Astronomer Dates Caesar's Invasion of Britain

"Thank heaven for scientists who never mind their own business. Prof Donald Olson is an astronomer at Texas State University who has, for more than a decade, taken on a summer job as a historian. You may have heard him on Radio 4, delivering an update on his latest research: the date and place of Julius Caesar's momentous landing on the coast of Britain in 55 BC.

According to Olson, who reports on his intellectual adventures for Sky and Telescope magazine, there has always been an argument about quite when and where this happened. Caesar himself had no Ordnance Survey map, had sketchy local knowledge, and of course expected a lot of resentment from the locals. But he described, in book four of the Gallic Wars, the white cliffs of Dover, and a detour to find a more level playing field for the grim game ahead.

He did not give a precise date ("only a small part of the summer was left") but he was particular about the time he saw the hostile cliffs ("about the fourth hour of the day"), about how long he waited ("until the ninth hour"), and the distance he had to go ("about seven miles"). He also mentions, a bit later on, that on his fourth day in Britain as an illegal immigrant the cavalry reinforcements from Gaul were delayed by a storm, a full moon and an unusually high tide.

Such clues were enough to give scholars a crack at dating the invasion. Just as astronomers can predict future full moons, so they can confidently time them far in the past. So, they calculated, Caesar saw Dover but turned north-east and sailed around the South Foreland and landed at either Walmer or Deal on August 26 or 27. Latin scholars might have been happy with this conclusion, but hydrographers and astronomers were not; they calculated that the tides would be running the wrong way at the ninth hour of those days and take Caesar to the south-west.

So the team from Texas made the Julian date with destiny their summer assignment. They read all the texts, checked the tidal patterns, turned up in Dover in August 2007 just when the equinox and lunar cycle coincided to replicate the tidal conditions that Caesar reported, and figured that the problem could be sorted by assuming an easily-made clerical error by someone who copied the original manuscript. If so, time and tide would have been just right for a landing at Deal on August 22 or 23. Case closed - possibly.

This is arcane science applied to ancient history, and it makes both subjects lively. To get a more complete picture of the problem, Olson and his colleagues and students had to read Dio Cassius, and study Valerius Maximus, a chronicler from the first century AD. They had to consult classicists and archaeologists, match the Julian and Gregorian calendars, examine the verdicts of Victorian astronomers and naval hydrographers and use global positioning satellites to measure their own progress around the Channel coast."