1 The Birth of Science - MSRI

74 3. Other Hellenistic Scientific Theories
rotation of the earth, 105 and the same is said of the Pythagoreans Hicetas
and Ecphantus. 106 These are important precedents, of which Aristarchan
heliocentrism is a natural development. If the alternation of day and night
can be explained by a movement of the earth, it is natural for astronomers
to explain the retrograde motion of planets in an analogous way. Saying
for the first time that the earth moves, on the other hand, requires a profound
transformation in the concepts of space and motion. It is not by
accident that Ptolemy, who shares Aristotle’s conception of space, rejects
not only heliocentrism but the rotation of the earth as well; nor that, in
modern times, the two were accepted at the same time. 107
People in prescientific societies have always talked about “rest” and
“motion” of bodies in an absolute sense, without ever feeling the need
to specify with respect to what reference system the motion is considered.
From the modern point of view this was possible through the implicit
use of a reference system fixed on the earth. Aristotle still believed that
space is absolute and comprises places of differing natures, and that one
such place, having the property of attracting heavy bodies, had become
(thanks to that property) the center of the world. This concept of space is
closely connected to the idea that the absolute state of motion of bodies is
observable.
If, while accepting the idea of absolute space, you dare hypothesize an
(absolute) motion for the earth, as Heraclides did before anyone, the very
idea of absolute motion cannot but face a crisis. For then you must accept
that you are moving at over 1000 km/h (the order of magnitude of the
speed due to terrestrial rotation at the latitudes of Hellenistic cities) without
noticing it. You realize then that your observations do not indicate
the “true” state of motion of bodies, but only the relative motion between page 111
observer and observed. 108
Did the attribution of movement to the earth lead eventually, in Hellenistic
times, to a relativistic concept of motion? I believe a positive answer
can be deduced from many sources.
105
The main extant source is Simplicius, In Aristotelis De Caelo commentaria, [CAG], vol. VII, p. 541.
This and other sources are discussed in [Heath: Aristarchus], pp. 249–283. See also [Neugebauer:
HAMA], pp. 694–696.
106
See, for example, [Heath: Aristarchus], pp. 187–188, 251–252. The meager information we have
about these Pythagoreans is due to Diogenes Laertius, Aetius and Hippolytus.
107
Recall that the objection to Copernicism based on biblical authority did not apply to terrestrial
revolution, only to rotation.
108
Of course in our classical mechanics not all reference systems are equivalent, and a reference
system fixed on the earth is not inertial. But since the motions of the earth, compared with those
experienced in everyday life, have enormous velocity but small acceleration, this subtlety can be
neglected to a first approximation.
Revision: 1.13 Date: 2002/10/16 19:04:00