From Chapter I., The Milesian School
3. The Eclipse Foretold by Thales
Now it is possible to predict eclipses of the moon approximately without knowing their true cause, and
there is no doubt that the Babylonians actually did so. It is generally stated, further, that they had made
out a cycle of 223 lunar months, within which eclipses of the sun and moon recurred at equal intervals
of time.11 This, however, would not have enabled them to predict eclipses of the sun for a given spot on
the earth's surface; for these phenomena are not visible at all places where the sun is above the horizon
at the time. We do not occupy a position at the centre of the earth, and the geocentric parallax has to
be taken into account. It would only, therefore, be possible to tell by means of the cycle that an eclipse
of the sun would be visible somewhere, and that it might be worth while to look out for it, though an
observer at a given place might be disappointed five times out of six. Now, if we may judge from
reports by Chaldaean astronomers which have been preserved, this was just the position of the
Babylonians in the eighth century B.C. They watched for eclipses at the proper dates; and, if they did
not occur, they announced the fact as a good omen.12
To explain what we are told about Thales no
more is required. He said there would be an eclipse by a certain date; and luckily it was visible in Asia
Minor, and on a striking occasion.13
8. Herod. i. 74.
9. For the theories held by Anaximander and Herakleitos, see infra, §§ 19, 71.
10. Diog. i. 23, δοκεῖ δὲ κατά τινας πρῶτος ἀστρολογῆσαι καὶ ἡλιακὰς ἐκλείψεις καὶ τροπὰς προειπεῖν, ὥς φησιν Εὔδημος ἐν τῇ Περὶ τῶν ἀστρολογουμένων ἱστορίᾳ, ὅθεν αὐτὸν καὶ Ξενοφάνης καὶ Ἡρόδοτος θαυμάζει. The statement that Thales "predicted" solstices as well as eclipses is not so absurd as has been thought. Eudemos may very well have meant that he fixed the dates of the solstices and equinoxes more accurately than had been done before. That he would do by observing the length of the shadow cast by an upright (γνώμων), and we shall see (p. 47) that popular tradition ascribed observations of the kind to him. This interpretation is favoured by another remark of Eudemos, preserved by Derkyllides (ap. Theon. p. 198, 17 Hiller), that Thales discovered τὴν κατὰ τὰς τροπὰς αὐτοῦ (τοῦ ἡλίου) περίοδον, ὡς οὐκ ἴση ἀεὶ συμβαίνει. In other words, he discovered the inequality of the four seasons which is due to the solar anomaly.
11. It is wrong to call this the Saros with Souidas; for sar on the monuments always means 602=3600, the number of the Great Year. The period of 223 lunations is, of course, that of the retrograde movement of the nodes.
12. See George Smith, Assyrian Discoveries (1875), p. 409. The inscription which follows was found at Kouyunjik:—
"To the king my lord, thy servant Abil-Istar.
"Concerning the eclipse of the moon of which the king my lord sent to me; in the cities of Akkad Borsippa, and Nipur, observations they made, and then in the city of Akkad, we saw part . . . . The observation was made, and the eclipse took place.
"And when for the eclipse of the sun we made an observation, the observation was made and it did not take place. That which I saw with my eyes to the king my lord I send." See further R. C. Thomson, Reports of the Magicians and Astrologers of Nineveh and Babylon (1900).
13. Cf. Schiaparelli, "I primordi dell' Astronomia presso i Babilonesi" (Scientia, 1908, p. 247). His conclusion is that "the law which regulates the circumstances of the visibility of solar eclipses is too complex to be discovered by simple observation," and that the Babylonians were not in a position to formulate it. "Such a triumph was reserved to the geometrical genius of the Greeks."
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