In 48 B.C.E., Julius Caesar and Pompey the Great were locked in desperate combat. The two generals led huge armies against each other in a civil war to decide the fate of the Roman Republic.

At Dyrrhachium in what is now Albania, Caesar attacked Pompey's supply base on the coast of the Adriatic Sea. Because of the vagaries of the wind, Caesar sent supply ships to several destinations across the Mediterranean Sea to ensure his own troops could be fed and outfitted in the coming campaign.

"For every day a large number of ships was gathering from every quarter to bring up stores, nor could any wind blow without their having a favorable course from some direction," Caesar later wrote in his book "The Civil War."

The reason for all this redundant planning had to do with a problem that has plagued Mediterranean mariners for at least 3,000 years. In the summer, prevailing westerly winds severely hampered the movement of sailing ships loaded with crops and other goods from the east back to Rome.

Yet the flow of food and supplies to the Italian peninsula continued unabated. Historians have wondered for decades how ancient mariners pulled it off.

An Israeli researcher wanted an answer. So first, he did what any academic might: He studied wind patterns and ancient texts about the weather. And then he did something more unusual. He and a team of experts built a replica of a 5th century B.C. boat and sailed it across part of the Mediterranean to test his theory.

The researcher, David Gal, a PhD candidate at the University of Haifa, published the results of his study this summer in the Journal of Archaeological Method and Theory.

"We started with a trivial question: How did Roman ships visiting the Levant return to Rome?" Gal said. "One would simply say, 'Oh, they turned them around and sailed the other direction.' However, a windward journey was not practical in the kind of ships they used. So how did they accomplish these voyages?"

Gal believes these superannuated seafarers took advantage of brief reverses in wind patterns to sail to Rome and other western destinations. In addition, by examining Roman and Greek texts about the weather, he discovered that those breeze cycles are virtually unchanged over the past three millennia.

Gal said the sailors' lives depended on anticipating weather patterns, so they knew when to begin a journey and when to find a safe port. They often waited days before catching the right winds to begin or resume travel.

"There's an ancient story of two friends who are departing and going in opposite directions," Gal said. "The blessing they give each other is, 'May the Gods grant us both favorable winds,' which is a contradiction. Waiting for favorable winds was a big part of ancient seafaring."

To understand how mariners managed to make their way across the Mediterranean, Gal and other researchers undertook a two-step process. First, they built a replica of a typical boat that sailed the sea between Europe and Africa three millennia ago, which they named Ma'agan Mikhael II. Its design was based on a shipwreck discovered off the coast of Israel in 1983. Rigged with a square sail, the new version was built by a team of experts led by Yaacov Kahanov, professor emeritus at the Department of Maritime Civilizations of the University of Haifa.

"It's an exact replica of a 2,400-year-old ship," Gal said. "We've learned a lot from sailing it, including the difficulties of windward sailing." They sailed from Israel to Cyprus with a crew of six over the course of 74 hours in 2018.

The second phase of the study involved understanding the weather. In addition to reading 3,000-year-old texts, Gal reviewed modern records of the winds and waves around the Mediterranean. He collected data points from 7,000 different locations, taken every hour over the past 15 years. He compared these findings with the ancient data and made a surprising discovery.

"The wind and wave oscillations are the same as they were 3,000 years ago," he said. "Once we could establish that modern winds equal ancient winds, we could use the data to analyze sailing mobility. We were able to look at routes the ships took with grain from Alexandria in Egypt, and found that in July and August they had to first sail northeast toward Turkey instead of west toward Rome."

Gal found that ancient vessels were able to locate brief breezes blowing to the west that usually occurred in the early mornings and late evenings. Those light airflows would enable the ships to sail for a short time toward Rome. Once the winds stalled, the crews would drop anchor and wait until they started again.

Gal cited the biblical example of Paul the Apostle. The New Testament records how he was transported from the town of Caesarea in Judea to Rome for trial by Emperor Nero on charges of sedition. The Acts of the Apostles records a protracted trip that involved several vessels.

"It could take weeks to make the journey from the Levant to Rome," Gal said. "Mariners did a lot of waiting in those days."

Using computers, Gal crunched all the numbers - old and new - to run cruise simulations. He discovered hundreds of possible trade routes ancient seafarers may have used to crisscross the eastern Mediterranean during the summer months when winds were unfavorable.

Modern mariners can tack against the wind by setting the sails at sharp angles. That wasn't feasible 2,400 years ago because sails were fixed then.

Gal spent 20 years as a pilot in the Israeli Air Force before taking an interest in sailing and meteorology. He said this research offers new insight into the complexities of sailing in ancient times and the impressive knowledge base of the sailors who plied those waters.

"In the summer, they had no option but to crawl their way across the Mediterranean and then start moving westward very slowly," he said. "Coastal sailing was difficult and dangerous. You might sit for 10 days waiting for a favorable breeze. It took tremendous expertise to do what they did back then."