1 The Birth of Science - MSRI

268 10. Lost Science
Thirdly, tides having been a subject of study in Hellenistic exact science
as early as Eratosthenes, if Seleucus carried out his study of tides in a page 334
purely empirical light it would be hard to see why the tradition regarded
him as a “mathematician”. 128
Since Seleucus’ essential contribution to tidal theory seems to have been
the study of the yearly cycle of diurnal inequality, the preceding discussion
leads one to think that Seleucus recognized the influence of the sun on
tides; 129 in other words, that he accepted or discovered himself the gravitational
effect of the sun, thus extending Eratosthenes’ theory. In this case
he would have been in a position to formulate the dynamical justification
of heliocentrism that we reconstructed at the end of Section 10.5 based on
the passage from Seneca.
Now, Plutarch writes:
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? 130
The passage refers to two types of terrestrial motion, rotation and revolution.
130a The verb appearing at the end of the passage allows
different possibilities for what Seleucus actually did, but the contrast with
“as a hypothesis” clearly implies that he found new arguments in support
128 See for example Strabo, Geography, XVI, i, 6, where Seleucus is linked to the Chaldeans, famous
among “mathematicians”.
129 Of course, the monthly cycle also depends on the sun. But deducing that influence from the
fact that tides correlate with the phases of the moon does not seem to be an obvious step. For
example, chapter 12 of Le monde de M. Descartes, ou Le traité de la lumière ([Descartes]) contains
a clear statement of the relationship of phases of the moon with tides, but explains it using the
moon alone. In contrast, is seems totally impossible to explain an annual cycle of any sort without
involving the sun.
130 Plutarch, Platonicae quaestiones, 1006C.
130a Thus [Schiaparelli], p. 36 and [Heath: Aristarchus], p. 305. Nevertheless, Dreyer and after him
Neugebauer took the words , which we have translated “turning and
revolving about”, to mean only the earth’s daily rotation. They offered no arguments for this position,
and indeed avoided translating the expression altogether, replacing it by a single word, perhaps
to avoid raising doubts in the reader’s mind ([Dreyer], p. 140; [Neugebauer: HAMA], p. 611).
It is true that, taken in isolation, each of these two Greek verbs might refer either to rotation around
an axis or revolution about an external point (as can “rotate” and “revolve” in English, outside the
narrowest astronomical convention). But if Plutarch had meant rotation only, it is hard to see why
he would have used two verbs. The idea of rotation and revolution is further clarified by the contrast
with the possibility of an earth that is fixed (¡) and confined (). Finally,
Plutarch specifies that the motions referred here are the ones already attributed by Aristarchus to
the earth, which we know from an unequivocal passage (Plutarch, De facie. . . , 6 = Moralia, 923A) to
be both rotation and revolution.
It is interesting that while the second of Plutarch’s verbs, , is a direct counterpart of the
Latin revolvo, which presumably was chosen by Copernicus exactly for that reason.
Revision: 1.11 Date: 2003/01/06 02:20:46