In the heliocentric (Sun-centered) model, apparent retrograde motion is due to
A) relative motion between the Earth and another planet in its orbit
B) the Earth reversing the direction of its orbit about the Sun
C) a planet moving in a small circle, the center of which moves in a larger circle about the Earth
D) a planet reversing the direction of its orbit about the Sun

Question

Quizplus · Accepted Answer

Apparent retrograde motion occurs when a planet appears to move backward in its orbit, compared to the background stars, as observed from Earth. This is because of the relative motion between the Earth and the other planet in its orbit. As the Earth orbits the Sun, it may pass by another planet. When this happens, the other planet may appear to move backward in the sky for a time, due to the difference in orbital speeds between the two planets. This is an apparent motion caused by the observer's point of view and not an actual change in the planet's orbit.

In the Heliocentric (Sun-Centered) Model, Apparent Retrograde Motion Is Due