1 The Birth of Science - MSRI

76 3. Other Hellenistic Scientific Theories
Lucretius about the passengers of a ship to whom it seems that the ship is
stationary and the land is moving. 113
It should be noted that, just as ascribing motion to the earth leads naturally
to “relativistic” views, such views, in turn, can make it seem as if
the question of whether the earth moves has little relevance. Thus the preceding
paragraphs can explain why post-Aristarchan Hellenistic sources,
starting with Archimedes, appear to modern eyes so indifferent to the
question of heliocentrism as to generate the belief that heliocentrism was
suddenly abandoned. 114
Confirmation for this explanation is provided by a passage of one of
the greatest modern scholars of ancient astronomy, J. L. E. Dreyer, who
studied with great care all the testimonia about Aristarchus and later astronomers:
Aristarchus is the last prominent philosopher or astronomer of the
Greek world who seriously attempted to find the physically true system
of the world. After him we find various ingenious mathematical
theories which represented more or less closely the observed movements
of the planets, but whose authors by degrees came to look on
these combinations of circular motion as a mere means of computing
the position of each planet at any moment, without insisting on the
actual physical truth of the system. 115
This passage is very instructive. Dreyer evidently thinks of the physical
truth of an astronomical theory as something other than its ability to
predict the observable position of each planet at any moment. This suggests
that the idea of judging the validity of a theory solely on the basis of
its power to “save the phenomena” (i.e., in the case of astronomy, represent
the observable positions of celestial bodies) 115a had not yet been fully
recovered in Dreyer’s time (the History of astronomy from which the quotation
is taken is from 1906). What does Dreyer regard as the physical truth
of an astronomical system? Its ability to determine the “true motion” of
planets, we surmise, given the prevailing belief at the turn of the twentieth
century in an absolute space with respect to which motions should
be identified by astronomers. 116 Dreyer, not finding the same concept of
113 Lucretius, De rerum natura, IV, 387–390.
114 According to Sextus Empiricus the motion of the earth was accepted by the “followers of
Aristarchus” ( ), so that Aristarchus was not isolated (Adversus dogmaticos, IV,
174). We will return to the developments of heliocentrism in the second century B.C. in the next
section and in Chapter 10.
115 [Dreyer], p. 149.
115a See Section 6.3 for a more extensive discussion.
116 Galilean relativity, as is well known, was refuted by Newton. The idea of absolute space was
then reinforced by the theory of the ether, which still held sway in the early 1900’s. Relativistic
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