1 The Birth of Science - MSRI

11.2 The Renaissance 299
was following Aristarchus was quite obvious to his contemporaries, who
could only do mathematics and astronomy by studying attentively the
Hellenistic works that had resurfaced. Note that Copernicus, in putting page 369
forth again the heliocentrism of Hellenistic times, at the same overcame
the Aristotelian concept of gravity, by co-opting the polycentric theory (capable
of explaining the roundness of the sun, moon and earth) mentioned SL: make sure page range
is nontrivial.
by Plutarch. 38
Copernicus did much more than overcome Ptolemy’s geocentrism and
views on gravity by taking up Aristarchus’ ideas. From the perspective
of science the key point is that he was able to create an algorithm, based
on a system of epicycles, to calculate the apparent motion of planets: thus
he was the first person able to reconstruct Ptolemy’s mathematical astronomy.
39 This reconstruction was far from trivial, because the Almagest contains
no hint about how to derive the algorithm whose use it describes,
and moreover the algorithm needed fine tuning in view of errors accumulated
in the intervening 1400 years.
Because of the choice of accepting the “hypotheses” of Aristarchus, this
fundamental work of recovery took on the odd guise of a battle against
Ptolemy. Copernicus’ opponent was not in fact Ptolemy, but the syncretic
image derived from his system by European culture. The Ptolemaic system
was not understood in its true function as an algorithm to predict
the motion of planets, because nobody before Copernicus had been able
to use it that way. Ptolemy’s name had instead become associated with
a complex cosmology, such as we see in Dante, blending certain aspects
of Ptolemaic astronomy (geocentrism, the use of circles to describe motions)
with features from Aristotelian natural philosophy and Christian
religious tradition. Thus the cultural battle for heliocentrism acquired a
huge ideological charge, since it was necessary to overcome that cosmology
and, by recognizing the earth as a “heavenly” body, conversely affirm page 370
the “earthly” nature of astronomical phenomena.
Kuhn has called attention to the fact that only after the Copernican revolution
took hold was it possible to observe the appearance of new stars
and the motion of comets across the putative planetary spheres. Though
visible to the naked eye, these phenomena had been ignored so long as the
Ptolemaic paradigm (with which they are incompatible) held sway. 40 Since
we know that novae were observed and recorded in Hellenistic times, as
38 Copernicus, De revolutionibus orbium caelestium, I, 9. For the polycentric theory and the
Plutarchan passages where it is mentioned, see pages 264–265. Recall that Copernicus cites Plutarch
already in the preface to his work.
39 This is underlined in [Neugebauer: ESA], pp. 241–242, but Neugebauer is totally indifferent to
the ideological import of the “Copernican revolution”.
40 [Kuhn: SSR], p. 115–116; [Kuhn: CR], pp. 206–209.
Revision: 1.11 Date: 2003/01/06 07:48:20