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Retrograde Motion
Note: Uses Mars orbit SWF — see Ptolemaic Orbit of Mars for the full demo.
Earth laps Mars. Line of sight then swings backward for a few weeks — retrograde motion — with no epicycle required.
Dashed line: Earth → Mars. Outer tick marks the apparent sky direction.
Before you start
Earth laps Mars. The line of sight then swings backward for a few weeks. No epicycle required.
Key idea
Retrograde is a perspective effect of two orbits, fastest when Mars is at opposition.
What you see
- Sun, Earth, Mars
- Line of sight extending to the ‘sky’
Try this
- Play through opposition
- Watch the sky-direction tick reverse
Common mistakes
- Thinking the planet physically stops and reverses in its orbit
Glossary
- Opposition
- Earth between Sun and outer planet.