Galileo discovered that Venus goes through a cycle of phases like the Moon. This suggested that

• A. Venus moved about the Sun.
• B. Venus orbits the Moon.
• C. Venus remains stationary in the sky.
• D. Venus orbits the Earth.

Answer: (B)

Observing Venus go through a full set of illuminated phases, from crescent to nearly full, shows how its illumination depends on its angle relative to the Sun as it moves. This pattern only makes sense if Venus orbits the Sun; as it travels around the Sun, our viewpoint on Earth sees different amounts of the day-lit side, just like the Moon does. If Venus orbited Earth (or the Moon), we wouldn’t expect it to display the complete range of phases—the geometry would keep Venus close to the Sun in our sky and phases would be limited. So the phase progression provides evidence that Venus moves around the Sun, not around the Earth, which is why the Venus-phase observation supports a solar-centered arrangement rather than Earth-centered.