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Commentary on Aristotle's text

And since many of this type, no less striking is the reason why the not move as many movements as are more distant from the first revolution, but more intermediates. It would seem logical that, by moving the first body with a single translation, the closest he moved with the fewest possible moves, say two the following three, or any other similar organization. In fact the opposite is true: for the Sun and the Moon move with less motion Some of the wandering stars, and yet are farther from the center and closer to those first body.

Aristotle About Heaven, Book II, 12, 291b-292nd.

Among the works of Aristotle (384 - 322 BC) About the sky belongs to all treaties dedicated to the study of nature. It seems that the origin of this book is a commentary to the dialogue of Plato (427 - 347 BC) Timaeus, where the teacher of Aristotle, shows a consistent cosmology in a universe made up of two areas. The area outside corresponds to the celestial sphere, which is equipped with a circular motion. The earth also is spherical, is suspended above the center of the field. The sun, moon, planets and stars revolve around the Earth describing circular orbits, uniform and regular. This regularity demanded by Plato to the Sun and planets, in contrast to the apparent irregularity of their movement observed from Earth: the retrograde of the outer planets, the speed changes, the phases of the moon, etc.. The only possible explanation was that these irregularities had to be the result of a composition of uniform circular motion.

therefore had to make a work of reconciliation between cosmological postulates observed celestial motions. How Greek astronomers such as Eudoxus of Cnidus (390 to 337 BC), Caliper Cízipo (370 to 310 BC) and later Ptolemy (100 - 170), directed their efforts to devise a series of geometric constructions in order to save phenomena that contradicted the Platonic view of the universe, full of order and perfection. Thus these mathematical solutions to the celestial movements must be subordinate to a physical view of the universe.

Foundations of Aristotle's cosmology is mostly on the second book about heaven. But this conception of the universe can not be understood without his ideas on natural philosophy, as stated in other treaties, such as Physics, Metaphysics and On the generation and corruption. What Aristotle meant by change, coupled with the material composition of the substances that form, its eternity and finitude makes the Aristotelian universe is rational in nature, where nothing that happens is the result of improvisation or chance.

Aristotle divides the universe into two worlds: the supralunar and the sublunary. The Earth, which has a spherical shape, and is at rest in the center of it, and around it are superimposed concentric spherical layers for the remaining three elements: water, air and fire (sublunary world). In this area the natural motion is rectilinear. Above, are the celestial spheres (supralunar world), where the fixed stars and planets, which are incorruptible and experience no changes other than the local circular motion. To explain these celestial movements, uses a geometric method proposed by Eudoxus, in which each planet was linked to a number of areas homocentric, ie, all with the same center, the Earth, whose orderly and regular movements combined to form the movement of each of the heavenly bodies separately. Just admit the there are three areas for the Sun and the Moon, and four areas for each of the five known planets (Mercury, Venus, Mars, Jupiter and Saturn), 26 areas in total. Calipo later, a pupil of Eudoxus, perfected the system by adding a total of 33 areas.

But the areas were to Eudoxus and Calipo geometric appeal to save the phenomena, they lacked physical consistency, its purpose was to explain and predict the motion of celestial bodies. Thus each sphere system was independent of the others, did not need the contact between the outermost sphere of a planet and the innermost of the following, for transmitting motion, only the areas that ordered el movimiento de cada planeta debían actuar en conjunto . Aristóteles proporcionará realidad física a estas esferas, compuestas de éter, que es material diferente a los cuatro elementos del mundo sublunar, este quinto elemento es superior a los demás, ya que, al contrario de los otros, solo experimenta un cambio el movimiento local circular .

Para explicar el origen de los movimientos planetarios, Aristóteles pensó en un primer motor que transmitía el movimiento a todas las esferas desde la más externa, la esfera de las estrellas fijas, a la más interna, la esfera de la Luna . Sin embargo esta idea implicaba, ampliar de 33 a 55 las esferas:

Su única función era la de proporcionar mechanical ties necessary to keep rotating the entire set of concentric shells, ie all areas transformed into a celestial timepiece driven by the field of stars. Since the universe was filled, all areas were in contact, and rubbing each other transmitting motion to the whole system. The sphere of the stars dragged which was closer, the outermost of the seven shells homocentric, which brings to Saturn. This shell dragged internal nearest neighbor in the set of Saturn, and so on, until the movement was finally transmitted to the lower field the set, responsible for the movement of the moon. This last area is the most ethereal interior of the shells, the lower limit of the region of the sky or supralunar.

Therefore, Aristotle's universe was eternal and finite, where the stars are incorruptible and unchanging number. The irregularity is the result of appearances, as shown in the text under review. This is an excerpt from Chapter 2 of Book II on Heaven. Aristotle asks why the rotations of the spheres do not follow an increasing order from the fixed stars to the sphere of the moon? That is, the reason for the apparent irregularity in the field of stars still moves in one motion, so that then the planets (wandering stars), do it with various movements, and the Moon and the Sun require fewer rotations and the Earth stands still. Aristotle's answer, is nothing to indicate that in addition to law enforcement bodies are endowed with activity and life, and therefore depending on the activity you need a body like this:

It seems that, in that it possesses the perfection , that is the fine without activity, in that it is very close is given by a small, single business and things away, through multiple activities

is a way of saying that nature strives for perfection by roads that are beyond us, but necessary. Nothing is vain in nature, everything has its reason and purpose is the pursuit of good. Therefore

Aristotle's text clearly expresses what the rationality of the Cosmos, which based on assumptions about the reason given by the apparent irrationality of the observed phenomena is that nature is still hiding the roads apparent order. This idea coupled with the first engine fitted in very well with the theological image of God developed during the Middle Ages, this is one reason why the Aristotelian cosmology lasted until the sixteenth century.