Astronomy

Plato treats astronomy as among the mathematical sciences that prepare the mind for higher knowledge. In the Republic, he assigns it a place in the curriculum of the philosopher-rulers, alongside arithmetic, geometry, and harmonics, because the study of heavenly motions disciplines the mind to attend to order and number rather than to mere appearance. The visible circuits of the stars are useful for practical purposes, Plato acknowledges, but the study of the actual motions in the visible heavens is of secondary importance compared with the mathematical order those motions imperfectly represent. The bearing of astronomical study on the education of those who are to govern is discussed more fully in the chapter on Education.

In the Timaeus, Plato offers a cosmological account in which the Demiurge fashions the heavens as a moving image of eternity. The sun, moon, and planets mark off the periods of time, and their regular circuits reflect the rational order of the cosmos. God gave us sight, Timaeus says, so that we might observe "the courses of intelligence in the heaven" and apply them to the courses of our own intelligence. On this account, astronomy is as much a moral discipline as a natural science; its proper end is the ordering of the soul, not the prediction of celestial positions.

Plato is also concerned with those who oppose astronomical inquiry on religious grounds, who fear that men who study celestial phenomena by natural methods will conclude that all things happen by necessity rather than by an intelligent will. His answer is that the order of the heavenly motions is itself one of the arguments for the existence of divine intelligence, and that a false understanding of necessity, not astronomy rightly pursued, is what gives rise to atheism.

Ptolemy, Copernicus, and Kepler can be found, in varying degrees, echoing Plato's conviction that the study of the heavens is not merely a practical science but one that bears on the mind's relation to truth and, for some, to God. Whether the Platonic view of astronomical knowledge as moral discipline is compatible with an astronomy that seeks only to describe and predict motions is a question the later tradition tends rather to assume than to examine directly.

Aristotle shares Plato's view that the heavens are a worthy and elevated object of inquiry, but he grounds his account in physical theory rather than in mathematical ideals. In On the Heavens, he argues that the celestial bodies are composed of a fifth element, aether, which neither comes into being nor perishes and admits only of circular motion. The four terrestrial elements (earth, water, air, and fire) move naturally in straight lines toward or away from the center of the world and undergo generation and corruption; the heavenly bodies, by contrast, move perpetually in circles and show no variation in size, shape, or quality. For Aristotle, this physical difference between the two realms appears to follow from what observation reveals, and it issues in a cosmology in which the heavens and the terrestrial region are governed by fundamentally different principles. The question of what matter the heavenly bodies are composed of is discussed more fully in the chapter on Matter.

The physical consequences follow directly from these principles. Because earthen matter sinks naturally to the center of the cosmos, the earth is necessarily stationary; and because celestial matter moves only in circles, the orbits of the heavenly bodies must be perfect circles, with any observed irregularities in their motions explained by the combination of circular movements. These conclusions are for Aristotle not hypotheses offered for mathematical convenience but propositions of natural science whose truth follows from the principles of his physics. On this account, the astronomer cannot simply adopt any mathematical arrangement that saves the appearances; the arrangement must also conform to what the physics of each element permits.

Ptolemy built his mathematical system within the constraints of Aristotle's physical cosmology, and the division between a celestial and a terrestrial physics was preserved in medieval astronomy and natural philosophy. When Aristotle observes in the Metaphysics that wonder at the phenomena of the moon and sun and stars was among the first occasions of philosophy, he expresses an attitude that the later tradition inherited along with his cosmological scheme.

The modification of Aristotle's celestial physics required not only new mathematical devices but a revision of the doctrine of matter itself. Copernicus retained circular orbits and did not directly address the composition of celestial matter; it was left to Galileo's telescopic observations of mountains on the moon and to Newton's argument for a uniform physics of all bodies to dissolve the Aristotelian distinction between two realms of nature.

