Among historians, it is generally accepted that the first marathon runner, Phidippides, died after reaching Athens in 491 B.C.E. What hasn’t been determined is why. Thousands of people successfully run 26.2 miles every year, but Phidippides died after crossing the finish line.

Now a pair of Texas professors believe they may have solved the mystery of the first marathon death 2,500 years ago. If the Battle of Marathon took place a month earlier than previously thought, in August rather than September, that would make it 8 degrees hotter in Greece, and much less favorable to running a long distance in the arid mountains.

The conventionally accepted date of the battle is September 12, 491 B.C.E., calculated by a 19th-century historian named August Bloeckt. A well-respected classicist of the period, Bloeckt found a way to equate two lunar months from the Athenian and the Spartan calendars in 413 B.C.E., and he worked backward to a Spartan festival that is believed to coincide with the battle.

Earlier this year, astronomer Donald Olson read an article about Bloeckt’s work and decided to research the original Spartan calendar. He and his fellow researcher, Russell Doescher, soon discovered that the Spartan calendar begins on the fall equinox, while the Athenian calendar begins in the summer. They calculated the lunar cycles going back to 491 B.C.E. and discovered that the Spartan calendar was a month ahead of the Athenian calendar at the time.The correct date for the battle of Marathon had to be August 12.

“It helps make it more plausible that the runner, after running, would die,” said Mr. Doescher.

For the unfortunate runner, August was a much worse month to make the long, dry trek to Athens. The average temperatures in that area of Greece in August is 91 to September’s 83. Near Athens itself, temperatures can reach 102.

“The real critical cutoff is the 80/80 point, where it’s 80 degrees Fahrenheit and the humidity is 80%. Where temperatures range above that, we call it the red zone,” said an expert in sports medicine at Beth Israel, Dr. Robert Gotlin. August is “likely the 80/80 month, so the relative risk is huge for that time of the year.”

This year’s Boston Marathon in April had temperatures that reached 83 degrees and high levels of humidity. The two finishers from 2003 dropped out from heat exhaustion, and the medical tent treated triple the number of patients from 2003 – 969 racers, or 5% of the total race.

Phidippides didn’t have the benefit of water and Gatorade every mile. Herodotus’s account of the run suggests he didn’t stop at all, and there were few streams in the dry mountains between Marathon and Athens.

As his body became more dehydrated, he probably passed from heat exhaustion to heat stroke, a phase in which, according to Dr. Gotlin, he stopped sweating, his skin became very dry, and various bodily functions began to shut down. Even as he said nike, or victory, to his compatriots, his heart likely gave out on him, unable to sustain him under the strain in the August heat.