Ancient Astronomy Timeline

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Ancient Astronomy Timeline

• 3000 BCE Babylonians Egyptians become aware of
  heavens.
• 350 BCE Aristotles perfect Celestial Sphere

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Remember, all we can see from Earth is the
projection of the stars and planets on the
celestial sphere!
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The imaginary celestial sphere.
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Ancient Astronomy Timeline

• 3000 BCE Babylonians Egyptians become aware of
  heavens.
• 350 BCE Aristotles perfect Celestial Sphere

• 300 BCE Aristarchus offers the first
  heliocentric model

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Retrograde Motion
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What is really happening
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The Big Question
How did the geocentric (Earth-centered) model of
the solar system change to account for the
retrograde motion of the planets?
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Ancient Astronomy Timeline

• 3000 BCE Babylonians Egyptians become aware of
  heavens.
• 350 BCE Aristotles perfect Celestial Sphere
• 300 BCE Aristarchus offers the first
  heliocentric model

• 150 CE Ptolemaic Geocentric Model

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Epicycles and Deferents
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See It Works
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Not quite as elegant
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Ancient Astronomy Timeline

• 3000 BCE Babylonians Egyptians become aware of
  heavens.
• 350 BCE Aristotles perfect Celestial Sphere
• 300 BCE Aristarchus offers the first
  heliocentric model
• 150 CE Ptolemaic Geocentric Model

• 1543 Copernicus Heliocentric Model

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The heliocentric theory correctly described
retrograde motion!
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A more elegant explanation
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Astronomy Timeline

• 3000 BCE Babylonians Egyptians become aware of
  heavens.
• 350 BCE Aristotles perfect Celestial Sphere
• 300 BCE Aristarchus offers the first
  heliocentric model
• 150 CE Ptolemaic Geocentric Model
• 1543 Copernicus Heliocentric Model

BOTH of these models correctly describe
retrograde motion how do we determine which one
is correct??