Now, in order to make a map of one's own country, little astronomical knowledge is required.

As we have seen, savages are able to draw such maps; but when it comes to describing the relative positions of countries divided from one another by seas, the problem is not so easy.

An Athenian would know roughly that Byzantium (now called Constantinople) was somewhat to the east and to the north of him, because in sailing thither he would have to sail towards the rising sun, and would find the climate getting colder as he approached Byzantium.

So, too, he might roughly guess that Marseilles was somewhere to the west and north of him; but how was he to fix the relative position of Marseilles and Byzantium to one another?
Was Marseilles more northerly than Byzantium?
Was it very far away from that city?
For though it took longer to get to Marseilles, the voyage was winding, and might possibly bring the vessel comparatively near to Byzantium, though there might be no direct road between the two cities.

There was one rough way of determining how far north a place stood: the very slightest observation of the starry heavens would show a traveller that as he moved towards the north, the pole-star rose higher up in the heavens.