HPSC 326 Aristotelian and Medieval Cosmology 1.
The cosmos of Aristotle.
Aristotle.
Aristotle's cosmos is based around the ideas of natural place and natural motion. There is a centre
to the cosmos. Earth and water have their natural place there and their natural motion is towards the
centre. Fire and air have a natural motion away from the centre. Aether, making up the heavens, has
a natural motion around the centre. Inherently geocentric physics and cosmology. Aristotle is the
classic expression of a centrifocal cosmology. Not only are the heavens arranged around a central,
focal point (as in Plato). The central point is also key to the physics of motion. Heavens are entirely
unchanging (according to Aristotle). No change recorded by Babylonians/ Egyptians. Heavens are
therefore suitable for mathematical astronomy, and all change (comets, meteors etc.) are sub-lunar
phenomena.
Aristotle - Physical heavens.
Aristotle accepts Callippus on astronomy. He uses four and five sphere models. Aristotle not actually
very up on astronomy - accepts what experts say. Cosmological move - each of the circular motions
in Callippus is now a sphere of aether. The motions combine together to move the object, be it sun,
moon or planet. Note difference to Plato, Eudoxus or Callippus who considered the spheres to be
mathematical analyses of a planet's motion. Aristotle - each sphere of aether passes its motion on to
the next one. In order that the system for one planets does not interfere with that of another, there
are 'unwinding' spheres to. So for each motion of each planet, there is a counteracting sphere,
except for the moon, as it is next to the earth and borders only the fiery part of the terrestrial realm.
Callippus - four and five sphere models.
HPSC 326 Aristotelian and Medieval Cosmology 2.
Some Aristotle difficulties.
Apart from the hideous complexity of the system. It is not clear how the heavens can interact with the
terrestrial realm (though it is clear that they do as the sun is the key source of heat and light).
Aristotle - the motion of each heavenly body stirs up the terrestrial realm directly below it, and what
we see is not the heavenly body but a patch of ignited fir in the fiery part of the terrestrial realm.
Problem - counteracting spheres should stop this. Further problem. Consider eclipses of sun and
moon. If the moon produces its own light, as this theory suggests, why should there be lunar
eclipses ? And what happens when the moon is in front of the sun for solar eclipses ?
Cosmos - end of antiquity.
Note the slightly different order to
Aristotle
How the ancients spaced orbits.
The epicycles touch, but do not overlap, giving the
spacing.
Ptolemy (C2 AD).
A new astronomy within the Aristotelian cosmology - epicycles, eccentric and equants take the place
of the concentric spheres, giving a more accurate description of heavenly motions. Technically
geostatic rather than geocentric due to the use of eccentrics and equants. Need to rethink the
nesting spheres of aether. More complex model required. Note the tension between astronomy and
cosmology in the Aristotelian tradition. The cosmology can be represented in a relatively simple,
earth centred diagram. The astronomy cannot - it is far too complex. Problem that runs through
astronomy/ cosmology up to Kepler.
Scholasticism.
Grafts a Christian conception of God onto the Aristotelian world view. Key figure, St. Thomas
Aquinas c1250, this is the system that the Copernican revolution will have to try to displace.
Christian hierarchies of perfection (God, angels, humans, earth, hell, etc.) fit well with Aristotle’s
notion of greater perfection due to greater actuality. The dominant system - accepted by the church
and taught by the universities. Highly coherent system, difficult to alter part without altering whole,
and thus difficult to replace. No replacement as a whole until around 1640/50 (note; century after
Copernicus !). There are serious problems with the acceptance of the Copernican view that
everything orbits the sun - such a view requires a new physics of motion and a new explanation of
why and how the planets move.
Renaissance and neoplatonism.
Plato's views revived by neoplatonists. Keen to assert mathematical, geometrical and harmonic
aspects of the heavens. General view - a geometer God has created the best possible cosmos,
using maths, geometry and music (harks back to the Pythagoreans). Much struggling to understand
how maths, geometry and music relate to the heavens. What is clear to us is how to use maths in
physics.
Has not always been clear and our conception had to be conceived, honed and fought for. So no
great surprise that disciplines related to maths (music, aspects of numerology) were experimented
with - was this how the cosmos was put together ? Durer's angel - how do we use all these
possibilities to put together the best possible cosmos ?