Aristotelian physics

Very early Greek philosophers (the Pre-Socratics) had considered the possibility that the earth was NOT at the center of the universe and that the earth was NOT stationary. The philosopher Anaximander proposed that the cosmos was spherical and that the earth was stationary at its center, but his was not the only description of the universe. The Pythagoreans (a group associated with the philosopher Pythagoras) proposed that the cosmos was spherical, that there was a fire in the center of the cosmos, and that the earth, moon, sun and five visible planets rotated around this central fire. (Note that the sun and the central fire were not the same thing. For an image of this cosmos, see here.)  The astronomer and mathematician Aristarchus (310- 230 B.C.) actually proposed that the sun was the center of the cosmos and the earth rotated around the sun. Further, it was readily apparent to the ancient Greeks that the daily rising and setting of the celestial bodies could be accounted for by the earth rotating once every twenty-four hours around an axis running through the North and South Poles. So why did the Greeks (and the medieval and early modern philosophers and astronomers who built on their work) come to the conclusion that the earth was stationary at the center of the cosmos?

A major reason for the dominance of the geocentric and geostatic model of the cosmos was Aristotle’s physics. By “physics” I mean Aristotle’s account of the natural world. The word physics comes from the Greek word physis, which is usually translated as “nature.” Aristotle’s physics differed from our modern physics in two fundamental ways. First, Aristotle’s physics was qualitative not quantitative. In other words, Aristotle’s physics was not mathematical. He did believe that the universe was essentially mathematical or that mathematics was the proper “language” for describing the physical world. (There is some precedent for this modern view in the work of certain Greek philosophers, notably Pythagoras and Plato.) The second major difference between Aristotle’s physics and modern physics, is that Aristotle’s physics encompassed the entire natural world. He did not separate biology (the study of living things) from astronomy (the study of the heavens).

Aristotle posited that there were two separate realms in the cosmos, the terrestrial (or earthly) and the celestial (or heavenly). Everything in the terrestrial realm was made up of four elements: earth, water, air and fire. Each of these four elements was a combination of two of the four basic qualities: hot, cold, wet and dry. The natural motion of each of these elements was rectilinear (in straight lines), either up or down. Each element had a natural place, and sought to reach that natural place. The natural place of the element earth was at the center of the cosmos. Thus, earth naturally moved down. This is why, if you drop a rock, it always falls down. The rock is made (predominantly) of the element earth and it seeks to reach its natural place at the center of the cosmos. Earth is the heaviest element, because its natural place is at the center. The natural place of the element water is also around the center of the cosmos, but water is less heavy than earth, so it tends to rest on top of earth. If you drop a bucket of water, the water always moves down. But if you drop a rock in water, the rock will fall to the bottom of the water. Water and earth are both heavy elements, that is, they have a natural tendency to move down. Air and fire are light elements, because they have a natural tendency to move up. The natural place of air is above water, and its natural motion is upwards. If you blow bubbles underwater, the bubbles (filled with air) move up to the surface. Fire is the lightest element. Its natural place is at the periphery of the terrestrial realm, and it tends to move up to reach its natural place. If you strike a match, note that the flame always goes up, showing that fire is lighter than air. The examples I’ve given presume pure (or relatively pure) elements. However, most things in the terrestrial realm were made of combinations of two or more of the elements. Further, in compound bodies, the elements were always breaking apart and recombining. The terrestrial realm is characterized by constant change: things are born, they grow, they age, they decay, and eventually they die. Nothing is stable; everything is always in flux.

The image on the left is a detail from a depiction of the cosmos in a book by Peter Apian (scroll down for the complete image). This detail, from the center of the image, depicts the terrestrial realm. In the very center, you can make out land and water, representing the heavier elements, earth and water. Around the sphere of earth and water is a sphere of air. And above that is a sphere of fire. Elemental fire was actually transparent, like air, which is why we don’t see a great sphere of flame above us.

Just as an aside: Aristotle borrowed the idea of the four elements from the pre-Socratic philosopher Empedocles (490 – 430 B.C.). If you are interested in Empedocles, see here and here.

In Aristotle’s physics, the celestial realm is completely different from the terrestrial realm. The celestial realm is made up of just one element, the ether or quintessence (literally “fifth element”). The natural motion of the ether is uniform circular motion. The heavens are composed of concentric spheres of ether. The moon, sun, stars and planets are also made of ether, and they are embedded in the spheres. The spheres rotate around the earth with uniform circular motion, carrying the celestial bodies with them. It is the rotation of these celestial spheres that accounts for all the observed motions in the heavens. Recall that the Greeks believed that the sphere was the most perfect shape and that uniform circular motion was the most perfect motion. Aristotle shared these beliefs. He held that the heavens were perfect, regular and unchanging. Unlike the terrestrial realm, which was the site of constant change, the celestial realm never changed. That’s not to say there was no motion in the celestial realm – of course Aristotle knew that celestial bodies move across the sky. However, these motions are regular and they recur at predictable intervals. The sun always takes a year to rotate around the earth, never more and never less. Similarly, the periods of the moon and all the planets were always the same. The first sphere of the celestial realm is the sphere of the moon. This is directly above the sphere of fire in the terrestrial realm. (In fact, certain atmospheric phenomena are caused by the friction between the sphere of the moon and the sphere of fire. We will discuss this in our section on the Earth.) The next sphere was the sphere of Mercury, then Venus, then the sun, then Mars, then Jupiter, then Saturn, and finally the fixed stars.

The following image from a sixteenth-century book depicts the Aristotelian cosmos. The terrestrial realm is at the center: first a sphere of earth and water, then a sphere of air, then a sphere of fire. Above the terrestrial realm is the celestial realm, composed of concentric spheres of ether with the moon, sun and five visible planets (also made of ether) embedded in them. At the very edge is the sphere of the fixed stars, also composed of ether.

Further reading: Aristotle on the location and shape of the earth