Where Plato treats astronomical contemplation as a path to piety and the ordering of the soul, Lucretius inverts the relation entirely. The Epicurean poem On the Nature of Things presents the study of the heavens not as a religious discipline but as a cure for the religious fear that ignorance of natural causes produces. When men do not understand what causes eclipses, lightning, or the irregular motions of the planets, they attribute these phenomena to the anger of gods and are rendered perpetually anxious. Lucretius aims to strip away that anxiety by explaining what the heavens actually are: configurations of atoms in the void, moving according to natural necessity and nothing more. The argument is directed as much against the Platonic tradition of celestial piety as against popular superstition.

In Book V of On the Nature of Things, Lucretius offers natural explanations for the motions of the sun, moon, and stars, noting candidly that several different physical causes could account for each of the appearances. He does not insist on any single account: the point is that any natural explanation suffices to relieve the mind of religious dread, for as long as one probable natural cause can be identified, there is no reason to invoke supernatural agency. This methodological pluralism is deliberate. Lucretius is less interested in constructing the correct astronomical theory than in establishing the principle that all celestial phenomena, whatever their specific causes, are governed by natural law. The question of whether this pluralism represents scientific modesty or something closer to indifference to the precise truth is one that separates Lucretius from the later tradition of mathematical astronomy discussed under the ideas of Hypothesis and Mathematics.

Lucretius anticipates a line of modern scientific thinking that regards astronomical understanding as an instrument of human emancipation from superstition, rather than as an approach to the divine. This inversion of the Platonic framework runs through Enlightenment accounts of science and finds an echo in Freud's characterization of the Copernican revolution as a deflation of human self-importance. The deeper disagreement between Lucretius and Plato concerns not only the religious significance of the heavens but the nature of piety itself: whether it consists in a kind of conformity of the mind to the celestial order, as Plato holds, or in the mind's freedom from the fear of celestial powers, as Lucretius maintains.

Ptolemy's Almagest is the most complete and systematic work of mathematical astronomy that antiquity produced. Its apparatus of epicycles, deferents, and equants allowed astronomers to calculate planetary positions with a degree of accuracy that no rival system achieved for fourteen centuries, and it remained the standard reference throughout the medieval period for those who needed to predict the motions of the heavenly bodies. Ptolemy regarded his enterprise as primarily mathematical rather than physical: the aim was to construct geometrical models that reproduced the observed motions, and he made no strong claim that these models corresponded to the actual physical arrangement of the heavens.

This methodological restriction had certain advantages but also raised a question that Ptolemy himself acknowledged. He noted that the supposition of a rotating earth was "plausible" as a means of accounting for the appearances, but he rejected it on physical grounds drawn from Aristotle: if the earth rotated, bodies thrown upward would be left behind by it, and the surface of the earth would be subject to great disturbances from the motion. His argument accepted Aristotelian physics as a constraint on astronomical theorizing even while treating the detailed mathematical devices as instrumental. Whether a mathematical construction that accurately describes appearances can be indifferent to physical truth, or whether it must be judged by physical criteria as well, is a question his successors could not long evade.

Ptolemy also wrote the Tetrabiblos, a treatise on the influences of the heavenly bodies on terrestrial events and human character, which touches on the boundary between astronomy and astrology. The questions raised by that boundary, and by the use of celestial phenomena as signs and omens, are discussed in the chapters on Prophecy and Sign and Symbol.

Kepler's insistence that astronomy must be a part of physics, not merely a branch of mathematics, was in large measure a rejection of the Ptolemaic conception of the astronomer's task. The numerical accuracy of Ptolemy's predictions meant, however, that any rival construction had first to match them before it could plausibly claim to have improved upon the Almagest.

