As a response to the thread’s title rather than the OP and the discussion that followed:
Romans had grain ships bringing grain from Egypt to Rome that were several hundred tons and over one hundred feet in length with relatively high freeboard, unlike the low and narrow warships. They could have crossed the Atlantic, although they had almost no ability to tack, so they were at the mercy of the prevailing winds. (I am not sure that they would have stood up to an Atlantic storm.)
Eratosthenes calculated the circumference of the Earth in the third century BC. He got pretty close to the actual size (exactly how close depends on how long you think his stadium was).
Was a round Earth common knowledge, though? Pliny states that the Earth is a sphere in the Historia Naturalis, in the first century AD. On one hand, he states it as an obvious fact. On the other hand, he does feel the need to actually point it out, so there’s that.
I think the consensus is pretty much that most people in Roman times, the village idiot perhaps excepted, would know that you couldn’t fall off the edge of the world (at least those who took the time to contemplate the question). I’d be happy to be corrected, though, if I’m off the mark on that.
You’d be right.
Sailors were, apparently, the first to believe in a spherical earth - by direct observation: at sea, you can see the effect of the curvature of the earth (as a ship sails away from you, if the sea is calm and the weather is clear, you can see the hull dissapear below the horizon, while still seeing the mast). Strabo cites Homer as knowing this.
Sailors would not, in general, fear sailing off the edge. They had a lot of quite reasonable things to fear about sailing into the unknown, though.
Thanks for the replies fellas, sounds like it was technically possible although would require a whole lot of luck.
To my mind, the limiting factor would likely have been water. It is unlikely any ship would deliberately carry enough fresh water for its crew for weeks of sailing out of sight of land.
Say what?
…They were just aping their betters.
And even if they did, the water becomes increasingly foul the longer they are away from port. Of course, you could posit that the crew got lucky with rain and making sail/rain traps that allowed them to get across. It would be a lot of luck though to get the right kinds of storms, the right winds, etc etc to make it across.
They’d have plenty of reasonable things to fear about sailing into the known. :eek:
I love the way this is phrased: “deliberately rowed across”. Was the Pacific the first ocean to be accidentally rowed across? Whoops!
Makes “Wrong Way” Corrigan’s flight look easy!
Very true. 
Pirates, shipwreck, storms, oh my.
Hence, people usually didn’t sail at all, unless they could be assured of massive profits
It’s about 3000 KM from Aden to the Indian Coast, almost 2000 miles. Across seas that are very rough most of the time. They still did it, because of the profits and more to the point they knew there was land at the end of it.
They had no idea what was across the Atlantic.
Plus, of course, they could follow coastline most of the way… or simply set out straight when they knew they could count on a quick turn north took them to land. The longest stretch would be 200km across the Gulf of Oman, or if they were daring, from Oman to where Karachi is, 600km (370 miles) - never far from land.
I agree, the biggest issue was that ships sailing the Mediterranean or the coast of Europe did not normally carry anywhere near the 3 months’ supplies Columbus needed. Even the Portuguese going down the coast of Africa appear to have stopped occasionally for water until they got really adventurous.
The other issue is navigation. Did the Romans and Phoenicians have compasses (lodestones?) Otherwise, long distance navigation headings during cloudy periods would be interesting.
The earliest writings in Europe regarding magnetic compasses date to around 1200AD.
A few people conjecture that Norse sailors used sunstones to locate the sun on cloudy days. Those would postdate the classical period by quite a lot.
Karachi is a touch under 1000 KM from what is now Muscat. Not that it matters since it was not a large port until the 20th century anyway,. While you can follow the coastline that causes your trip to increase severalfold, most ships did go direct.
As it is, the Indonesian sailors and traders who first settled Madagascar early in the first millenium travelled nearly 7000 Km across the Ocean.
I’m curious - what’s the evidence for this?
Here.