A solar eclipse during the Battle of Halys led to a truce between the kingdoms of Media and Lydia, making it the earliest historical event that can be precisely dated.

The Eclipse of Thales was recorded in The Histories of the Greek historian Herodotus. He claims that the philosopher Thales of Miletus accurately predicted the eclipse in advance, marking what science writer Isaac Asimov later described as ‘the birth of science’.

Herodotus writes that the Lydians under King Alyattes II and the Medes under Cyaxares had been at war for five years over their competing interests in Anatolia. The spark had been a desire for revenge over the killing of one of Cyaxares’ sons by nomadic hunters who were subsequently given protection by the Lydians. Having fought a series of indecisive battles in the preceding years, the two armies met again in 585 BCE during which a solar eclipse took place.

There is some doubt over Herodotus’ claim that Thales predicted the eclipse in advance, especially as no records survive regarding exactly how he made his calculations. However, the eclipse was also recorded in other accounts. Herodotus describes how ‘suddenly the day became night’ and that the warring armies interpreted this as an omen to stop fighting. The peace was sealed by Alyattes’ daughter marrying one of Cyaxares’ surviving sons.

Later astronomers were able to pinpoint the exact date of historical eclipses, using the same calculations that help to predict future ones. By combining data of these ancient events with contextual knowledge of the Battle of Halys, 28 May 585 BCE was consequently identified as the most likely date. This makes the day of the battle a cardinal date, meaning it provides a waypoint from which numerous other dates in the ancient world can be calculated.

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