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The revolutionary Copernicus
on 30 December 2011
I regard this as a second masterpiece from Dava Sobel that matches her "Longitude" (1995) in importance. I did not know that Copernicus was actively persuaded to publish by Rheticus, a young Lutheran scholar. I appreciated none of the history of ducal Prussia, although I did realise that in 1543 when "De Revolutionibus" was published the Counter Reformation was in full swing. In particular I did not know that that Copernicus' city of Frauenburg had a particularly schismatic bishop who had proscribed all Lutherans in his jurisdiction.
Having set the scene, not so easy for this (to us) little-known corner of Europe with a political complexion which is both very complicated and very unfamiliar to us, Sobel proceeds to tell the main story with scintillating verve in the form of a two-act play, bringing home to us some idea of the depth and intricacy of what was involved.
But then, Sobel brings her overview up to date, summarising the results of the "Annotated Census of Copernicus' De Revolutionibus" by Owen Gingerich (2002, Harvard; see also "The Book Nobody Read", Gingerich 2004). Gingerich found 277 surviving copies of the first (1543, Nuremberg) edition and 324 of the second (1566, Basel). Contrary to Arthur Koestler's assertion in his (otherwise brilliant) book "The Sleepwalkers" (1959), everybody had read Copernicus's book! I really did not know this.
In particular, Kepler had a copy originally owned by Schreiber (a classmate of Rheticus), which he purchased in 1598 and annotated heavily. Kepler found, against a discussion by Copernicus about "computing the apparent sun" (Book III ch.25) a one-word annotation by Schreiber: "ellipse". Kepler's masterpiece, "Astronomia Nova" (1609), demonstrated that the orbit of Mars was elliptical. Moreover, in it he demonstrated that using Ptolemy's "equants" construction, the construction that Copernicus had proved was superfluous, the orbit of Mars could not be calculated with a maximum error less than 8 minutes of arc, half an order of magnitude larger than Tycho's average measurement error. Kepler had not tried using the ellipse earlier since he considered that previous astromomers would not have overlooked such a "simple" geometrical figure! These guys were seriously clever! Of course, it was the ellipse that Newton subsequently proved was the natural orbit in a two-body system under a central inverse-square-law force. So Newton built on Kepler, and Kepler built on Copernicus.
Interestingly, Sobel also discusses (rather obliquely) the mediaeval doctrine of "saving the phaenomenon". In scholastic philosophy, "physics" described the essences of things, where "mathematics" was viewed as a calculational tool for mundane purposes. So astronomers were "mathematicians" since they were able to calculate when eclipses and other such celestial phaenomena would occur, but they could not be "physicists" since nobody believed that the fun and games that the planets got up to according to Ptolemy really existed. This is completely opposite to the way we think, where the mathematicians are in a world of their own but the physicists accurately describe reality. Copernicus was revolutionary also because he believed that the earth really did go round the sun: it was not just a mathematical fiction that "saved the phaenomena". It is this new attitude that finally allowed the scholars (and, eventually, us) to escape from the deterministic shadow of Aristotle.
Thank you so much, Dava Sobel!