Half the journeys on this map are sea voyages, and the ancient Mediterranean was shut for a third of the year. Conybeare and Howson, who wrote their voyage chapter with a serving admiral’s papers in hand, say the sea was "technically said to be open" in March; the closed season they give only inside an untranslated line of Vegetius, from the third day before the Ides of November to the sixth before the Ides of March. ⚠ The reason was not the weather. They quote Smith of Jordanhill: not voyaging after summer arose in great measure from the obscurity of the sky in winter, and not from the gales. With no compass and no charts, a ship steered by sun and stars, and an overcast sky closed the sea more surely than a storm. They also demolish the idea that such ships could not work to windward. A Roman merchantman carried one mast and one great square sail on an enormous yard; it could not lie nearer than seven points off the wind where a modern one managed six — but it could beat, and Pliny says so. With a fair wind behind it the same rig was fast: **seven knots**, which is their own conclusion from the passage times they collect, and which they then apply twice and check. That is the figure to hold against any sea leg here. It is also why a straight line between two ports is a floor and not a distance: the ship sailed the wind it had, in the months it was allowed.
The Life and Epistles of St Paul · vol. I p. 164 and n. 5 (the season opens, Vegetius v.9); vol. II pp. 300–301 (Smith on the winter sky), 301–302 (the rig), 305 (seven points), 306 (seven knots), 321 n. 4 (Vegetius again)