While it has been highly improbable in establishing the exact chronology of the Battle of Thermopylae since it occurred approximately 2,500 years ago, several authors have promulgated certain theories as to the date of the battle in 480 B.C. Based on their hypotheses, the following authors have concluded that the battle commenced on the following day:
August 2nd or 3rd, 480 B.C.
Charles Hignett - Xerxes Invasion of Greece (1963)
August 17th, 480 B.C.
Paul Cartledge - Thermopylae - The Battle that Changed the World (2006)
August 18th, 480 B.C.
Peter Green - Xerxes at Salamis (1970) , revised edition The Greco-Persian Wars (1996)
Ernle Bradford - The Year of Thermopylae, (1980) U.S. edition The Battle for the West, Thermopylae
August 19th or 20th, 480 B.C.
Philip Steele - Thermopylae (1993)
September, 480 B.C.
A. R. Burn - The Persian Wars (1962), revised edition Persia and the Greeks (1984)
While this is in no way a comprehensive list, it shows that there isn't unanimity amongst the authors and academics as to the dating of the battle.
Appearance of Thermopylae, past and present
Any account of the Battle of Thermopylae must make mention of the pass which according to Herodotus was approximately 50 feetwide in 480 B.C. Strategically speaking, the width of the pass was of paramount importance since it negated the strength of the Persian army which has been conservatively numbered on average at 200,000 warriors.
While Herodotus wrote that the Persian army consisted of 1.7 million warriors, academics have reduced this number to a much more realistic figure, however, it still should be emphasized that these forces still overwhelmingly outnumbered the Greek garrison of 7,000 hoplites led by King Leonidas and his 300 Spartiates.
It is difficult to envision how the pass looked approximately 2,500 years ago since the coastline has receded several miles due to the alluvial deposits. Therefore, for comparative purposes, a reconstruction (courtesy of the Perseus Project) of how Thermopylae may have appeared at the time of the battle is included next to an image of the status quo.
Digital reconstruction of Thermopylae 480 B.C., courtesy of the Perseus Project
Thermopylae, present day
As can be seen from viewing the two images above, the battlefield of Thermopylae would have extended from the mountains as they appear on the left to the modern road which can be viewed in the second photo.