1 The Birth of Science - MSRI

10.9 Precession, Comets, Etc. 271
ment of or theoretical explanation for the polar flattening (for one thing,
there was longstanding debate over whether the ellipsoid was flattened or
elongated 142 ). If the idea that the earth is ellipsoidal arose neither from experimental
data nor from theoretical arguments, what led to it? Couldn’t
it have been the assiduous reading of Strabo attested among seventeenthcentury
geographers?
10.9 Precession, Comets, Etc.
The main result of Hipparchus mentioned by Ptolemy is the discovery
of the precession of the equinoxes. 143 The precession is so slow that any
available observational data would have given Hipparchus displacements
of only a few degrees. 144 Nevertheless, the astronomer, who is known to
have been very rigorous in his use of experimental data, 145 dared to extrapolate
from a tiny arc the existence of a circular uniform motion with
a period of 26,000 years. If his astronomy was “dynamical”, any old top
might have given him the idea of sifting through observational data for
the existence and periodicity of precession. 146
Phenomenologically, comets have very little in common with planets;
their appearance is quite different and even if the periodicity of a comet
can be noticed (which is far from easy), the trajectory remains for the most
part hidden and is different from that of any other comet. A purely de- page 338
scriptive astronomy precludes a theory of comets. On the other hand, a
“dynamical” astronomy based on some sort of gravitational theory might
naturally ask whether there are things besides the planets going around
the sun, perhaps with very long orbits. It is not an accident that there is no
theory of comets in the Almagest and that the modern theory was created
by Halley only after the development of Newton’s theory of gravitation.
But Seneca writes:
For he [Apollonius Myndius] says that the Chaldeans reckon comets
among the planets and know their orbits. 147
142
The second possibility was still maintained by the famous Giacomo Cassini (1677–1756).
143
Ptolemy, Syntaxis mathematica, III, i, 192 (ed. Heiberg).
144
Ptolemy says that Hipparchus noticed a difference of less than three degrees between his own
observations and those of Meton, of 431 B.C. (Syntaxis mathematica, VII, ii, 15–16, ed. Heiberg).
145
Ptolemy (Syntaxis mathematica, IX, ii, 211, ed. Heiberg) concedes this much to Hipparchus in
the midst of his efforts to deny him credit for a planetary theory (compare page 253).
146
Tops display precession clearly. They were common toys; see, for example, Callimachus’ epigram
in Anthologia graeca, VII, 89.
147
“Hic enim ait cometas in numero stellarum errantium poni a Chaldaeis tenerique cursus eorum”
(Seneca, Naturales quaestiones, VII, iv, 1). Recall that Seleucus was one of the Chaldeans (see
note 128 above).
Revision: 1.11 Date: 2003/01/06 02:20:46