5.3 Aristotle's cosmology and astronomy

Aristotle's cosmology placed Earth at the universe's center, with celestial bodies revolving around it. He divided the cosmos into two realms: the changeable sublunary realm and the perfect superlunary realm, each with distinct characteristics and elements.

Celestial objects were thought to be made of aether, moving in perfect circles. Aristotle's model explained phenomena like planetary retrograde motion through complex systems, influencing astronomical thought for centuries despite its eventual disproval.

Aristotelian Cosmology

Geocentric Model and Realms of the Universe

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• Geocentric model placed Earth at the center of the universe
• Earth remained stationary while celestial bodies revolved around it
• Sublunary realm encompassed the area below the Moon's orbit
  • Consisted of four elements: earth, water, air, and fire
  • Subject to change, generation, and corruption
• Superlunary realm extended beyond the Moon's orbit
  • Composed of a fifth element called aether
  • Characterized by perfection and unchanging nature

Structure and Boundaries of the Cosmos

• Finite universe with a definite boundary
• Spherical shape considered the most perfect form for the cosmos
• Outermost sphere contained the fixed stars
• Nested spheres within held the planets, Sun, and Moon
• No concept of empty space or void beyond the celestial spheres

Celestial Objects and Motion

Composition and Nature of Celestial Bodies

• Celestial spheres carried planets, stars, and other heavenly bodies
• Aether (quintessence) composed all celestial objects and spheres
  • Transparent, weightless, and incorruptible substance
  • Explained the perceived perfection of heavenly bodies
• Perfect circular motion characterized movement in the superlunary realm
  • Reflected the divine nature of celestial objects
  • Contrasted with the imperfect linear motion in the sublunary realm

Explaining Celestial Phenomena

• Planetary retrograde motion puzzled ancient astronomers
  • Appeared as backward movement of planets against the fixed stars
  • Explained through complex system of epicycles and deferents
• Crystalline spheres concept developed to support the celestial sphere model
  • Invisible, solid spheres carrying celestial bodies
  • Explained the fixed distances between celestial objects
  • Accounted for the transmission of motion from outer to inner spheres