The heliocentric model helps explain which of the following?

• A. The shapes of comets.
• B. The orbit of the Moon around Earth.
• C. The motion of planets and their varying brightness as seen from Earth.
• D. The distance to the stars.

Answer: (C)

In a Sun-centered system, all planets orbit the Sun at different distances and speeds, which explains how they move across the sky and why their brightness changes over time. The varying motion you see, including occasional apparent backward looping (retrograde motion), comes from the way Earth overtakes or is overtaken by other planets in their orbits. The brightness changes because a planet’s distance from Earth and Sun changes as it travels, and for the inner planets we also see phases that affect how much of the sunlit side we observe. This combination matches the observed behavior of planets like Venus and Mars far better than other scenarios. The Moon’s motion around Earth is a separate two-body interaction, and distances to stars rely on parallax on a much larger scale, not the planets’ motion around the Sun.