Retrograde Motion of Planets

For a few weeks every year or two, Mars appears to stop, reverse course, and drift backward against the background of stars before resuming its normal path across the sky. Ancient observers noticed the same behavior in Jupiter and Saturn, and for centuries this apparent reversal, called retrograde motion, was one of astronomy's most stubborn puzzles. The planet never actually reverses direction in its orbit -- the illusion comes entirely from the shifting vantage point of an observer on a moving Earth. Because Earth orbits the sun faster than Mars, it periodically catches up to and passes Mars on the inside track, much like a faster car on a highway briefly makes a slower car appear to drift backward against the distant scenery, even though both cars are still moving forward. As Earth swings past, the angle to Mars shifts rapidly enough that Mars appears to slow, stop, and briefly reverse before Earth pulls far enough ahead for the normal forward motion to resume. Explaining retrograde motion was a genuine problem for the older, Earth-centered model of the solar system, which needed an increasingly elaborate system of circles-within-circles to account for it. The heliocentric model proposed by Copernicus explained the same observation far more simply, since retrograde motion falls naturally out of two planets orbiting the sun at different speeds.