Ancient Models of the Universe Prof' Jim Miller

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Ancient Models of the UniverseProf. Jim Miller
http//content.answers.com/main/content/img/scitec
h/HSptolem.jpg
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Thales model of the cosmos (620s - 546 BC)

• Believed water to be the essential element
• May have thought that the Earth was flat and
  circular (disk-shaped)(see www.iep.utm.edu/t/thal
  es.htmSH6a)
• Said to have predicted a total solar eclipse in
  585 BC
• Attempted to estimate the distance to the Sun

• http//www.iep.utm.edu/t/thales.htm

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Platos model of the cosmos (400s BC)

• Stars revolve once a day on one axis
• Sun, moon and planets have same motion
• - second circular motion moves them relative to
  stars
• As stars revolve, sun, moon and planets in
  different positions

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Eudoxus model of the cosmos (400-350 BC) Nested
homocentric spheres (non-physical, mathematical
entities)

• 27 spheres
• 3 for Sun andfor moon
• 4 for each of 5 planets
• 1 for the fixed stars

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Aristotles model of the cosmos (350 BC)
55 spheres
real made of aether
- no space between them
Celestial bodies attached
Rotate at different velocities
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The Aristotelian world system
Why a geocentric-geostatic model?
Earthly
- corruptible
- earth absolutely heavy
- reaches proper place and stops
Celestial

• - Heavens perfect, unchanging

- Only motion in Heavens is uniform circular
motion
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Problems

• inaccurate didnt match observations
• cant explain how the apparent size /
  brightness of the planets vary
• cant explain how the apparent speeds of
  planets vary
• didnt model retrograde motion well

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Aristarchus (310-250? BC)

• Heliocentric model
• Estimated Earth Sun distance as 5 million miles
• Earth both rotated and revolved around the Sun

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Ptolemy (85-165) Egyptian-born Greek astronomer
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Almagest
Text of mathematical astronomy
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Ptolemaic Model

• spherical, unmoving Earth in central region of
  Universe ? its natural place
• not strictly geocentric - model used eccentrics
  but was geostatic
• stars fixed on celestial sphere that rotated once
  every 24 hours.
• Sun and five planets had their motions explained
  by combinations of epicycles, deferents and
  eccentrics.
• seventy circles and spheres required

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Celestial spheres

• Assumped no voids between the planets
• Each celestial body seated in sphere of aether
  just as in Aristotles system
• Sphere for each celestial body thick enough to
  include the epicycle.

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Order of Celestial Bodies

• Generally accepted Moon was closest, but various
  opinions as to the order of the other spheres
• Consensus at the time that Saturn was furthest
  away, followed by Jupiter, and then Mars
• no such consensus regarding Mercury and Venus

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Planetary Motion in the Ptolemaic model
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Strictly Uniform Circular Motion

• the speed of the yellow object along the circle
  is uniform

• the circle shares a center with the Earth

• the object sweeps out equal angles in equal times

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Basic Construction - Epicycle
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Epicycle on Deferent

• Deferent produces eastward motion seen most of
  the time

• Epicycle produces retrograde motion

• explains the varying size brightness of the
  planets

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Basic Construction - Eccentric
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Eccentric Circle
The speed of the green object along the circle is
uniform, just like the previous yellow object
But the circle isnt centered on Earth
From the Earth, the object appears to move faster
in the bottom half of its motion than in the top
half
because the Earth does not coincide with the
center of the motion 
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Basic Construction - Equant
Basic Construction - Equant
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Equant Circle
The speed of the red object along the circle
varies it moves faster in the bottom half,
slower on the top half
Motion is uniform in that red object sweeps out
equal angles in equal times from a point inside
the circle the equant point (not the center of
the circle or the center of Earth
Speed of object is nonuniform from perspective of
equant point, center of equant circle, and center
of Earth 
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Combinations of three constructions
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Ptolemys Model

• Centers of epicycles of the inferior planets
  between Earth and sun move on an imaginary line
  connecting the Earth and the Sun

• This modification keeps these planets close to
  the Sun at all times, as observed

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The Ptolemaic Model was highly complex

• Epicycles within the epicycles existed (as many
  as 28 for some planets)

• The epicycles of the planets account for
  retrograde motion

• Sun-linked epicycles of the inferior planets
  explain why they always remain close to the Sun
  in the sky

• The equants account for the fact that the planets
  speed up slightly when they are close to the
  Earth and slow down when they are further away

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but successful standard for 1,500 years

• It placed the Earth in its natural place at the
  center of things, satisfying Aristotelian
  philosophy
• It matched with common sense. We do not feel the
  Earth move and Ptolemy's model had a static Earth
  
• It accounted fairly well for the movements of the
  known celestial objects (to within about 2o)
• But the eccentric spheres and the equants
  violated Greek concepts of the Heavens