On the Heavens

Summary

On the Heavens (Greek Peri ouranou, Latin De caelo) is Aristotle's account of the physical universe as a whole: what the heavens are made of, why they move as they do, where the earth sits and what shape it has, and why stones fall and flames rise. Its four books proceed from first principles to particular bodies. Book I begins from the three dimensions of body and the two simple kinds of motion, straight and circular. Because straight motion belongs to the four familiar elements (earth and water moving down, air and fire moving up), Aristotle argues that circular motion must belong naturally to some other simple body, which he identifies with the aither of the poets and calls the 'first body'. This substance neither comes to be nor passes away, neither grows nor alters, and so the region of the stars is ungenerated, unaging and eternal. The rest of Book I argues that the universe is finite, that there cannot be more than one world, and that it had no beginning and will have no end, against Plato's Timaeus, which Aristotle reads as describing a cosmos that came to be. Book II turns to the heavens in detail. The cosmos has a right and a left, an up and a down; the stars are spherical, fixed in revolving spheres rather than moving by themselves, and produce no music, against the Pythagorean harmony of the spheres. Aristotle then asks the strange question why the planets have more complex motions than either the outermost sphere or the sun and moon, and proposes that the stars be thought of as living and acting. The last chapters of the book argue that the earth is at rest at the centre, against the Pythagorean central fire, and that it is a sphere of no great size, citing the curved shadow in lunar eclipses, the change in visible stars between Egypt and the north, and the mathematicians' estimate of its circumference. Books III and IV leave the heavens for the sublunary world, criticising the elemental theories of Empedocles, the atomists and the Timaeus, and explaining heaviness and lightness by each element's tendency toward its natural place. The treatise matters to students twice over. It shows Aristotle reasoning carefully from observation, testimony and argument, and correcting his predecessors by name. It also supplied the framework of nested spheres, a changeless heaven and a central earth that Ptolemy refined, medieval Christian writers adopted with corrections, Dante turned into poetry, and Copernicus, Kepler and Galileo dismantled. Much of its physics is mistaken, yet its demand that a theory answer to the observed facts is the very standard by which it was later overturned.

Plot or Argument

• Book I: a fifth body that moves in a circle

  Starting from the completeness of three-dimensional body and the division of simple motions into straight and circular, Aristotle argues (Parts 2-4) for a primary body whose natural motion is circular, exempt from generation, growth and alteration. He links the name aither to 'running always' (Part 3) and notes that belief in a divine upper region seems to confirm the argument.

• Book I: one finite, eternal cosmos

  Parts 5-7 argue that no body is infinite, since an infinite body could not rotate and infinite weight is impossible; Parts 8-9 that there cannot be several worlds, because each element would have to tend toward one centre; Parts 10-12 that the world is neither generated nor destructible, against the Timaeus and against cyclical cosmogonies such as Empedocles'.

• Book II: the order and shape of the heavens

  The heaven moves without effort, so no Atlas is needed to hold it up (Part 1). It has directions like a living body (Part 2); its sphere is perfectly round (Part 4); the stars are made of the first body, are spherical, and are carried by their spheres (Parts 7-8). The Pythagorean music of the spheres is rejected because so vast a noise would shatter things (Part 9).

• Book II: two puzzles and a proposal

  Part 12 asks why the planets between the outermost sphere and the earth show the most complex motions, and why the outermost sphere carries so many stars while each planet carries one. Aristotle offers a tentative answer: we should think of the heavenly bodies as sharing in life and action, attaining their good by more or less complex activity.

• Book II: the earth at rest and round

  Part 13 surveys earlier views of the earth (the Pythagorean central fire and counter-earth, Plato, Thales, Anaximenes, Anaxagoras, Democritus). Part 14 argues that the earth is motionless at the centre, that heavy bodies thrown straight up return to their starting point, and that its shape is spherical: eclipse shadows are always curved, stars visible in Egypt and Cyprus vanish further north, and mathematicians estimate its circumference at 400,000 stades.

• Books III-IV: the elements, weight and natural place

  Book III criticises those who make bodies out of planes (the Timaeus), atoms (Leucippus and Democritus) or a single element, and argues that the elements change into one another. Book IV defines heavy and light absolutely and relatively, explains why a large mass of earth falls faster than a small one by its tendency toward the centre, and attributes floating and sinking to shape and the resistance of the medium.

