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Modeling Physical Situations

Does the earth orbit the sun or the sun orbit the earth?

To believe the earth orbits the sun requires us to disbelieve our instincts, that we are standing still.

To believe the sun orbits the earth requires us to believe in epicycles.

  • From an earth man's perspective, Mars looks like it rotates around the earth on a spirograph.

Only when we analyze the planetary motions using the sun centered model do we understand that this spirograph motion is an illusion caused by the relative motion of Earth and Mars.

When people discovered this..that the sun was at the center of things, the orbits still didn't fit exactly. This is because the orbits are not exactly circular but a tiny bit elliptical.

This illustrates another idea about scientific models--your model often ignores some of the details and just gets the general behavior right.

In fact people later found that the orbits weren't precisely ellipses but that planets tugged on each other due to gravity when they came near each other.

This is how we discovered Uranus (pronounced Your Anus) and illustrates an additional feature. Another point of a good scientific model--deviations from the behavior predicted by the model often lead to new discoveries.

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