Aristotle

Aristotle was a Greek philosopher who believed that celestial bodies are spheres. Aristotle used several methods to prove that the Earth is a sphere. One was the observation that during a lunar eclipse, the Earth's shadow on the Moon is always a circle. Also, upon seeing the Moon pass in front of Mars, Aristotle concluded that Mars is higher up in the heavens than the Moon. He developed many physical works which had a bearing on the study of astronomy, including the physics which explained change, motion, void and time. Aristotle was born in Stagira in Thrace and studied in Athens, where he became a distinguished member of the Academy founded by Plato. While there, he opened a school at Assos. At this time he regarded himself as a Platonist, but his subsequent thought led him further from the traditions that had formed his early background and he was later critical of Plato. In about 344 BC he moved to Mytilene in Lesvos, and devoted the next two years to the study of natural history. Meanwhile, during his residence at Assos, he had married Pythias, niece and adopted daughter of Hermeias, ruler of Atarneus. His major writings on astronomy are brought together in the four-volume Peri ouranou/On the Heavens. Aristotle rejected the notion of infinity and the notion of a vacuum. A vacuum he held to be impossible because an object moving in it would meet no resistance and would therefore attain infinite velocity. Space could not be infinite, because in Aristotle's view, the universe consisted of a series of concentric spheres which rotated around the centrally placed, stationary Earth. If the outermost sphere were an infinite distance from the Earth, it would be unable to complete its rotation within a finite period of time, in particular within the 24-hour period in which the stars, fixed, as Aristotle believed, to the sphere, rotated around the Earth. Aristotle's work in astronomy also included proving that the Earth was spherical. He observed that the Earth cast a circular shadow on the Moon during an eclipse and he pointed out that as one travelled north or south, the stars changed their positions. Aristotle overestimated the Earth's diameter by only 50%.