The Eclipse of Thales stopped a battle on 28 May 585 BC

A total solar eclipse on 28 May 585 BC stopped a battle between the armies of Lydia and the Medes in Anatolia, according to Herodotus, and the two sides made peace. It is known as the Eclipse of Thales, after the Greek thinker said to have predicted it.

Herodotus says the war between King Alyattes of Lydia and King Cyaxares of the Medes had run for five years with neither side gaining the upper hand. In the sixth year, in the middle of a battle, the day was suddenly turned to night.

“So when the Lydians and Medes saw the day turned to night, they stopped fighting, and both were the more eager to make peace.” (Herodotus, Histories 1.74, trans. A. D. Godley)

Mediators from Cilicia and Babylon sealed the truce with a marriage: Alyattes’s daughter Aryenis wed Astyages, the son of Cyaxares.

Herodotus names no year, so astronomers had to pick the date from the sky:

• 30 September 610 BC: An eclipse in Alyattes’s reign, but it appears not to have been total in Asia Minor.
• 28 May 585 BC: The other eclipse in his reign, and a total one: NASA’s catalogue gives up to 6 minutes 4 seconds of darkness on the central line. This is the accepted date.

It is often called the oldest event that can be dated to the day, but it is not: the same NASA list includes earlier ones, such as the Assyrian eclipse of 15 June 763 BC.

How Thales managed it is the bigger puzzle. Herodotus says only that he fixed the eclipse “within the year”. His method is unknown; he may have used the Babylonian Saros cycle, a period of 223 lunar months (about 18 years and 11 days) after which similar eclipses recur. True or not, the prediction is celebrated as an early milestone in Greek science.