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Home Demo 02

Early astronomers believed that, as Mars moved through its orbit, it would stop, go in reverse, and then go forward again. Today that sounds like a crazy way for a planet to move, but that's still the way it looks from Earth, despite the fact that we now know Mars travels in an elliptical orbit around the Sun without backing up. Why does Mars appear to go backwards?

What You Do

  1. Go to a park and find a long, clear straightaway with few pedestrians.
  2. Put on the inline skates, or get your bike ready.
  3. Pick a starting point and end point for your experiment, and then find a stationary object around the middle to focus on.
  4. Both begin at your starting point, with your friend walking at a steady pace the whole way to the finish. When you start moving, begin by moving slightly slower than your friend, watching the focus point.
  5. Keep your eyes on the focus point as you speed up and pass your friend.
  6. What did you see?

What Did You Find?

Your friend never stopped moving forward, but from your point of view, she appeared to back up! The same thing happens when Earth moves faster and passes Mars on its way around the Sun-Mars only appears to go in reverse. This type of apparent motion is called retrograde motion. The odd retrograde motion of Mars helped to clue some astronomers onto the flaws inherent in early models of the Solar System-why would this one planet go backwards for such a short period when the others move steadily in one direction? Mars going in reverse may have put astronomy in fast-forward!

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  What You Need

  • A friend
  • A bike or inline skates

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Figure 1