We have been at the beach this week on the Georgia coast, and none of us is doing much more than playing in the waves and reading. Anticipating a late high tide and an approaching storm, the boys and I went out yesterday evening for some twilight body-surfing, and after we came back and had a very fine dinner of shrimp and pasta, I began to look over Caesar’s Gallic Wars, which I’m teaching in the fall.
Since we’ll be in England later this summer, I have been particularly focused on Caesar’s account of the invasion of Britain. His first attempt, in August of 55 B.C., began well enough. Employing catapults, Caesar drove back the Britons from the coast at Deal and established a beach-head, where he dragged his ships ashore. Alas, it was not smooth sailing from that point (Gallic Wars, 4.29):
It happened that night there was a full moon, which usually causes great tides in that ocean. This matter was unknown to our men. So, at the same time, the tide began to fill the warships which Caesar had drawn up on the strand, and a storm began to dash the cargo-ships which were tied together at anchor. Our men had no way of managing these things, or of being any help. Many ships were wrecked …
Caesar was a brilliant tactician, but the difference between ocean tides and those of the Mediterrean was not something he was banking on. I guess the tide would count for Caesar as one of those “unknown unknowns” that Donald Rumsfield identified in the context of a more recent invasion.
One cannot read of Channel crossings without thinking of the Normandy invasion, of course, and I was surprised to look at my computer this morning and realize it was June 5th, a day short of D-day. (Vacation tends to make you forget what day it is). I remember coming across an article in Physics Today last fall about the careful consideration of tides in the Allies’ calculations. Rommel was convinced the invasion would come when the water was at its highest, and had planted all kinds of nasty hull-destroying objects along the coast to be hidden by the tide. Knowing this, the Allies planned for a low-tide invasion. Absolute accuracy was key to their planning, and they used a nineteenth-century tide-predicting machine (a sort of analog computer) designed by Lord Kelvin to obtain it.
In our day and age, the iPhone can give you all the accuracy you need about tides. Caesar’s first invasion of Britain would surely have succeeded had he possessed such information, and D-day likely would have failed in its absence. As for me, a knowledge of the tides is of greatest use these days when I decide to look up from my books and screens and head down to the beach with the kids. As, I believe, I will do right now.
Shakespeare, Julius Caesar 4.3.218-224:
There is a tide in the affairs of men.
Which, taken at the flood, leads on to fortune;
Omitted, all the voyage of their life
Is bound in shallows and in miseries.
On such a full sea are we now afloat,
And we must take the current when it serves,
Or lose our ventures.