1 The Birth of Science - MSRI

254 10. Lost Science
The five stars are of course the planets. Shortly before, Seneca had said
about the fixed stars that the Greeks had started to name them “less than
fifteen hundred years ago”. It is clear, therefore, that the “few years” of the
present passage should not be taken literally, but in the context of a very
long time scale.
It is generally believed that Seneca’s Naturales quaestiones draws from
essentially a single source, dating from the first century B.C. Therefore in
the first century B.C. there was still the living memory of a new theory
through which scientists had “started to understand” planetary motions.
The lack of astronomers in the period between Hipparchus and Seneca
makes it very unlikely that the theory that Seneca refers to was unknown
to Hipparchus. Thus Seneca’s passage strengthens the case that, Ptolemy
notwithstanding, Hipparchus had at least started to fashion a new planetary
theory.
Seneca tells us more about this “new” theory in the sequel:
There are some who have told us: “You are mistaken in thinking
that any star stops on its track or turns backward. Heavenly bodies
cannot be detained or turned back; they forever move forth; as
they once were sent on their way, so they continue; their path does
not end but with their own end. This eternal work has irrevocable
motions: if ever [these bodies] stop, they will fall upon one another,
for it is constancy and evenness that preserve them now. Why is it
then that some of them seem to turn back? The appearance of tardiness
is caused by the intervention of the sun (solis occursus), and by
the nature of their paths and circles, so arranged that for a certain
time they deceive the observer: just as ships, though moving under page 317
full sail, appear stationary.” 90
Planets cannot reverse their motion: heavenly bodies are kept in their
orbits by the regularity of their motion; they cannot stop because, if they
did, they would fall on one another (alia aliis incident). It sounds like the
same idea presented more pointedly by Plutarch in the De facie for the
case of the moon, but with a significant difference: gravity here seems to
be viewed as a mutual action between bodies.
90 “Inventi sunt qui nobis dicerent: ‘Erratis, quod ullam stellam aut supprimere cursum iudicatis
aut vertere. Non licet stare caelestibus nec averti; prodeunt omnia: ut semel missa sunt, vadunt;
idem erit illis cursus qui sui finis. Opus hoc aeternum irrevocabiles habet motus: qui si quando
constiterint, alia aliis incident, quae nunc tenor et aequalitas servat. Quid est ergo cur aliqua redire
videantur? Solis occursus speciem illis tarditatis imponit et natura viarum circolorumque sic positorum
ut certo tempore intuentes fallant: sic naves, quamvis plenis velis eant, videntur tamen
stare’ ”. (Seneca, Naturales quaestiones, VII, xxv, 6-7).
Revision: 1.11 Date: 2003/01/06 02:20:46