Question

What is some of the evidence used by Galileo to support Copernicus’s model and disprove Aristotle and Ptolemy’s?

Each planet spins in an epicycle.
Planets appear to move backward.
Jupiter has moons orbiting it.
The religious and cultural beliefs supported this model.

Answer 1

Correct choices:
- Planets appear to move backward (retrograde motion) — yes, this observed behavior is explained naturally by the Copernican model as an apparent effect of relative motion of Earth and the other planets, whereas the Ptolemaic system needed complex epicycles to account for it.
- Jupiter has moons orbiting it — yes. Galileo’s discovery of four moons around Jupiter (the “Galilean satellites”) showed that not everything orbits the Earth, directly contradicting the Aristotelian/Ptolemaic idea that Earth was the unique center of all celestial motion.

Incorrect or not evidence:
- Each planet spins in an epicycle — no. Epicycles are part of the Ptolemaic (and even Copernican refinements) descriptive apparatus, not evidence for heliocentrism; Copernicus reduced the need for some epicycles but did not eliminate them.
- The religious and cultural beliefs supported this model — no. Religious or cultural beliefs are not scientific evidence; in fact, many religious authorities opposed heliocentrism.

Additional Galileo evidence (useful to know): phases of Venus (showing Venus orbits the Sun) and observations of sunspots and lunar mountains (showing the heavens were not perfect and unchanging) were also important.