Chapter 6.

Aristotelian Science and Cosmology

6.1. Socrates of Athens (470 - 399 b.c.)

• Ex stone cutter turned philosopher
• Represents a watershed in Greek philosophy; turned philosophical discussion from cosmological questions of the 5th and 6th century to questions of political and ethical concerns

6.2. Plato of Athens (428 - 348 B.C.)

• Two-sphere concept, Earth as one sphere, surrounded by second set of spheres - the celestial realm
• Set task, according to Simplicius, of finding out "the uniform and ordered motions by the assumption of which the apparent movements of the planets can be accounted for"
• Aesthetic argument based on symmetry; apparently reasoned that celestial realm was arrayed on perfect geometric figure, the sphere, with planets moving in perfectly symmetric plane figure, the circle
• Generations of astronomers thought that planetary movements must be accounted for by combinations of uniform circular motions with Earth at center
• The ultimate product of geocentric cosmology was the Ptolemaic system.

6.3. Eudoxus of Cnidus (ca. 400 - ca. 347 B.C.)

• Associate of Plato's Academy; among ancient astronomers and mathematicians, few surpass Eudoxus in creativity
• Explained motions of Sun, Moon, and planets by means of coupled concentric spheres with adjustable radii, rates of rotation (but not variable, must be uniform), and inclination to each other - Eudoxian spheres
• Used 4 coupled concentric spheres for each planet
• Used 3 coupled concentric spheres for Sun and Moon
• Number of spheres to represent motion of various objects: 1 for stars, 3 for Sun, 3 for Moon, 20 for five naked-eye planets (4 for each), for total of 27
• Reproduced retrograde motion; however, planets at constant distance from Earth and thus no variation in brightness
• Devised a computational device to account for complex motions of planets; even had predictive capability.
• Homocentric spheres is superior mathematical construction; however, is that the same as being "real"?

6.4. Aristotle (384 - 322 B.C.)

• Adopted basic Eudoxian system as modified by Callippus (ca. 370 - ca. 300 B.C.)
• Major change, reasoned that system was physically real; not obvious that earlier Greek natural philosophers believed it to be physical, but only mathematical convenience
• In treatise, Physics, he explained that celestial motions must be circular because they are closed figures and can be eternal
• Made sharp distinction between terrestrial realm which was corruptible, and celestial realm which was incorruptible
• All terrestrial matter composed of four elements: earth, water, air, fire
• Natural terrestrial motion was "like seeking like," such as a rock seeking earth; concept of natural place, earth and water go down, air and fire up; also forced motion
• Celestial motion perfect and caused by Prime Mover acting from outside sphere of stars
• Sphere of stars large compared to terrestrial sphere, but not infinite