Characters

• The Pythagoreans, opponents

  Credited with the central fire, the counter-earth and the harmony of the spheres; Aristotle accuses them of fitting observations to theories rather than the reverse (Book II, Parts 9 and 13).

• Plato (the Timaeus), principal interlocutor

  Aristotle reads the Timaeus as teaching that the cosmos was generated but will never perish, and rejects its construction of the elements from triangles (Book I, Part 10; Book III).

• Empedocles, opponent

  Poet-philosopher of the four roots, Love and Strife; Aristotle criticises his alternating cosmos and his explanation of why the earth stays in place.

• Leucippus and Democritus, opponents

  Atomists whose primary bodies move in the void; Aristotle asks what their natural motion could be and argues that indivisible magnitudes overthrow mathematics (Book I, Part 5; Book III).

• Anaxagoras, opponent

  Criticised for misusing the name aither as if it meant fire, and for his account of the earth resting on air.

• The mathematicians, witnesses

  Unnamed astronomers whose estimate of the earth's circumference Aristotle cites as evidence that the earth is small compared with the stars (Book II, Part 14).

Lessons

• Testing a cosmology against eclipses and travellers' skies

  Aristotle's proof that the earth is a sphere rests on facts anyone could check: the moon's phases take many shapes but the edge of an eclipse is always curved, and travellers to Egypt and Cyprus see stars that never rise in Greece. Students can follow an ancient argument that is still sound and learn what makes evidence decisive.

  • Why does Aristotle think the eclipse shadow tells us about the earth rather than about the moon?
  • What does the change in visible stars prove that the eclipse alone does not?
  • Which of Aristotle's arguments in Book II, Part 14 would you still use today, and which would you drop?

• A small first error grows a thousandfold

  In Book I, Part 5 Aristotle warns that a slight mistake at the level of principles becomes enormous in its consequences, and he gives the example of atomism undermining mathematics. The remark applies to his own system: his assumption that circular motion is natural to a special heavenly body shaped two thousand years of astronomy. The lesson concerns humility about starting points.

  • What example does Aristotle give of a small initial error with large consequences?
  • Which of Aristotle's own first assumptions turned out to matter most for later astronomy?
  • Where in your own studies has a small early mistake grown into a larger one?

• Fitting theories to facts, not facts to theories

  Aristotle charges the Pythagoreans with forcing their observations to fit opinions they already held, such as the central fire and the counter-earth (Book II, Part 13). The same standard, applied to Aristotle by later astronomers, eventually overturned his central earth. The treatise thus teaches a method stronger than many of its conclusions.

  • What exactly does Aristotle think the Pythagoreans did wrong?
  • Is Aristotle himself guilty of the same fault anywhere in the treatise?
  • How would Galileo use Aristotle's own principle against Aristotle?

• The modesty of a tentative answer

  Before tackling the puzzles of planetary motion in Book II, Part 12, Aristotle asks to be judged as one whose love of wisdom makes him grateful for slight indications, a sign of modesty rather than overconfidence. Elsewhere (Part 5) he distinguishes the certainty proper to human reasoning from a stricter kind. Students see a great thinker marking how sure he is.

  • How does Aristotle signal that his answer in Book II, Part 12 is only probable?
  • Why might it be better to offer a tentative answer than none at all?
  • What is the difference between intellectual modesty and refusing to take any position?

• An eternal heaven and the doctrine of creation

  Aristotle argues that the cosmos never began and will never end, and that the highest region is unchanging and divine. Medieval Christian readers such as Aquinas welcomed his order and his first mover but denied the world's eternity on the authority of Genesis, arguing that reason alone could not settle the question. The treatise lets students see faith and philosophy in real conversation.

  • What reasons does Aristotle give in Book I, Parts 10-12 for thinking the world has no beginning?
  • Why did Christian thinkers accept much of Aristotle's cosmos while rejecting this conclusion?
  • Does Aristotle's appeal to what 'all men' believe about the gods strengthen or weaken his argument?

