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.
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 through space 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 astēr planētēs, 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.
Ptolemy's model was convoluted, but it could accurately predict the positions of the planets a few years out. His model was also backed by the church, who found it consonant with scripture (after all, Joshua's command was that the sun stand still in the sky, not that the Earth stop rotating).
Overall, Ptolemy's system was popular, precise, and plausible, and, in the words of astronomer Carl Sagan it "effectively prevented the advance of astronomy for 1,500 years."
Until Copernicus came along, that is. The Polish polymath was far from being a secular iconoclast who set out to undermine scriptural authority. Indeed, as the Monitor's Steph Solis points out, Copernicus, who took holy orders, remained in good standing with the Catholic church throughout his life. What bothered Copernicus the most about Ptolemy's theory was its ad hoc addition of the equant point, and, most of all, its failure to have every planet moving in an Aristotelean uniform circular motion.
In a short manuscript he distributed in 1514, Copernicus called Ptolemy's system "neither sufficiently absolute nor sufficiently pleasing to the mind."
Copernicus's alternative system proposed that the Earth was a planet, and that all planets moved in perfect circles around the sun, which was close to the center of the universe. The Earth also rotates daily, which accounts for the apparent motion of the sun. He correctly noted that the apparent loop-de-loop of the other planets occurs when our planet overtakes another one on its trip around the sun.
But Copernicus's theory was no simpler than Ptolemy's. Indeed, in his quest to remove the equant point and to restore those perfect Aristotelean circles, Copernicus was forced to have every planet moving along two additional epicycles. What's more, his system was no more accurate than Ptolemy's, in part because Copernicus relied on Ptolemy's 1,500-year-old dataset.
But despite these shortcomings, evidence gradually mounted for a sun-centered universe. In 1609, just as Galileo was pointing his new device, the telescope, at the sky, the German mathematician Johannes Kepler published his first two laws of planetary motion.
Kepler's first law did away with those perfect Aristotelean circles, replacing them with seemingly messy ellipses. But Kepler's second law uncovered a mathematical regularity that was far more sublime than anything Aristotle had imagined. Observing that planets move faster when they are closer to the sun, and slower when they are farther away, Kepler determined that a planet sweeps out an equal area over an equal time. Kepler's discovery was among the first of what would come to be known as laws of physics.
By 1687, when Isaac Newton reformulated Kepler's laws in his "Philosophiæ Naturalis Principia Mathematica," science was ready to abandon the Aristotelean notion that the stars and planets consisted of a unique substance that followed its own rules, and the quasi-religious idea that the universe must always act in harmony with human aesthetics. By uniting the motions of heaven and earth under the principles of gravity and inertia, Newton brought the Copernican Revolution full circle.