Retrograde motion was a major source of difficulty in designing a theoretical model of the solar system. What is the explanation for retrograde motion?
A) The planets actually stop and reverse direction in their orbits.
B) The planets move not in uniform circular motion about the Sun but rather in circular orbits (epicycles) about points which themselves move about the Sun in circular orbits.
C) Retrograde motion is an apparent motion of other planets, which we observe only because we are viewing their motion from Earth, itself in motion around the Sun.
D) Retrograde motion is an apparent motion caused by the precession of Earth's rotation axis.

Question

Quizplus · Accepted Answer

Retrograde motion is an optical illusion that occurs because we observe the planets from Earth, which is also moving around the Sun. This makes it seem like the planets are moving backward in their orbits at certain times, but in reality, they are not.

Retrograde Motion Was a Major Source of Difficulty in Designing