Greek Science II

1. Greek Science II Fundamental Issues and Theories

2. First Proposition: The earth does not move. 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…for 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 [and toward the center of the cosmos]. but this is utterly ridiculous, for the rotation of the earth would be more violent than any (1000 mph).

3. “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.. Second Proposition:The earth is at the center of the cosmos.

4. Third Proposition: all objects with ‘weight’ will naturally fall to the center 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…those that do not must be of a different substance (aether, like the stars, and their natural movement is circular and eternal)

5. Hence… Given[?!?!] that the earth is at the center of the cosmos and that the movements of the stars are eternal and ipso facto circular, Plato posed the question that was to affect astronomy for centuries "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.

6. Assumptions and Complications • Implicit and reflecting ‘common sense’ (the ‘cultural component’), is the intuitive 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. Implications about natural motion? • 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?

7. 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.

8. 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?

9. 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).

10. 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.

11. 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!)