A popular post on /r/todayilearned claims that Julius Caesar was able to defeat Pompey due to his superior knowledge of the Roman calendar. Specifically, that due to the calendar drift, months in which it was thought unsafe to sail were actually fine to sail in. This knowledge, according to the Reddit post, gave Caesar a military advantage that helped him win the war.
On the face of it, this sounds like absolute BS to me. Farmers (and the nobility who held farmland) would know the seasons even if a formal calendar had drifted. Sailors would be caught at sea at the wrong times. Calendar drift happens slowly enough that people would notice.
Is there any truth to this story?
Link to post: https://www.reddit.com/r/todayilearned/comments/zgeetb/til_that_caesar_used_the_knowledge_that_the/
Yeah, this story is utter BS. It doesn't show up in any actual academic sources, and just gets passed around the Internet from place to place.
For one, in this instance, we don't actually know if the dates used are relying on the older calendar, or if they've been updated with later reforms, so even saying that Caesar set out in mid-winter would be contested.
The first thing to remember, and the thing that so many people seem to forget is that ancient people weren't morons. They didn't go "Well, it's warm and temperate outside, but this calendar says that it's cold and inhospitable, so my senses must be wrong". People depended on the seasons for their livelihoods, from farming to sailing. If Caesar knew, a lot of people knew.
There's also just the fact that the idea of Romans ceasing all maritime travel during the winter is a myth. They had experienced and skilled navigators who could handle it, and the winter winds were actually better for certain kinds of ships, especially those Caesar used. Caesar's tactic would have been completely understood at the time.
Not to mention, the reason that Bibulus drew most of his ships into port wasn't just because he thought the weather was too dangerous to sail. Caesar's plan had relied on amassing a fleet up ships, but he had been unable to do so, all while being tied down with political struggle. Bibulus then saw no reason to expose his ships to the risk of being on constant patrol at open sea, exhausting his men, so he had many of them in port. Galleys could be used in winter, but the prolonged usage of them increased risk. That's why Caesar's plan only relied in a brief voyage.
Probably the best evidence against this was that Caesar discovered a large convoy of Pompey's grain ships with eighteen galleys in Oricum. If Pompey genuinely didn't know it was safe to sail, he'd never have risked such a vital supply mission, along with a large number of valuable ships. This is also supported by the fact that Bibulus reacted extremely quickly to Caesar's maneuver, and set out within days, managing to catch and destroy some of Caesar's ships.
Sources:
This is a good paper. Primary sources include Caesar's own Civil War and Pharsalia (which is more of a dramatic narrative, but still has some general basis in fact).