Ptolemy and Medieval Astronomy

I found this recent podcast clarifying in a number of ways:

We won’t have time to discuss all the implications of this week’s astronomical reading (!), and so I should offer some additional remarks.  One of the main points to emphasize as we examine several cosmological models (what Plato in Timaeus called “likely stories”), which the intellectual elite knew were inventions — some allegorical, others  geometrical — is that the earth is not such a fixed and central point as is commonly proposed.  Nor is it unmoved in the other sense, i.e., unaffected or untouched. The now commonplace cross-sectional diagram, showing all the spheres as perfectly concentric, is a gross simplification handed down by the encyclopedias and anthologists over the centuries.  In reality, several planets could not have the earth at their center without falsifying what was then regularly observed in astronomy and admitted about the limitations of human knowledge.  That much should be clear.  Moreover, Ptolemaic and Copernican universes are both fair descriptions of the deviating courses of planets – those wily wanderers in Greek – roaming across the night sky.  The point has been reiterated since but is often forgotten.  “Although it is not uncommon for people to say that Copernicus proved Ptolemy wrong,” as Stephen Hawking and Leonard Mlodinow write, “that is not true. . . . for our observations of the heavens can be explained by assuming either the earth or the sun to be at rest.”  They go to say, “the real advantage of the Copernican system is simply that the equations of motion are much simpler in the frame of reference in which the sun is at rest” (The Grand Design, pp. 41-2).  Third, neither system when simplified is an advance over centrism.  Nicholas of Cusa might be a better example for proposing a “centerless” universe, nearing the real complexity of the matter, although his remarks seem highly metaphorical.  He advocated the idea that the world has a center everywhere and circumference nowhere. He undersood what many other scholars knew about the relativity of space and motion: “we cannot discover motion unless it be by comparison with something fixed, that is [by referring it to] the poles or the centers and assuming them in our measurements of the motions [as being at rest]” (cited in Alexander Koyré, From the Closed World to the Infinite Universe). Says Nicole Oresme, “It is apparent, then, how one cannot demonstrate by any experience whatever that the heavens are moved with daily movement, because . . . if any observer is in the heavens and he sees the earth clearly, it (the earth) would seem to be moved” (excerpted in The Scientific Achievement of the Middle Ages, p. 137).

Premodern science often held their models at an ironic distance, phenomenalizing them in these ways – imagining what things look like from different angles of vision, only coming to rest in articles of faith that cannot themselves be demonstrated. They are “likely stories.”