The sixteenth and seventeenth centuries witnessed a dramatic growth of interest in the natural world, sparked off not least by the European discovery of the Americas. For the first time since antiquity there was an enthusiastic commitment to what it was possible to know empirically about the world around us. The search to understand the sources of knowledge intensified, made easier by new instruments such as the telescope and microscope, which allowed more accurate observations. Mathematics was to play an important role in contributing models for the universe that was now seen as acting under uniform laws, as would be brilliantly defined by Isaac Newton. The elements of Aristotle, earth, fire, air and water, were replaced by an understanding that material might be made up of small particles of different chemical ''elements''. Scientists--or ''natural philosophers'' as they were then known--were eager to find ways of proving their theories and thereby to provide the foundations for the development of new ones. Experimentation thus became the rage. Of course, it is only in hindsight that one can see which experiments bore fruit in the sense of providing accurate representations of reality and which led nowhere.

This would make the development of scientific knowledge a cumulative process, a crucial development in the history of European thought and one that has proved enduring. What unites the figures to be discussed in this chapter is a deeprooted desire to understand the nature of the universe. The roots of this desire are hard to define, since they varied from individual to individual, but all of these astronomers, physicists and mathematicians of the era of the Scientific Revolution were united by their commitment to observing and calculating. As we shall see, some were committed Christians and some were not. Religion in its varied forms does not appear to have been a powerful force in encouraging or hampering the progress of science (see the end of Chapter 30 for further discussion of this issue). Telling the story of the transformation of natural philosophy into what might be seen as science involves many complex issues, not least because so many natural events appeared inexplicable unless it was assumed that they were controlled by forces beyond human observation. Were there sources of ancient wisdom--the works of Hermes Trismegistus championed by Marsilio Ficino, for instance--that might hold the secrets of knowledge? An influential work on magnetism by the Englishman William Gilbert (1544-1603), De Magnete (1600), raised the question of how one object, the lodestone, could influence another without any physical connection. Gilbert compared the action of the lodestone with the effect that certain substances, if ingested, can have on the human body--such as the laxative influence of rhubarb--and with the relationship between the phases of the moon and the tides.

Was God acting in some way through objects or were there invisible forces underlying the natural world which were present in certain substances? Gilbert made his own suggestion: ''We consider that the whole universe is animated, and that all the globes, all the stars, and also the noble earth, have been governed, since the beginning by their own appointed souls and have motives of self-preservation. In contrast to Gilbert''s ''animated'' universe, Descartes put forward the notion of the universe as a machine, in which every particle was in contact with each other (see p. 668). Others, such as the astronomer Johannes Kepler (see below, p. 660), attempted to classify natural phenomena in relation to their creation by God. The alchemists remained influential--if esoteric--investigators of the physical world and were valued for their skills in working with metals, in particular. Meanwhile, the findings of those who used empirical methods often contradicted classical sources, raising concerns about the authority of each. Studies of the classification systems of the private libraries that were now a feature of cultural life for the elite reveal that it was only slowly that the study of nature and the universe sorted itself out into recognizable disciplines.

Rather than attempting to define a coherent sequence of events, traditionally termed the ''Scientific Revolution'' and taking place somewhere in the seventeenth century, some scholars prefer to identify--to quote the historian of science Steven Shapin--''a diverse array of cultural practices aimed at understanding, explaining, and controlling the natural world, each with different characteristics and each experiencing different modes of change''.3 The relatively coherent term ''science'', ''the disciplined enquiry into the phenomena and order of the natural world'', that we know today struggled to be born from many different philosophical and empirical sources. Many histories of ''modern'' science begin in 1543 with Copernicus''s hypothesis that the Earth and the other planets move around the sun. He had his predecessors. Aristarchus of Samos (c.310-c.230 bc) had posited a sun-centred system and Copernicus knew of this. However, the sophisticated, Earth-centred description of the cosmos by the Greek geographer and astronomer Ptolemy had held sway since the second century ad.

In the Almagest (see Chapter 7) Ptolemy used this geocentric model of the known universe to explain the movements of the six known planets (Mercury, Venus, Earth, Mars, Jupiter and Saturn), in addition to the sun and the moon. Ptolemy insisted that all planetary movements were in perfect circles. Once this and the central position of the Earth had been accepted, Ptolemy had had to construct elaborate models to explain the recorded observations of the planets. In the geometrically based Ptolemaic universe, the planets move clockwise in a small circle, the epicycle, which at the same time moves clockwise around a larger circle, the deferent. This model had in fact been used by Greek astronomers for centuries, but the problem for Ptolemy was that it did not entirely square with available observational data relating to the speed of planetary orbits. In order to reconcile these planetary movements with his hypothesis of uniform circular motion, Ptolemy developed the mathematical concept of the equant, a point close to but outside the Earth, from which point a planet would always appear to be moving at a uniform speed. This did actually account for the many astronomical observations. Understandably, very few medieval astronomers had managed to grasp the breadth of Ptolemy''s work in its Latin translations.

The accepted explanation of planetary movement was the Aristotelian cosmos, in which the planets moved in concentric spheres. Several centuries earlier than Ptolemy, and with less observational data to go on, Aristotle had argued that the passage of the planets was perfect and unchanging. Once put in motion by the Supreme Mover, they simply went on moving, each planet travelling entirely within its own sphere. The Earth was thus surrounded by concentric spheres, one for each planet, beyond which, in the Christian interpretation of the system, was heaven itself. While Aristotle had talked of the aether above the fire, air and water that surrounded the Earth, there had been much speculation what the substance of each celestial sphere actually was. One suggestion was that it consisted of a form of invisible crystal, within which the planet travelled. What was agreed was that no planet or other star could break out of its sphere. The universe was in itself finite, with the fixed stars in a sphere of their own, not that far beyond the planets.

(The Aristotelian system is shown in the sky above Dante on pp. 238-9.) However complicated, Ptolemy''s system claimed superiority over the simpler Aristotelian system because it was based on a mass of observations, some adopted from Babylonian sources; Ptolemy had listed 1,022 stars in 48 constellations. Gradually, however, its problems had become apparent. In the Epitome, or abridgement, of the Almagest, completed by the German astronomer Johannes Müller, known as Regiomontanus, in 1462 (although not printed until 1496), Regiomontanus had noted that, following Ptolemy''s logic, the moon should have appeared much larger as it neared the Earth in the course of its orbit. He began speculating as to whether Ptolemy''s geometrical models actually worked. The study of astronomy had been stagnant for many centuries, the university texts inadequate and there had been more concentration on astrology, notably the prediction of the course of diseases based on star charts for an individual. The German astronomer Johannes Kepler would later remark that astrology was like a foolish but well-off daughter without whose help astronomy proper would starve.

Even in the papal court, star charts were still being drawn up in the seventeenth century (and an auspicious date picked for the laying the cornerstone of the new St Peter''s in Rome). Copernicus (1473-1543) became interested in astronomy while at the University of Cracow in his native Poland, although his doctorate, from the university of Ferrara in Italy, was in canon law.* While in Italy, he also studied the humanities in Bologna and medicine in Padua, but he was already committed to understanding the motion of the planets and, rather than being worried by the size of the moon, he seems to have been frustrated by the lack of coherence in Ptolemy''s system. It appeared to him to have been built up piece by piece rather than having a harmonious simplicity. To put it bluntly, it was inelegant, and Copernicus, who had absorbed Platonism, found this unacceptable. In 1503 Copernicus returned home to Poland. Here, despite a busy life as a cleric, he continued his investigations into astronomy and postulated for the first time that a system based on.