Nicolaus Copernicus, whose 540th birthday is celebrated on Google's homepage Tuesday, kicked off the Scientific Revolution. ?Enlarge
It's astonishing to think that anatomically modern humans walked on this planet for nearly two hundred millennia before they realized that it was moving.?Skip to next paragraph
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Then again, you can't really blame us. Even though the equator is spinning at more than a thousand miles per hour, even though our planet is hurtling around our sun at about 66,000 miles per hour, even though our solar system and everything in it is careening around our galaxy at nearly half a million miles per hour, and even though our galaxy is whirling around at a mind-blowing 1.2 million miles an hour,?the very fact that you're not currently clinging to the ground for dear life makes it only natural?to think of the Earth as 16th century astronomer Tycho Brahe did, as a "hulking, lazy body, unfit for motion."
Nicolaus Copernicus's book, titled "On the Revolutions of the Heavenly Spheres," was published the year the Polish astronomer died, in 1543. It marked the beginning of an eclipse of a worldview dominated by the ideas of Aristotle, who believed that?the Earth remained at rest while the sun, moon, other planets, and all the?stars, which were made of an unchanging substance called aether, revolved around it in perfect circles.
Of course nothing is ever that simple. Our prehistoric ancestors observed five celestial bodies moving independently from the rest of the lights in the sky. These bodies would move slowly across the sky, loop backward for a few months, and then loop forward again.?The ancient Greeks called them?ast?r?plan?t?s, or "wandering stars," and thought them to be living beings.
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.