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Early records indicated that planets moved from west to east among the stars but from time to time, one would seem to move backwards and appear brighter halfway along the backward path. You can see how an earth-centered theory might explain the phenomenon.

Put a clay a ball on one spoke of an umbrella to represent a planet, then hold the umbrella above your head and slowly
rotate it while you walk in a circle around another student, who represents the earth. This is what tests your balance.

Note when the planet goes in the same direction as you and
when it goes in the opposite direction. Discuss when the
planet would seem to be brightest to an earth observer if it reflected light.

According to an earth-centered theory, the planet moved along a complicated path called an epicycle. Research how a sun-centered theory explained the same phenomenon differently and in a less complicated way.

You're going to have to look in the encyclopedia yourselves. I'm ready for a nap.

While I snooze, try this:

Use empty cardboard boxes and styrofoam balls or clay to make models of the Solar System according to an earth-centered theory and a sun-centered theory. Research and compare the two theories and discuss why astronomers now universally accept the sun-centered theory.

Research the work of Claudium Ptolemy (second century A.D.), an Alexandrian astronomer who made the last major refinement of a centuries-old earth-centered (geocentric) theory.

Compare Ptolemy's work with the work of Nicolaus Copernicus
(1473-1543), a Polish astronomer who is credited with the the sun-centered (heliocentric) theory.

Look at that! The word, "egocentric" is merely a misspelled "geocentric!" That little item might explain why the earth-centered theory is wrong. !!! Use a 40 foot length of string and chalk to draw a circle on the playground. Within the circle draw eight more circles at intervals that represent the average distance of the planets from the sun.

Ask nine students, in addition to you, to line up in a row,
each on a different line. At a predetermined signal, move in counterclockwise orbits around the center of the large
circle. You will see that the outer planets fall behind and that the various planets change in their distances from the earth. When a planet makes one complete orbit around the sun, the orbit is called a revolution.

You might be interested in researching the revolution periods of the planets. The periods can be compared to the distances each planet is from the sun.


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