Monday

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Mondays Virtual Astronomy Lecture

1. The Aristotelian Universe
2. The Contribution of Ptolemy to the Aristotelian
   Universe

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Interpreting the Apparent Motion of the Stars,
Sun. Moon and Planets

• Mankind does not long live without developing a
  model of the Universe around him. The model
  explains the physical relation between mans
  habitat and the rest of nature. This model of
  the Universe provides mankind with a world-view
  which permeates and gives meaning to his every
  action, practical and spiritual.
• Primitive conceptions of the Universe displayed
  considerable variation, but all were shaped
  primarily by terrestrial events, the daily
  patterns of life.

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Interpreting the Apparent Motion of the Stars,
Sun. Moon and Planets

• For example, the Ancient Egyptians developed a
  model of the Universe as an elongated platter.
  The platters long dimension paralleled the Nile
  its flat bottom was the alluvial basin to which
  ancient Egyptian civilization was restricted, and
  its curved rippled rim represented the mountains
  bounding the terrestrial world. Above the
  platter was air supporting an inverted
  platter-dome which carried the stars. The
  platter was supported from underneath by water.
  The Sun was Ra, the principle Egyptian God,
  supplied with two boats, one for his daily
  journey through the air as a second for his
  nocturnal trip through the water under the
  platter.

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Interpreting the Apparent Motion of the Stars,
Sun. Moon and Planets

• This conceptual model provided the ancient
  Egyptians with a mental picture of where they
  were. This in turn made them feel at home in the
  larger Universe. Their platter model of the
  Universe provided them with a sense of security
  and explained to them where they as a
  civilization fit in the larger framework. The
  model did not try to account for such details and
  the annual motion of the Sun or the wanderings of
  the planets. This failure of ancient models of
  the Universe to account for these physical
  characteristics was common.

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Interpreting the Apparent Motion of the Stars,
Sun. Moon and Planets

• Primitive conceptual models are only schematic
  sketches against which the play of nature takes
  place, very few details or explanations of
  astronomical phenomena are incorporated into
  these models.
• The requirement that a conceptual model of the
  Universe supply both a psychologically satisfying
  world-view and an explanation of observed
  phenomena like the daily change in the position
  of sunrise has vastly increased the power of
  these models.

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Interpreting the Apparent Motion of the Stars,
Sun. Moon and Planets

• Only in our Western civilization has the
  explanation of such astronomical details been
  considered a function of the conceptual model of
  the Universe.
• Only in pan-Hellenic Greek society has a model of
  the Universe also been required to explain
  physical details of astronomy.

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Interpreting the Apparent Motion of the Stars,
Sun. Moon and Planets

• We would reject a model like the ancient Egyptian
  platter as inadequate because it fails to account
  for observed phenomena.
• Aristotle had developed a geocentric model of the
  Universe that was able to explain the daily
  observations of astronomers and provide a setting
  in which mankind could interact with his God.

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Interpreting the Apparent Motion of the Stars,
Sun. Moon and Planets

• We have been studying the principle observations,
  all of them accessible to the naked-eye, upon
  which depend the two main scientific models of
  the West, the Aristotelian/Ptolemaic and the
  Copernican.
• Let us examine the key ideas of the Aristotelian
  Universe that dominated Western Europe from 300
  B.C. to, perhaps, 1700 A.D.

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Interpreting the Apparent Motion of the Stars,
Sun. Moon and Planets

• In class, we will examine the innovation of
  Copernicus whose re-interpretation of the same
  observations led to a complete revision not only
  in astronomy, but also in the basic underlying
  beliefs of society.
• I have taken material in the notes that follow
  directly from the on-line encyclopedia Wikipedia
  from the subjects Aristotle and Ptolemy. In
  addition I have included entire paragraphs from
  the reserve reading On the Copernican Revolution
  by Thomas S. Kuhn.

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Who was Aristotle?

• The three most influential ancient Greek
  philosophers were Aristotle, Plato (a teacher of
  Aristotle) and Socrates (ca. 470 BC-399 BC),
  whose thinking deeply influenced Plato. Among
  them they transformed Presocratic Greek
  philosophy into the foundations of Western
  philosophy as we know it. Socrates did not leave
  any writings, possibly as a result of the reasons
  articulated against writing philosophy attributed
  to him in Plato's dialogue Phaedrus. His ideas
  are therefore known to us only indirectly,
  through Plato and a few other writers. The
  writings of Plato and Aristotle form the core of
  Ancient philosophy.

