1 The Birth of Science - MSRI

3.7 From the Closed World to the Infinite Universe 77
absolute space in the ancient astronomers, draws the facile (and unwarranted)
conclusion that this absence was a serious limitation of Hellenistic
astronomy.
The attitude of historians of science changed after the idea of absolute
space was definitively laid to rest. Neugebauer, after collecting many ancient
passages that illustrate the relativity of motion, concludes that “statements
about obvious cinematic equivalences are a commonplace in ancient
literature”. 117 But the oldest reference he found regards Heraclides
of Pontus. Neugebauer calls Heraclides’ idea “relativistic” and considers
it “obvious”, but ideas of this type were certainly not obvious before Heraclides,
and they stop being obvious again from the end of the Hellenistic
period down to Dreyer’s time at least. Clearly, notions like the possibility
of a free choice of reference system are not only extremely hard to acquire:
once acquired, it is also extremely hard to shake them off in order to appreciate
their depth.
3.7 From the Closed World to the Infinite Universe
As everyone knows who has observed the night sky for a few hours, the
stars seem to move all together, keeping fixed their mutual distances and
so the shape of the constellations. This naturally suggests the thought that
what is going around, making a full turn per day, is the whole sky, imagined
as a material sphere in which the individual stars are embedded. The
rotating sphere of fixed stars, besides giving a straightforward explanation
for the most obvious of astronomical observations, seems to provide
also a natural limit for the extension of the cosmos, imagined as a sphere
whose center is the earth. This image of an enclosed and spherical universe,
which goes back perhaps to Pythagoras and was certainly held by
Parmenides, is also present in Plato’s and Aristotle’s works, and was accepted
by Ptolemy, who handed it down to the Arabic and European Middle
Ages. Nevertheless, this was not the cosmology of all the “Ancients”,
as many think.
In the Hellenistic era the idea that the earth moved had important cosmological
consequences, which modified profoundly the picture just described.
If, indeed, one is bold enough to imagine that the motion of the
stars is merely apparent, and that it is the earth that turns around daily,
views reacquired the upper hand only thanks to Einstein, whose first work on special relativity
(Zur Elektrodynamik bewegter Körper, 1905) came out just before Dreyer’s History of Astronomy and,
given its technical character and difficulty, could hardly have had an immediate influence on the
ideas presented in a historical work such as Dreyer’s.
117 [Neugebauer: HAMA], p. 695.
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