ABSTRACT
Diogenes Laertius reports that Thales divided the year into 365 days and ‘found’ the seasons of the year. There is no counter-indication to these converging voices; and we should keep in mind that, except Anaximander no one else showed any interest in these findings for a century or more after Thales. The hypothesis put forward by Charles Kahn tries to ignore this need. Kahn started from the observation that, on the day of the equinox, and on no other day of the year, the morning shadow and the evening shadow together form a right angle. A page of Cleomedes of Astypalaea – an astronomer who lived six or even ten centuries after Thales, and whose Caelestia is expressly dedicated to spreading the astronomical knowledge of the Stoics – has the singular merit of describing a method of detection very similar to the one hypothesized above.
