Which planet's phase observations provided evidence for heliocentric model?

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

Answer: (D)

Observing how a planet’s light changes as it orbits tells us whether it goes around the Sun or around Earth. Venus shows a full range of phases—from new to full—just like the Moon. That pattern can only happen if Venus orbits the Sun and sometimes lies between us and the Sun or on the far side of its orbit. In a geocentric view where everything revolves around the Earth, Venus would never reach a full phase because it would always stay too close to the Sun in our sky. Galileo’s telescope images of Venus displaying those full phases provided strong, convincing evidence that the planets orbit the Sun. The other options don’t provide that same decisive constraint.