ABSTRACT
The sources differ in detail; but they are reporting a rather complex idea. The three statements in Diogenes Laertius cohere rather well: Thales managed to predict the tropai, discovered the passage from one trope to the other, and discovered the times of the year – likely based on the dates of both tropai and equinoxes. It is obvious, then, that Thales must have done more than to teach what many people already knew, that is, that the tropai take place twice a year. As an important mathematician, his name was closely associated with, among other things, the idea of concentric heavenly spheres, whose movement could explain anomalies which seemed to affect the motion of the planets. The equinoxes too are phenomena with a high level of regularity. It is therefore very difficult to identify a plausible reason for speaking of irregularities in relation to tropai.
