Greek Cosmology

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Greek Cosmology

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Ptolemy
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Assumptions

• Perfect circles
• Uniform speeds
• Earth at Centre

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The Sequence of Planets

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Problems

• Variable eclipses (moon sometimes too small to
  block sun ? farther away?)
• Variable speeds of moon and sun across starry
  background
• Retrograde motion of outer planets
• Peculiar behaviour of Mercury and Venus

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Retrograde Loops
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• Ptolemys
• Solution

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Like a Spirograph
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Even More Complex

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The Real Explanation

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Crucial Ingredients

• Planets farther out move more slowly
• We overtake them on the inside track
• They appear briefly to drift backwards

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How About Venus and Mercury?

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Walking a Dog- consider the actual leash
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Predictive Power

• http//www.dudleyobservatory.org/Exhibits/alfonso.
  htm
• After hearing an explanation about Ptolemys
  theory of epicycles he Alfonso X said
• "If the Lord Almighty had consulted me before
  embarking on creation I should have recommended
  something simpler".

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Question How to Disprove?

• We have already met some answers! we learned
  how to prove the Earth moves through space,
  rather than standing still.
• But how did this develop historically?