Introduction

In the year 1600, the the man about to become physician to Queen Elizabeth I of England published a long treatise summarizing his two decades of experimentation on magnetism. After disposing of such issues as whether garlic causes magnets to “lose their virtue” William Gilbert recounted his observations upon moving a compass over the surface of a permanent magnet that had been specially fashioned in the form of a sphere (Gilbert, 1893, 1958). The similarity between the compass readings on the surface of his magnet and those recorded in mariners’ charts led Gilbert to conclude that his magnet was a terrella, a little earth, and that our big earth is (among other things) a giant magnet. Gilbert’s little earth organized the pattern of compass readings not only on its surface but also in the space surrounding it. From this, he boldly asserted that the big earth’s magnetic influence continues far into empty space, where no mariner of his day could ever go. The profundity of this remark was not lost on Gilbert’s younger contemporary, Johannes Kepler, who found in it an explanation of the earth’s annual motion around the sun. Kepler reasoned more or less as follows (in modern language): Since the earth and the sun are both celestial bodies, they both should rotate, and they both should have magnetic fields surrounding them in space. Their two rotating fields interact somehow, somewhere, in the space between them, communicating the sun’s rotational motion to the earth and pushing the earth around its orbit. In this curious way, Kepler might have been the first to perceive that the sun acts upon terrestrial magnetism. He was not the last. In 1580, Kepler’s teacher, Michael Maestlin, had recorded an observation of a distinct region of oscillating luminosity in the northern sky, an aurora. The aurora had been a topic of scientific interest since Graeco-Roman antiquity [of particular importance was Aristotle’s (384-322 B.C.) discussion of it in his Meteorology], but it had become an object of superstition in the European Middle Ages, and scientific interest in it only began to re-emerge in the second half of the 16th century (Link, 1957).