Greek Science II

1. Greek Science II Fundamental Issues and Theories What intuition and ‘everyday experience’ suggest is ‘true’ about the cosmos…

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, see below).

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 naturallyfall 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 that 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 (Ptolemy, is referring to aether, the fifth element, which is like the stars and whose natural movement is circular and eternal)

5. A moment of reflection… • In other words: Given [?!?!] that the earth is at the center of the cosmos and that the movements of the stars are different, eternal and ipso facto circular, the stars must be a different substance. • Hence, in reflecting on the movements of the planets, 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" and preserve the culturally preferred uniform circular movement?

6. Assumptions and Complications • Implicit and reflecting ‘common sense’ (the ‘cultural component’), is the intuitive perception • that the earth does not move and • that ‘appears’ to be 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. Consider the 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 explication of retrograde motios was made by Eudoxus. He proposed to explain retrograde motion with using concentric circles. Alternate view • The second and more enduring solution was developed by the astronomer Claudius Ptolemy, namely the use of epicycles. • In both cases, circles are perfect and the earth remains unmoved at the center of the cosmos.

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 would have to move at 1000 mph. Should we not sense that ‘violent’ level of motion? • C (earth’s orbit)=2 π r [where r = 93million]= 584,400,0000 • 8760 hour year • Hence, the earth moves through space at ≅ 73,000 m/h And that is a counter-intuitive notion.

9. Some awkward problems... • how to account for the change of seasons? if the earth is 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 there is no compelling evidence against the notion the earth remains at the center and is unmoved. • Moreover, how could the helio-centric model explain why the moon and planets (which are not of aither) did not ‘fall’ to the earth. What kept them ‘in place/orbit’?

11. Reconciliation I ?? • Both theories, the geo-centric and the helio-centric had awkward points. • To some thoughtful people, to those who admitted that no stellar parallax could be demonstrated, there were two awkward models. • “preserving the phenomena" (i.e., the intuitive) was widely accepted because such people preferred the explanation that was consistent with their observations and intuition that the earth did not move; theyaccepted that there were anomalies ‘to be explained’, • The alternative, assume that the kosmos was so very much larger than it was perceived to be; so large that no parallax could be measured. And that seems also improbable • In brief, there may then be a consensus about the cosmos, but the consensus is just that ("consensus" means "convergent trend of opinion"; not exactly objective truth!)

12. Reconciliation II • For “others” the anomalies were very important because they challenged the notion the cosmos was orderly. Like Herodotus, an explanation was needed to explain that the ‘anomalies’ were truly apparent, and that the system was ‘orderly’. • Both of these trends will dominate popular perception and scientific debate into the Renaissance and beyond.