Augustine's position on astronomy is formed in part by a biographical episode recounted in the Confessions. As a young Manichaean, he had been told that his sect's cosmological doctrines were confirmed by precise knowledge of the heavens. When he eventually encountered astronomers who could demonstrate the inaccuracy of the Manichaean astronomical claims, Augustine drew a lesson that remained with him: religious teaching should not be staked on astronomical claims that natural inquiry can test and overturn. The problem was not that Faustus, the Manichaean leader, had made errors in natural philosophy; it was that he had pretended religious authority where none was warranted. From this experience Augustine developed the principle that theology and astronomical science have quite separate competences, and that entangling them serves neither well.

In On Christian Doctrine, Augustine states the separability more directly. A Christian who does not know "even the circuits of the Great Bear" is not thereby worse off spiritually than an astronomer who can measure the heavens but neglects God. The scriptures speak of natural things in language suited to the understanding of common people, not in the technical idiom of natural philosophy; to insist on the literal astronomical accuracy of scriptural language is to misread the purpose of scripture. Augustine allows that precise knowledge of astronomical matters is not religiously harmful and may even be useful for fixing the dates of Christian festivals, but he insists that such knowledge is not necessary for salvation and that the authority of scripture does not depend on it. The relation between scripture and natural philosophy is treated more fully in the chapters on Religion and Theology.

Augustine's position anticipates the argument that Cardinal Barberini is reported to have put to Galileo during the controversy over the Copernican hypothesis: that astronomy and theology have separate tasks, the one teaching how the heavens go, the other how to go to heaven. This formula, whether or not it accurately represents Barberini's actual views, captures the Augustinian distinction as later interpreters understood it. Where Plato had made astronomical contemplation a form of spiritual discipline, and where Lucretius had made it an instrument of liberation from religious fear, Augustine refused both connections: the heavens neither purify the soul nor threaten it, and their scientific investigation has no direct bearing on faith.

Aquinas inherits Aristotle's two-realm cosmology and integrates it within a theological account of creation. The heavenly bodies, composed of incorruptible matter, are instruments of divine providence, and their regular motions manifest the order that God has imposed on the whole of nature. In treating the work of the six days in the Summa Theologica, Aquinas addresses the creation of the luminaries on the fourth day and their function as signs of seasons and sources of light for the lower world. He also considers the question of whether the celestial spheres are moved by angelic intelligences, a topic he treats with some care, noting that the mode of this relation is not fully determined by reason alone. The connection between the celestial motions and the activity of the angels is discussed in the chapter on Angel.

On the question of what astronomy can claim to establish, Aquinas draws a distinction between matters of natural philosophy and matters of faith. The particular hypothesis an astronomer adopts (whether the spheres are arranged in one way or another, how many there are, what moves them) is a question for natural inquiry, and scripture does not settle it. Aquinas observes that scripture speaks of the heavens in language suited to the understanding of common people, not as a scientific treatise, and that when what reason establishes in natural philosophy does not contradict faith, one ought to follow reason's findings. He also notes that when several explanations are consistent with the text of scripture, one should not insist on any single one, since a defender of scripture who too firmly commits it to one view of natural things may expose it to ridicule if that view is later shown to be false.

Aquinas follows Aristotle in holding that the sun, as the principal heavenly body, acts as a cause in the generation of terrestrial living things; he considers the possibility that the sun's influence may even contribute to spontaneous generation from putrefying matter. This connection between celestial causation and biological generation belongs to a broader range of questions about the relation between astronomical and terrestrial science.

Aquinas thus distinguished between the theological and the scientific aspects of cosmology in a way that afforded some latitude to astronomical inquiry. Whether the boundary he drew between natural philosophy and scriptural interpretation could be maintained in practice depended on the willingness of all parties to observe it. The relation between astronomical theory and scriptural authority, which the controversy over Copernicus and Galileo would make acute, is treated more fully in the chapters on Religion and Theology.

