Russo, L., 2004, The Forgotten Revolution: How Science Was Born in 300 BC and Why it Had to Be Reborn, 311-2.
Seleucus of Babylon, already encountered on page 88 in connection with the infnity of the universe, was an astronomer from the second century B.C. about whom not much else is known. But Plutarch offers a very interesting testimonium, whose import appears to have been neglected by historians of science:
Was [Timaeus] giving the earth motion . . . , and should the earth . . . be understood to have been designed not as confned 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?
The passage refers to two types of terrestrial motion, rotation and revolution. The verb ἀποφαίνομαι 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 of these motions.
To state, as Seleucus did, that the sun really is fixed and the earth is moving is equivalent to stating that planetary stations and retrogressions don't just disappear under the assumption that the sun is stationary, as Aristarchus said, but that they really don't exist. That retrogressions and stations are merely apparent is repeated by pre-Ptolemaic Latin sources, including Pliny and Seneca, suggesting that the notion of heliocentrism as a physical reality, far from being exceptional, was well-known. Thus we might hope to find traces of Seleucus' proof in the literature.
One argument in favor of heliocentrism is what we reconstructed in Section 10.7 based on a passage of Seneca. With the sun as the reference, the planets' motion admits a simple dynamical description, where centrifugal force balances attraction. In a geocentric model this is not so easy to do: if the planets are attracted by the earth, why wouldn't they fall when they stop in the sky? And if not attracted by the earth, why don't they go off forever? One is tempted to deduce that only the motion around the sun is real. Since classical literature contains no other arguments in favor of heliocentrism, it is reasonable to conjecture that the proof that Plutarch attributes to Seleucus is based on the argument just given, which is reported by Seneca.
Russo, L., 2004, The Forgotten Revolution: How Science Was Born in 300 BC and Why it Had to Be Reborn. Berlim/ Heidelberg/ Nova Iorque: Springer Verlag)
