He backed up the theory with empirical observation and was the first person to prove that the earth was indeed round. He observed a lunar eclipse (when the Earth casts its shadow on the moon) and noticed that the shadow of the earth on the moon was curved. As only a round object could cast a curved shadow, it could be inferred that the earth was round. (Fowler)
Aristotle theorized that not only was the earth the center of the universe, it was stationary in contrast to other planets. He "proved" this theory by stating that if the earth was moving, an observer on it would see the fixed stars as moving, just as he can see the planets moving. Since this is not the case, Aristotle deducted that the earth must be at rest. This theory about the earth being stationary and the center of the universe remained an accepted fact for centuries before Galileo and Copernicus proved it wrong through more scientific observation.
In Astronomy, Aristotle accepted the theory prevalent at the time that there were four basic elements -- earth, air, fire and water and proposed that the central region of the universe was composed of these four elements. He, however, added the theory that the celestial bodies (the heavens) were made of a fifth element called aether which was eternal and unalterable. It was found in the purest form in the outermost (celestial) regions but was contaminated in the region below the moon.
Aristotle's theory of motion is rather complicated and has been interpreted in different ways. He used this theory to explain the movement of objects on the earth and beyond. According to Aristotle, the universe consists of two distinct worlds -- the sub-lunar which was always changing and in which objects moved in rectilinear motion and the super-lunar, the world of the unchanging celestial eternal bodies (made of aether) that moved in a circular motion. He further maintained that the four elements of the sub-lunar world tend to move in straight lines but the direction of their movement would depend on their specific gravity, i.e., the earth being heavy would move downward, fire upward, and water and air would hang in between.
Aristotle also believed that the objects in the universe including the celestial bodies could not have started moving by themselves and had to be set in motion by something else. He, therefore, introduced the concept of a "prime mover" -- an outside force purportedly placed behind the "fixed" stars which caused the first motion. The prime mover was a transcendent and eternal object without magnitude that caused circular movement -- the most perfect movement without a beginning or an end.
Aristotle's concept of the "prime mover" was accepted in theology and used to explain various religious concepts. Christian theologist Saint Thomas Aquinas re-interpreted the prime movers as angels while the Muslim theologists who studied Aristotle interpreted the concept "prime mover" as the eternal power (God) that created the heavens and the earth. Such religious sanction of Aristotle's astronomy contributed to its longevity and it remained unchallenged as the accepted model for a long time.
Aristotle." Article in Microsoft Encyclopedia Encarta. Online Version. 2004. October 10, 2004. http://encarta.msn.com/encyclopedia_761557129/Aristotle.html
Aristotle." From Wikipedia, the Free Online Encyclopedia. 2004. October 10, 2004. http://en.wikipedia.org/wiki/Aristotle
Fowler, Thomas. "Aristotle's Astronomy." Paper Written for a Greek Science Course at Tuft University, Spring 1995. October 10, 2004. http://www.perseus.tufts.edu/GreekScience/Students/Tom/AristotleAstro.html
O'Connor, J.J. And E.F. Robertson. "Greek Astronomy." University of St. Andrews Website. April 1996. October 10, 2004. http://www-history.mcs.st-andrews.ac.uk/HistTopics/Greek_astronomy.html
The others being Socrates and Plato.
Others before him (including Plato) had theorized about the roundness of the earth but had not been able to offer any proof for the theory.
Aristotle was also the first person to use a simple device called "camera obscura" to observe the sun indirectly. The 'device' was a screen with a small hole placed between the sun and the ground that created an image of the sun on the ground. It is still used to observe solar eclipses.