Greek Science II

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Greek Science II

• Fundamental Issues and Theories
• As applied to the Motions of the Planets

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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.
• (remember, the circle is perfect)

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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 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 where retrograde motion can be explained
  using nested concentric circles.
• The second and more enduring solution was
  developed by the astronomer Claudius Ptolemy,
  through the use of epicycles

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Postulate 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

• By the 3rd Cent BC the Greeks had not only a
  good idea of that the earth was a sphere, but
  devised a method to calculate its diameter. This
  calculation leads to the 1000 mph rotational
  velocity and the idea that cows should be flying
  off the surface of the earth yet no such flying
  cows were ever observed. Therefore the Earth
  could not be rotating and it was the sun
  revolving around the earth that caused day/night.

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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 (with naked eye)
  therefore the earth remains at the center and
  unmoved.

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Reconciliation??

• Both theories, the geo-centric and the
  helio-centric had inconsistencies with respect to
  observed data ? so what do you choose?
• 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 primary observation that the earth did
  not move.
• This primary observation then forms the basis for
  scientific consensus and the adopted geocentric
  model of the Solar System