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Ptolemaic Orbit of Mars
Ptolemy put Mars on a small circle (epicycle) whose center rides a large circle (deferent). The extra loop makes retrograde motion.
Before you start
Ptolemy’s Mars: a deferent plus a large epicycle to manufacture retrograde loops.
Key idea
The loop is real on the sky. The extra circle is an accounting device, not a second physical orbit.
What you see
- Earth at centre
- Deferent and epicycle
- Mars on the small circle
Try this
- Play and watch the loop
- Compare with Retrograde Motion (heliocentric)
Common mistakes
- Thinking Mars literally rides a carnival loop in space
Glossary
- Retrograde
- Apparent westward drift against the stars.