1 The Birth of Science - MSRI

258 10. Lost Science
movement or holding them back, the sun does not allow them to go
forth but [forces them] to return to it. . . 100
Pliny has a parallel passage:
[The planets] are struck in the position we have said and are prevented
by a triangular solar ray from following a straight path and
are lifted on high by the burning force [of the sun]. 101
The overlaps between the two texts are significant above all in view of
the matching contexts: both authors are explaining the motion of the three
outer planets. Both passages clearly display the idea of the sun’s pull on
the planets; Pliny includes the particularly interesting idea that this pull
has the effect of preventing the planets from moving on a straight line.
The continuation of the Vitruvius passage is very obscure. It reads:
and to be in the signum of the other triangle. It may be asked why the
sun in its hotness pulls [the planet] back in the fifth signum, rather
than in the second or third, which are closer. Therefore I will explain
why this seems to happen. Its rays extend out into the universe using
lines in the shape of a triangle with equal sides. This however page 322
does not happen either more or less [than] to the fifth signum from it.
Therefore. . . 102
To understand this passage it is essential to understand the sense in
which Vitruvius, and above all his source, used the term signum, which we
have not translated. It is generally assumed that signum means a zodiacal
sign and this interpretation is supported by the fact that elsewhere, including
in earlier passages in this chapter, Vitruvius uses the word in that
sense. This might seem to settle the question, but notice that in the previous
passage, about the strange “darkness”, Vitruvius demonstrates difficulty
in understanding his astronomical sources. Immediately after the
passage just quoted he curtails the argument by appealing to Euripides’
authority, another sign of his lack of familiarity with the subject. More-
100 “solis impetus vehemens trigoni forma porrectis insequentes stellas ad se perducit et ante currentes
veluti refrenando retinendoque non patitur progredi sed ad se regredi. . . ” (Vitruvius, De
architectura, IX, i, 12).
101 “Percussae in qua diximus parte et triangulo solis radio inhibentur rectum agere cursum et
ignea vi levantur in sublime” (Pliny, Naturalis historia, II, 69).
102 “in alterius trigoni signum esse. Fortasse desiderabitur, quid ita sol quinto a se signo potius
quam secundo aut tertio, quae sunt propiora, facit in his fervoribus retentiones. Ergo, quemadmodum
id fieri videatur, exponam. Eius radii in mundo uti trigoni paribus lateribus formae liniationibus
extenduntur. Id autem nec plus nec minus est ad quintum ab eo signum. Igitur. . . ” (Vitruvius,
De architectura, IX, i, 13). The penultimate word quoted is a generally accepted emendation of
the manuscripts’ “signo”, but the text as received also has much the same translation (“to the fifth
signum from that one”, i.e., the one where the sun is).
Revision: 1.11 Date: 2003/01/06 02:20:46