1. Planetary Motion

2. Wanderers The Sun, Moon and planets appear to move relative to the stars. Planetary motion is confined to the ecliptic. –Generally west to east –Not exactly on the ecliptic

3. Conjunction A conjunction occurs when two objects line up as seen from Earth. Inferior planets have two solar conjunctions each cycle. –Mercury, Venus An opposition occurs when a planet is directly opposite the Sun. Superior planets have one conjunction and one opposition. –Mars, Jupiter, Saturn

4. Retrograde Planets sometimes move east to west. –Backwards against the stars This retrograde motion was difficult to explain. wikipedia

5. Geocentric means the Earth is at the center and motionless. Described as the Ptolemaic view (after Ptolemy). Stars orbit the Earth on a very distant celestial sphere. The Sun and other planets orbit the Earth on circles within circles. Mars epicycle deferent Earth Geocentric View Ptolemaic Simulator

6. Heliocentric View Heliocentric means the Sun is at the center and motionless. Described as the Copernican view. The Earth revolves on its axis once per day. All planets move about the Sun on circles and epicycles. The Catholic church adopted the Ptolemaic view as the “revealed truth” in the 13th Century. The Copernican model was published in 1543 with detailed comparisons to observations.

7. Heliocentric motion has the Earth move faster than the superior planets. –Retrograde motion at opposition. This is a simpler solution – Occam’s razor. Mars Earth Sun Opposition Copernican Simulator