"That during the battle the day was suddenly turned to night. Thales of Miletus had foretold this loss of daylight to the Ionians, fixing it within the year in which the change did indeed happen."
Herodotus, The Histories, Book 1, Chapter 74, Section 2
Imagine yourself as an ancient warrior. You rise at dawn with your fellow soldiers and gather together on the field of battle. You engage in combat with the enemy for most of the day. The fighting is fierce. Then suddenly, the day darkens and turns into night. For many frightening minutes, you and everyone else in the battle pause unsure what is going on. The light returns. You decide this is an omen, and you throw down your weapon. After five years of fighting, the commanders of the two opposing armies decide to call a truce.
The Greek historian Herodotus describes such a battle. In his book The Histories, over a hundred years after the event, Herodotus tells the story of a war between two kingdoms: the Lydians (located in modern Turkey) and the Medes (the center of their empire was in Iran). Both sides were equally matched. Each side won key battles, but could not totally claim victory. In the sixth year of the war, a battle occurred (Herodotus gives us no clue exactly where) and during the engagement, it is interpreted that an eclipse happened. Upon seeing the eclipse, the two sides decided to conclude peace and thereby ending the war. Then Herodotus adds an interesting detail. He claims a certain Thales from Miletus predicted such an eclipse would occur.
Thales was a Greek philosopher who lived during the latter half of the 6th century B.C.E. He lived in the Greek city of Miletus on the Ionian coast of present-day Turkey. He was noted for his work in mathematics and astronomy. He was one of the first thinkers to separate myth from reason and broke away from using mythical deities to explain natural phenomena. Later classical writers considered him the founder of classical science. There is no extant writing by Thales therefore much of what we know about him has come to us from later authors. He is known as the discoverer of what is called Thales Theorem in geometry. He demonstrated the practical side of mathematics when he calculated the height of the pyramids and how far ships were from the coastline. He predicted the weather favorable for a good olive harvest. In one version of the story, based on this he bought olive presses and then rented them out at high prices. In another version, he bought the presses at a discount, then he rented them out when demanded increased. Therefore, he is credited for being the first to successfully earn money from financial futures or options. He is noted for determining accurately when the solstices and equinoxes occur, and he made the relationship between them and the seasons and the length of the year. He suggested using the constellation of Ursa Minor, which is where the North Star is located, for navigation instead of neighboring Ursa Major. Overall, as per Aristotle, Thales wished to show that by understanding the basic principles of nature then problems could be solved.
But did Thales actually predict a solar eclipse? Despite his renown as a natural philosopher, there is some question as to if he performed this feat as Herodotus tells us. Part of the problem is with Herodotus himself. Much of what he wrote in his Histories has been verified, but there are many events that Herodotus relates that are doubtful. The historian recounts many events that happened prior to his lifetime. These he heard either by hearsay, oral tradition, or third party sources. He hardly verifies the sources and often is simply retelling the story. This may or may not be the case in this instance.
Modern astronomers have determined there was an eclipse that occurred on May 28, 585 B.C.E. Researchers believe that this is most likely the eclipse Herodotus describes if his account is accurate. It is suspected that the battle he describes took place along the Halys River in central Turkey. The path of totality for the eclipse would have been over the battle. (Thales, himself, in Miletus would have only witnessed a partial eclipse.) Based on the reigns of Medean kings, there are many classical scholars who doubt that the battle Herodotus refers to took place on the date of this particular eclipse.
Assuming that this is the battle and then the question remains how Thales was able to predict this eclipse. Historians have muddled over this with various degrees of speculation. There is a good many who believe that this is simply a fabrication. Given Thales' reputation as a mathematician and astronomer, it is quite possible that this is just a story handed down over generations until it reached Herodotus. As far as we know, Thales had no access to scientific data or instruments that would allow him to make such a prediction. Herodotus only points out that Thales predicted the year of the eclipse. This odd, as mathematician Dmitri Pachenko points out: "if one can predict an eclipse at all, one can predict it to the day.” Astronomy is a very precise science.
Others have pointed out that it is possible that Thales had access to the records of the Babylonians. The Babylonians had kept accurate records, not only of eclipses but of many other celestial events. Miletus was a major commercial center in the Greek world, and ideas and goods flowed from East to West through the city. The Babylonians were aware of a cycle governing solar eclipses. One that we today call the Saros cycle. In the Saros cycle, eclipses are grouped into a series lasting 223 months. After three of these series, a solar eclipse will reoccur in a particular geographic region. But they are much more complicated than that. At any given time, there may be as many as 40 such cycles carrying on for over 1000 years. Old cycles end and new ones begin. Most of all, for the Saros cycle to be useful in predicting eclipses, one must have the knowledge that the Earth is round. There is no evidence that shows Thales understood this basic fact. From what we can interpret of his philosophy, he seems to have held the belief that the Earth was a flat disc that floated in water. (According to Thales, this is why we have earthquakes.) The Babylonians knew about the cycles, but there is no record of them successfully predicting an eclipse. Modern scientists have tried to predict the 585 B.C.E. eclipse using the Saros cycle but found that it was not feasible given the data Thales would have had at the time.
If Thales indeed predicted this eclipse, then his method of doing so has been lost. In 1969, scientist Willy Hartner created a list of 29 eclipses. These would have been during the same time period and Thales would have been able to observe them. There are patterns among those, but it is doubtful, according to Hartner, that Thales would have understood them enough to make any kind of predictions. At the least, he might have been able to predict the eclipse one year later on May 18, 584 B.C.E.. When an eclipse occurred a year early, Thales might have just shrugged it off as being close enough. Another scientist, Dirk Couprie has a simpler answer. Thales' prediction was just "lucky coincidence." Regardless, whatever method Thales used, it apparently worked only once. It would be several centuries before anyone else would be able to come close to predicting a solar eclipse.
I think a major point is lost in the discussion of whether Herodotus' story is true or not. What is important is that Thales, and many of his contemporaries, were not seeking answers in the spiritual world, but in nature. They believed that the real world operated on underlying principles that could be deduced through the use of observation and reason. Once those principles were known, then they could be applied to everyday life from commerce to war. Thales may have tried to predict solar eclipses. He may or may not have been successful at it. But he had the vision to see an eclipse not as a mythological monster eating the sun, but as a natural phenomenon that can be studied, controlled, and made to seem less ominous.
Further reading:
Was the First Eclipse Prediction an Act of Genius, a Brilliant Mistake, or Dumb Luck: Natasha Frost (Atlas Obscura)
Thales Casts a Long Shadow: How a Solar Eclipse Heralded Classical Education: Marc Hayes (Classical Conversations)
Early Greek Science: Thales to Aristotle: Geoffrey E.R. Lloyd