1 The Birth of Science - MSRI

10.8 The Shape of the Earth: Sling or Ellipsoid? 269
of these motions. Since no other argument in support of heliocentrism has
ever been found in classical literature, apart from the one reconstructed in
Section 10.5, it seems reasonable to suppose that Seleucus based both his
theory of tides and his proof of heliocentrism on the sun’s gravitational
effect.
Note that if one thinks (as suggested by the passages in Plutarch and
Seneca) that lunar attraction and centrifugal force act on the earth jointly page 335
and in opposite directions, it is not hard to accept that high tide takes
place simultaneously at antipodal points. Thus the dynamical explanation
of heliocentrism turns out to open up avenues for justifying theoretically
a fundamental observed property of tides.
In trying to track the origin of the ideas about gravity that seem to
be present in Hipparchus, we end up with Seleucus: probably a contemporary
of Hipparchus, but older. One could perhaps conjecture that the
younger scientist accepted and developed the notion, first advanced by
Seleucus, of a gravitational interaction with the sun. This conjecture cannot
be proved, but it is consistent with Strabo’s remark that Hipparchus
regarded Seleucus as an authority on tides. 131
10.8 The Shape of the Earth: Sling or Ellipsoid?
Some histories of geography say that one of the shapes given the earth in
Antiquity was a sling. Behind this strange claim are obscure statements
in late sources. Agathemerus, writing probably in the third century A.D.,
says that Posidonius described the ge (earth or land) as sling-shaped. 131a
The twelfth-century Byzantine archbishop Eustathius writes in a commentary
on Homer: “Posidonius the Stoic and Dionysius say that the ge is
shaped like a sling.” 132
We don’t know what Posidonius really said. As to Dionysius, a poet
from the third century A.D. (nicknamed Periegetes from the title of his
short geographical poem, Oikoumenes periegesis or Sketch of the world), the
expression he uses is like a sling (¦ ).
Commentators have tried to save the day by taking ge to mean the dry
land, a natural meaning of the word but one that makes the analogy no
less enigmatic. As for a sling-shaped earth, the idea is just too odd to
131 Strabo, Geography, I, i, 9. See [Russo: Seleuco] for an analysis of all the testimonia on Seleucus,
and in particular of a passage of Aetius ([DG], 383, 9) that has, in my opinion, led astray many
scholars, including Galileo.
131a ¥ © ¦ and he continues
© (Agathemerus, I, i).
132 Eustathius, In Homeri Iliadem, vi, 446.
Revision: 1.11 Date: 2003/01/06 02:20:46