Greek Science II

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Greek Science II

• Fundamental Issues and Theories

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Plato set the problem that was to influence the
study of astronomy for generations

• "By the assumption of what uniform and ordered
  motions can the apparent (i.e., "erratic")
  motions of the planets be accounted for." That
  is, how is one to "save the phenomena" in terms
  of uniform circular movement.

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Assumptions and Complications

• Implicit and reflecting common sense (the
  cultural component), is the perception that the
  earth must be unmoving and at the center of the
  universe.
• It is also manifest that everything either moves
  (falls) toward the earth naturally or
  revolves eternally around it in perfect circles.
• But there were problems the planets do not move
  in perfect circles. They exhibit retrograde
  motion getting brighter and dimmer, moving
  forwards and going backwards. How to explain and
  save the theory?

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Two solutions

• The earliest attempt to resolve the conflict
  between cultural expectations about perfect
  circles and retrograde motions was made by
  Eudoxus. He proposed to explain retrograde motion
  with using concentric circles.
• The second and more enduring solution was
  developed by the astronomer Claudius Ptolemy,
  namely the use of epicycles.

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The earth is the center of the cosmos and unmoved

• Ptolemy, Almagest I, 7...it may be proved that
  the earth cannot make any movement whatever...or
  ever change its position at all from its place at
  the center of the cosmos.
• it is manifest to any observer that the earth
  occupies the middle place in the cosmos, and that
  all weights move toward it..that the earth is
  spherical and situated in the middle of the
  cosmos...and it is a simple fact that in all
  parts of the earth without exception the
  tendencies and the motions of bodies which have
  weight operate at right angles to tangent drawn
  through the point of contact where the object
  falls...That is, objects that have weight
  "naturally" fall to the center of the cosmos.

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The second argument

• if the earth moved away from the center of the
  universe, heavier objects would still all toward
  that center and the animals and all separate
  weights would have been left behind floating in
  air at the center of the cosmos, while the
  earth at its great speed would have fallen
  completely out of the cosmos itself. but this is
  utterly ridiculous, for the rotation of the earth
  would be more violent than any (1000 mph).

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The ancients did know the circumference of the
earth

• Already in the 3rd Cent BC the Greeks had not
  only a good idea of that the earth was a sphere,
  but could calculate its size. If the earth
  rotated moved on its axis, it had to move at
  1000 mph. We ought feel that motion?

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Some awkward problems

• how to account for the change of seasons if the
  earth was absolutely immobile?
• So, too, if the earth turns on its axis it turns
  east toward the rising sun, so we should observe
  a strong wind "east wind" but that is not the
  case (prevailing winds blow from the west).

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And the response

• And if the earths rotates in an orbit around the
  sun, one ought to observe a change in stellar
  parallax (a change in the angle of the "fixed"
  stars as the earth moves from (say) winter to
  summer.
• But that is not observed therefore the earth
  remains at the center and unmoved.

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Reconciliation??

• Both theories, the geo-centric and the
  helio-centric had awkward points.
• The important point here is that, faced with two
  awkward theories, preserving the phenomena"
  (i.e., the intuitive) won out, that is people
  preferred the explanation that was consistent
  with their observations that the earth did not
  move. There may then be a scientific consensus,
  but the consensus is just that ("consensus" means
  "convergent trend of opinion" not exactly
  objective truth!)

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Galen and Experimental Methodology

• Galen want to demonstrate the irreversability of
  the flow of urine from the bladder "south".
• Text and Model go to the linked site
• Can we describe this procedure as scientific
  and why?