Based on information from Herodotus's The Histories Book VII, the date of Ephialtes' betrayal and use of the mountain path by the Immortals can be narrowed to a few days in September of 480 BC, as follows. Not knowing the terrain, they would have needed some form of light, but torches would have given away their intent. They therefore traversed the path when light from the moon would be the greatest - the full moon. In Book VII Herodotus mentions the solar eclipse that occurred at the crossing of the Hellespont by the Persians. By estimating the distance the Persian army could move each day, it can be established that the battle took place around September of 480 BC. Tracing back via a lunar calendar, the date of the betrayal can be narrowed to September 18, 19, or 20, 480 BC.