In their port cities, the Phoenicians saw the mast tips of the returning ships on the horizon. When the boats got closer, they could see the sails and finally the whole ship. At sea, a sailor could see distant landmarks on the mast, that you couldn't see down on deck. So the earth couldn't be flat, as was believed in other parts of the Old World; the Phoenicians concluded, that the earth is a sphere. It was not until many centuries later that the Greeks and Romans used this knowledge, to take measurements at sea.