Retrograde Motion

• Like the Sun, planets usually appear to move eastward relative to the stars.
• But as we pass them by in our orbit, they move west relative to the stars for a few weeks or months.
• Noticeable over many nights; on a single night, a planet rises in east and sets in west…
• Earth is at center
• Sun orbits Eartth
• Planets orbit on small circles whose centres orbit the Earth on larger circles [the small circles are called epicycles]
• This explained retrograde motion
• Inferior planet epicycles were fixed to the Earth-Sun line
• This explained why Mercury & Venus never strayed far from the Sun!