Engraving of Tycho Brahe at the mural quadrant, from his book Astronomiae instauratae mechanica (1598). The engraving depicts Brahe, in the centre with arm upraised, and the work of his observatory at Uraniborg, on the island of Ven. In the background, assistants perform astronomical observations, work in Brahe's study, and do chemical experiments. Behind Brahe are a globe and portraits of his patrons, King Frederick II and Queen Sophia of Denmark. The hound at his feet symbolizes loyalty.Courtesy of the Joseph Regenstein Library, University of Chicago
At the same time Galileo was searching the heavens with his telescope, in Germany Johannes Kepler was searching them with his mind. Tycho’s precise observations permitted Kepler to discover that Mars (and, by analogy, all the other planets) did not revolve in a circle at all, but in an ellipse, with the Sun at one focus. Ellipses tied all the planets together in grand Copernican harmony. The Keplerian cosmos was most un-Aristotelian, but Kepler hid his discoveries by burying them in almost impenetrable Latin prose in a series of works that did not circulate widely.
Kepler, JohannesJohannes Kepler, oil painting by an unknown artist, 1627; in the cathedral of Strasbourg, France.Erich Lessing/Art Resource, New York
What Galileo and Kepler could not provide, although they tried, was an alternative to Aristotle that made equal sense. If the Earth revolves on its axis, then why do objects not fly off it? And why do objects dropped from towers not fall to the west as the Earth rotates to the east beneath them? And how is it possible for the Earth, suspended in empty space, to go around the Sun—whether in circles or ellipses—without anything pushing it? The answers were long in coming.
Galileo attacked the problems of the Earth’s rotation and its revolution by logical analysis. Bodies do not fly off the Earth because they are not really revolving rapidly, even though their speed is high. In revolutions per minute, any body on the Earth is going very slowly and, therefore, has little tendency to fly off. Bodies fall to the base of towers from which they are dropped because they share with the tower the rotation of the Earth. Hence, bodies already in motion preserve that motion when another motion is added. So, Galileo deduced, a ball dropped from the top of a mast of a moving ship would fall at the base of the mast. If the ball were allowed to move on a frictionless horizontal plane, it would continue to move forever. Hence, Galileo concluded, the planets, once set in circular motion, continue to move in circles forever. Therefore, Copernican orbits exist. Galileo never acknowledged Kepler’s ellipses; to do so would have meant abandoning his solution to the Copernican problem.
Frontispiece to Galileo's Dialogo sopra i due massimi sistemi del mondo, tolemaico e copernicano (1632; Dialogue Concerning the Two Chief World Systems, Ptolemaic & Copernican). From left to right are Aristotle, Ptolemy, and Copernicus. Ptolemy holds an astrolabe, Copernicus a model of a planet orbiting the Sun.Courtesy of the Joseph Regenstein Library, The University of Chicago
Kepler realized that there was a real problem with planetary motion. He sought to solve it by appealing to the one force that appeared to be cosmic in nature, namely magnetism. The Earth had been shown to be a giant magnet by William Gilbert in 1600, and Kepler seized upon this fact. A magnetic force, Kepler argued, emanated from the Sun and pushed the planets around in their orbits, but he was never able to quantify this rather vague and unsatisfactory idea.
By the end of the first quarter of the 17th century Aristotelianism was rapidly dying, but there was no satisfactory system to take its place. The result was a mood of skepticism and unease, for, as one observer put it, “The new philosophy calls all in doubt.” It was this void that accounted largely for the success of a rather crude system proposed by René Descartes. Matter and motion were taken by Descartes to explain everything by means of mechanical models of natural processes, even though he warned that such models were not the way nature probably worked. They provided merely “likely stories,” which seemed better than no explanation at all.
Armed with matter and motion, Descartes attacked the basic Copernican problems. Bodies once in motion, Descartes argued, remain in motion in a straight line unless and until they are deflected from this line by the impact of another body. All changes of motion are the result of such impacts. Hence, the ball falls at the foot of the mast because, unless struck by another body, it continues to move with the ship. Planets move around the Sun because they are swept around by whirlpools of a subtle matter filling all space. Similar models could be constructed to account for all phenomena; the Aristotelian system could be replaced by the Cartesian. There was one major problem, however, and it sufficed to bring down Cartesianism. Cartesian matter and motion had no purpose, nor did Descartes’s philosophy seem to need the active participation of a deity. The Cartesian cosmos, as Voltaire later put it, was like a watch that had been wound up at the creation and continues ticking to eternity. Newton
The 17th century was a time of intense religious feeling, and nowhere was that feeling more intense than in Great Britain. There a devout young man, Isaac Newton, was finally to discover the way to a new synthesis in which truth was revealed and God was preserved.
Isaac Newton, portrait by Sir Godfrey Kneller, 1689.© Bettmann/Corbis
Newton was both an experimental and a mathematical genius, a combination that enabled him to establish both the Copernican system and a new mechanics. His method was simplicity itself: “from the phenomena of motions to investigate the forces of nature, and then from these forces to demonstrate the other phenomena.” Newton’s genius guided him in the selection of phenomena to be investigated, and his creation of a fundamental mathematical tool—the calculus (simultaneously invented by Gottfried Leibniz)—permitted him to submit the forces he inferred to calculation. The result was Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy, usually called simply the Principia), which appeared in 1687. Here was a new physics that applied equally well to terrestrial and celestial bodies. Copernicus, Kepler, and Galileo were all justified by Newton’s analysis of forces. Descartes was utterly routed.
Title page from Isaac Newton's De Philosophiae Naturalis Principia Mathematica (1687; Mathematical Principles of Natural Philosophy). Courtesy of the Joseph Regenstein Library, The University of Chicago
Newton’s three laws of motion and his principle of universal gravitation sufficed to regulate the new cosmos, but only, Newton believed, with the help of God. Gravity, he more than once hinted, was direct divine action, as were all forces for order and vitality. Absolute space, for Newton, was essential, because space was the “sensorium of God,” and the divine abode must necessarily be the ultimate coordinate system. Finally, Newton’s analysis of the mutual perturbations of the planets caused by their individual gravitational fields predicted the natural collapse of the solar system unless God acted to set things right again. The diffusion of scientific method