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How Nicolaus Copernicus moved the Earth

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The looping behavior of these wanderers – whom today we know as the planets Mercury, Venus, Mars, Jupiter, and Saturn – presented a problem for ancient astronomers, many of whom would have loved nothing more than to have observed them circling above in a simple, orderly fashion, just as Aristotle said they did. But the heavens demanded a more nuanced explanation.

That explanation was provided by the Alexandrian astronomer Claudius Ptolemy, who in the 2nd century AD marshaled more than 800 years of detailed astronomical descriptions to produce a model of the cosmos. In Ptolemy's model, the Earth lay at the center of a series of eight concentric crystalline spheres, each of which had celestial bodies attached to them. Nearest was the sphere holding the moon, followed by Mercury, Venus, the sun, Mars, Jupiter, Saturn, and finally the sphere of fixed stars.   

As for those very inconvenient loop-de-loops, Ptolemy's system had the planets moving not just in large circles, but along smaller circular "epicycles" that were attached to the larger circles. As a result each planet moved not in a circle, but in a clover-shaped path called an epitrochoid. 

Ptolemy also noted that the planets failed to appear to move at a uniform speed. So he postulated a place called the equant point, located some distance from the Earth. A hypothetical observer stationed at the equant point, according to Ptolemy, would see the planets moving at a steady pace.

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