“Thales’s Spherical Earth
Modern commentators assume that Thales regarded the earth as flat, thin, and circular, but there is no ancient testimony to support that opinion. On the contrary, Aristotle may have attributed knowledge of the sphericity of the earth to Thales, an opinion which was later reported by Aëtius (Aët. III. 9-10) and followed by Ps.-Plutarch (Epit. III.10). Aristotle wrote that some think it spherical, others flat and shaped like a drum (Arist. Cael. 293 b33-294 a1), and then attributed belief in a flat earth to Anaximenes, Anaxagoras, and Democritus (Arist. Cael. 294 b14-15). If following chronological order, Aristotle’s words, ‘some think it spherical’, referred to the theory of Thales. Aristotle then followed with the theory of Thales’s immediate Milesian successor, Anaximander, and then reported the flat earth view of Anaximenes, the third of the Milesian natural philosophers.
There are several good reasons to accept that Thales envisaged the earth as spherical. Aristotle used these arguments to support his own view (Arist. Cael. 297 b25-298 a8). First is the fact that during a solar eclipse, the shadow caused by the interposition of the earth between the sun and the moon is always convex; therefore the earth must be spherical. In other words, if the earth were a flat disk, the shadow cast during an eclipse would be elliptical. Second, Thales, who is acknowledged as an observer of the heavens, would have observed that stars which are visible in a certain locality may not be visible further to the north or south, a phenomena which could be explained within the understanding of a spherical earth. Third, from mere observation the earth has the appearance of being curved. From observation, it appears that the earth is covered by a dome. When observed from an elevated site, the sky seems to surround the earth, like a dome, to meet the apparently curved horizon. If observed over the seasons, the dome would appear to revolve, with many of the heavenly bodies changing their position in varying degrees, but returning annually to a similar place in the heavens. Through his work in astronomy Thales would almost certainly have become familiar with the night sky and the motion of the heavenly bodies. There is evidence that he gave advice to navigate by Ursa Minor, and was so involved in observation of the stars that he fell into a well. As a result of observations made over a long period of time, Thales could have realized that the motions of the fixed stars could not be explained within the idea of the observable hemispherical dome. During the determination of the size of the rising sun, and again while watching its risings and settings during his work on fixing the solstices, Thales may have realized that much natural phenomena could be explained only within the understanding of the earth as a sphere.
From the shore, a ship can be seen to be descending, gradually, below the horizon, with the hull disappearing from view first, to be followed by masts and sails. If one had a companion observing from a higher point, the companion would see the ship for a long period before it disappeared from view.
Aëtius recorded the different opinions of the shape of the earth that were held by Thales, Anaximander and Anaximenes (III.9-10; III.10; and III.10). Cicero attributed to Thales the earliest construction of a solid celestial globe (Rep. I.XIII.22). Thales’s immediate successors proposed theories about the shape of the earth which were quite different from each other, but that is no reason to reject the view that Thales hypothesized a spherical earth. It is not the only occasion on which Anaximander and Anaximenes failed to follow the theories of Thales. That they did not do so is the main argument in favour of accepting that the scientific method commenced in the Milesian School. There is testimony that Thales knew the earth to be spherical, but no evidence to suggest that he proposed any other shape.”