Context

On the Heavens belongs to the cluster of Aristotle's physical writings that includes the Physics, On Generation and Corruption and the Meteorology, and it presupposes the Physics' analysis of motion and nature. It was written in a Greek world where Eudoxus of Cnidus, a contemporary connected with Plato's Academy, had recently modelled planetary motion with nested concentric spheres; Aristotle's Metaphysics XII adopts a refined version of that scheme. The treatise argues with earlier natural philosophers by name, especially the Pythagoreans, Empedocles, Anaxagoras and the atomists, and with Plato's Timaeus.

Composition

Like Aristotle's other surviving treatises, it was not a polished publication but a set of lectures or working notes from his teaching, probably revised over years; the traditional date of c. 350 BC is approximate and some scholars place it earlier, in or soon after his Academy years. The four-book arrangement, and the common Latin title De caelo, come from the later editorial tradition. The library's text is J. L. Stocks's Oxford translation (1922), transcribed by the MIT Internet Classics Archive, with Book and Part divisions as in that transcription.

Reception

Late ancient commentators, notably Simplicius in the sixth century and Philoponus, who attacked the eternity of the world and the fifth element on Christian grounds, debated the treatise closely. Averroes wrote commentaries on it, and in the thirteenth century Thomas Aquinas began a commentary on it, accepting its spherical earth and ordered spheres while denying that reason proves the world eternal. Nicole Oresme's fourteenth-century French commentary seriously considered a rotating earth. Its spheres shaped Dante's Paradiso, and its physics of natural place was the chief target of Galileo's Dialogue and Two New Sciences. It was included in the Great Books of the Western World (2nd ed., 1990), volume 7.

Comparisons

• Almagest

  Ptolemy keeps Aristotle's central, motionless, spherical earth and repeats several of his arguments, but replaces concentric spheres with epicycles and eccentrics designed to predict planetary positions, a mathematical aim Aristotle's physical treatise does not pursue.

• On the Revolutions of the Heavenly Spheres

  Copernicus answers On the Heavens directly in Book I of his own work, rebutting the argument that a moving earth would fling objects off and leave thrown stones behind, and moving the centre of the cosmos from the earth to the sun.

• Timaeus

  Plato's dialogue is Aristotle's main target: Aristotle rejects its generated cosmos, its elements built from triangles and its world-soul, while sharing its conviction that the heavens are ordered, spherical and in some sense divine.

• The Divine Comedy

  Dante's Paradiso climbs through the concentric spheres that descend from Aristotle's cosmology, but makes their motion the work of love for God, turning Aristotle's physics into a theology of ascent.

• Dialogues Concerning Two New Sciences (Second to Fourth Days)

  Galileo's study of falling bodies attacks the claim in On the Heavens that a larger mass of earth moves faster toward its place, replacing natural place with uniformly accelerated motion.

• Meteorology

  The Meteorology continues where On the Heavens leaves off, applying the theory of the four elements and their natural places to comets, the Milky Way, winds, rivers and earthquakes in the region below the moon.

Quotes

Creative Prompts

• Dialogue

  Write a conversation on a ship sailing from Athens to Egypt in which a sailor notices new stars rising in the south and Aristotle's student explains what this proves about the earth.

• Essay

  Using Book II, Part 13, explain what Aristotle means by forcing observations to fit theories, then argue whether his own fifth element passes or fails his test.

• Classroom

  During or after a lunar eclipse (or using photographs of one), sketch the shadow's edge at several stages and write up Aristotle's argument from Book II, Part 14 in your own words, beside your drawings.

• Poetry

  Write a short poem in two voices, one speaking from Aristotle's changeless sphere of the fixed stars and one from the changing world below the moon.

Further Reading

• Physics, Aristotle

  The analysis of nature, motion, place and the infinite that On the Heavens presupposes.

• Metaphysics, Book XII (Lambda), Aristotle

  Counts the celestial spheres following Eudoxus and Callippus and argues for unmoved movers.

• Commentary on Aristotle's De caelo, Simplicius

  The fullest ancient commentary, preserving much earlier Greek astronomy and Simplicius' replies to Philoponus.

• The Discarded Image, C. S. Lewis

  An introduction to the medieval model of the universe built largely on Aristotle's cosmology.

• The Copernican Revolution, Thomas S. Kuhn

  Explains the two-sphere universe of Aristotle and Ptolemy and how it gave way.