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.
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