Miscellaneous announcements

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Miscellaneous announcements

• Homework 2 due Wednesday
• Astrophotographers Email me info about your
  schedule (which nights youre available) by
  Wednesday.

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Review of the Two-Sphere Universe
Whats wrong with this picture?
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Alternative speculations

• Could the earth be spinning?
• Could the center of the universe be somewhere
  other than earth?
• Could the earth be one of the heavenly bodies?

It is not hard to realize why the ancients
believed in the two-sphere universe. The problem
is to discover why the conception was given
up. Thomas Kuhn
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Motions of the Planets

• 11 September 2006

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Today

• Appearance and motion of the five naked-eye
  planets (Mercury, Venus, Mars, Jupiter, Saturn)
• Ancient explanations of this motion
• Summary of earth-centered cosmology

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Days of the week
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Saturn in Saggitarius
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Mars in Taurus
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Summary

• Like the sun and moon, the other five planets
  slowly wander among the stars (always near the
  ecliptic).
• Like the sun and moon, the other five planets
  normally drift eastward with respect to the
  stars.
• However, the other five planets periodically
  undergo retrograde motion (westward with
  respect to the stars).

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The Aristotelian Universe
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The Aristotelian Universe
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What about retrograde motion?
Epicycle
Planet
Earth
Deferent
(This mechanism invented by Hipparchus, 190-120
BC improved by Ptolemy, 87-150 AD.)
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Epicycle Motion
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Ups and downs of epicycles

• Can accommodate any size and rate of retrograde
  motion.
• Correctly predicts that planets (especially Mars)
  should be brighter during retrograde motion.
• Precise predictions (especially over long time
  periods) require further complexity compounded
  epicycles, etc.
• Despite 1700 years of use, the theory was never
  quite perfected.