1 The Birth of Science - MSRI

10.6 Rays of Darkness and Triangular Rays 257
tion of planets, and concludes by saying: “This is the theory of the outer
planets, and it is harder than the rest; never before me has it been divulged.”
96 Together with his later remarks about Hipparchus, this makes
one think that the planetary theory that Pliny is “divulging” is that of
Hipparchus. It is very unlikely that Pliny would have dared to read any
of the astronomer’s works directly, but his statements prove that a planetary
theory attributed to Hipparchus was explained in books that were
still available in Rome in the first century A.D.
As to the contents of the theory, it is admittedly not easy to reconstruct it
based on the pages of the Natural history. But it is clear that Pliny’s sources
include at least one that is scientific, because some of the data mentioned,
such as the periods of planetary revolutions, are reasonably accurate.
Pliny, like Seneca, says that planetary stations are merely appearances, 97
which suggests that his source too espoused heliocentrism. Vitruvius adds
a bizarre explanation (which he says he does not agree with) of the retrogressions
and stations of the outer planets Mars, Jupiter and Saturn, involving
a “darkness” that certain people supposedly claimed was caused
by the sun. 98 It is not very likely that someone actually claimed that the
sun sends out rays of darkness. It seems more plausible that Vitruvius
misunderstood the heliocentric argument transmitted by Seneca and alluded
to by Pliny. The source may have contained some remark (similar
to the one found in Seneca) to the effect that the motion of the planets is page 321
obscured by the sun — that is, cannot be perceived directly from earth —
an explanation which Vitruvius could not grasp and took to mean a literal
darkness (obscuritas). The heliocentrism of Vitruvius’ source is corroborated
by his statement that “the planets Mercury and Venus nearest the
rays of the sun move round the sun as a centre”. 99
Vitruvius’ most interesting passage is probably this:
. . . the sun’s powerful force attracts to itself the planets by means of
rays projected in the shape of triangles; as if braking their forward
96
“Haec est superiorum stellarum ratio; difficilior reliquarum et a nullo ante nos reddita” (II,
71). This discussion of the motion of outer planets starts with “aperienda est subtilitas immensa et
omnes eas complexa causas” (II, 67).
97
“Hoc non protinus intellegi potest visu nostro, ideoque existimantur stare, unde et nomen
accepit statio” (Pliny, Naturalis historia, II, 70).
98
“Id autem nonnullis sic fieri placet, quod aiunt solem, cum longius absit abstantia quadam,
non lucidis itineribus errantia per ea sidera obscuritatis morationibus impedire. Nobis vero id non
videtur.” (De architectura, IX, i, 11). Vitruvius then contradicts the explanation on the grounds that
the sun sheds light, not darkness, and that the outer planets are visible even while retrogressing.
99
Vitruvius, De architectura, IX, i, 6, Gwilt translation.
Revision: 1.11 Date: 2003/01/06 02:20:46