The belief of early astronomers that the Earth was the center of the universe stemmed from limited astronomical tools and geocentric attitudes. The Ptolemaic Model, developed around 100 A.D., presented the Earth-centered solar system in which most early Roman astronomers believed.
In the early 1500s, Nicolaus Copernicus published the first major theoretical work that challenged the notion of an Earth-centered universe. Copernicus theorized a sun-centered universe, or a heliocentric solar system. In 1610, Galileo Galilei was able to use the first telescope to study the solar system. He strengthened the argument for a heliocentric system by noting changing levels of brightness during the rotations of certain planets as well as the Earth's moon and the sun.