- The motions of the
Solar-System bodies were reproduced by
the motions of compounded
celestial spheres.
They turned at constant rates (i.e., uniformly) on offset axes.
If you adjusted the rates and axes just right, you qualitatively made the
planets,
Sun,
and Moon
in model
match the actual angular motions seen in the sky.
In fact, the even qualitatively,
yours truly thinks
Aristotelian cosmology
did NOT match the actual motions.
Note that every point on a celestial sphere
executed a uniform circular motion.
The motions of the
Heavens were made up of compounded
uniform circular motions
was a principle of
Aristotelian cosmology
and later epicycle models.
- The outermost of the celestial spheres
was the celestial sphere of the stars
which rotated around the
Earth once per day.
- Aristotle also postulated
a radical distinction between Earth and
Heavens.
The Heavens
were a realm of eternal cyclic motions of perfect bodies.
- The celestial spheres
and all matter from the
Moon's sphere outward
were made of the 5th element,
aether
or quintessence.
- The celestial spheres
were real, solid objects in
Aristotelian cosmology.
But they were invisible, and hence have sometimes
been called the crystalline spheres.
- It's sometimes said that Aristotle used
47
celestial spheres and sometimes
55
celestial spheres.
He is just
unclear on this point.
But 55 seems to be the right number.
See Celestial Spheres: Antiquity
for NO explanation.
See
John North 1994, The Norton History of Astronomy and Cosmology, p. 83.
for some clarification---but some extra confusion too.
- The celestial spheres
were kept in motion by
unmoved movers or
prime movers.
Aristotle managed to be
ambiguous as to whether there was a single
unmoved mover
or as many unmoved movers
as celestial spheres
(see Wikipedia:
Unmoved mover: The number of movers).
- The outermost
celestial sphere was the
celestial sphere of the stars on which
the stars were pasted.
Beyond the celestial sphere of the stars
was nothing, NOT even empty
space---which is something that
many people, even
dyed-in-the-wool
Aristotelians, had a hard time
buying.
So Aristotle thought the
universe was a finite
sphere.
It was also eternal.
He had arguments for these theories, but
there NOT worth knowing.
- As a kinematic
description of the celestial motion,
Aristotelian cosmology
was only very roughly qualitatively valid.
It's hopeless to try to make it quantitatively predictive for the
writing of ephemerides
(tables of predictions of the motions of
astronomical objects).
Maybe someone tried to make it quantatively predictive, but they couldn't get very far.
- Aristotle
considered the
unmoved movers
to be Gods
(non-anthropomorphic gods).
In later monotheistic
versions of Aristotelian cosmology,
the unmoved movers were
interpreted as angels.
- In later Greco-Roman antiquity
and then in western Eurasia up until
circa year 1600,
Aristotelian cosmology become
a philosophical dogma for
Aristotelians.
They contended that
Aristotelian cosmology was
physically correct,
but that the actual motions of the Heavens
were too complicated to be explained by it.
So you needed the
Ptolemaic system or the like
as calculational devices.
This is an unhappy compromise in our modern view, but
Aristotelians were happy enough with it.
Importantly,
Nicolaus Copernicus (1473--1543) was
NOT happy with the compromise.
- Yours truly believes that
Aristotelian cosmology was always
a pretty bad
scientific theory in any
time period.
It's long vogue is probably partially accidental and partially due to the
great authority that Aristotelianism
in toto
acquired: Aristotelian cosmology was
accepted as just part of the package.
- Remarkably, Aristotelian cosmology
has a similarity to our
modern concept of the
observable universe.
The observable universe
is geocentric: it's a really big sphere
centered on us.
Beyond the
observable universe is
NOT nothing: it's probably much like the
observable universe for a long way,
but probably NOT to infinity.
But we are phyiscally detached from that "beyond" and cannot observe it though we can understand
it theoretically.
So Aristotle was right---a little bit.