The Aristotelian Universe Emerges

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 than 18 centuries from its inception (around 350 BCE) 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.

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 so, no Physics !

Movement is therefore endowed to bodies. Is this science?

Some bodies naturally move in straight lines, others naturally are at rest ( but what does that mean? ). But there is yet another natural movement: the circular motion.

By logic: since to each motion there must correspond a substance, there ought to be some bodies that naturally move in circles. Aristotle then proclaims that such things are the heavenly bodies as they are made of a more exalted and perfect substance than all earthly objects.

Note the descriptors here: Exalted and Perfect

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''.

Aristotle's universe is not infinite .

Aristotle also concluded the atomists were wrong, stating that matter is in fact continuous and infinitely divisible.

The Aristotelian Business Card

(The Finite Crystalline Sphere Universe)

The initial motion of these spheres was caused by the action of a prime mover which acts on the outermost sphere of the fixed stars; somehow, however, this action is communicated to the other spheres and they move as well, but at different rates, since that is what the observations demand.

Aristotle also 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 .

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.

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 (and, for which, of course, no reconciliation is possible).

Retrograde motion of Mars vs Aristotelian Universe

The crystalline sphere universe of Aristotle makes a simple prediction all the wheels move in the same direction.
This means that any planet should always move in the same direction, with respect to the background stars (which are the coordinate system here).
Yet, the retrograde motion of Mars was well known at the time and completely violated this principle.

Mars generally moves west to east (right to left) on the background of stars. But every 780 days it goes through period of 83 days during which it moves east to west against the stars, the retrograde motion.

The retrograde period is centered around the time when Mars is in opposition - Mars is directly opposite the Sun. Thus, this phenomena could be established as repeatable and reliable after only a decade's worth of observations.

Aristarchus and a Sun-Centered Universe

Plato gave his students a major problem to work on. Their task was to find a geometric explanation for the apparent motion of the planets, especially the strange retrograde motion.

One key observations is that near and during the time of retrograde motion, Mars appears significantly brighter in the sky than at other times What might this mean?

Plato and his students were, of course, also guided by the Pythagorean Paradigm. This meant that regardless of the scheme they came up with, the Earth should be at the unmoving center of the planet motions.

One student named Aristarchus violated that rule and developed a model with the Sun at the center. His model was not accepted because of the obvious observations against a moving Earth. These were all intuitive arguments based on the observation that a) one didn't feel any motion, b) the clouds in the sky didn't seem to be flying off and c) why would the center of the Universe be moving?