1 The Birth of Science - MSRI

11.6 Terrestrial Motion, Tides and Gravitation 317
11.6 Terrestrial Motion, Tides and Gravitation
Galileo, whose main scientific goal was to prove how the earth moves,
surely studied with attention the few known ancient passages relevant
to heliocentrism, including those in Plutarch (an author that, as we recall,
was quoted from the preface onward by Copernicus). One Plutarchan passage
especially must have aroused his interest, the one we discussed on
page 268:
Was [Timaeus] giving the earth motion . . . , and should the earth
. . . be understood to have been designed not as confined and fixed
but as turning and revolving about, in the way expounded later by
Aristarchus and Seleucus, the former assuming this as a hypothesis
and the latter proving it? 101
Our scientist must have felt that the way to solve the problem most dear
to him was to reconstruct Seleucus’ proof, thus completing what Copernicus
had started when he revived Aristarchus’ idea. In reconstructing such
a proof the obvious starting point would have been a thorough search for
all that was said about Seleucus in the literature; this would soon have
turned up a passage in a work that at the time was also thought to be by
Plutarch, though now we know it to be by Aetius. The passage, part of a
list of explanations for the phenomenon of tides, reads:
Seleucus the mathematician (also one of those who think the earth
moves) says that the moon’s revolution resists the rotation and the
motion of the earth. Because the pneuma that lies between the two
bodies is pulled in both directions and falls onto the Atlantic Ocean,
the sea is in turmoil. 102
The importance of Seleucus’ studies on tides was also attested by Strabo
(see page 266). The conclusion that Seleucus’ proof involved a recognition
of tides as an effect of the motion of the earth would have leapt out at the page 391
reader. Thus it is not too surprising that, even before Galileo, several scholars
strove to place the cause of tides in the motion of the earth, though
none of them was able to fashion a theory capable of accounting for observations.
There was, for example, Celio Calcagnini, who broaches the
101 Plutarch, Platonicae quaestiones, 1006C.
102 ¥
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([DG],
383a, 17–25 = Pseudo-Plutarch, De placitis philosophorum, III, xvii; this work was included in the
edition of Plutarch’s Moralia published by Stephanus in 1572). The same report, with minor variations,
is found in Stobaeus ([DG], 383b, 26–34). A book now in preparation will provide a full
discussion and interpretation of the passage, in the context of a study of the two-thousand year old
history of the astronomical theory of tides; meanwhile, see [Russo: Seleuco] and [Bonelli, Russo].
Revision: 1.11 Date: 2003/01/06 07:48:20