What is the cause of retrograde motion?

Question

BlackTom AI · Accepted Answer

To approach the question, consider what retrograde motion actually is: an apparent reversal in the path of a planet as seen from Earth. 
Option 1: The planets moving at different speeds. This aligns with the standard explanation that planets orbit the Sun at different velocities, and because Earth orbits faster than some outer planets, the relative position of those planets appears to drift backward for a time. The effect is an optical illusion caused by our viewpoint on a moving Earth, not a true reversal of the planet’s orbit. 
Option 2: The Earth's rotation reversing direction. This is incorrect; the Earth’s rotation does not reverse, and even if it did, that would not produce a planetary retrograde loop in the sky. 
Option 3: The Earth's orbital speed changing throughout the year. While Earth’s orbital speed does vary slightly due to orbital dynamics (Kepler’s laws), this variability alone does not create retrograde motion; retrograde is about relative motion from our viewpoint, not a secular change in Earth’s speed. 
Option 4: Different constellations being visible throughout the year. This describes the changing background stars as Earth orbits, not the backward motion of planets themselves. It explains inclination and seasonal sky changes but not retrograde loops. 
In summary, the most accurate cause among the options is that the apparent retrograde motion results from the planets moving at different speeds and the resulting relative motion observed from Earth, which creates the illusion of backward motion during certain periods.

ASTR-1-45046-F25 Lecture: The Copernican Revolution

What is the cause of retrograde motion?

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