Theories of Planetary Motion

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Theories of Planetary Motion

• Part 1 Geocentric Theory

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Why should they study Astronomy

• One main reason
• Survival
• Prepare for climate changes
• Know when to plant crops
• Know when to harvest crops

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Geocentric Model

• Plato 400 BC
• Created idea of a Celestial Sphere
• Sun, Moon, Planets on separate spheres (26
  total)
• Earth is center of all spheres (center of
  universe)

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Geocentric Model

• Plato (cont.)
• (Geocentric Acceptance)
• Earth wasnt part of the heavens
• Earth wasnt bright
• Little change is the sky night after night
• Our senses say we arent moving (no wind, objects
  not left behind, nothing flies off)

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Geocentric Model

• Aristotle 300 BC
• Supported Geocentric Model
• Added 29 more Celestial Spheres

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Geocentric Model

• Aristarchus 300 BC
• Student of Plato proposed idea of the Sun as
  center of universe.
• Not accepted because
• It violated common sense
• Went against religious beliefs
• He couldnt explain lack of parallax (shift /
  movement of an object due to a position change)

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Geocentric Model

• Ptolemy 85 165 AD
• Provided theories (mathematically based) to
  support Geo. Theory
• Most importantly, he explained retrograde motion
• -retrograde motion the apparent backward
  motion of a planet in the sky.

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Geocentric Model

• Ptolemy (cont.)
• 3 new theories to explain retrograde motion
• Epicycle rotational path of planet while
  circling around earth
• Equant center of deferent (not earth, earth
  slightly to the side), earth is eccentric (off
  center)
• Deferent circular path around earth

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Ptolemys Theory
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Ptolemys Theory (cont.)

• Retrograde Motion explained by Ptolemy
• Retrograde Motion Animation