- The diagram is exactly geocentric
as Aristotelian cosmos
demands.
- However, all the compounded
celestial spheres of the astronomical objects
with their offset rotational axes
are NOT explicitly shown.
Showing them was too tricky and much more than most people wanted to see.
Yours truly gets the feeling that only
mathematical astronomers and
natural philosophers cared about
the compounded celestial spheres
much anyway.
- You see the main celestial spheres
of the planets,
Sun, and
Moon in a
cross section through
the ecliptic.
However, they are offset from the astronomical objects
as in the
Ptolemaic physical model of the Solar System
(see Wikipedia: Ptolemy: Astronomy;
ptolemaic_physical_model.html),
and NOT as in pure Aristotelian cosmos.
- The celestial spheres
carry, relatively slowly, the astronomical objects
eastward or
(counterclockwise in the
diagram)
relative to the celestial sphere of the stars
and the zodiac
which rotate westward once per day.
- Both the
zodiac constellations
(duplicated with slight offsets for reasons beyond
yours truly's understanding
in the 2nd and 3rd outermost circular panels)
and the
zodiac signs
(in the outermost circular panels)
are illustated.
- The zodiac signs are 30° segments
of the ecliptic starting at the
vernal equinox
which is also called is the
First Point Aries
(which is the start of the
sign Aries
represented by the astronomical symbol
♈ in the
diagram).
- Circa year 500 BCE, the
zodiac signs approximately enclosed the
zodiac constellations for which
zodiac signs are named.
The axial precession
has shifted the zodiac signs
by ∼ 30° westward
from zodiac constellation
since then.
- The Coelum empireum habitaculum dei et omnium electorum (the
Empyrean of
God and the
Elect)
either is or is beyond the
celestial sphere of the stars:
it's unclear to yours truly---and maybe everyone else---which it is.
- Shown are the astronomical symbols for
the Solar-System
astronomical objects
known then:
and for the zodiac
(zodiac constellation
linked by name;
zodiac sign by
linked by
astronomical symbol):
- The ordering of the
planets
(which include Sun
and Moon)
and the celestial sphere of the stars
going outward from the
Earth is the usual one
for ancient Greek astronomers
including Aristotle (384--322 BCE)???:
Earth:
Moon,
Mercury,
Venus,
Sun,
Mars,
Jupiter,
Saturn,
celestial sphere of the stars.
The ordering of the planets
is by decreasing
angular velocity
around the Earth
NOT counting the daily rotation of the whole
sky:
- The argument for the ordering of the
planets etc.
was that if the spatial speed
of the planets etc.
was the roughly the same
for all the planets etc.
(NOT counting the daily rotation of the whole
sky),
then the
angular velocity
should decrease with increasing circumference and radius of the
orbit.
- This argument is a half truth.
For, superior planets
the angular velocity
should decrease with increasing distance from the
Earth which relative to the
superior planets
is close to the approximate center of their
orbits the Sun.
The argument is sort of right for the
and the Moon which does actually
physically orbit the Earth.
- For the inferior planets
and the Sun
the argument is NOT true in any straightforward way
since the mean distance to Mercury and
Venus
is the
Earth-Sun distance
and it's unclear how to apply the argument
since the angular velocities
of Mercury and
Venus have large variations.
In fact, the ordering of
Mercury,
Venus, and
Sun
was disputed by the ancient Greek astronomers
and sometimes different orderings of
Mercury,
Venus, and
Sun
were suggested I think????.
- The outermost of the celestial spheres
of Aristotelian cosmology
was the celestial sphere of the stars.
The
celestial sphere of the stars
evolved into the modern
celestial sphere which is
NOT a physical thing, but a sky mapping tool.
- There is no conclusive argument for the location of
the celestial sphere of the stars
known ancient Greek astronomers.
But since the motions of the
planets
are measured relative to the
fixed stars,
it is natural to think of the
fixed stars as forming a background.
The ancient Greek astronomers
just followed this natural path without dissent as far as yours truly knows.
- The ancient Greek astronomers
could NOT find the real ordering or structure of
the Solar System
because they could NOT measure
Solar System distances,
except that they did eventually determine that
the Moon was
∼ 60 Earth radii away from
the Earth.
Ptolemy (c.100--c.170 CE) found an
apogee value
of 64 and 1/6 Earth radii of some kind
(see Wikipedia: Lunar distance:
Lunar eclipse).
The modern
perigee distance = 55.89
mean orbital radius = 60.2687
and apogee distance = 63.76
all in
Earth equatorial radii (R_eq_⊕ = 6378.1370 km)
Earth equatorial radii
(see Wikipedia: Lunar distance: Value
but yours truly's values are more
accurate/precise).
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