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Miscellaneous announcements…

Explore the "Two-Sphere Universe," motions of planets, retrograde motion, and Aristotelian Universe theories. Unveiling the ancient explanations of planetary motion and the quest for cosmic understanding.

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Miscellaneous announcements…

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  1. Miscellaneous announcements… • Homework 2 due Wednesday • Astrophotographers: Email me info about your schedule (which nights you’re available) by Wednesday.

  2. Review of the “Two-Sphere Universe” What’s wrong with this picture?

  3. 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

  4. Motions of the Planets 11 September 2006

  5. 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

  6. Days of the week

  7. Saturn in Saggitarius

  8. Mars in Taurus

  9. 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).

  10. The Aristotelian Universe

  11. The Aristotelian Universe

  12. What about retrograde motion? Epicycle Planet Earth Deferent (This mechanism invented by Hipparchus, 190-120 BC; improved by Ptolemy, 87-150 AD.)

  13. Epicycle Motion

  14. 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.

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