It's not every day that a famous historical event, scrutinized by generations of classical scholars, can be re-dated by two astronomers and their college honors class. But that's exactly what Donald W. Olson and Russell Doescher of Texas State University did, with the help of students Kellie Beicker and Amanda Gregory. They report their findings in the August 2008 Sky & Telescope, which has just hit the newsstands.

Tipped off by Don in advance, I was fortunate to be able to join the team's research trip to the southern coast of England last summer. The white cliffs of Dover, subject of a memorable song from World War II (listen), were also the setting for a much earlier clash of civilizations. Along this very shore, Julius Caesar first landed with two legions of Roman soldiers in 55 BC.

Caesar, in his first-hand account of the invasion, carefully noted the phase of the Moon, the approach of the equinox, and above all the unexpected ocean tides his fleet encountered. So it's a simple matter for any astronomer to determine the precise date of the invasion, right?

Wrong! No lesser astronomers than Edmond Halley and George B. Airy carefully studied the astronomical aspects of 55 BC in hopes of letting historians know the exact date and location where Caesar and his legions came ashore. But Airy and Halley disagreed with each other. And what's more, they both got it partly wrong, as Olson's Texas State team found out on their research trip.

Some years back, Don realized that the summer of 2007 offered a unique chance to settle this tricky problem once and for all. In 55 BC the full Moon came about three days before lunar perigee and about 3.5 weeks before the equinox, just as in 2007, so the key tidal factors would be virtually identical. On less than a dozen dates in the last 2,061 years has this match been so good. August 2007 offered the perfect chance to find out just where and when Caesar came ashore in 55 BC.

What Needed to Be Determined

There were two top uncertainties to answer about the ocean currents when the Roman fleet arrived off the white cliffs of Dover:

 (1) Which way was the current flowing on the traditionally accepted invasion date on the afternoon of August 26 or 27, 55 BC?

 (2) Which way was the current flowing on an invasion date four days earlier, one that the Texas State researchers had already started to focus on?

To address the first question, our group went to the coastal town of Deal, the area historians have long believed to be the Roman fleet's eventual landing spot because it's roughly seven miles north of the stretch of white cliffs Caesar says he first encountered. That beach is indeed "open and flat," just as Caesar described. I noticed it wasn't sandy at all, being thickly paved with golf-ball-size pebbles its entire length, and wouldn't have been an easy place for Roman warriors to scramble ashore as they dodged a hail of spears and arrows from Britain's hostile Celtic tribes.

On the date in 2007 that corresponded closely to August 26 or 27, 55 BC, we walked out to the end of the Deal pier, which sticks out hundreds of feet into the English Channel. There Don tossed an apple into the ocean at roughly the same time of afternoon Caesar described the movement of the fleet. Sure enough, the apple drifted southwest toward Dover. No way could an invasion fleet, arriving in oar-powered triremes and other ancient warships, have come up from Dover on that particular afternoon.

To address the second question (the current's direction on the revised invasion date, August 22 or 23), we chartered a sightseeing boat that normally takes tourists around the Dover inner harbor. The skipper agreed to take us well beyond the breakwater, into the open Channel and northward along the white cliffs. To view my not-quite-ready-for-YouTube video clip as we pulled away from the Dover dock, click here (QuickTime player required).

Once we were out in the open sea, the skipper turned off the boat's engine. Kellie and Mandy began noting GPS readings and times, crucial data for determining the current's rate and direction by the drift of our boat. And yes — we were drifting northeast toward Deal. So on that afternoon, with lunar conditions so nearly matching those of 55 BC, the Roman fleet would have had no trouble making its way along the coast toward Deal.

I don't know about the others in our group, but I was starting to feel a little queasy as our small boat bobbed around in the choppy seas. I was glad when we got back ashore at Dover. The Roman fleet, its mission only just begun, had no such option.

For more more about the Texas State researchers' findings, check their press release as well as the August Sky & Telescope.

So You Never Knew All This About Caesar?

Time was when all high-school students translated Caesar's Commentary on the Gallic War in second-year Latin class. You know, the famous narrative that begins, "Gallia est omnis divisa in partes tres...." The other night I dug out my old textbook. (And Mr. Dalton, if you're out there, I saved a great caricature of you that was surreptitiously drawn by a fellow classmate of mine back in 1961!)

Flipping those old pages to Caesar's Book IV, I saw that I'd underlined the words, "Eadem nocte accidit, ut esset luna plena...," meaning "That night, it happened that there was a full Moon...." Caesar was from the Mediterranean, where there is very little tide, and he didn't know that true ocean tides have nearly their maximum range whenever the Moon is full. As a result, his invasion fleet faced unexpected challenges as they looked for a suitable landing beach on the British shore.

Picking up on subtle astronomical clues like these has been a hallmark of the many projects undertaken by Don Olson and his Texas State researchers for past articles in Sky & Telescope — whether in a well-known painting, the original marathon, or a famous photograph by Ansel Adams.

The Roman army's historic landing on the coast of Britain in 55 BC involved perhaps 100 ships and 10,000 men. But this was a rather limited incursion, by Caesar's own standards. Buoyed by the sensation his exploits caused back home, Caesar returned to Britain the following spring (54 BC) with an invasion fleet perhaps 10 times larger. It was like a D-day in reverse.

Caesar's crossing of the Channel, nearly 21 centuries ago, would alter the course of British history forever.