1. The geocentric model of the universe made the claim that the Earth was the center of the universe. The other planets, objects in space, and even the Sun would rotate and revolve around the Earth. There were two reasons that people believed that the Earth was at the center of the universe was: they believed that the stars, the Sun, and the planets all appeared to revolve around the Earth each day. In ancient Greece, while some types of telescopes were available, the technology did not exist to allow people to fully understand the workings of space and the functions of the planets and the Sun. The second reason is that people did not believe that the Earth was moving, given how solid and stable it is. From their point of view, watching the night skies and seeing the comets streak past them, the people of ancient Greece quickly believed that the Earth was motionless and that everything was passing by them.
2. The heliocentric model of the universe states that the sun is the center of not only the solar system, but of the universe. The Earth, the rest of the planets, and objects in space revolve and rotate around the Sun. This theory first came to light in the third century BC by Aristarchus. It was not until the sixteenth century that a mathematical model of the heliocentric model was created, allowing Nicolaus Copernicus to redefine the heliocentric model. This model was later elaborated upon and expanded by Johannes Kepler and Galileo Galilei. Once Greek philosophers realized that the Earth was round, they began to figure out the rest of the workings of the universe. Once they figured out the shape of the Earth, they attempted to understand how big it is; once this one day, they discovered that the Sun was bigger than the Earth, thus only making sense that small Earth would revolve around the big Sun.