1 The Birth of Science - MSRI

10.10 Ptolemy and Theon of Smyrna
10.10 Ptolemy and Theon of Smyrna 273
The preceding sections have argued that Hipparchus, and perhaps other
astronomers in the second century B.C., reached a sort of “dynamical heliocentrism”
based on the principle of inertia and the equilibrium between
centrifugal force and the gravitational pull of the sun.
The greatest obstacle to accepting this picture is that these ideas are
nowhere to be found in the Almagest. But the Almagest itself offers indirect
evidence for the proposed reconstruction, in the form of remnants
of scientific ideas not understood by Ptolemy and the absence of other
ideas whose existence in Hellenistic science is well documented. Dennis page 340
Rawlins makes a strong case that several technical elements of Ptolemaic
astronomy can only be explained as derivatives of an earlier heliocentric
model. 152
Among the consequences of the cultural gap separating Ptolemy from
Hellenistic astronomy was the loss of the idea of relative motion. Indeed,
the idea that one can choose the reference system, present for example in
Euclid, Herophilus and one of Seneca’s sources, 153 was completely foreign
to Ptolemy. Nevertheless he reported recognizably relativistic opinions, 154
which he himself understood simply as a convenience for describing observed
motions, with no consequences to his concept of space and motion,
which is squarely Aristotelian. In just the same way physicians like Rufus
Ephesius, though diligently transmitting Herophilus’ neologisms, could
no longer grasp their nature.
Ptolemy not only did not use gravity (or any other dynamical idea) in
astronomy; he also approached the earth’s sphericity in a purely geometric
and descriptive way, seemingly unaware of the explanation for it that was
well known in Hellenistic times. 155
The Almagest mentions (and contests, of course) the opinion that the
“fixed” stars have a uniform linear motion. 157 To Ptolemy the motion in page 341
question was the apparent movement in the sky, but we may conjecture
that his source was not making such a bizarre claim; it was instead referring
to the “real” motion whose existence Hipparchus suspected on the
basis of general principles and for whose detection he sought to enlist the
help of posterity.
152
[Rawlins: AHPE].
153
See pages 153, 163 and 256.
154
See page 75.
155
Ptolemy, Syntaxis mathematica, I, iv, 14–16 (ed. Heiberg); for the explanation of sphericity see
Section 10.8.
157
Ptolemy, Syntaxis mathematica, I, iii, 11 (ed. Heiberg).
Revision: 1.11 Date: 2003/01/06 02:20:46