When Julius Caesar arrived off the coast of Britain with his hundred-ship force in August, 55 b.c., he was greeted by a host of defenders poised to hurl spears down on his invading army from the towering Dover cliffs. Seeking a better landing site, he sailed on a strong afternoon current and landed his troops at a beach seven miles away, according to his own account.

Caesar neglected to mention, however, whether he sailed southwest or northeast.

The only shoreline within seven miles of Dover that matches Caesar's description lies to the northeast, near present-day Deal. That would settle it, except that the current flowed southwest from Dover on the afternoons of August 26 and 27 — four days before the full moon, as Caesar obliquely reported the landing date. (It's unknown whether he counted the day of the full moon itself.) For centuries, the paradox has provoked debate among historians and astronomers.

Enter forensic astronomer Donald W. Olson of Texas State University in San Marcos. With a colleague and two honors students, Olson traveled to Britain in August 2007, when astronomical conditions almost exactly duplicated those of 55 b.c. They confirmed that on August 26 and 27, the afternoon current ran southwestward. But on the 22nd and 23rd, it flowed strongly northeastward, toward Deal.

So that's where, and when, Caesar landed.

Could the great warrior have erred by four days? Probably not, says Olson, but his original manuscript is long vanished, and only copies of copies, made centuries later, survive. At some point, Caesar's handwritten VII or VIII — indicating August 22 or 23, seven or eight days before the full Moon—was likely mistranscribed as IIII.

The research was detailed in Sky & Telescope.