Galileo's telescope observations provided evidence for the heliocentric model by revealing which planetary phase?

Galileo's telescope observations provided evidence for the heliocentric model by revealing which planetary phase?

• A. Mars
• B. Jupiter
• C. Venus
• D. Saturn

Answer: (C)

The idea being tested is how the changing shape of a planet’s visible illumination reveals its orbit relative to the Sun. Galileo’s observations of Venus showed a full range of phases—from new to full—similar to the Moon. This makes sense only if Venus orbits the Sun and we view it from Earth at different angles as it goes around, causing the sunlit portion we see to change. In a geocentric model with Earth at the center, Venus would always stay close to the Sun in the sky, so it could not display the full sequence of phases. The heliocentric arrangement—Venus circling the Sun—explains the varying illumination clearly, providing strong evidence that the Sun is at the center of the system.

The idea being tested is how the changing shape of a planet’s visible illumination reveals its orbit relative to the Sun. Galileo’s observations of Venus showed a full range of phases—from new to full—similar to the Moon. This makes sense only if Venus orbits the Sun and we view it from Earth at different angles as it goes around, causing the sunlit portion we see to change.

In a geocentric model with Earth at the center, Venus would always stay close to the Sun in the sky, so it could not display the full sequence of phases. The heliocentric arrangement—Venus circling the Sun—explains the varying illumination clearly, providing strong evidence that the Sun is at the center of the system.