The Eclipse of Thales was a solar eclipse that, according to The Histories of Herodotus, was accurately predicted by the Greek philosopher Thales of Miletus. If Herodotus’s account is accurate, this eclipse is the earliest recorded as being known in advance of its occurrence. Many historians believe that the predicted eclipse was the solar eclipse of 28 May 585 BC. How exactly Thales predicted the eclipse remains uncertain; some scholars assert the eclipse was never predicted at all. Others have argued for different dates, but only the eclipse of 28 May 585 BC matches the conditions of visibility necessary to explain the historical event.
According to Herodotus, the appearance of the eclipse was interpreted as an omen, and interrupted a battle in a long-standing war between the Medes and the Lydians. The fighting immediately stopped, and they agreed to a truce.
The Battle of the Eclipse or Battle of Halys was fought between the Medes and the Lydians in the early 6th century BC. Herodotus writes that in the sixth year of the war, the Lydians under King Alyattes and the Medes under Cyaxares were engaged in an indecisive battle when suddenly day turned into night, leading to both parties halting the fighting and negotiating a peace agreement. Herodotus also mentions that the loss of daylight had been predicted by Thales of Miletus.
Herodotus’ The Histories 1.73–74 states that a war started in the period between the Medes and the Lydians. There were two reasons for the war: the two sides had clashing interests in Anatolia, but also there was a motive of vengeance: some Scythian hunters employed by the Medes who once returned empty-handed were insulted by King Cyaxares. In revenge the hunters slaughtered one of his sons and served him to the Medes. The hunters then fled to Sardis, the capital of the Lydians. When Cyaxares asked for the Scythians to be returned to him, Alyattes refused to hand them over; in response, the Medes invaded.
Afterwards, on the refusal of Alyattes to give up his suppliants when Cyaxares sent to demand them of him, war broke out between the Lydians and the Medes, and continued for five years, with various success. In the course of it the Medes gained many victories over the Lydians, and the Lydians also gained many victories over the Medes. Among their other battles there was one night engagement. As, however, the balance had not inclined in favour of either nation, another combat took place in the sixth year, in the course of which, just as the battle was growing warm, day was on a sudden changed into night. This event had been foretold by Thales, the Milesian, who forewarned the Ionians of it, fixing for it the very year in which it actually took place. The Medes and Lydians, when they observed the change, ceased fighting, and were alike anxious to have terms of peace agreed on.
As part of the terms of the peace agreement, Alyattes’s daughter Aryenis was married to Cyaxares’s son Astyages, and the Halys River (now known as the Kızılırmak River) was declared to be the border of the two warring nations.
If one reads the description by Herodotus of the event as a solar eclipse, then based on modern astronomical calculations it can be identified with the solar eclipse of May 28, 585 BC (known as Eclipse of Thales), hence yielding the exact date of the battle. For the location of the battle, some scholars assume the Halys River as it was located in the border region between both kingdoms.
However, such a reading is for a variety of reasons rather problematic and hence disputed by various scholars. For example, the known astronomical knowledge available of that time was not sufficient for Thales to predict the eclipse. Also, the eclipse would have occurred shortly before sunset at any plausible site of the battle, and it was very uncommon for battles to take place at that time of day. Furthermore, based on the list of Medean kings and their regnal lengths reported elsewhere by Herodotus, Cyaxares died 10 years before the eclipse.
While doubt has been cast on the truth of the story, there are other accounts of it besides that of Herodotus. Diogenes Laërtius says that Xenophanes, who lived in the same century as Thales, was impressed with the prediction, and he also gives additional testimonies from the pre-Socratics Democritus and Heraclitus.
At the time of Thales’ purported prediction it was not yet known that eclipses were caused by the Moon coming between the Earth and the Sun, a fact that would not be discovered until over a century later by either Anaxagoras or Empedocles.
If the account is true, it has been suggested that Thales would have had to calculate the timing of any eclipse by recognizing patterns in the periodicities of eclipses.
It has been also postulated that Thales may have used the Saros cycle in his determination, or that he may have had some knowledge of Babylonian astronomy. However, Babylonians were far from being able to predict the local conditions of solar eclipses at that point, which makes this hypothesis highly unlikely. In fact, there is no known cycle that can be reliably used to predict an eclipse for a given location and, therefore, any accurate prediction would have been down to luck.
Because astronomers can calculate the dates of historical eclipses, Isaac Asimov described this battle as the earliest historical event whose date is known with precision to the day and described the prediction as “the birth of science“.