The asymmetric sunrise effect on Thales' alleged measurement of the Sun angular size.

This article critically examines the historical claim that Thales of Miletus, a sixth-century BC Greek philosopher, was the first to measure the angular size of the Sun by timing the duration of sunrise, a method later described by Cleomedes in the second century AD. It introduces the "asymmetric sunrise effect," which shows that the proportionality between the Sun's angular diameter and the sunrise duration varies significantly with the observer's latitude and the time of year, causing systematic overestimation of the Sun's size by 26%–46% at Thales' likely location. Given that spherical trigonometry needed to correct for this effect was not developed until centuries later, the article concludes that Thales could not have obtained the roughly accurate solar diameter attributed to him using Cleomedes' timing method. It also discusses plausible experimental setups for timing sunrise with water clocks and argues that while the timing method could achieve about 10% accuracy, the asymmetric sunrise effect presents a fundamental limitation to its use in early Greek astronomy.