Lecture 3 Geocentrism vs. Heliocentrism

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Lecture 3Geocentrism vs. Heliocentrism

• Dennis Papadopoulos
• ASTR 340
• Fall 2006

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Aristotles Universe
Planets on physical spheres each moving the
other earth and sphere of prime mover
stationary
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Aristotle-Ptolemy and the
Church
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Aristotles influence Science roots of
cosmology

• Pythagoras and Plato described the world through
  mathematical relationships. Many common ideas
  Aristotle started from these with focus on
• Universe is dynamic, living, organic structure
  perfectly ordered on musically mathematical
  proportions
• Earth is center of the universe and planets
  revolve in perfect circles
• Aristotle used Mathematics and Observations for
  his cosmos
• Cosmology based on the fact that there are
  natural laws that describe how objects behave in
  the Universe
• Using these laws and sufficient observations we
  can understand the universe

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Aristotles laws of motion

• Motion Kinematics-On Earth
• Why do objects move the way the do on earth?
• Why do objects fall when dropped?
• Why stones fall while bubbles rise on water
• Answer They move till they found their natural
  state- rest Different elements move differently
• Earth towards the center of the earth
• Fire away from center of the earth
• Water and air occupy the in between space
• Motion linear
• Concept of force Force(Resistance)x(speed)
• Continuous motion required application of force -
  inertia

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Laws of motion The heavens - Ether

• Motion kinematics heavens
• Fifth element ether
• Objects in heavens (above moon) move in circles
• Why? Universe finite linear motion would force
  them to fall off the end of universe
• Why earthly objects cannot reach the end of
  universe? they move in circles as soon they
  approach heavens
• Aristotles cosmology inextricably linked to laws
  of motion (physics) as todays cosmology

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Aristotle vs. Aristarchos
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Geocentric vs. heliocentric
Pros and cons
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Medieval Universe
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Enter Copernicus - 1543
Copernican Principle THE EARTH IS NOT LOCATED
IN A PRIVILEDGED POSITION
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Tycho Brahe Johannes Keppler
Tycho Brahe aristocrat/nobleman observer naked
eye Observations from island observatory Amassed
data with extreme precision In 1572 observed
first supernova in the constellation of
Cassiopeia. Could not measure parallax but he
realized that it was not an atmospheric
brightening but it belonged to the realm of fixed
stars -gt Heavens not immutable Used parallax to
show that comet orbits lay outside the orbit of
the moon. Another strike against Aristotle
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Brahe on observing
Supernova

• Amazed, and as if astonished and stupefied, I
  stood still, gazing for a certain length of time
  with my eyes fixed intently upon it and noticing
  that same star placed close to the stars which
  antiquity attributed to Cassiopeia. When I had
  satisfied myself that no star of that kind had
  ever shone forth before, I was led into such
  perplexity by the unbelievability of the thing
  that I began to doubt the faith of my own eyes.

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Brahes Universe
Earth at the center orbited by moon and
Sun Planets orbit Sun Failed to confirm
Copernicus by testing parallax change of Mars,
Jupiter due to inaccurate knowledge of earths
orbit (by a factor of 20)
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Keppler Elliptical paths
Conic section Foci Major axis Minor axis
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Circular vs. elliptical motion
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KEPLERS LAWS
The orbits of the planets are ellipses, with the
Sun at one focus of the ellipse.
R is semi-major axis
(P/yr)2(R/1 AU)3
1 AU150 million km
The line joining the planet to the Sun sweeps out
equal areas in equal times as the planet travels
around the ellipse.
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From ellipse to parabola
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Keppler vs. Copernicus

• Wrong assumptions of Copernicus
• Planets move in perfect circles
• Planets move in constant speed
• Sun at the center of these orbits

• Keppler showed
• Planets move in ellipses
• Planetary speed varies constantly
• Sun is not quite at the center of the planetary
  orbits

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Galileo Galilei
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Galileo Looking at the sky
PHASES OF VENUS
MOON
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(No Transcript)
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Geocentric vs. heliocentric
Post Galileo
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Galileos drawing
Huyghens drawing
Hubble image
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Zeno Achilles and the turtle
Atomists Zeno-Democritus-Lefkippos Universe
made up of little particles atoms- indivisible
and eternals quanta They move in vacuum
Aristotle -gt no vacuum
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Stoic Universe