We all know that the Earth orbits around the Sun in a cycle that lasts 365 days, or one year. We also know that the Sun is part of a larger system called the Milky Way Galaxy and that our galaxy is part of an even greater system called the Universe. But back in the times of the classic Greek period, the early astronomers did not have advanced observatories like Kit Peak or our Hubble Space Telescope that could show us images or proof of the order of the Universe. They had to rely on their ground based observations that limited their view. By these observations they concluded that the Earth remained motionless and stable while all of the heavens, including the Sun, rotated around them. This idea of an Earth centered universe is called a Geocentric model, which originated around the fourth century BC. The famous student of Plato and philosopher, Aristotle supported this model in his teachings and studies.
Another philosopher named Anaximander was said to have thought that the Earth itself was a celestial object. This means that he pictured the Earth as just another part of the heavens that orbited the center of the cosmos. This idea was very radical for the time period, but was later taken up and studied by astronomers in the age of the scientific revolution, which is another interesting historical period of Europe that began in the 16th century A.D.
Thales, discussed on the Sun page, theorized that the Earth rested on water, while Anaximander believed that it needed nothing to support it. Still another philosopher named Anaximenes envisioned the Earth as a flat disk that floated on the air beneath it.
Around 200 B.C., Eratosthenes calculated the size of the Earth within 1% accuracy. What a phenomenal project! He accomplished this by using simple geometry to calculate the circumference and the radius of the Earth. It is believed that he used the same principles to calculate the size of the Sun and Moon as well. We still use his geometrical reasoning to measure distances outside of our Solar System.
Socrates described the Earth as situated under the heavens and that a substance called aether surrounded the Earth and the rest of the universe. The aether (an idea perfected by Aristotle) was believed to be made up of small, invisible particles that enveloped the heavens, much like the air around us takes up available space. This idea is not particularly unreasonable. In fact, the idea of aether was accepted all the way up to the times of Einstein in the 18th century A.D. when an experiment named the "Michalson-Morely" finally disproved its existence. Socrates also wrote of the Earth as being a twelve-panneled sphere of different colors that each served a different purpose to sustain life. Socrates passed the idea of the Earth as a sphere to his student Plato. Though Plato never directly documented his idea that the Earth existed as a sphere, his writings implied it. We are certain that Plato's student, Aristotle, accepted the concept of a spherical Earth, resting immovably at the canter of all the heaven which moved around the Earth. He tried to prove that the Earth was itself stationary by applying his observations to his "doctrine of natural motion".
Aristotle was also the first to offer proof that the Earth was indead round. Others before him had theorized the shperical or round shape of the Earth, but Aristotle used the scientific method (observation, theory, and testing) to prove his idea. While observing a lunar eclise (when the Earth casts its shadow on the moon) he noticed that the shadow of the Earth was curved. He theorized that this would happen durring all lunar eclipses and cuncluded that the Earth must be round in order to have produced the curved shadow.