For centuries it was believed that the Earth was the center of the universe.

This belief, known as the geocentric model, held that the Earth was stationary while the sky revolved above it on a set of celestial spheres.

Aristotle eloquently argued how the Earth must be stationary and thus the center of the universe because you could not feel the Earth move;
there was no wind due to a moving Earth, hence it must be stationary; the birds and the clouds would be left behind if the Earth were moving; and if the Earth were moving the stars would display a parallax effect.

This was to be the common belief for over 1800 years, not only because of the persuasive arguments of Aristotle, but also because a feature of the Greek philosophy was that the universe was perfect. So, they believed that the celestial spheres the stars moved on were perfect, and everything in the sky was perfect.

Later, as the Catholic Church rose to prominence, this would be interpreted as a philosophy of a perfect sky as an indication of a perfect God. Therefore, any philosophy that did not agree with the geocentric belief was considered heretical. Anyone professing such beliefs risked facing the wrath of the Church. This understandably made people reluctant to present new ideas.

In 1543, Nicolaus Copernicus published Concerning the Revolutions of Celestial Spheres. He was very elderly, and in fact received the first copy of his book the day he died. This undoubtedly played a role in his decision to publish, because his book presented the then-controversial idea of what we call the heliocentric model.

What Copernicus claimed was that the Sun, not the Earth, was the center of the universe, and the Earth, stars, and planets all revolved about the Sun. It is this book that marks the beginning of modern astronomy and modern science. Because of the enormous importance of the work, the heliocentric model is commonly called the Copernican System.

Copernicus' work was initially rejected and his book was banned by the Church. But the damage had been done, and Johannes Kepler devoted himself to the task of mathematically describing the orbits of the planets about the Sun. He was able to solve this problem by 1619 with what is now known as Kepler's Three Laws of Planetary Motion.

Galileo declared in the early 1600s that he believed in the Copernican System, but he didn't have the necessary tools to prove it. However, by the end of 1610, Galileo had access to a new tool to prove his beliefs: the telescope. In that year, or early 1611, Galileo became the first person to observe the heavens through a telescope.

He immediately made several amazing discoveries, including mountains on the Moon and moons orbiting Jupiter (now known as the Galilean moons), and also discovered that the Milky Way was made of thousands of small stars and Venus went through a whole set of phases like the Moon. He presented his findings in a book called Sidereus Nuncias (The Messenger of the Stars) in 1611.

His beliefs would eventually bring him before the Inquisition, where he was forced to recant his beliefs before the court and later placed under house arrest, although it was not rigorously enforced.

But before long, the popularity of the Copernican System grew until it was the accepted model. Does this mean that it was no longer believed that we were the center of the universe? No. The belief was that the Sun, not the Earth, was at the center of the universe.

So we were still focused on the idea that there was something special about us. We had simply moved the universal center from our planet to our star. One of the reasons for this is because astronomers saw basically the same numbers of stars in all directions. If we were not the center, they reasoned, then we would see more stars in one direction than another. Since this was not the case, it must be because we were at the center.

By the latter part of the nineteenth century, a number of star groups called globular clusters had been identified. Globular clusters are large, spherical groups of stars, numbering in the hundreds of thousands, or even millions of stars, and orbit the Milky Way.

Distances to these globular clusters could be calculated, and the astronomer Harlow Shapley reasoned that if we were at the center of the universe, there should be an even distribution of globular clusters in all directions. However, when he plotted the locations of the globular clusters, he found we were not at the center of the Milky Way, and he was able to make a good estimate of the distance to the true center.

Then it was established that the Sun was not the center of the universe, but it was still thought that the center of the Milky Way was the center of the universe. It was not until the twentieth century that it was finally established that neither we nor our galaxy formed the center of the universe.