Aristotelian Astrophysics

⊕ Aristotle builds on the work of Eudoxus of Cnidus and Callippus of Cyzicus (Greek mathematicians and astronomers, 4th century BCE). Eudoxus was among the greatest of the Ancient Greek mathematicians, perhaps second only to Archimedes (3rd century BCE).

Systems of epicycles (developed by Hipparchus in the 2nd century BCE and by Ptolemy in the 2nd Century CE) replace the homocentric theories in subsequent theories of the heavens in astronomy in the Ancient world. Aristotle understands heavenly motion in terms of homocentric spheres.

The Homocentric Spheres

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The stars and the planets are eternal sensible substances. Unlike human beings and the sublunary sensible substances, the stars and the planets do not come into or go out of existence. "[T]here are three kinds of substance. One is sensible (and may be either eternal or perishable; the latter, e.g. plants and animals, is universally recognized).... Another is immutable..." (Metaphysics XII.1069a). The first unmoved mover causes the fixed stars in the first sphere to rotate.

The spheres in heaven are nested. How Aristotle understood this nesting is difficult to grasp, but we can understand the case in which one sphere is nested within another. The outer sphere rotates. The inner sphere is attached to it. This causes the inner sphere to rotate along with the outer sphere, but the inner sphere also rotates on its own. (The demonstrations in the side notes make this a little clearer.) ⊕


"As regards the number of these motions, we have now reached a question which must be investigated by the aid of that branch of mathematical science which is most akin to philosophy, i.e. astronomy (ἀστρολογίας); for this has as its object a substance which is sensible but eternal.... That there are more spatial motions than there are bodies which move in space is obvious to those who have even a moderate grasp of the subject, since each of the non-fixed stars has more than one spatial motion" (Metaphysics XII.1073b).



Heliocentric (Copernican) Retrograde Motion

Two Nested Spheres, Rotations Applied Sequentially

Three Nested Spheres, Hippopede Drift West to East If this rotation of the inner sphere is in the opposite direction of the outer sphere, a point rotating on the inner sphere traces a "hippopede" (ἱπποπέδη).

It is possible, then, to use nested spheres to model the retrograde motion the planets (the wandering stars) appear to follow when we on earth look to the heavens above us.

The Unmoved Movers

Aristotle seems to think that there are multiple unmoved movers.

We can begin to understand this view if we think about the motion of the first sphere. Aristotle's view seems to be that the first sphere moves and that the fixed stars move with it. The first unmoved mover causes the motion of the first sphere and the motion of the fixed stars in this sphere, and because there are spheres nested with the first sphere, the motion of the first sphere moves these spheres too and thus explains all the motion we see.

In this homocentric model, to account for the retrograde motions of the wandering stars, one or more spheres rotate within an outer sphere. If the outer sphere ultimately gets its motion from the first sphere, it remains to explain the motion of the inner spheres.

Aristotle seems to have thought that there are unmoved movers for these spheres.

"We must not disregard the question whether we should hold that there is one substance of this kind or more than one, and if more than one, how many (Metaphysics XII.1073a).

"The principle (ἀρχὴ) and primary reality (πρῶτον τῶν ὄντων) is immovable, essentially and accidentally, but it excites the primary form of motion, which is one and eternal. Now since that which is moved must be moved by something, and the first mover must be essentially immovable, and eternal motion must be excited by something eternal, and one motion by some one thing; and since we can see that besides the simple spatial motion of the universe (which we hold to be excited by the primary immovable substance) there are other spatial motions—those of the planets—which are eternal... then each of these spatial motions must also be excited by a substance which is essentially immovable and eternal. ... Thus it is clear that the movers are substances, and that one of them is first and another second and so on in the same order as the spatial motions of the heavenly bodies" (Metaphysics XII.1073a).

⊕

"A tradition has been handed down by the ancient thinkers of very early times, and bequeathed to posterity in the form of a myth, to the effect that these heavenly bodies are gods, and that the divine pervades the whole of nature. The rest of their tradition has been added later in a mythological form to influence the vulgar and as a utilitarian expedient; they say that these gods are human in shape or are like certain other animals, and make other statements consequent upon and similar to those which we have mentioned. If we separate these statements and accept only the first, that they supposed the primary substances to be gods, we must regard it as an inspired saying.... [and that to this extent] the views of our forefathers and of the earliest thinkers are intelligible to us" (Metaphysics XII.1074b).

"We are inclined to think of the stars as mere bodies, occurring in a certain order and completely lifeless [if we follow Anaxagoras]; whereas we ought to think of them as partaking of life and initiative. Once we do this, the events will no longer seem surprising" (On the Heavens II.12.292a).

"Man is superior to the other animals, but ... there exist things far more divine in their nature than man, for instance, to mention the most visible, the things that compose the cosmos" (Nicomachean Ethics VI.7.1141a). These unmoved movers are gods whose existence is eternal bliss. The spheres and heavenly bodies rotate in imitation of the perfect existence these unmoved movers enjoy.

Perseus Digital Library

Aristotle, Metaphysics

Henry George Liddell, Robert Scott, A Greek-English Lexicon

"Aristotle clearly envisages that the being which is moved is a being which has desires, and hence a soul, and a being which has thoughts and hence a rational soul or an intellect. It clearly plays some role in Aristotle's argument that we have to distinguish among desires quite generally (ὄρεξις) specific forms of desire, namely rational desire (βούλησις) and non-rational desire (ἐπιθυμία), as we can see in [Metaphysics XII.]1072a27-8. The suggestion is (cf. 1072b29-30) that, at least in the cases we are interested in, the desire which explains the motion is rational desire which has its origin in thought. It is because the object of thought is seen to be attractive that a rational desire is felt for it. To put the matter admittedly prejudicially, a desire to move is generated by a thought. Now one can imagine a notion of desire, and in particular of rational desire, such an intellect desires or wants to do what it does anyway, namely think. This is why the immaterial intellects enjoy a life of eternal bliss: they enjoy in an unimpeded way what they like doing and are good at doing, namely thinking. But I take it that the notion of desire involved here is the notion of an appetite for something which motivates an agent to do something to attain a goal. It is a desire for something which one does not already have and which one only attains by doing something. The immaterial intellects have all along attained their goal in having all along thought whatever it is they have to think. Their desire is not a desire for something which they do not already have. It is rather a desire that is already fulfilled. It is their unqualified satisfaction with what they already have. A desire which motivates something in the sense of making it move or change by contrast is a desire for something which it does not yet have, which it will only attain by changing in a certain way. This means that the beings that are moved in this way by their desire are not the unchanging immaterial intellects. It also strongly suggests that the immaterial intellects are not just the rational souls or intellects of the moving spheres. For the rational souls of the spheres not only have thoughts and desires like the immaterial intellects, they also, unlike the immaterial intellects, have triggered by their thoughts desires in the sense characterized above, desires which can only be satisfied if the spheres change place in the appropriate way, that is to say, if the desire produces the appropriate motion. Now this yields an unsatisfactory complex picture of stars, animate and intelligent spheres, and disembodies intellects, which raises further questions one does not know how even to begin to answer" (Michael Frede, "Introduction," 38-39. Aristotle's Metaphysics Lambda, 1-52).