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.

Top 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.

• 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.

• 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.

• 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.

• 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.

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