Copernicus was not the first to propose that the earth moves around the sun: the hypothesis is attributed to Aristarchus of Samos by Archimedes, and Ptolemy himself acknowledged that a rotating earth could account for certain appearances before rejecting it on physical grounds. What Copernicus undertook was to develop the heliocentric supposition into a complete mathematical system comparable in scope and detail to the Almagest. His primary dissatisfaction with the Ptolemaic arrangement concerned the equant, a device requiring a planet to move uniformly about a point other than the center of its circle, which seemed to violate the ancient principle that celestial motions must be perfectly uniform and circular. Copernicus held that a heliocentric arrangement would restore the mathematical order that Ptolemy's construction had compromised.

The result, presented in On the Revolutions of the Heavenly Spheres and dedicated to Pope Paul III in 1543, was a system in which the earth was treated as one planet among several orbiting the sun, with the apparent retrograde motions of the other planets explained by the relative orbital motion of the earth rather than by the superposition of one epicycle upon another. Copernicus retained circular orbits and required smaller epicycles to match the observations precisely; his system was not substantially more accurate in its numerical predictions than Ptolemy's, but it offered what seemed to him a simpler and more fitting account of the structure of the whole. He presented the work partly as a mathematical hypothesis and partly as a physical claim, without fully resolving the relation between the two. The dedication to the Pope and the general spirit of the preface suggest that Copernicus regarded his hypothesis as consistent with a reverence for astronomical order of the kind Plato and Ptolemy had expressed.

The physical implications of the heliocentric hypothesis, if taken as literally true rather than as a mathematical device, extended well beyond the question of planetary positions. If the earth moves, the Aristotelian physics of natural place and natural motion, which had been among the main reasons for treating the earth as stationary, could no longer stand without revision. Copernicus did not himself develop this consequence at length; he left the physical interpretation to those who came after him.

Kepler and Galileo supplied what Copernicus had not: the physical arguments and the observational evidence that gave the heliocentric hypothesis a status beyond that of a mathematical alternative to Ptolemy. What is sometimes called the Copernican revolution belongs in its full significance to the sequence of inquiry from Copernicus through Newton rather than to Copernicus alone.

Montaigne uses the history of astronomical theory as one of his sharpest examples of human intellectual presumption. In the Apology for Raymond Sebond, the longest and most philosophically concentrated of the Essays, he surveys the succession of cosmological systems and finds in their variety not a record of progress but evidence that the human mind is not equipped for the certainty it claims. Astronomical systems are revised and overturned; those who held the Ptolemaic arrangement with confidence were wrong; those who hold Copernican heliocentrism with confidence may be no better placed. What is presented in each age as the settled account of the heavens turns out to be one more attempt whose inadequacy becomes apparent to the next generation. Montaigne is not hostile to the inquiry itself, but he insists that a modest awareness of its limits ought to accompany it. The question of what kind of knowledge astronomical hypotheses can claim is treated at length in the chapter on Hypothesis.

Montaigne's humanist argument draws on an anecdote he finds in the tradition. The servant girl who saw Thales fall into a ditch while gazing at the stars serves as his emblem of the philosopher who loses sight of what is near and pressing while pursuing the remote and speculative. When Anaximenes was asked why he troubled himself with the secrets of the stars while death and slavery lay before him, the question was not merely a rebuke to stargazing but a statement of priority: the disorders of the soul, the proximity of mortality, the uncertainties of political life are more urgent objects of attention than the motions of bodies that can never be reached, much less fully known. This preference for self-knowledge over cosmic speculation is the humanist alternative to Plato's prescription that the study of the heavens orders the soul; for Montaigne, it is precisely our inability to know ourselves that makes the presumption of celestial knowledge so striking.

The skeptical reading of astronomical history that Montaigne develops was available to a generation that had watched the Ptolemaic system give way to the Copernican. Kant would subsequently argue that the Copernican revolution, properly understood, confirmed the limits of human knowledge at the cosmological scale: the antinomies of pure reason show that reason cannot determine whether the universe is finite or infinite, whether it had a beginning or not. Kant's restraint recalls Montaigne's, though it is founded on a different kind of argument. What is common to both is the conviction that the starry heavens, however they fill the mind with awe, are not fully at the disposal of human understanding.

