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#### Updated on Feb 07, 2012

Grade Level: 5th - 8th; Type: Physics

To understand why planets appear to move backwards at times.

The purpose of this experiment is to recreate the movement of planetary bodies in order to learn why they sometimes appear to move backwards when observed from the surface of the earth.

• What were the early models of the universe like?
• Why did humans believe the Earth was in the center of the universe?
• What evidence led them to believe that the Earth was at the center of the universe?
• What evidence led them to believe that the Earth was not at the center of the universe?
• Why do planets appear to move retrograde?

When observed from the surface of the Earth, the sun, moon, planets and stars appear to circle around us. Early models of the universe placed the Earth in the center and everything else in orbitals around it. It took human beings a long time to understand that the Earth and the planets orbit the sun. Though we stuck to our geocentric notions for a long time, the Earth-centered model of the universe was not without problems. One of these was the apparent retrograde motion of the planets. Periodically, planets appear to move backwards in the sky. Early astronomers had a hard time accounting for this motion and it wasn’t until humans accepted the sun-centered model of the universe that the principles of retrograde motion were finally understood.

• A few volunteers
• A yard stick
• A protractor
• A notebook and a pencil
1. For the first part of the experiment you will take the role of the planet Earth. One of your friends will play the sun and the other will play Mercury.

2. Have you friend, the sun, stand in the center of a large, open space.

3. Have your friend, Mercury, stand next to the sun and take 10 large steps away from it.

4. Stand next to the sun and take 20 large steps away from it, in the same direction as Mercury. Bring the protractor, yard stick and a notebook and pencil with you.

5. Mercury should be about half way between you and the sun. This is not to scale, but that is okay.

6. Face the sun and place the protractor on the ground with the flat edge against your toes.

7. Take the yard stick, stand it up on its thin edge and point it at Mercury.

8. Record the angle that Mercury is from you now. (It should be 90 degrees for this first measurement.)

9. Both you and Mercury take ten large steps in a circle around the sun. Make sure you walk in the same direction.

10. Repeat steps 6-8. (The angle will now deviate from 90 degrees.)

11. Continue in this manner until you have traveled completely around the sun. Mercury should have traveled around twice in this time.

12. Next, have your friend who was Mercury play the role of Mars.

13. Have Mars stand next to you and then take 20 large steps away.

14. Stand with your back to the sun and place the protractor on the ground with the flat edge against your toes.

15. Record the angle that Mars is from you now. (It should be 90 degrees for this first measurement.)

16. Both you and Mars take ten large steps in a circle around the sun. Make sure you walk in the same direction.

17. Repeat steps 6-8. (The angle will now deviate from 90 degrees.)

18. Continue in this manner until you have traveled completely around the sun twice. Mars should have traveled around once in this time.

19. The angular data you have recorded shows how your perception creates an apparent backwards motion when you observe other planets in their orbit around the sun.

20. (optional) Repeat the experiment from the point of view of the sun.

21. (optional) Repeat the experiment from the point of view of a geo-centric Earth. In order to observe retrograde motions, how do the planets need to move? Hint: early models suggested circles within circles!)

Terms/Concepts: Orbit; Geo-centric model; Sun-centered model; Solar system; Planet; Retrograde

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Writer and educator Crystal Beran is rarely seen without a pen. Her adventures have brought her to four continents and her quest for answers has led her to discover more questions than she could fill all the pages with. She currently resides in Northern California, where she can be found sipping tea and writing books.