Aristotle

Aristotle lived from 384 BCE to 322 BCE, and is considered one of the greatest of all Greek philosophers. Aristotle was a student of Plato, and a teacher of Alexander the Great. As a philosopher, he made many contributions to a variety of subjects.Aristotle's contribution to astronomy lies in his codification and extension of previous theories and speculations about the movements of celestial bodies. Like his teacher Plato, Aristotle believed that the Earth was the center of the Solar System, and by extension, the Universe (as understood by him and others at that time).One argument made by Aristotle was that the Earth was too big to move, so it must be the object around which all other bodies move. Simple visual observations seem to back up this notion. Another argument he made was that if the Earth was moving very fast, there should be enormous winds, which there were not.Aristotle was correct in his surmise that the Earth was a sphere. He pointed to the shadow cast by the Earth on the Moon during a lunar eclipse as proof of that speculation. Eratosthenes would later try to determine the radius of the Earth.A more perfidious notion espoused by Aristotle (and his teacher Plato) was that the Earth was corrupt and the heavens perfect. The farther away an object was from Earth, the more perfect was that object. It "followed" that the more perfect an object, the more perfect must be its motion. Since Plato considered circular motion to be the most perfect, the circular motion of the so-called "fixed" stars indicated that they were far away. By contrast, the wandering movement of the planets, specifically the planets' occasional retrograde motions, indicated that they were closer to the imperfect, corrupt Earth.

For close to two millennia, these notions of Aristotle held sway. The mechanical theories of planetary motion espoused by Ptolemy would not be seriously challenged until the Polish cleric Copernicus published, on his death bed, his astounding treatise "De Revolutionibus Orbium Coelestium".