Miscellaneous announcements…

• Homework 2 due Wednesday

• Astrophotographers: Email me info about your schedule (which nights you’re available) by Wednesday.

Review of the “Two-Sphere Universe”

What’s wrong with this picture?

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

Motions of the Planets

11 September 2006

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

Days of the week

English Spanish French Planet

Sunday Domingo Dimanche Sun

Monday Lunes Lundi Moon

Tuesday Martes Mardi Mars

Wednesday Miércoles Mercredi Mercury

Thursday Jueves Jeudi Jupiter

Friday Viernes Vendredi Venus

Saturday Sábado Samedi Saturn

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).

What about retrograde motion?

Earth

Deferent

Epicycle

Planet

(This mechanism invented by Hipparchus, 190-120 BC; improved by Ptolemy, 87-150 AD.)

Epicycle Motion

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.