Why Mars Moves Backward - This is the retrograde motion of Mars. So why does Mars sometimes appear to move backward in the night sky? Retrograde motion is an optical illusion that happens when Earth, moving faster in its orbit, overtakes Mars. As we pass it, Mars appears to slow down, stop, and move westward temporarily — even though it's still moving forward in space. Because Mars orbits farther from the Sun than Earth does, it takes longer to complete one orbit. When Earth "laps" Mars about ev