Kepler's central departure from his predecessors lay in his insistence that astronomy is a part of physics and not merely a branch of mathematics. Where Ptolemy had been content to construct geometrical devices that reproduced the observed motions without claiming to describe physical causes, and where Copernicus had offered a mathematically simpler arrangement while leaving the physical interpretation largely to others, Kepler maintained that astronomical hypotheses must conform to physical reality. The astronomer, on this view, cannot postulate any motion or arrangement he finds mathematically convenient; he is bound by the same considerations of physical cause that constrain the natural philosopher in other parts of science.

His three laws of planetary motion were developed within this framework. The first, that planets move in ellipses with the sun at one focus, required the abandonment of the circular orbits that every astronomer from Plato through Copernicus had treated as a fixed condition of any acceptable theory. The second, that a line drawn from the sun to a planet sweeps equal areas in equal intervals of time, replaced uniform circular motion with a motion variable in speed according to the planet's distance from the sun. The third related each planet's period of revolution to its mean distance from the sun in a fixed numerical proportion that held across the whole solar system. These results were derived from extensive calculation with the observational data assembled by Tycho Brahe, and in particular from the study of the orbit of Mars, for which circular models were tried and found insufficient.

Kepler also argued, against those who thought the heliocentric arrangement unfitting for man's habitation, that it was more appropriate for man, who was to be the dweller in and contemplator of the world, to move around it in his earthly home than to sit confined at the center. In proposing that the heavenly machine operates by a physical force rather than by the will of living intelligences, he contributed to the transformation of cosmology examined in the chapter on Mechanics.

Newton's unification of celestial and terrestrial mechanics in the Principia presupposed Kepler's three laws, which provided the empirical constraints that any physical theory of planetary motion had to satisfy. Kepler's insistence that astronomical theory must answer to physical reality rather than merely to geometrical convenience made it possible to ask what single force could account for the motions his laws described.

Galileo applied the telescope to the heavens and obtained results that bore on several central doctrines of Aristotelian cosmology. The surface of the moon, rather than being perfectly smooth and uniform as Aristotle's account of celestial matter implied, displayed mountains and valleys analogous to those of the earth. Jupiter was observed to have four satellites in orbit about it, showing that not all celestial revolution was centered on the earth. Venus was found to pass through a complete set of phases, a result compatible with a Copernican arrangement but difficult to reconcile with a Ptolemaic one. The Milky Way, under telescopic observation, resolved into a multitude of individual stars. Each of these findings had implications for a different aspect of the traditional cosmological scheme.

In the Dialogue Concerning the Two Chief World Systems, Galileo examined the arguments for the Ptolemaic and Copernican arrangements through a conversation among three characters: the Copernican Salviati, the Aristotelian Simplicio, and the independent Sagredo. The work examined the physical arguments for a moving earth and questioned the Aristotelian distinction between celestial and terrestrial physics, arguing that no adequate reason had been given for supposing that the principles governing motion in the terrestrial region were different from those applicable to the heavens. In arguing against this distinction, Galileo was pressing toward the unified physics that Newton would later establish. The controversy that followed the publication of the Dialogue, and the question of the relation between astronomical theory and religious authority, are discussed in the chapter on Religion.

Galileo also maintained that scripture and natural science have separate domains: scripture teaches the way to salvation, not the structure of the heavens, and is not to be interpreted as an astronomical treatise. This position drew on a distinction that Aquinas had already made and that Cardinal Barberini had acknowledged; the difficulty was that not all parties were prepared to observe it consistently.

Galileo's telescopic observations removed certain empirical supports from the Ptolemaic system, and his physical arguments pressed the case for treating the heavens and the earth as subject to the same laws of motion. The construction of a single physics adequate to both fell to Newton.

