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Aristotle (384 BC-322 BC) was a Greek polymath, one of the most imaginative and systematic thinkers in history, whose writings embraced virtually every aspect of contemporary thought, including cosmology. Aristotle was born at Stagirus, a port on the Chalcidic peninsula of Macedonia, in 384 BC. His father, Nichomachus, was court physician to Amyntas III (sometimes called Amyntas II), King of Macedonia, and it seems probable that he introduced Aristotle to the body of medical and biological knowledge at an early age. Nichomachus died in Aristotle' s youth and Aristotle was placed in the care of a ward, who sent him to Athens in 367 BC to study at Plato's Academy. Plato's death in 348/347 BC coincided with a wave of anti-Macedonian fervour in Athens, a combination of events which induced Aristotle to leave the city and go on an extensive tour of Asia Minor, where for the first time he engaged in a serious study of natural history. In 342 BC King Philip II invited Aristotle to the Macedonian court to become tutor to the crown prince, the future Alexander the Great. Shortly after Alexander came to the throne in 336 BC, Aristotle returned to Athens, where he established his own school, the Lyceum (known also as the Peripatetic School from Aristotle's habit of lecturing while walking in the garden) in 335 BC. At the Lyceum Aristotle established a zoo (stocked with animals captured during Alexander's Asian campaigns) and a library. The latter formed the basis of the great library established in Alexandria by the Ptolemies. The death of Alexander in 323 BC and another upsurge of anti-Macedonian sentiment in Athens suddenly made Aristotle's position uncomfortable. Largely because of his association with Antipater, the Macedonian regent and general, Aristotle was politically suspect. He was charged with impiety and, rather than suffer the fate of Socrates, he withdrew to Chalcis (now Khalkis), north of Athens, where he died in 322 BC. Aristotle's writings, which have come down to us only in later, edited versions of his notes, lectures and publications, cover philosophy, logic, politics, physics, biology and cosmology. Among his many scientific and philosophical treatises are the Organon, a collection of treatises on logic; the Physica, on natural science; the Historia animalium, a classification of animals; and De incessu animalium, on the progression of animals. His major writings on cosmology, or astronomy, are brought together in the four-volume text, De caelo ('Of 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, adopted from the work of Eudoxus (c. 460 BC - c. 370 BC) and Callippus (c. 370 BC-c. 300 BC), 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 cosmos-geocentric and broadly speaking mechanical, not dynamic- differed only in details from the model proposed by Eudoxus and Callippus. Callippus posited 33 spheres; Aristotle added 22 new spheres, then amalgamated some of them, to reach a total of 49. This clumsy model, which was unable to account even for eclipses, was partly replaced by the Ptolemaic system based on epicycles. In Aristotle' s system the outermost sphere contained the fixed stars. Then followed the spheres of Saturn, Jupiter, Mars, the Sun, Venus, Mercury and, closest to the Earth, the Moon. Each of these had several spheres in order to account for all their movements. The outermost sphere was controlled by divine influence and indirectly it determined the movement of all the inner spheres. The original motive power of the Universe was thus removed from the centre, where the Pythagoreans had placed it. According to Aristotle's laws of motion, bodies moved upwards or downwards in straight lines. Of Empedocles' four natural elements in the Universe, earth and water fell, air and fire rose. To explain the motion of the heavenly spheres, therefore, Aristotle introduced a fifth element, ether, whose natural movement was circular. Aristotle thus posited that the laws of motions governing the celestial bodies above the Moon were different from the laws which governed bodies beneath the Moon. 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 traveled north or south, the stars changed their positions. Since it was not necessary to travel very far to observe this effect, it was clear that the Earth was a sphere, and a rather small one at that. As a result Aristotle was able to make a tolerably fair estimate of the Earth's diameter, overestimating it by only 50%. Author not available, Aristotle (384 BC-322 BC). , The Hutchinson Dictionary of Scientific Biography, 01-01-1998. |
