetic compass is Meng Chhi Pi Than (Dream Pool Essays) written by Shen Kua at about 1088 AD. In this book, it is said that “Magicians rub the point of a needle with the lodestone; then it is able to point to the south. But it always inclines slightly to the east, and does not point directly at the south.” The text explains how to make a magnetic needle, its south-seeking property, and moreover the fact that there is slight difference between the true south and its pointing direction (i.e., the first mention of the declination). Shen Kua further says that “It is best to suspend it by a single cocoon fibre of new silk attached to the centre of the needle by a piece of wax the size of a mustard-seed – then, hanging in a windless place, it will always point to the north.” This is the earliest written record about the magnetic compass using a magnetic (magnetized) needle. Regarding the oldest compass, existence of an even earlier record was pointed out also by Wang (1948). The text was found in Wu Ching Tsung Yao (Collection of the Important Military Techniques), which is a compendium of military technology edited by Tseng Kung-Liang and completed in 1044. In this, it is said that “When troops encountered gloomy weather or dark nights, and the directions of space could not be distinguished, they let an old horse go on before to lead them, or else they made use of the south-pointing carriage, or the south-pointing fish to identify the direction.” Ключевые слова: e, r, o