Tuesday, October 18, 2011

New Copernicus biography

Dava Sobel wrote a new biography of Copernicus:
She ends her book with a lyrical reflection on what Copernicus might make of the universe we know today: “He had initiated a cascade of diminishments. The Earth is merely one of several planets in orbit around the Sun. The Sun is only one star among 200 billion in the Milky Way” — itself just one galaxy “surrounded by countless other galaxy groups stretched across the universe.”

Sounding forgivably Carl Sagan-ish, she concludes, “All the shining stars of all the galaxies are as nothing compared to the great volume of unseen dark matter.” Dark matter itself “is dwarfed by the still more elusive entity, dark energy,” she writes. “The very notion of a center no longer makes any sense.”
This is silly. The ancient Greeks debated the motion of the Earth, and it had nothing to do with diminishment. Copernicus put the Sun at the center of the universe. Saying that there is no center does not validate Copernicus.

The book has a lot of fictionalized dialog about how Copernicus might have collaborated to publish his book.

I think that it is a mistake to make such a big deal out of what Copernicus did. He did not really prove his predecessors wrong. Ptolemy did argue against the motion of the Earth, but his reasoning was not refuted.

Here is what a recent scholarly paper says:
Harald Siebert.11 In what follows, I will restrict my analysis mostly to textual aspects that I found problematic and especially significant. The extrusion effect of the diurnal rotation of the Earth is presented by Galileo in the Dialogue as follows. Now there remains the objection based upon the experience of seeing that the speed of a whirling has a property of extruding and discarding material adher- ing to the revolving frame. For that reason it has appeared to many, including Ptolemy, that if the Earth turned upon itself with great speed, rocks and animals would necessarily be thrown toward the stars, and buildings could not be attached to their foundations with cement so strong that they too would not suffer similar ruin.12

The question immediately arises of Galileo’s attribution to Ptolemy of a similar argument. The implicit reference seems to be to Almagest Book 1, Chapter 7. Here is G. J. Toomer’s translation of the relevant passage from the original Greek (on the basis of Heiberg’s text).
If the Earth had a single motion in common with other heavy objects, it is obvious that it would be carried down faster than all of them because of its much greater size: living things and individual heavy objects would be left behind, riding on the air, and the Earth itself would very soon have fallen completely out of the heavens. But such things are utterly ridiculous merely to think of.13
A more literal reading of the passage has been suggested to me by James G. Lennox, as follows:14
But if there were some motion of the Earth that was one and the same and shared with the other heavy bodies, it is clear that it would overtake everything in descent on account of its much greater magnitude, and the animals and individual heavy bodies floating on the air would be left behind, and the Earth would very quickly fall from the very heaven itself. But even contemplating such things would appear the most laughable thing of all.
This text from the Almagest is highly problematic. It is not obvious, at least to my mind, what the meaning conveyed by the image of an Earth’s falling from the heaven exactly is. The beginning of the passage highlights a common motion. The phrasing is consistent with both a rectilinear and a circular motion. Presumably, however, given the general context of the initial discussion in Chapter 7, a rectilinear motion is intended by Ptolemy. The subsequent portion of Chapter 7 focuses on circular motion explicitly and eventually goes on to dismiss the possibility of a diurnal rotation of the Earth around its polar axis.
What I get out of this is that scholars are unsure of Ptolemy's exactly reasoning. Ptolemy did not just say that the Earth was in the center because Man is the most important thing in the universe. He had technical reasons for doubting the Earth's motion.

At any rate, I think that it is bizarre to devote so much attention to what is just a couple of sentences out of Ptolemy's treatise. The treatise is hundreds of pages long, and was used to predict the appearance of the sky for centuries. Those couple of sentences could not have had that much to do with the rest of the book, or people could have figured out what he meant by how the ideas were used elsewhere. The fact is that his model had little to do with whether the Earth moved or not. That was just a detail where he had some superfluous opinions.

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