The Revolutionary Astronomers
Since the second century B.C., when Ptolemy wrote his astronomical manual, The Almagest, until the seventeenth century A.D., a geocentric conception of the universe prevailed in Europe under Ptolemy's authority, which was itself founded on Aristotle's fourth-century work, the Physics. No other cosmological model was considered by European astronomers until the late sixteenth century, so thoroughly had Ptolemaic astronomy permeated the European mind and imagination. Like Darwinian evolution in the twentieth century, Ptolemaic astronomy, and particularly the geocentric model of the universe, was accepted reflexively and informed the imaginations of professional astronomers and educated people throughout Europe. Aristotle's authority dominated European philosophy as well as the science of this period; even in the theology of the Church and Protestant sects Aristotle's profound influence was received through the adaptive work of St. Thomas Aquinas. To dislodge the geocentric conception of the universe would upset a good deal else in the intellectual framework of Europe.
Ptolemaic astronomy involved a complex geometry of explanations of the planetary and sidereal movements, of epicycles, deferents, and equants, many of which were unworkable. The discrepancies between the growing body of observational data and Ptolemy's calculations, as well as the many inclarities in his mathematics, became increasingly apparent to astronomers by the mid-fifteenth century. Though Ptolemy's authority was not questioned, it became necessary to clarify the mathematics of his system to make them more useful. It was this intense review of the Ptolemaic calculations that led to the work of Nicolaus Copernicus a century later. Copernicus is known as the father of heliocentrism, the new astronomy that set the sun, not the earth, at the center of the universe, but historians generally agree that his purpose was conservative, not revolutionary. Working conscientiously within the established tradition, Copernicus's aim was to simplify and clarify in the extremely complex Ptolemaic calculations what had become unworkable in applied astronomy. Although his De revolutionibus orbium coelestium fundamentally altered the accepted structure of the universe by supposing the earth to be in motion with the planets around the sun, it did so from very orthodox assumptions and is even dedicated to Pope Paul III. Copernicus's theory left unchallenged such fundamental assumptions of the old model as the sphericity of the universe, the circular motion (with epicycles) and regularity of the planetary orbits, and the finiteness of the universe, being bound by the sphere of the fixed stars. Even the idea of the sun's centrality derives from the pythagorean-platonic tradition of sun-worship, which, in Copernicus's mind, was its chief virtue. Copernicus postulated the earth's orbiting the sun not because of the force of his observational data, but because, believing in the pythagorean idea that the sun belongs at the center of the cosmos, the earth's motion was a necessary corollary. The mathematics were then worked with little effort at making new observations, the result being a theory that was scarcely more accurate than Ptolemy's, which is why Copernicus himself did not publish it, trying until the end of his life to make it work.
Published after Copernicus's death in 1543, De revolutionibus was widely recognized as an important hypothesis. Twenty-five years later it was being enthusiastically promoted in England by a prominent mathematician, Leonard Digges, as a revolutionary theory describing the real physical character of the universe rather than as the purely abstract hypothetical mathematical theorem as it was thought to be. Digges even went so far as to insist that the Copernican theory allowed for an infinite universe, though Copernicus himself only suggested it was much larger than the Ptolemaic universe. This came at the same time as a certain constriction and solidification of opinion in conservative circles that necessarily reacted to Copernicanism as it was being interpreted and would have nothing to do with it. Opinion was divided and Copernicanism was now in the hands of the innovators of astronomy.
Now that a viable alternative to Ptolemaic astronomy existed, it became necessary for those astronomers who did not wholly dismiss it to make variations and adaptations to the Copernican theory. The Dutch astronomer Tycho Brahewas one of the most important of these. Still, he is less renowned for his own cosmological hypothesis than for his design and use of the most accurate and powerful astronomical instruments of the time. Tycho's observational data were methodically planned and recorded and remarkably accurate. They helped discredit Aristotelian physics and were material in producing the Rudolphine Tables (1627), published after his death, which established Copernicanism as a workable theory. Tycho observed and recorded a nova's appearance in 1572 and a widely discussed comet in 1577 and demonstrated that Aristotle (and Ptolemy who followed him) was wrong that such things only occur beneath the sphere of the moon, that is, between earth and moon, beyond which nothing changes. Tycho had carefully calculated that both nova and comet occurred at distances from the earth far beyond that of the moon. The work in which he set forth his observations and his own geocentric hypothesis, De Mundi aetherei recentioribus phaenomenis, was published in 1588.
The stage was set for the coming of Galileo Galilei, who in the 1590s, still early in his career, was already a confirmed opponent of Aristotelian physics and convinced that the Copernican theory was a physical reality. Galileo was perhaps the first modern scientist, for he was an empiricist, basing his conclusions, not upon ancient authorities and mere logic, but upon his own observations made with the best instruments available. In 1609 and 1610, using a telescope he redesigned with improvements, Galileo recorded several observations that would make several Ptolemaic assumptions about the universe indefensible. He observed the surface of the moon to be mountainous and not perfectly spherical as supposed; he saw four of Jupiter's moons, spots on the sun, and the phases of Venus. All of these observations were explainable by the Copernican model but not by the Ptolemaic, which fact sealed Galileo's confidence. Galileo first published his findings in his Sidereus Nuncius (1610), but this work had little effect on the debate as Galileo was then too politically cautious to press his convictions. His Dialogo (Dialogues Between the Ptolemaic and Copernican World Systems), published in 1628, late in his career, garnered the most attention. In this work he supported the heliocentric theory, but in an unconventional mock-socratic format that satirized the traditional view. The Dialogo, with its strong arguments and the force of the authors' reputation, did much to propagate the Copernican theory and, in the view of many historians, assured that it would ultimately supplantthe Ptolemaic-Aristotelean model of the universe. Galileo's subsequent trouble with the Vatican had to do with the theological implications of the theory as Galileo taught it.
The fate of the Ptolemaic theory already sealed—at least among professional astronomers, Johannes Kepler's three laws dealt the coup de grace. It was Kepler, a contemporary of Tycho Brahe and his collaborator for some years, who would establish that the planetary orbits are neither circular nor regular, but elliptical and move faster when closer to the sun. The first two of Kepler's three laws exploded two Ptolemaic (and Copernican) assumptions about celestial motion: that it is necessarily circular, circularity being the perfect form of motion, and also necessarily regular for no irregularity can occur in the celestial regions. Both in the way of mathematical theory and speculation, Kepler's three laws of planetary motion and the Rudolphine Tables helped astronomers accept the Copernican model of a heliocentric universe as a physical fact by improving on its mathematical accuracy and thus, at the very least, a theory to contend with. In the fifty years after Kepler's death (1630), it was the ambition of many astronomers to either disprove or modify Kepler's principles. So much attention for so long inevitably made Copericanism a commonplace and thus so much the easier to accept. Kepler indeed did more than anyone to establish the Copernican model in the European mind. For he paved a direct path to Newton whose discovery of the law of universal gravitation—by applying Kepler's laws in the light of his own discoveries—introduced a new physics that forever laid to rest the physics of Aristotle and the astronomy of Ptolemy. Until the twentieth century, Newtonian physics stood in the place of Aristotle and Ptolemy, holding sway in its ability to explain observable astronomical phenomena.