1 The Birth of Science - MSRI

256 10. Lost Science
not only here but in several other ancient passages: Lucretius develops it
quite explicitly. 91
Finally, we know that Seneca’s source envisaged the possibility that the page 319
earth moves, because in an earlier passage Seneca mentions the rotatingearth
explanation for the daily motion of the skies. 92
We must conclude that Seneca’s source applied to the revolution of
planets around the sun the same idea mentioned by Plutarch in the case
of the moon: the cancellation between gravitational attraction and centrifugal
force. Because heliocentrism can explain planetary motion in this
way, overcoming the difficulty inherent in planetary stations, the result
was a dynamical justification of heliocentrism. Since Seneca’s source must
be talking about ideas from Hellenistic astronomy of the second century
B.C. (the time of Hipparchus), the conclusion reached is consistent with
the thesis that the De facie passage discussed in Section 10.3 was based on
Hipparchus’ book on gravity. 93
10.6 Rays of Darkness and Triangular Rays 94
Although we have no true astronomical works from the period between
Hipparchus and Ptolemy, the Roman literature of the first centuries B.C.
and A.D. contains, besides the Seneca passages, at least two expositions
of astronomical arguments with some pretense of systematicity; they occur
in Book II of Pliny’s Natural history and in Book IX of Vitruvius’ De
architectura. Since astronomical activity in Rhodes went on after it ceased
in Alexandria in 145 B.C., and since Rome maintained important trade
and cultural ties with Rhodes down to the mid first century B.C., it would
not be too surprising to find in these works traces of ideas of Hipparchus
unknown to Ptolemy — the more so because, when Pliny mentions the page 320
foreign sources for Book II of the Natural history, Hipparchus heads the
list and garners lavish and enthusiastic praise. In fact Pliny regrets that
no one was left to carry on the astronomer’s scientific legacy. 95 He briefly
discusses an “immensely ingenious” theory capable of explaining the mo-
91 “Qua vehimur navi, fertur, cum stare videtur; / quae manet in statione, ea praeter creditur
ire. / et fugere ad puppim colles campique videntur, / quos agimus praeter navem velisque volamus”
(De rerum natura, IV, 387–390). This passage parallels Seneca’s, as shown in [Russo: Lucrezio]
based on the respective contexts.
92 Seneca, Naturales quaestiones, VII, ii, 3.
93 It has been generally assumed that this work of Hipparchus on gravity bore no relation to his
astronomical interests. But already Heath wrote: “It is possible that even in this work Hipparchus
may have applied his doctrine to the case of the heavenly bodies” ([Heath: HGM], p. 256).
94 The material in this section is drawn largely from [Russo: Vitruvio] and [Russo: Hipparchus].
95 Pliny, Naturalis historia, II, 95. Vitruvius, too, names Hipparchus (De architectura, IX, vi, 3).
Revision: 1.11 Date: 2003/01/06 02:20:46