Greek Science IIGreek Science II

Fundamental Issues and Fundamental Issues and TheoriesTheories

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).

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

““it is manifest to any observerit is manifest to any observer that the earth occupies the middle that the earth occupies the middle place in the cosmos, and that all place in the cosmos, and that all weights move toward it...that the weights move toward it...that the earth is spherical and situated in the earth is spherical and situated in the middle of the cosmos..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…those that do not must be of a different substance (aether, like the stars, and their natural movement is circular and eternal)

Third Proposition: all objects with Third Proposition: all objects with ‘weight’ will ‘weight’ will naturally naturally fall to the fall to the

center of the cosmos center of the cosmos ..

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.

Hence…Hence…

Assumptions and ComplicationsAssumptions and Complications Implicit and reflecting ‘common sense’ (the Implicit and reflecting ‘common sense’ (the

‘cultural component’), is the intuitive ‘cultural component’), is the intuitive perception that the earth must be perception that the earth must be unmovingunmoving and at the and at the centercenter of the universe. of the universe.

It is also manifest(!!) that everything either It is also manifest(!!) that everything either moves (‘falls’) toward the earth ‘naturally’ moves (‘falls’) toward the earth ‘naturally’ oror revolves eternally around it in perfect circles. revolves eternally around it in perfect circles. Implications about natural motion? Implications about natural motion?

But there were problems: the planets do not But there were problems: the planets do not move in perfect circles. They exhibit move in perfect circles. They exhibit ‘retrograde motion; getting ‘retrograde motion; getting brighter brighter and and dimmerdimmer, moving forwards and , moving forwards and backwardsbackwards. . How to explain and save the theory?How to explain and save the theory?

Two solutionsTwo solutions

The earliest attempt to resolve the conflict The earliest attempt to resolve the conflict between cultural expectations about between cultural expectations about perfect circles and retrograde motions perfect circles and retrograde motions was made by Eudoxus. He proposed to was made by Eudoxus. He proposed to explain retrograde motion with using explain retrograde motion with using concentric circles.circles.

The second and more enduring solution The second and more enduring solution was developed by the astronomer was developed by the astronomer Claudius Ptolemy, namely the use of Claudius Ptolemy, namely the use of epicycles. .

The ancients did know the The ancients did know the circumference of the earthcircumference of the earth

Already in the 3Already in the 3rdrd Cent BC the Cent BC the Greeks had not only a good idea Greeks had not only a good idea of that the earth was a sphere, of that the earth was a sphere, but could but could calculate its size. its size.

If the earth rotated [‘moved’] on If the earth rotated [‘moved’] on its axis, it had to move at 1000 its axis, it had to move at 1000 mph. We ought ‘feel’ that mph. We ought ‘feel’ that motion? motion?

Some awkward problemsSome awkward problems

how to account for the change of how to account for the change of seasons if the earth was absolutely seasons if the earth was absolutely immobile? immobile?

So, too, if the earth turns on its axis it So, too, if the earth turns on its axis it turns east toward the rising sun, so turns east toward the rising sun, so we should observe a strong wind "east we should observe a strong wind "east wind" but that is not the case wind" but that is not the case (prevailing winds blow from the west). (prevailing winds blow from the west).

And the response…And the response…

And if the earths rotates in an orbit And if the earths rotates in an orbit around the sun, one ought to observe a around the sun, one ought to observe a change in stellar parallax (a change in change in stellar parallax (a change in the angle of the "fixed" stars as the the angle of the "fixed" stars as the earth moves from (say) winter to earth moves from (say) winter to summer.summer.

But that is not observed; therefore the But that is not observed; therefore the earth remains at the center and earth remains at the center and unmoved.unmoved.

Reconciliation??Reconciliation?? Both theories, the geo-centric and the helio-Both theories, the geo-centric and the helio-

centric had awkward points.centric had awkward points. The important point here is that, faced with The important point here is that, faced with

two awkward theories, “preserving the two awkward theories, “preserving the phenomena" (i.e., the intuitive) won out, phenomena" (i.e., the intuitive) won out, that is people preferred the explanation that that is people preferred the explanation that was consistent with their observations that was consistent with their observations that the earth did not move. There may then be the earth did not move. There may then be a scientific consensus, but the consensus is a scientific consensus, but the consensus is just that ("consensus" means "convergent just that ("consensus" means "convergent trend of opinion"; not exactly objective trend of opinion"; not exactly objective truth!) truth!)