In his study of the heavens, the ancient Greek philosopher Aristotle placed Earth at the center of the universe. Since then, his geocentric model became the official doctrine of the Roman Catholic Church. Most scientists at that time used a geocentric model to explain a planet’s apparent backward, or retrograde, motion. However, a geocentric view made retrograde motion extremely complicated to explain. In fact, it was so difficult to follow that some people wondered whether something this complicated could actually occur naturally.
Astronomer Nicolas Copernicus found a simpler way to explain the movement of planets and other objects in the night sky. His model placed the Sun, not the Earth, at the center of the universe. Unfortunately, his heliocentric model not only contradicted the beliefs of most scientists of his day, it also went against the teachings of the Catholic Church.
Copernicus was well aware that his ideas could bring him into conflict with church leaders. He was trained in theology, or the study of religious belief.
Copernicus was born in 1473 in a part of Europe that is now the country of Poland. He was sent to school to prepare for a career in canon law, or the laws used to govern the Church. Although he did complete his studies in canon law, he never became a Catholic priest.
As part of his education, Copernicus learned about what was then called “the science of stars.” This science consisted of what people now consider two fields of study: astronomy and astrology. Astronomy is the study of objects in space such as planets, moons, and stars. Astrology, in contrast, studied the movement of these objects to predict upcoming events. Today, astrology is no longer considered a science. Its predictions cannot be tested, and its core ideas lack evidence. However, in Copernicus’s day, a scientist of the stars would be expected to provide astrological forecasts.
Instead of the priesthood, Copernicus worked as a secretary and physician to his uncle in Poland. His living quarters were located in a tower that contained an observatory. In his spare time, Copernicus took the opportunity to study the night sky.
Aristotle’s geocentric model of the universe remained dominant in Copernicus’s day, even though it had been developed hundreds of years earlier. Although scientists now know his ideas are not true, it is easy to see why Aristotle concluded that Earth did not move. If Earth moves, Aristotle reasoned, then there must be signs that it is moving. He did not see any. For example, there was no wind that continuously traveled across Earth’s surface. Aristotle also noticed that when people threw a stone into the air, it traveled straight up and landed near the thrower. If Earth moved, Aristotle reasoned, a stone tossed into the air would actually land a distance
away from the thrower. Thus, Aristotle reasoned that planets move around a non-moving Earth within invisible spheres at fixed distances. This conclusion placed Earth at the center of the universe.
However, even in Copernicus’s day, scientists noticed that Aristotle’s geocentric model did not explain everything that happened in the night sky. They noticed, for instance, that planets could be more or less bright on different nights. If it was true that the planets traveled at fixed distances from Earth, as Aristotle maintained, their brightness should not vary. There was also the problem of retrograde motion. Aristotle’s model did not allow for planets to move backwards, which they indeed seemed to do. Scientists needed to find a way to explain these irregularities in a way that did not also conflict with Church doctrine.
Scientists turned to the ancient Egyptian astronomer Ptolemy. Like Aristotle, Ptolemy’s model of the universe was geocentric. However, Ptolemy addressed the problem of retrograde motion by suggesting that there were actually a number of other circles, called epicycles, contained within a planet’s path. While Ptolemy’s explanation worked, it was very complicated. In some cases, it took as many as seven circles to explain the path of a single planet. It was so complicated that many scientists doubted whether it was possible for such a process to develop in a natural system.
Copernicus’s model was simpler than Ptolemy’s. The Sun, not Earth, was at the center of the universe. While it appears as though the Sun moves by rising and setting daily, its apparent movement is actually because the Earth itself rotates. Earth’s movement also explained the motion of stars across the night sky. While Earth rotated on its own axis, Copernicus explained, it also orbited the Sun. This yearly orbit accounted for Earth’s seasons. In contrast to Ptolemy’s complicated system of epicycles, Copernicus said that the retrograde motion of planets was caused by the movement of Earth itself.
With his religious training, Copernicus knew full well that his ideas conflicted with Church doctrine. When he first shared his heliocentric model in 1514, it was with his friends only. When he eventually finished writing his book On the Revolutions of the Heavenly Spheres in 1532, even then, he chose not to release it. Instead, Copernicus waited until 1543—just two months before he died of a stroke at age 70—to publish it.
Surprisingly, at first, the Church seemed not to be bothered by Copernicus’s book. One reason was likely the fact that Copernicus’s ideas were hard to understand by anyone untrained in mathematics and astronomy. Another reason was that the book’s printer had added a note to the text that stated that Copernicus’s model would help scientists solve equations which described the movements of heavenly bodies, so it did not matter if it was actually true or not.
The Church did eventually ban the book in 1616, 73 years after the astronomer’s death. However, by the 1700s, educators insisted that both the geocentric and heliocentric models be taught in schools. As more professional and amateur astronomers continued to look at the skies, they collected more and more data about the movement of the planets and stars. Time
and time again, their observations were better explained by Copernicus’s heliocentric model. Over time, fewer and fewer scientists believed the geocentric model. The Church finally lifted its ban on Copernicus’s ideas in the early 1800s.
While Copernicus’s ideas revolutionized science, they would also have an enormous impact on how people viewed their place in the universe. In Copernicus’s model, human beings could no longer claim to be at the center of everything. “Of all discoveries and opinions, none may have exerted a greater effect on the human spirit than the doctrine of Copernicus,” wrote German poet Johann Wolfgang von Goethe. “The world has scarcely become known as round and complete in itself when it was asked to waive the tremendous privilege of being the center of the universe.”