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Ptolemaic System (NAAP)
Geocentric NAAP model. The Sun rides a deferent; planets ride epicycles. Mercury is included (it was often omitted in classroom Flash copies).
Yellow: Sun · Grey: Mercury · Cream: Venus · Rust: Mars
Before you start
Ptolemy put Earth at the centre. Planets ride small circles (epicycles) on larger circles (deferents). This copy includes Mercury.
Key idea
Epicycles can copy the sky’s appearance without being physically true. Copernicus later moved the centre to the Sun.
What you see
- Earth fixed in the middle
- Sun, Mercury, Venus, and Mars on nested paths
- Checkbox to hide Mercury
Try this
- Play the animation and watch Mercury hug the Sun
- Hide Mercury, then turn it back on
- Compare with the heliocentric Retrograde Motion lab
Common mistakes
- Thinking a model that ‘fits the data’ must be the true layout of space
Glossary
- Deferent
- Large circle around Earth in Ptolemy’s system.
- Epicycle
- Small circle whose centre rides the deferent.