The true date of Meton’s observation of the summer solstice

The date assigned to Meton’s highly reputed observation of the summer solstice in the Almagest implies June 27, 432 BC. Since the solstice took actually place a day later, such an inaccuracy presents a puzzle. It can be demonstrated, however, that Meton’s observation was in fact accurate, for he made it on June 28, 433 BC. This follows from adequate interpretation of chronological indications in Thucydides and finds support in various data of the ancient sources. The mistaken date arose, and was maintained, because of the misleading assumption according to which the Athenian archon year invariably began after the summer solstice. It was wrongly decided that Meton had observed the summer solstice at the end of the year of the archon Apseudes and not at its initial part. The true date of Meton’s solstice presents in new light the observation of the summer solstice by Aristarchus of Samos, Eudoxus’ preoccupation with the octaeteris and early Greek astronomy in general. The beginning of the Peloponnesian War is now firmly established at early April (almost certainly, April 6), 431 BC.