The Boston Marathon, New York City Marathon, and all the other marathon races held worldwide trace their origin to this battle in ancient Greece and the well-known story of a messenger who ran about 26 miles from the battlefield back to Athens and died when he arrived. The ancient calendars and uncertain records have made it difficult to date these events precisely. But the ancient sources describe the lunar phase near the time of the battle, and we can calculate when the Battle of Marathon occurred using these astronomical clues.