Observing the Sky: Observing the Planets - Retrograde MotionJust as the Sun seems to drop backward against the stars as the Earth moves forward in its orbit, the planets seem to move eastward against the stars. Every now and then, the superior planets also do something odd. Over the course of a few days you might notice them slow down, stop, and start moving in the opposite direction. This backward movement is called retrograde motion. Eventually, the planet will slow down and reverse direction, heading east once again. In reality, the planets aren't doing anything strange, they are orbiting the Sun as they always do. But our perspective on their motion changes. The Earth is moving around the Sun faster than the outer planets, so there comes a point in our orbit where we pass them. Think of riding in a car and watching the cars you pass. You know those other cars are moving in the same direction as yours, but they seem to move backward as you pass them. |
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CyberScientist@onr.navy.mil