Physics

• Geocentric model


  The astronomical model with Earth at the center of the Universe. The Sun, Moon, stars, and planets all orbit Earth
• Heliocentric model


  The astronomical model in which the Earth and planets revolve around the Sun at the center of the Universe
• Supporters of the Geocentric Model

  • Eudoxus
  • Aristotle
  • Ptolemy
• Supporters of the Heliocentric Model

  • Aristarchus
  • Copernicus
• Eudoxus


  • Created the first model of a geocentric universe (380 B.C.)
  • Series of cosmic spheres (stars, the sun, and the moon)
  • Daily motions of the celestial objects
  • Retrograde motion of the planets
• Eudoxus' model


  1. 27 Spheres: 1 for the fixed stars, 3 each for the Sun and Moon, 4 each for the 5 planets (Mercury, Venus, Mars, Jupiter, and Saturn)

  2. Spheres within spheres in perfect circular motion combine to give retrograde motions

  3. The rotation of the spheres on their poles once every 24 hours accounts for the daily rotation of the heavens
• Aristotle

  • Considered the model proposed by Eudoxus (spheres as physical entities)
  • Thought the spheres were filled with the divine and eternal "ether" that caused the spheres to move
  • Introduced the Prime Mover, as the cause of the movement of the spheres
  • His model was composed of 56 spheres that guided the motion of the Sun, the Moon, and five known planets
• Ptolemy


  • Proposed his own geocentric (Earth-centered) model of the universe
  • Accounted for the apparent motions of the planets around the Earth by assuming that each planet moved around a sphere called an epicycle
  • The center of the epicycle then moved on a larger sphere called a deferent
• Deferent


  Refers to a circular path in which the planet moves around the Earth
• Epicycle


  Pertains to a smaller circle along which a planet moves
• As seen from Earth, planet moves westward (retrograde motion)


  As seen from Earth, planet moves eastward (direct motion)
• The Sun, Moon, stars, and planets were believed to move in a uniform circular motion - the "perfect" motion
• Aristarchus


  • Hypothesized that the Sun is the center of the universe
  • Visualized that the Moon orbits around a spherical Earth which then revolves around the Sun
  • Believed that the stars are very far away from the Earth
• Aristarchus concluded that the Sun is 20 times farther from the Earth than the Moon is to the Earth; the Earth is about three times larger than the Moon, and the Sun is 20 times larger than the Moon
• Copernicus


  • Proposed that the Sun, not the Earth, is the center of the universe
  • Concept - the motion of spherical heavenly bodies is uniform, eternal, and circular
  • Earth is spherical= motion is circular
  • Earth has three different motions: daily rotation on its axis, yearly motion around the Sun, and the precession, or change in orientation, of its axis every 26 000 years
  • Proposed that the fixed stars are immovable and at immeasurable distances from the Earth
  • Placed the Sun at the center of the universe and the orbits of Mercury and Venus in between the Sun and the Earth, eliminating the need for epicycles