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Who was Aristotle?

• Aristotle was born in 384 BCE. at Stagirus, a
  Greek colony and seaport on the coast of Thrace.
  His father Nichomachus was court physician to
  King Amyntas of Macedonia. At age 17 his
  guardian, Proxenus, sent him to Athens, the
  intellectual center of the world, to complete his
  education. He joined the Academy and studied
  under Plato, attending his lectures for a period
  of twenty years. In the later years of his
  association with Plato and the Academy he began
  to lecture on his own account, especially on the
  subject of rhetoric.

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Who was Aristotle?

• At the invitation of Philip of Macedonia he
  became the tutor of his 13 year old son Alexander
  (later world conqueror) he did this for the next
  five years. He then set up his own school at a
  place called the Lyceum. When teaching at the
  Lyceum, Aristotle had a habit of walking about as
  he discoursed. It was in connection with this
  that his followers became known in later years as
  the peripatetics, meaning "to walk about." For
  the next thirteen years he devoted his energies
  to his teaching and composing his philosophical
  treatises.

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Who was Aristotle?

• He is said to have given two kinds of lectures
  the more detailed discussions in the morning for
  an inner circle of advanced students, and the
  popular discourses in the evening for the general
  body of lovers of knowledge. In the first year of
  his residence at Chalcis he complained of a
  stomach illness and died in 322 BCE.
• Aristotle is known for being one of the few
  figures in history who studied almost every
  subject possible at the time. In science,
  Aristotle studied anatomy, astronomy, embryology,
  geography, geology, meteorology, physics, and
  zoology. In philosophy, Aristotle wrote on
  aesthetics, economics, ethics, government,
  metaphysics, politics, psychology, rhetoric and
  theology. He also dealt with education, foreign
  customs, literature and poetry. His combined
  works practically comprise an encyclopedia of
  Greek knowledge.

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The Aristotelian Universe

• The Terrestrial World
• Four elements comprised all matter on the Earth
  in various proportions. Those elements were
  earth, water, air and fire
• For example a human body obviously had a solid
  component in its bones and muscle this was the
  earth component. In addition, there was a
  liquid component seen in blood and urine dubbed
  water. The heat from the body was from the
  fire component and we all know of a unsociable
  gaseous component of the body that is represented
  by the air element.

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The Aristotelian Universe

• The Terrestrial World
• All terrestrial matter was characterized by
  change through decay or death. Terrestrial matter
  was considered imperfect due to its changing
  nature
• The natural state of motion for terrestrial
  matter was a state of rest. A person only need
  look around and see that all objects are
  stationary (at rest) unless forced into motion
  by some unnatural cause.
• The Earth sat at the center of the Stellar Sphere
  immovable (fixed and central).

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The Aristotelian Universe

• The Celestial World
• The stars, Sun, Moon and other planets were
  considered to be made of a fifth element not
  found in Earthly matter.
• All celestial matter was characterized by its
  unchanging nature. The stars and planets never
  changed their appearance or motion. They seemed
  eternal.
• Celestial matter was considered perfect due to
  its unchanging incorruptible nature.

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The Aristotelian Universe

• The Celestial World
• The natural state of motion for celestial matter
  was constant motion, never ceasing, in a circular
  pattern. The apparent motion of the stars best
  typifies this motion.
• The stars sat on a very large stellar sphere
  which contained the whole Universe and was set
  into rotation (once every 23h56m 4.09s) by a
  prime mover (i.e. God).
• God lives on the Celestial Sphere and Heaven was
  a place on the celestial sphere.

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The Aristotelian Universe

• Terrestrial Matter
• Temporary
• Corruptible
• Naturally at rest
• Physically imperfect

• Celestial Matter
• Eternal
• Incorruptible
• Naturally in constant circular motion
• Physically perfect