Newton's Principia presented a single mathematical account of motion applicable without distinction to terrestrial bodies and to the planets. His law of universal gravitation held that every body in the universe attracts every other body with a force proportional to the product of their masses and inversely proportional to the square of the distance between them. From this principle, together with the three laws of motion, he derived Kepler's three laws of planetary motion, an account of the tides, and explanations of such phenomena as the precession of the equinoxes and the paths of comets. What had been the separate results of Galileo and Kepler were shown to follow from a single set of mechanical principles.

The unification had a bearing on philosophy as well as on natural science. Aristotle had maintained that the heavens and the terrestrial region were governed by different physical principles and composed of different kinds of matter. Newton's Third Rule of Reasoning asserted that "qualities of bodies which are found to belong to all bodies within the reach of our experiments are to be esteemed the universal qualities of all bodies whatsoever." On this principle, celestial and terrestrial matter are of the same kind, and the same gravitational force governs both. The Aristotelian distinction between two realms of nature was not so much argued against as rendered unnecessary by a theory that had no use for it. The question of what the matter of the heavenly bodies consists in, which Aristotle had treated as requiring a special fifth element, is discussed in the chapter on Matter.

Newton himself was reluctant to propose a physical cause for the gravitational force; he insisted that he would frame no hypotheses about what produced it, since such hypotheses, not being deduced from the phenomena, had no place in experimental philosophy. Whether a science that establishes the mathematical law governing a force while declining to explain the nature of that force is fully satisfactory is a question that Newton's successors have continued to examine.

After Newton, astronomy was no longer a discipline with its own separate principles but a part of a general mechanics whose laws applied equally to terrestrial and celestial bodies. Whether this unification exhausted what could be said about the structure of the cosmos, or whether certain questions about its origin, extent, and the conditions of possible astronomical knowledge lay beyond the reach of Newtonian mechanics, was taken up by Kant.

Kant engaged with astronomy on two levels. In the Universal Natural History and Theory of the Heavens of 1755, written before the critical philosophy, he proposed that the solar system had formed through the condensation of a diffuse rotating cloud of matter under Newtonian gravitational attraction. This extension of Newtonian mechanics to the question of the origin of the celestial order addressed a topic Newton himself had not treated as within the scope of gravitational theory. Laplace subsequently developed a similar hypothesis on independent grounds. Whether such a cosmogony is consistent with the demands of Newtonian physics, or whether it introduces assumptions that go beyond what the phenomena warrant, is one of the questions that the critical philosophy would later press.

In the Critique of Pure Reason, Kant addressed the epistemological problem that astronomical inquiry raises. The Antinomies of Pure Reason examine pairs of contradictory propositions about the cosmos: whether the world has a beginning in time and a boundary in space, and whether it does not. For each pair, he finds that apparently valid arguments can be constructed on both sides. This situation reveals, on Kant's account, not a failure of astronomical reasoning but a structural feature of human reason: the concepts and principles that make knowledge of phenomena within experience possible cannot be legitimately extended to determine the character of the cosmos as a whole. The questions of the finitude or infinity of the universe in time and in space are discussed in the chapters on Eternity, Time, Space, and Infinity.

Kant's well-known passage connects the moral and the astronomical perspectives in a way that recalls Plato's account of the heavens as a model for the soul, though on different grounds. The starry heavens, contemplated in their immensity, reduce man to insignificance as an animal creature; the moral law within reveals a dignity that is independent of that physical order. Kant does not advise forsaking astronomical study; on the contrary, he recommends it both for its scientific value and for the moral effect it produces in the reflective mind.

The investigations of the astronomers have shown us, Kant writes, the abyss of our ignorance in relation to the universe; yet this acknowledgment of ignorance does not, for him, counsel the abandonment of the inquiry. Reason must be honest about the boundaries of its competence, and astronomy, rightly understood, contributes to that honesty.