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Illustration of retrograde motion

A schematic of how retrograde motion works when Earth (T) passes an outer planet (P) as they both orbit the sun (S). The changing viewing angle from Earth makes the projection of the planet against the celestial sphere (A) move backwards (A2-A4) as we pass the slower planet. Credit: Wikipedia user Rursus

Illustration of retrograde motion

Illustration of retrograde motion

Christopher Crockett

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