Even though the heliocentric model has advantages over the Ptolemaic model, it suffered from the assumption that the planets moved in perfect circles about the Sun. What was this assumption?

• A. Planets moved in perfect circles about the Sun
• B. Planets moved in ellipses about the Sun
• C. The Sun orbited the planets
• D. The planets stopped moving at some point

Answer: (A)

The idea being tested is that early heliocentric thinking assumed planets move in perfect circles around the Sun. This circular-orbit assumption looked elegant and simple, but when astronomers compared predictions to precise observations, it didn’t match the actual motions. That mismatch is what made the circular model “suffer.” It wasn’t until Kepler showed that orbits are ellipses with the Sun at a focus that the model fit the data much better, explaining why planets sped up and slowed down along their paths. In essence, the correct idea is that the planets were thought to move in perfect circles about the Sun.