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The Moving Earth | Nicolaus Copernicus - In 1543, the Polish astronomer Nicolaus Copernicus caused a great controversy when he published a book proposing that the Earth revolved around the Sun.* This idea conflicted with the popular view that the Earth was the center of the universe. Copernicus’s concept of a Sun centered solar system was the result of years of studying the planets. He had kept his theory from the public for two reasons. The first reason was that he feared persecution: 

a theory so completely different from common opinion would surely be taken as an attack on the established order. The second reason was that he had reservations about it himself: he could not reconcile the idea of a moving Earth with the prevailing ideas of motion. The concept of inertia was unknown to him and to others of his time. In the final days of his life, at the urging of close friends, he sent his manuscript, De Revolutionibus Orbium Coelestium, to the printer. The final copy of his famous exposition reached him on the day he died May 24, 1543.

The idea of a moving Earth was much debated. Europeans thought about the universe much as Aristotle had, and the existence of a force big enough to keep the Earth moving was beyond their imagination. They had no concept of inertia. One of the arguments against a moving Earth was the following:

Consider a bird sitting at rest on a branch of a tall tree. On the ground below is a fat, juicy worm. The bird sees the worm and drops vertically below and catches it. It was argued that this would be impossible if the Earth were moving. A moving Earth would have to travel at an enormous speed to circle the Sun in one year. While the bird would be in the air descending from its branch to the ground below, the worm would bswept far away along with the moving Earth. It seemed that catchin a worm on a moving Earth would be an impossible task. The fact that birds do catch worms from tree branches seemed to be clear evi dence that the Earth must be at rest. Can you see the mistake in this argument? You can if you use the concept of inertia. You see, not only is the Earth moving at a great speed, but so are the tree, the branch of the tree, the bird that sits on it, the worm below, and even the air in between. Things in motion remain in motion if no unbalanced forces are acting on them So when the bird drops from the branch, its initial sideways motion remains unchanged. It catches the worm quite unaffected by the motion of its total environment. We live on a moving Earth. If you stand next to a wall and jump up so that your feet are no longer in contact with the floor, does the moving wall slam into you? Why not? It doesn’t because you are also traveling at the same speed, before, during, and after your jump. The speed of the Earth relative to the Sun is not the speed of the wall relative to you. Four hundred years ago, people had difficulty with ideas like these. One reason is that they didn’t yet travel in high-speed vehicles. Rather, they experienced slow, bumpy rides in horse-drawn carts. People were less aware of the effects of inertia. Today, we can flip a coin in a high-speed car, bus, or plane and catch the vertically moving coin as easily as we could if the vehicle were at rest. We see evidence of the law of inertia when the horizontal motion of the coin before, during, and after the catch is the same. The coin always keeps up with us.