1 The Birth of Science - MSRI

252 10. Lost Science
discussed in the Almagest and in the Optics, 83 but the two explanations
offered are completely different — one based on refraction and the other
psychological — and neither passage mentions the other.
One particular, about geography, will illustrate Ptolemy’s methodology.
Both Eratosthenes and Hipparchus chose Alexandria for defining zero
longitude, 84 much as British astronomers adopted Greenwich. Ptolemy, page 314
in his Geography, prefers instead to reckon longitudes from the faraway
and shadowy “Blessed Islands”. Why on earth? Evidently in view of these
islands’ property of being on the leftmost edge of the map. This choice of
a reference meridian is not particularly useful for compiling new maps,
nor for navigation, but once established it is the most convenient for data
transfer and armchair geography, since it avoids the need to specify east
or west.
The Almagest presents a system for predicting the motion of the planets
but no explanation about how the system was obtained. In other words,
the book gives a recipe or algorithm, which depends on certain parameters
also given, but it does not say how the parameters can be derived from
experimental data. As in other fields, so in astronomy too: knowing how
to build theories no longer matters, only how to use them.
For it to be true that the Almagest incorporated all the astronomical
knowledge present in earlier works, it would have been necessary for
Ptolemy to have known them and mastered their methods thoroughly.
The considerations above cast doubt on whether this second condition
was satisfied. As to the first, we have mentioned evidence of Ptolemy’s
incomplete knowledge of Hipparchus’ works, regarding instrumentation
(Ptolemy shows no knowledge of the dioptra described by Heron, which
in all likelihood goes back to Hipparchus) and geography (Ptolemy is ignorant
of the length of a degree of the meridian, which was measured by
Eratosthenes and which Hipparchus knew well). 85 More direct evidence
is provided by Ptolemy himself, who writes:
Hipparchus did not even begin to formulate theories for the planets,
at least in the works that have reached me. 86
The disclaimer, which may seem due to plain conscientiousness, 87 gains
its full import in the light of Ptolemy’s awareness of the titles of all works
by Hipparchus; indeed, earlier in the Almagest he had cited verbatim a
passage from a Catalogue of my own works by Hipparchus. 88
83
Ptolemy, Almagest, I, iii, 13 (ed. Heiberg); Optics, III, 59 = 115, 16 – 116, 8 (ed. Lejeune).
84
We know this from Strabo (Geography, I, iv, 1).
85
See pages 61 and 239.
86
Ptolemy, Syntaxis mathematica, IX, ii, 210 (ed. Heiberg).
87
Toomer takes it so: [Ptolemy/Toomer], p. 421, note 10.
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