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Aristotelian Cosmology

Aristotle's cosmological work On The Heavens is the most influential treatise of its kind in the history of humanity. It was accepted for more that 18 centuries from its inception (around 350 B.C.) until the works of Copernicus in the early 1500s. In this work Aristotle discussed the general nature of the cosmos and certain properties of individual bodies.

Aristotle believed that all bodies are made up of four elements: earth, water, air and fire (see Fig. 2.7). These elements naturally move up or down, fire being the lightest and earth the heaviest. A composite object will have the features of the element which dominates; most things are of this sort. But since the elements in, for example, a worm, are not where they belong (the fiery part is too low being bound by the earth part, which is a bit too high), then the worm is imperfect. All things on earth are thus imperfect.

The idea that all bodies, by their very nature, have a natural way of moving is central to Aristotelian cosmology. Movement is not, he states, the result of the influence of one body on another

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Some bodies naturally move in straight lines, others naturally stay put. But there is yet another natural movement: the circular motion. Since to each motion there must correspond a substance, there ought to be some things that naturally move in circles. Aristotle then states that such things are the heavenly bodies which are made of a more exalted and perfect substance than all earthly objects.

Since the stars and planets are made of this exalted substance and then move in circles, it is also natural, according to Aristotle, for these objects to be spheres also. The cosmos is then made of a central earth (which he accepted as spherical) surrounded by the moon, sun and stars all moving in circles around it. This conglomerate he called ``the world''. Note the strange idea that all celestial bodies are perfect, yet they must circle the imperfect Earth. The initial motion of these spheres was caused by the action of a ``prime mover'' which (who?) acts on the outermost sphere of the fixed stars; the motion then trickles down to the other spheres through a dragging force.

Aristotle also addresses the question whether this world is unique or not; he argues that it is unique. The argument goes as follows: earth (the substance) moves naturally to the center, if there world is not unique there ought to be at least two centers, but then, how can earth know to which of the two centers to go? But since ``earthy'' objects have no trouble deciding how to move, he concludes that there can only be one center (the Earth) circled endlessly by all heavenly bodies. The clearest counterexample was found by Galileo when he saw Jupiter and its miniature satellite system (see Fig. 2.11), which looks like a copy of our ``world''. Aristotle was wrong not in the logic, but in the initial assumptions: things do not have a natural motion.


Figure 2.11: Montage of Jupiter and the Galilean satellites, Io, Europa, Ganymede, and Callisto. 
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It is interesting to note that Aristotle asserts that the world did not come into being at one point, but that it has existed, unchanged, for all eternity (it had to be that way since it was ``perfect''); the universe is in a kind of ``steady state scenario''. Still, since he believed that the sphere was the most perfect of the geometrical shapes, the universe did have a center (the Earth) and its ``material'' part had an edge, which was ``gradual'' starting in the lunar and ending in the fixed star sphere. Beyond the sphere of the stars the universe continued into the spiritual realm where material things cannot be (Fig. 2.12). This is in direct conflict with the Biblical description of creation, and an enormous amount of effort was spent by the medieval philosophers in trying to reconcile these views.


Figure 2.12: A pictorial view of the Aristotelian model of the cosmos. 
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On the specific description of the heavens, Aristotle created a complex system containing 55 spheres(!) which, despite it complexity, had the virtue of explaining and predicting most of the observed motions of the stars and planets. Thus, despite all the bad publicity it has received, this model had all the characteristics of a scientific theory (see Sect. 1.2.1): starting from the hypothesis that heavenly bodies move in spheres around the Earth, Aristotle painstakingly modified this idea, matching it to the observations, until all data could be accurately explained. He then used this theory to make predictions (such as where will Mars be a year from now) which were confirmed by subsequent observations.

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Jose Wudka