Before the 16th century, the common belief was that the sun and all the other planets revolved around the Earth. This theory had been developed by the ancient Greek astronomer Ptolemy around A.D. 150, but the Polish astronomer Copernicus found that the Ptolemaic system could not explain the observed motions of the planets.
In 1543, he published a book calledThe Revolutions of the Heavenly Orbs, which argued that the Earth was not the center of the universe but was one of the planets and, like them, revolved around the sun. This model of the solar system is known as the Copernican system.
Copernicus also said that the Earth spins on its axis once per day, which accounts for why the sky appears to revolve around the Earth.
Copernicus’s theory explained why superior planets—those further from the sun than Earth—sometimes appear to be moving backward (inretrogrademotion) with respect to the stars, while those closest to the sun—Mercury and Venus—always appear to move in only one direction. The Earth is moving in a faster orbit around the sun than the more distant planets—periodically “passing” them, making them appear to move backward. But Mercury and Venus are moving in even faster orbits closer to the sun. Copernicus was right about the basic arrangement of the solar system, but he had no real proof.