Lacking instruments​—even a compass—​navigators in the first century operated strictly by sight. Therefore, travel was the safest when visibility was the best​—generally from late May to mid-September. During the two months before and after that time, merchants might chance sailing. But during wintertime, mist and clouds often obscured landmarks and the sun by day and the stars by night. Navigation was considered closed (Latin, mare clausum) from November 11 to March 10, except in cases of absolute necessity or urgency. Those traveling late in the season ran a risk of having to winter in a foreign port.​—Acts 27:12; 28:11.

Paul was evidently aware of the dangers of out-of-season sea travel. He even advised against sailing in late September or early October, saying: “Men, I perceive that navigation is going to be with damage and great loss not only of the cargo and the boat but also of our souls.” (Acts 27:9, 10) However, the army officer in charge ignored these words, and this resulted in the shipwreck at Malta.