Retrograde Motion Was a Major Source of Difficulty in Designing

question 203

Multiple Choice

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.