Retrograde motion is when the Earth catches up with an outer planet, such as Mars or Jupiter. As the Earth nears the other planet, it appears to move across the sky over a period of several weeks. When the Earth meets the planet and begins to move away from it, the planet appears to do a sort of loop-dee-loop in the sky before continuing its path across the sky down into the horizon.
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think of two cars on a racetrack... the car on the inner track has a camera in it.... as it catches up to and passes the outer car, that car "appears" to move "backwards" even though BOTH cars are travelling along at hundreds of miles an hr. The outer car, falling back, is retrograde
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Because we pass them on our orbit.