1 The Birth of Science - MSRI

80 3. Other Hellenistic Scientific Theories
Ptolemy’s work 126 and so handed down until such a time when a variation
in the positions of the “fixed” stars could be detected. Changes were
first noticed in 1718 A.D. by Halley, who, probably without realizing that
he was completing an experiment consciously started two thousand years
earlier, recorded that his measured coordinates for Sirius, Arcturus and
Aldebaran diverged noticeably from those given by Ptolemy.
Because one can hardly relinquish the sphere of fixed stars without concluding
that the daily motion of stars is merely apparent — that is, without
recognizing that the earth moves 127 — the testimonia mentioned above
suggest that the earth’s motions were not discarded by Aristarchus’ Hel- page 117
lenistic successors, and in particular that Hipparchus did not consider the
earth immobile.
Once the stars are conceived as extremely distant bodies not all at the
same distance, they can be ascribed other important properties, above all
an enormous size. We cannot follow the debate on this point in astronomical
works of the time, but we can perhaps hear some distant echoes of it.
For example, Cleomedes, though a believer in geocentrism and the sidereal
sphere, 128 wonders how the earth would appear from a star; since he
knows that the sun is much bigger than the earth and that the stars are
vastly more distant than the sun, he deduces that seen from the sun the
earth would appear minuscule, and that from a star it would not be visible
at all. It follows that the stars, which we can see, must be much bigger
than the earth. Cleomedes also says that the sun, seen from a star, would
look as the stars look to us. 129 The statement that the stars are larger than
the earth is also found in other authors. 130
The notion of the universe as a multicentered structure, where a great
many (or infinitely many) worlds coexist, was held also on other grounds.
We will return to this in Section 10.7.
3.8 Ptolemaic Astronomy
The only well known Greek astronomical theory is the one Ptolemy expounds
in the Almagest. We defer to Chapter 10 a comparison between
126
See page 251 in Section 10.4 for the relationship between the catalogues of Hipparchus and
Ptolemy.
127
The examples of Aristarchus, Copernicus and Kepler show that the reverse implication is false.
The earth’s motions can be imagined to coexist with a rigid sidereal star, kept for tradition’s sake
though stripped of its function of explaining the rigid motion of the heavens.
128
However, Cleomedes insists that the void is infinite, and regards it as somehow real and existing
beyond the sky.
129
Cleomedes, Caelestia, I, 8, 19–31 (ed. Todd).
130
Compare Cicero, De re publica, VI, xvi; Proclus, In Platonis rem publicam, II, 218, 5–13 (ed. Kroll).
Revision: 1.13 Date: 2002/10/16 19:04:00