Aristotle made an additional conceptual leap by
proposing that Celestial Matter was not just
physically perfect, but morally perfect also,
while Terrestrial Matter (including people) were
physically and morally imperfect.
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The Aristotelian Universe
Aristotle explained mankinds painful life and
inevitable death as a direct consequence of being
at the center of the Celestial sphere farthest
removed from the eternal and perfect nature of
God (The center of a sphere is the single most
distant point in the interior from the surface).
Man suffered because he was physically far or
distant from God. I hope you can see how the
story in Genesis of Adam and Eves expulsion from
the garden of Eden symbolizes the physical
distance of man from God first codified by
Aristotle. Many other Aristotelian concepts were
incorporated into the belief structure of the
early Christian church.
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Ptolemys Contribution to the Aristotelian
Universe
Claudius Ptolemaeus (ca. 100 ca. 178), known in
English as Ptolemy, was an Ancient geographer,
astronomer, and astrologer who probably lived and
worked in Alexandria in Egypt. Ptolemy was the
author of several scientific treatises, two of
which have been of continuing importance to later
Islamic and European science. One is the
astronomical treatise that is now known as the
Almagest (in Greek ? µe???? S??ta???, "The Great
Treatise"). The other is the Geography, which is
a thorough discussion of the geographic knowledge
of the Greco-Roman world.
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Ptolemys Contribution to the Aristotelian
Universe
In the Almagest, one of the most influential
books of classical antiquity, Ptolemy compiled
the astronomical knowledge of the ancient Greek
and Babylonian world he relied mainly on the
work of Hipparchus of three centuries earlier. It
was preserved, like most of Classical Greek
science, in Arabic manuscripts (hence its
familiar name) and only made available in Latin
translation (by Gerard of Cremona) in the 12th
century. Ptolemy formulated a geocentric model
(see Ptolemaic system) of the solar system which
remained the generally accepted model in the
Western and Arab worlds until it was superseded
by the heliocentric solar system of Copernicus.
The Almagest also contains a star catalogue,
which is probably an updated version of a
catalogue created by Hipparchus.
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Ptolemys Contribution to the Aristotelian
Universe
An Epitome of the Almagest (Epitome in Ptolemaei
Almagestum) was written between 1460 and 1463 by
the Austrian astronomer Georg Peurbach and his
famous pupil Johannes Regiomontanus at the
suggestion of Cardinal Bessarion. It gave
Europeans the first sophisticated understanding
of Ptolemy's astronomy, and was studied by every
competent astronomer of the 16th century. Unlike
earlier systems (such as 'the stars move because
that is the will of the gods', or the model of
concentric spheres), the Ptolemaic model
explained all phenomena in the sky, while holding
to Plato's dictum which states that all motions
in the heavens can be explained with uniform,
circular motion, and obeying Aristotelian physics.
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Ptolemys Contribution to the Aristotelian
Universe
According to the Ptolemaic model, the spherical
Earth is at the center of the universe. All
heavenly bodies are attached to crystal spheres
which rotate around Earth. The Moon is on the
innermost sphere, and touches the realm of Earth,
thereby contaminating it, and causing the light
and dark spots and the ability to go through
phases. It is not perfect like the other heavenly
bodies, which shine by their own light.
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Ptolemys Contribution to the Aristotelian
Universe
The planets are actually attached to 2 spheres
one sphere which is centered on Earth (the
deferent), and another sphere (the epicycle)
embedded within the deferent. The epicycle
rotates within the deferent, causing the planet
to move closer to and farther from Earth at
different points in its orbit, and even to slow
down, stop, and move backward (in retrograde
motion). The epicycles of Venus and Mercury are
always centered on a line between Earth and the
Sun (Mercury being closer to Earth), which
explains why they are always near it in the sky.
The order of spheres from Earth outward is
Earth, Moon, Mercury, Venus, Sun, Mars, Jupiter,
Saturn, Stars.
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Ptolemys Contribution to the Aristotelian
Universe
Unfortunately, the system still did not quite
match observations. Sometimes the size of a
planet's retrograde loop (most notably that of
Mars) would be smaller, and sometimes larger.
Ptolemy could not explain this even when he moved
deferents off-center, for the change in loop size
did not match with the change in speed. This
prompted Ptolemy to come up with the idea of an
equant. The equant was a point near the center of
a planet's orbit which, if you were to stand
there and watch, the center of the planet's
epicycle would always appear to move at the same
speed. Therefore, the planet actually moved at
different speeds at different points in its
orbit.
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Ptolemys Contribution to the Aristotelian
Universe
By using an equant, Ptolemy claimed to keep
motion which was uniform and circular, but a lot
of people didn't like it because they didn't
think it was true to Plato's dictum of "uniform,
circular motion." The resultant system which
eventually became Catholic dogma was an unwieldy
one, using two sets of epicycles, revolving on a
deferent, offset by an equant which was different
for each retrograde planet (then known to be only
Mars, Jupiter, and Saturn), but it predicted the
beginnings and ends of retrograde motion far more
accurately than either earlier Platonic spheres
or early (and falsely perfect) Copernican systems.