One of the Earth-shaking moments in human history was the Copernican revolution. For millennia, humans had assumed that Earth was at the center of the physical world. Above it rotated a set of crystalline spheres. Each sphere contained a “planet.” Above the planetary sphere was the fixed, starry sphere, which rotated at its own speed.
Our ancient forebears defined the term “planet” differently than we do. They included the sun and the moon because they rotated at a different speed than the sphere of stars above them. The planetary spheres rotated at various speeds to justify their differing motions.
As propounded by Aristotle in the 4th century BCE, those motions had to be perfect. The planets had to revolve in perfect circles if they were embedded in perfect spheres. As the spheres dragged the planets across the sky, they had to be uniform in their rotational speed. Our forebears could not abide the notion of any speeding up or slowing down.
The ancients were looking for perfection, but they didn’t get it. As astronomers got better at plotting the planets, they discovered that the moon’s and sun’s speed did not seem to vary. So far, so good.
The other planets were a different story. They all seemed to slow down, reverse their direction, and then speed up to resume their original course and speed. What accounted for that strange phenomenon, which came to be called retrograde motion?
By the second century CE, astronomers began to create increasingly complex systems to account for the planets’ retrograde motion. To retain the notion of uniform, circular motion, the astronomer Ptolemy created his system of epicycles, wheels within wheels. The planets still moved around the Earth. As they did, they traced a smaller circle around a point in their orbit.
But if they did so, they could not be embedded in a crystalline sphere. To keep the Aristotelian notion of uniform, circular motion, the spheres had to go.
Ptolemy had shattered the spheres, and the notion of the orbit was born. The spheres no longer rotated. The planets revolved around Earth.
However, by the 16th century CE, astronomers like Tycho Brahe had become increasingly adept at plotting the positions of the planets.
Astronomers tend to evaluate the success of a model by seeing if it produces predictable results. Even with the addition of epicycles, astronomers could not accurately predict the planets’ exact position using an Earth-centered model.
To account for the actual positions of the planets, epicycles had to be added on top of epicycles — wheels within wheels within wheels — until the whole mess became ridiculously complicated and unbelievable.
And that’s when Copernicus entered the fray. In 1543, he simply suggested that the planets, Earth now included, orbited the sun.
But the case was not closed. Copernicus’s planetary orbits did not deviate from Aristotle’s uniform, circular motion. In the Copernican system, the planets orbited in circles at unvarying speeds. As a result, the Copernican sun-centered model was no better at predicting the planets’ positions than the old Earth-centered model.
An excruciating 76 years passed before Johannes Kepler saved the day for the Copernican sun-centered model. Using the planetary data generated by Tycho Brahe, he determined that the planets orbited the sun in stretched-out orbits called ellipses.
Stretch out a circle into an ellipse, and you’ll end up with two “centers” to the original center’s left and right. Kepler placed the planets at one of those two points. Consequently, a given planet orbits at varying distances from the sun. As it gets closer to the sun, it speeds up. As it moves away from the sun, it slows down.
The orbits were neither circular nor uniform in their speed. Kepler’s new system predicted the positions of the planet with great accuracy. Aristotle’s Earth-centered model was finally dead.
However, the motion of the planets accounts for only half of the Copernican revolution.
Something had to be done about the strange rotation of the starry sphere.
If you stay up all night, you will inevitably notice that the stars move across the sky from east to west, just like the sun. Go back at the same time the next night, and a given star will be roughly in the same location. The starry sphere appears to rotate once every day.
And from that starry motion arose the often neglected second part of the Copernican revolution. Copernicus resurrected an old idea — that Earth, not the starry sphere, was rotating on its axis once every day.
If Earth’s rotation seems obvious, consider that most people, including me, talk and act like the Earth is not rotating. We react like astronomical objects are moving.
Tonight, I will go out and watch the sunset. I will be intellectually aware that the sun is not actually moving. I will still think, “The sun is setting” and not “Earth is rotating.”
Still, except for a few internet crazies, the idea of Earth’s rotation doesn’t sound radical to us. But our ancient forbears would have scoffed. To rotate once every day, Earth has to be zipping along at about 1,000 miles per hour at the equator. At 40 degrees latitude, where we are, our planet is “only” rotating at about 670 miles per hour.
Wouldn’t a spinning Earth cause objects to fly off unto space? Wouldn’t such fast motion create high winds and knock us right off our feet?
Besides, if Earth is rotating that fast, wouldn’t a dropped object land at a different location than straight down? If you were on a high mast of a ship and dropped a hammer, wouldn’t the hammer fall into the sea?
Despite those reservations, Copernicus suggested that the Earth was rotating. Because of those reservations, people did not widely accept the idea until the mid-17th century, 100 years after he proposed it in 1543.
In fact, Earth’s rotation had been proposed almost two millennia earlier by the Greek philosophers Heraclides Ponticus and Hicetas of Syracuse in the fourth century BCE.
But the Aristotelian model of an unmoving Earth dominated so completely that the idea died until the early Middle Ages when it was briefly revived by Arabic, Persian, and Indian astronomers.
Notable among them was an Indian astronomer Aryabhata, who wrote his about the issue around 500 CE. In his book called the Aryabhatiya, he argued that the stars were stationary. Their apparent motion from east to west was an illusion created by Earth rotation in the opposite direction.
While he was at it, Aryabhata solved the conundrum of Earth’s position in the solar system by ignoring it. He is the father of the cyclic system for predicting the positions of the planets.
Observe the planets’ motions for long enough, and you will eventually see the complex patterns of their motions. It doesn’t matter what’s at the center so long as you observe long enough to see the cyclic patterns. As any diehard stargazer will tell you, observe the planets’ motions for a few decades, and the repeated cycles will become evident.
But I digress. The model of the spinning Earth still has to answer the questions asked by the skeptics.
As with many astronomical questions, the answer came with Isaac Newton’s notion of gravity. I always gave the following sage advice to the kids at Perkins Observatory field trips: If they didn’t know the answer to a question on an astronomy test, they should answer “gravity.” Gravity is the answer to half the questions in astronomy.
Why don’t objects fly off into space? Gravity holds them to the surface.
Why doesn’t Earth’s rapid spin create fierce, unabating winds? Earth’s gravity holds the air in place the same way it holds a loose grain of sand to the ground. To put it another way, gravity means that the atmosphere is as much a part of Earth as you are.
And why does the hammer dropped from a high ship’s mast fall straight down? Grav … No, wait. You got me on that one. The answer is the conservation of angular momentum.
Let’s say the ship is moving at 20 knots toward the east. The sailor at the top of the mast is moving at 20 knots. But so is the hammer she’s holding. When she drops the hammer, she, the ship, and the hammer are all moving at 20 knots. The hammer drops straight down because everything is moving at the same speed in the same direction.
Of course, those unalterable laws of physics will not stop anyone from saying that the moon is setting in the west. Nobody is going to say, “Look, the moon is stationary. Can you feel the Earth move from west to east as the sun appears to set?”
Well, nobody but a diehard astronerd like me, anyway. See enough moonsets, and you will feel the Earth move as well.
Tom Burns is the former director of the Perkins Observatory in Delaware.