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Ptolemaic Phases of Venus
Note: Pair with Phases of Venus to compare heliocentric vs geocentric predictions.
In Ptolemy's model Venus's epicycle sits between Earth and the Sun. Venus never goes to the far side, so it should never look gibbous — contradicting Galileo.
Venus stays on the near side → only crescents in the geocentric model.
Before you start
In the geocentric model Venus’s epicycle sits between Earth and the Sun. Venus never goes ‘behind’ the Sun.
Key idea
That predicts only crescent Venus. Galileo saw otherwise, so the model fails.
What you see
- Earth, epicycle, Sun in a line
- Venus stuck on the near side
Try this
- Play a full epicycle — Venus never reaches the far side
- Open Phases of Venus next
Common mistakes
- Treating Ptolemy as ‘just another drawing’ instead of a testable prediction
Glossary
- Inferior planet
- Orbit inside Earth’s — Mercury or Venus.