Introduction to Aristotle's Celestial and Terrestrial Physics

This page last modified on November 7, 2012


For Aristotle (384-322 BCE), the Earth is in center of the physical universe, and stationary. Sun, moon and what we today call planets (the ones then known, Mercury, Venus, Mars, Jupiter, Saturn) orbit the center. Their orbits are determined by rotating spheres to which they are attached. Since planetary motion appears to be somewhat irregular, more than one sphere is required to explain the motion of a single planetary body. (Secondary spheres are attached to the primary ones; sometimes tertiary spheres are attached to secondary ones.)

Aristotle says there are 55 spheres corresponding to the "planets" (which includes the Sun and the Moon).

The sphere of fixed stars (bringing the total to 56) at periphery of the universe. Sometimes this sphere is simply called the kosmos, i.e., universe or world. There is no "place" and nothing material beyond this sphere.

There are five material elements. Things below the level of the moon--in the sublunary order--are composed of terrestrial matter, and contain the four EWAF elements. Each has its natural place, to which it returns if not impeded--fire tends to rise, earth to move to the center, while water and air are by nature intermediate.

Terrestrial matter moves discontinuously, tends to come to a stop upon reaching its natural place. Terrestrial bodies come to be and pass away; they are corruptible.

The celestial spheres and celestial bodies are composed of an altogether different kind of matter, the fifth element. By nature celestial bodies move continuously; their motion is circular, everlasting, without beginning or end. The heavenly bodies, including the mostly invisible spheres, are incorruptible.

Celestial Bodies are Characterized by     Terrestrial Bodies are Characterized by
Continous MotionDiscontinuous Motion
Circular MotionLinear Motion
Perfect Shape (Sphericity)Irregular Shape

For each of the spheres there is an intelligence, an unmoved mover: there are 56 such intelligences. The motions of the subordinate spheres are caused partly by the intelligences associated with them, partly by the motions of the spheres adjacent to them but farther from Earth. Aristotle's view seems to be that planetary motions are partly independent, and partly dependent on the motion of other spheres.

The intelligence associated with the sphere of fixed stars is an unmoved mover. Nothing moves him, but he somehow causes the sphere of fixed stars to move eternally. Since the motion of this sphere is entirely independent of the motion of other spheres, its mover is the first or prime (unmoved) mover.

The unmoved mover is an immaterial mind. Its only activity is to think about itself (To think of anything other than itself would be to think of something less perfect.) It does not cause the motion of the stars as a driver's hand causes the motion of a steering wheel she is turning but in a different way; rather it does so as a loved object, simply through its appearing beautiful, causes the motion of the lover.

The unmoved movers are divine (theios) but Aristotle does not regard them as gods.*

Natural movement must be distinguished from unnatural movement. When things move naturally, there is a cause of movement internal to them. The maturation of animals and plants are natural movements. Unnatural movement involves external causes; works of deliberate production, therefore, are products of unnatural movement.

* A lot of commentators get this wrong; Aristotle held that the unmoved mover is similar in various ways to "the god" or gods, but, unlike Thomas Aquinas, he did not claim that an unmoved mover (or the prime unmoved mover) is a god, much less God in the sense of the major Western monotheist traditions. He was reasoning by analogy, not inventing a theology to substitute for pagan religion.

However, Aristotle did accept the existence of gods. Like Plato, he conceived of them as immortal living beings, with incorruptible bodies; they are normally invisible to us but can reveal themselves to us when they choose. In his view, some gods probably concern themselves with the destinies of individuals and city-states. It is likely that some individuals, say, philosophers, are "beloved by the gods." However, we cannot have strictly scientific knowledge of the gods.

For the justification of the comments in this note on Aristotle's notions of the gods, see Richard Bodéüs, Aristotle and the Theology of the Living Immortals (Albany, NY: State University of New York Press, 2000).