By the light of the moon

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.

They also had to understand Roman time measurement (there were always 12 hours between sunrise and sunset so the hour had to be elastic, according to season), and of course, they had to put themselves, and the readers of Sky and Telescope, in the position of an invasion force, ironclad and sword-wielding, on a hostile strand.

Olson has been illuminating history and brightening the skies in such ways for many years. He stared at Van Gogh's White House at Night, identified the evening star, and used its position to track down not just the original house but the exact day and hour at which Vincent must have begun his painting. He found a reference to Tycho Brahe's 1572 supernova in Hamlet ("the same star that's westward of the pole") and he dated a freak tide (Brittany, December 19, 1340) mentioned in Chaucer's The Franklin's Tale. He used astronomical experience to provide new information about Edvard Munch's The Scream and Van Gogh's 1889 painting Moonrise.

He proposed a revised date for the battle of Marathon in 490 BC - from which the modern Olympic endurance test takes its name - reasoning that if experienced 20th century runners don't collapse and die, then why would Pheidippides the fateful messenger fall dead as he delivered the news? That bit of research involved poring over Herodotus and Plutarch, a fresh look at the lunar cycle and a bit of juggling with the Spartan and Athenian calendars, to provide an intemperately hot August date (with the extra risk of heat stroke) for the battle that saved Athens from the Persians, rather than a milder September one. And of course, with a showman's eye for timing, he cracked the question in 2004, the summer of the Athens Olympics.

Do any of these things matter? Yes, because they provide another way of making sense of the world around us. Olson is not the only such adventurer: one Italian physicist reported in the journal Nature that the great poet Dante had described, 350 years in advance, the Galilean principle of invariance (check it out in Canto XVII of Inferno, the bit where Dante and Virgil board the monster Geryon for an aerial tour of one circle of hell).

Other scientists have tried to crack the secrets of Damascus steel (remember the bit in Walter Scott's The Talisman where Saladin slices a cushion in half?) and recreate the beer that ancient Egyptians might have quaffed and explain the fall of the walls of Jericho (not so much Joshua's trumpet, more of a timely earthquake), and even conjecture the conditions on planet Krypton that would select for the superhuman musculature of Superman.

All these are games with science: attempts to apply rigorous reasoning to events that are either one step away from myth or even complete nonsense.

But why not? To enjoy the story of Caesar and the storming of Deal (or Walmer) you now have to think about the moon and the tides and the cliffs of Dover bristling with spear-carrying warriors and the slow progress around the South Foreland to that awful moment when, with a heavy shield, a spear and a sword, you have to splash through the water onto a strange land, under attack from a fierce foe painted with woad. It adds a dash of excitement to the mathematics you had to learn to calculate gravitational forces, and it colours the leaden classroom memory of translation from Caesar's Commentaries.

In the supermarket of scholarship, this is the kind of value-added thinking that brings the punters in: buy astronomy, and get a turning point in history for free; choose a slice of Roman imperial adventure and feel the wind in your face and the tide racing up the English Channel.