Retrograde Motion
All of the planets of our solar system move at different speeds as they move around the Sun. Those closest to the Sun move the fastest. As we on Earth pass each outer planet in turn, that planet appears to move backward. We observe this illusory backward motion as retrograde motion. For a planet such as Mars, the retrograde loop is fairly large and obvious to a patient observer over the course of several weeks. For a distant planet such as Neptune, the effect is hardly noticeable.
Some of my students seem to get the impression that retrograde motion is some theoretical observation that only a professional astronomer can appreciate. All it really takes is to spend a few weeks or months getting familiar with the sky and watching the planets as they dance among the constellations.
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