Researchers say that references to planets and constellations in the Odyssey describe a solar eclipse that occurred in 1178 B.C., nearly three centuries before Homer is believed to have written the story. If correct, the finding would suggest that the ancient poet had a surprisingly detailed knowledge of astronomy.

The Odyssey, commonly dated to near 800 B.C., describes the 10-year voyage of the Greek general Odysseus to his home on the island of Ithaca after the fall of Troy in approximately 1200 B.C. Toward the end of the story, a seer named Theoclymenus prophecies the death of a group of suitors competing for the affection of Penelope, the wife of Odysseus, who is believed to be dead. Theoclymenus delivers his prophecy as the suitors are sitting down for their noontime meal. He foresees them entering Hades and ends his speech with the statement, "The Sun has been obliterated from the sky, and an unlucky darkness invades the world." Odysseus dispatches the suitors not long thereafter.

Greek scholars Plutarch and Heraclitus advanced the idea that Theoclymenus's speech was a poetic description of an eclipse. They cited references in the story that the day of the prophecy was a new moon, which would be true of an eclipse. In the 1920s researchers speculated that Homer might have had a real eclipse in mind, after calculating that a total solar eclipse (in which the moon blocks out the sun) would have been visible on April 16, 1178 B.C. over the Ionian Islands, where Homer's poem was set. The idea languished, however, because the first writings on Greek astronomy did not come until centuries later.

Inspired by an incorrect reference to Homer's alleged eclipse in an astronomy textbook, biophysicists Constantino Baikouzis and Marcelo Magnasco, both at the Rockefeller University, pored over the Odyssey for additional clues. Sailing back to Ithaca on a raft, Odysseus navigates by monitoring the constellations Pleiades and Bootes, which share the sky twice a year in March and September. The morning he arrives in Ithaca, Venus rises in the sky before dawn, which happens on about one third of new moons. But the crucial clue came from a reference to the god Hermes flying west to the island of Ogygia. The researchers propose the god's voyage actually refers to the planet Mercury, which hangs low in the sky and reverses course from west to east every 116 days.

Baikouzis and Magnasco used commercial astronomical software to scan all 1,684 new moons between the years 1250 and 1125 B.C. for dates that matched those conditions. "Even though each event happens individually often, the pattern doesn't repeat that often," Magnasco says—only once every 2,000 years, he and Baikouzis report in Proceedings of the National Academy of Sciences USA. And one of those matches is April 16, 1178 B.C. Magnasco says that he had a hunch about Hermes, known as Mercury to the Romans, after coming across the idea that the Greeks used the stories of the gods as mnemonic devices to remember astronomical events. Taking Mercury out of the mix leaves 15 matching dates in the 135-year search period, he adds.

According to Magnasco, the references imply that Homer chose to set the slaying of the suitors on the day of the eclipse. (He notes that he has "no clue" whether similar events to those described in the Odyssey actually occurred on that day.)