LearnPlanetary scienceSome maths
Retrograde motion
The apparent backwards loop a superior planet traces against the stars as the Earth overtakes it on the inside.
Why it matters
Explaining it consumed astronomy for nearly two thousand years. Ptolemy's epicycles reproduced it; heliocentrism made it trivially obvious.
What people usually get wrong
The planet actually reverses direction.
It never does. The loop is a projection effect from a moving observer, which is why it happens exactly at opposition.
Rests on
4 concepts in the full chain back to first principles.
Leads to
Nothing here builds on it yet.